As the coupling of hydrodynamic fields in petroliferous basins, a geotemperature-geopressure system (Geo-TPS) directly controls the migration and accumulation of oil and gas. Studies have showed that characteristics of the Geo-TPS in China differ greatly from one petroliferous basin to another. Based on the relationship of Geo-TPS slopes in shallow and deep formations, three types of Geo-TPS models were put forward: the high-pressure multiple Geo-TPS (HP-MGeo-TPS), the low-pressure multiple Geo-TPS (LP-MGeo-TPS) and the single Geo-TPS (S-Geo-TPS). As for the HP-MGeo-TPS, the slope range of the Geo-TPS is narrower in shallow formations but wider in deep formations. On the contrary, as for the LP-MGeo-TPS, the slope range of the Geo-TPS is wider in shallow formations but narrower in deep formations. And as for the S-Geo-TPS, the slope range of the Geo-TPS is relatively concentrated and regionally identical in both shallow and deep formations. A wider slope range indicates a big variation of the Geo-TPS with different basins, while a narrower slope range means a small change of the Geo-TPS with different basins. The difference of Geo-TPS characteristics suggests that a vertical connectedness in subsurface is better in the S-Geo-TPS than that either in the HP-MGeo-TPS or in the LP-MGeo-TPS. A poor connectedness may lead to an upward migration tendency of deep fluids in the HP-MGeo-TPS but a downward migration tendency of shallow fluids in the LP-MGeo-TPS along vertical fractures. Statistic data show that in China, both the HP-MGeo-TPS and the LP-MGeo-TPS develop well in extensional petroliferous basins; the HP-MGeo-TPS is more common than the LP-MGeo-TPS in compressional basins; and either the LP-MGeo-TPS or the HP-MGeo-TPS occurs in craton basins.

Oil-gas migration is regarded as one of the most difficult problems encountered in researches of petroleum geology in the world and currently a frontier subject as well. Faults are an important pathway for oil-gas migration in rift basins. A thorough analysis of the success or failure of many exploratory wells in the Bohai oil province demonstrated that oil-gas migration is inefficient simply depending on faults, only in concert with sand bodies within source rocks serving as an “oil-gas transfer station”, can large faults provide traps above with an ample source of hydrocarbons. In sags, small faults without “oil-gas transfer station” at root can only form small oil reservoirs, while vertical strike-slip faults without “oil-gas transfer station” at root can not form oil reservoirs in upside. It will greatly improve the success ratio of exploration to make further studies on oil-gas migration capacity of faults.

Mid-deep lacustrine organic-rich shales in Member 2 of the Liushagang Formation (LS2) are the most important high-quality source rock in the Beibuwan Basin. These dark-grey or-brown shales with developed lamellation contain abundant non-marine dinoflagellate, rugasphaera and leiosphaeridia fossils, they are rich in organic matter, with TOC content ranging between 3.07%～10.35%, type II1 and I kerogen and Pr/Ph ratios varying from 0.6 to 2.5. The shales are characterized by a high abundance of 4-methyl steranes that are probably related to certain dinoflagellates thriving in lakes, relatively abundant pentacyclic triterpanes, tricyclic diterpanes and tetracyclic terpanes but a very tiny amount of oleanane indicative of terrigenous higher-plant input, and a “V”-shaped distribution of C₂₇, C₂₈ and C₂₉ regular steranes. All these characteristics indicated that the organic matter in those shales was mainly of algal origin. The data of various algal fossils, especially plenty of non-marine dinoflagellates and amorphous organic macerals suggested that they were formed in an anoxic and stratified environment of mid-deep brackish lakes in humid climate, where organic-rich shales of the upper LS2 occurred at the top of a high-stand system tract and those of the lower LS2 were distributed at the bottom of a transgressive system tract. In this area the sedimentation rate for Member 2 of the Liushagang Formation (LS2) was relatively low and the lake remained as a starved basin, meanwhile the bloom and death of algae near surface supplied a great deal of organic matter to lacustrine sediments, and the oxygen-depleted bottom water of palaeo-lakes, due to the water-column stratification, favored the accumulation and preservation of sedimentary organic matter, resulting in the formation of organic-rich shales, a high-quality source rock.

GC/MS analytical data of 60 saturated and aromatic fractions revealed that saturated hydrocarbons from Pinghu Formation coal-bearing source rocks in the Xihu Depression of the East China Sea Shelf Basin are characterized by abundant diterpenoid compounds originated from resins of coniferous trees and dominant C₂₉ in steranes（>45%）. Moreover, aromatic fractions are abnormally rich in terrigenous biomarkers, such as retene and 1,7-dimethylphenanthrene, indicating a predominant source of organic matter derived from terrigenous higher plants. Compared with coals and carbonaceous mudstones, dark mudstones show a relatively higher content of C₂₇ sterane and a bimodal distribution in n-alkanes, suggesting a certain proportion of aquatic organism contributions in the organic matter source. Source rocks as a whole are believed to be deposited in an oxic limnetic-facies environment, as reflected by high Pr/Ph ratios ranging mostly from 3.5 to 8.5 and abundant dibenzofuranes. However, some dark mudstones show relatively lower Pr/Ph values (round 1.5), suggesting a weakly reducing to suboxic semi-deep lake setting. Coals and carbonaceous mudstones can be distinguished from dark mudstones by a higher relative concentration of C₂₉  sterane (>80%) and lower gammacerane /C₃₀ hopane ratio (<0.05).

The lower sub-segment Es₁ in the Qikou Sag formed a specific lithology of several sets of thin bed of dolomite developed in dark sandstone during the overall expansion of the lake, which showed a strong brackish lacustrine environment. The author studied the trace element of different lithology and the results tasted that the intended interval is mainly the saltwater atmosphere with large salinity. The carbon and oxygen stable isotope test displayed that the carbonate δ¹³C tended to positive anomaly and the  Z  value distributed between 111~168.52; the allocation for strontium stable isotopes is slightly higher than the same period sea and far lower than the river and lake; the study indicated that brackish lake basin maybe caused by marine transgression after combined with the diagnostic glauconite and collophane found in the phase and the calcareous nannofossils discovered in the SEM.

Deep coalbed methane (CBM) is a new area in unconventional natural gas exploration of China.Thus, the paper analyzed geological conditions and basic principles of deep CBM reservoirs and discussed the specificity of deep CBM-accumulation effects in three respects including the state transition of deep geo-stress, the negative effect of geothermal field on deep coal adsorptivity, and specific physical properties of coals under temperature and pressure conditions of deep strata. The results show that the critical depth for deep geo-stress state transition is related to the maximum horizontal principal stress, which affects the permeability of deep coal reservoirs in various degrees. The negative effect of deep geothermal field on coal adsorptivity is greater than the positive effect of formation pressure, resulting also in a critical depth for deep CBM content, and the deep CBM content can not be simply predicted using the shallow CBM content gradient. Confining pressure is a major factor that affects mechanical properties of deep coal reservoirs, and the effect of temperature and fluid pressure on mechanical properties is rather complex, it profoundly affects the porosity, permeability and absorptivity of coal reservoirs. As a result of differences in the seepage capability of various surrounding rocks, the fluid pressure system of deep coal seams is significantly controlled by the sedimentary framework of coal-bearing strata, which may lead to different gas-bearing systems for coal and non-coal reservoirs even in the same set of coal-bearing strata. On this basis, an idea named as the four-step hierarchical coupling analysis on deep CBM-accumulation effects was proposed and it provided a basis for the establishment of an optimum method to seek deep favorable CBM zones.

Large quantities of natural gases have been found in the Upper Triassic Xujiahe Formation of the Sichuan Basin in recent years. At the same time, formation water with high salinity was also found widespread in the Xujiahe Formation and it contains not only the particularity of constant ion development, but also particularity of stable isotopes and trace element distributions. This paper established an evaporation-fractionation trend line of stable isotopes for the Jurassic to Permian systems in the middle-western Sichuan Basin, and preliminarily determined standards to discriminate origins of formation water in the study area. Influenced by gypsum-salt layers in the Leikoupo Formation, the isotopes evaporation-fractionation evolution of formation water was characterized by the dominance of a rapid increase of δD and δ¹⁸ O values, respectively, for Jurassic to the 3rd member of the Leikoupo Formation and the 2nd member of Leikoupo Formation to Permian. The formation water in the Xujiahe Formation was identified to be a metamorphic mixture of the high-salinity formation water from the upper Leikoupo Formation and the depositional water excreted by compaction after the burial of the Xujiahe Formation. Meanwhile, some gasfield water of the Xujiahe Formation was found to contain a considerable amount of condensate water, and the presence of this condensate water might also indicate a strong uplift of the Xujiahe Formation and some gas-accumulation events by exsolution of water-soluble gases.

Ordovician carbonate rocks in the Halahatang area of the northern Tarim Basin are important karstic reservoirs, where their reserving space is mainly corroded cavities and fractures formed by multi-episode tectonic movements. Based on the analysis of cores from six typical drilling wells, we studied genetic types, developmental episodes and characteristics of fractures in carbonate rocks of the Yijianfang Formation in the Halahatang area and their influences on the development of reservoirs. There are mainly five types of fractures in carbonate rocks, including near vertical, oblique crossing, horizontal, netted and undirectional fractures, of which the former two types of fractures are of structural genesis, the horizontal and undirectional fractures are diagenetic fractures while the netted fracture is of superimposed origin. All the fractures developed mainly in the Middle Caledonian, Late Caledonian, Early Hercynian and Middle-Late Hercynian structural periods. Most of them are effective fractures because of incomplete filling except for the undirectional fracture formed in the Middle Caledonian that is completely filled with sparry calcite. The development of fractures not only increases effective reserving space of carbonates and connects isolated caverns, but also further benefits the fluid migration and late alteration of reservoirs. A netted system of various fractures in carbonate rocks has effectively improved the porosity and permeability of reservoirs in the Yijianfang Formation.

Calculation formula for normal stress and shear stress on fault plane in a triaxial crustal stress state were deduced through establishing mathematical models, and three parameters: fault sealing coefficient, fault tightness index and fault shearing index were taken as indexes to quantitatively evaluate the fault sealing ability. Analysis results demonstrate that the fault sealing ability shows better when the fault sealing coefficient and fault tightness index get greater, while the relationship between the fault shearing index and fault sealing ability is relevant to the property of normal stress that acts on fault plane. When normal stress is a compressive stress, the greater the fault shearing index, the better is the fault sealing ability, while the opening extent of faults gets greater with the increase of the fault shearing index when normal stress is an extensional stress. Moreover, the influence of the magnitude and direction of crustal stress on the fault sealing ability was analyzed. The result indicates that when the included angle between fault strike and the maximum horizontal principal stress changes, both the variation trend of normal stress and shearing stress on fault plane and its influence on the fault sealing ability are contrary, thus in practical application this influence should be synthetically considered. Furthermore, the fault sealing ability of compressive and compresso-shear faults is superior to that of extensional and tenso-shear faults. Numerical simulations of the Tertiary tectonic stress field and analysis of the fault sealing ability in the mid-northern Raoyang Depression demonstrated that the Sha-2 and Sha-3 members should be major targets of exploration, and the Sha-1 member and the Dongying Formation came next. This conclusion is consistent with the exploration and development practice in the Raoyang Depression.

A shattered fault zone is defined as a geological body composed of fractures derived from fault movement and fragment fillers between two walls of a fault, it can be identified easily from outcrop but difficultly in subsurface. Based on repeated comparisons of well logging and related geological data, the authors identified 4 types of shattered fault zones in the Dongxin Oilfield of the Jiyang Depression, i.e. ①the symmetrical shattered fault zone with a configuration of induced fractures-fragment fillers-induced fractures, ②the unsymmetrical shattered fault zone with a configuration of induced fractures-fragment fillers, ③the incomplete shattered fault zone with either induced fractures or fragment fillers, and ④the complex shattered fault zone without fixed induced fractures or fragment fillers. A key coefficient （F_dr） was used successfully in the identification, namely if  F_dr is≥0.35, it is an induced fracture and if F_dr  is <0.35, it is a fragment filler. By examining structural and distributional characteristics of shattered fault zones with different sizes in the Yong-3 block, we found that the thickness, petrophysics and displacement pressure of induced fractures and fragment fillers were key factors for the fault sealing property. The distribution of shattered fault zones was then mapped and applied to the allocation of 3 horizontal wells so as to recover the remaining oil in fault blocks.

Steam channeling of high-permeability layers in the cyclic steam stimulation stage in heavy oil production will affect the development results. The development with horizontal wells in heavy-oil reservoirs can enhance an ultimate recovery by increasing the drain area and flow conductivity, however, it is more susceptible to water channeling, resulting in some development problems including uneven production of different sections and high water cut in horizontal wells. We applied a three-dimensional physical model in studying the enhanced oil recovery mechanism of steam nitrogen-foam flooding with a vertical and horizontal well combination and evaluated the development effect of temperature fields after transformation from steam flooding to steam nitrogen-foam flooding by using plane data and vertical 3D temperature field profile data of different flooding stages. The results showed that water cut first rose then fell down when the production under high water cut conditions was assisted by steam nitrogen-foam flooding, and the ultimate recovery was 63.3%, a net increment of 6.2% compared to that by steam flooding only. Therefore, steam nitrogen-foam flooding can reduce phenomena including steam breakthrough of an individual well and steam overlap, increase swept region between wells, enlarge the holistic swept volumes and improve the effect of steam flooding.

The PVT test method of foamy oil is different from that of conventional oil because there is a process in foamy oil during which gas bubbles are continuously generated and burst successively under different pressure conditions, therefore, the PVT test of foamy oils is a non-stable test. The measured PVT behaviors of foamy oil are quite variable under different pressure depletion rates. In order to characterize PVT behaviors of foamy oil under different operating parameters and depict PVT behaviors during a real production process of reservoirs, experimental analyses on key PVT parameters of foamy oil were carried out. A series of experiments on PVT behaviors were carried out using a Venezuelan extra-heavy foamy oil sample, which revealed a pseudo bubble point behaviors during foamy oil production, and a set of non-routine tests for PVT parameters of foamy oils was summarized. This approach can achieve accurate measurements on PVT behavior parameters under non-routine conditions and describe their influence factors as well.

Thin intercalations and internal structures within mono-sandbody play a more and more important role in injection flooding and remaining oil generation. Based on depositional models of subaqueous distributary channels in the fluvial-dominated delta, we established an internal structure model for subaqueous distributary channels in the fluvial-dominated delta and figured out a three-dimensional distribution mode of intercalations in a single subaqueous distributary channel using data of outcrop photos and well sections. The results show that there are three kinds of intercalations within subaqueous distributary channels. The first is a lateral accretion intercalation that is formed when channels shift laterally, the second is a foreset intercalation that is formed when channel progradation occurs in a front slope, and the third is a vertical accretion intercalation that is formed when channel accretion occurs vertically in a flat area. These different intercalations may objectively reveal a three-dimensional distribution mode of basic units within subaqueous distributary channels and veritably reflect internal structures of subaqueous distributary channel sands in the fluvial-dominated delta.

Water coning is an important issue in the development of bottom-water-drive gas reservoirs. Physical models for calculating critical production rates generally assume that formation fluids are Darcy flow in reservoirs, which ignores the pressure drop caused by non-Darcy flow. However, the fluid flow velocity near wellbore areas in high production gas wells is so great that the pressure drop caused by non-Darcy flow should not be ignored. We investigated the influence of non-Darcy flow on water coning in models for calculating critical production rates and derived a new calculation model with full consideration of non-Darcy flow by introducing a pseudo-skin factor. This model avoids complex calculations of the non-Darcy flow radius, adopts numerical simulation methods to quantify the influence of non-Darcy flow, and calculates critical production rates with full consideration of the non-Darcy flow bottom-water coning through establishing numerical models for low-, middle-and high-permeability gas reservoirs driven by bottom water, respectively, simulating deliverability testing processes and upbuilding deliverability equations. The calculation results indicated that the influence of non-Darcy flow on critical production rates could be ignored in a low-permeability gas reservoir, while an error of the critical production rate without consideration of non-Darcy flow reached 14.46% when the permeability was 30 mD, the perforation thickness was 208.3 m and the water avoidance thickness was 31 m. Therefore, a model for calculating critical production rates of water coning with consideration of non-Darcy flow should be applied so as to be more accordant with the actual flow law of reservoir fluids when the permeability is higher than 30 mD.

The gravity percolation mechanism was studied by establishing gravity percolation models including the mathematical model and mechanism model of layered reservoirs. The results show that the production by gravity drive is directly proportional to the permeability, reservoir thickness, fluid density of strata and the square of sine value of stratum dip, but inversely proportional to the fluid viscosity of strata. Gravity percolation occurs mainly in an area where the included angle to stratum dip is under 60°. Moreover，the bigger the angle，the weaker the percolation. The mechanism model indicates that the gravity percolation of water phase becomes obviously strong when the permeability is greater than 10 mD and stratum dip exceeds 5°. The injected water intrudes severely to a low position, so the water injection mode must be optimized to improve the development effect of such kind of reservoir. The practical application to typical fault blocks of the Hailar Basin showed that this approach could be used to both evaluate gravity energy and optimize water injection modes.

Based on Green’s function and the Newman superposition principle, we established a transient reservoirs/wellbore coupling model for fractured horizontal wells with consideration of fluid inflow from both base rocks and hydraulic fractures to wellbores and developed a new solution method combining QuasiNewton method and Particle Swarm Optimization (PSO) algorithms, which contains merits of the both mothods with great speediness and accuracy in solution. This model could accurately compute the deliverability of horizontal wells with or without hydraulic fractures. The computational solution for a real case was compared with the published previously. The influence of various factors that affect the production performance of fractured horizontal wells was analyzed and effects of fractures on the distribution of pressure and flux rates in wellbores were investigated. The results indicate that the permeability of hydraulic fractures is a major factor that affects the source of fluids in wellbores, the flux comes mainly from hydraulic fractures when the permeability of hydraulic fractures is very high, otherwise it comes mainly from base rocks.

In order to solve the negative skin problem of horizontal wells, we established a theoretical model for the transient well test of horizontal wells and for the analysis of production decline based on traditional analysis methods. The model was solved with modern mathematical methods including the Laplace transformation, separation of variables, eigenvalue and eigenfunction. Expressions for a dynamic response to bottomhole pressure in the Laplace space and for production decline rates were obtained. Finally, type curves of typical well test and the Blasingame production decline rate analysis were programmed and plotted, and flow stages were divided so as to analyze characteristics of type curves. In addition, type curves affected by negative skin were especially plotted and they were no longer oscillatory, indicating a successful settlement of the negative problem of horizontal wells.

It is common to calculate rheological parameters of a non-Newtonian fluid based on shear rates obtained by the Newtonian fluid formula, which gives a calculated result with considerable errors that goes against either an accurate description of rheological properties of drilling fluids or a high-precision hydraulic calculation. Therefore, it is necessary to amend shear rates derived from the Newtonian fluid formula when they are employed in a rheological parameter calculation of non-Newtonian fluids. Based on a constitutional equation of the four-parameter rheological model, we obtained an expression for rheological laws and shear rates of fluids in a horizontal rotation movement by means of the theoretical inference of power series. A calculation procedure of rheological parameters was programmed, in which shear rates are amended according to the iteration of shear-rate calculated modes and then rheological parameters are calculated through the regression of revised shear rates. This calculation model was rheologically analyzed and evaluated in terms of hydraulic calculation by utilizing various drilling fluid data and a set of measured hydraulic data. The results show that an unrevised shear rate shows a larger error, while rheological parameters obtained from revised shear rates can describe the rheological behavior of drilling fluids better. The result of hydraulic calculations obtained by using rheological parameters derived from revised shear rates shows a higher calculation accuracy of pressure drawdown by 0.5MPa, more consistent with measured data by the attached ECD at down hole, thus, this approach is of definite theory and application values.

The riser hang-off window is an extremum condition of environmental load to allow the riser hang-off operation, which is essential to the field operation. Here we put forward a method to analyze operating windows for soft or hard hang-off modes of the riser, which includes to establish a finite-element model for the riser axial dynamic analysis and to examine riser hang-off windows under different oceanic conditions based on the restraint criterion of riser hang-off operations. Analyses of operating windows of wave height and periods show that the hard hang-off window is jointly affected by the vessel heave movement and wave periods, and it gets narrower when the wave period falls into a range of 10~16 s or 20~24 s. The recommended vessel heading for the hard hang-off mode survival in case of extreme weather should be at a 45° wave direction angle. The soft hang-off window is restricted mainly by the telescopic joint stroke and it keeps approximately identical, independent of the wave direction angle. Analyses of operating windows of wave and oceanic current indicate that the maximum allowable wave height for the riser hang-off operation gets smaller when the oceanic current velocity increases. A comparison of operating windows between the two hang-off modes shows that the soft hang-off mode can significantly expand the riser hang-off operating window.

The borehole wall instability of hard-brittle shale formations is a very complex problem because after the invasion of foreign fluids, rocks will have a series of internal microscopic physical and chemical changes that affect the stability of formations. Here we proposed a new approach to studying the borehole wall instability of hard-brittle shale formations, which can microcosmically reveal the development law of cracks and the damage to rocks during a hydration process of hard-brittle shales by means of CT imaging technology, analyze its mechanism of and effects on the borehole wall instability with a combination of scanning electron microscope and other instrumentations, and then set down a practicable countermeasure for wall stabilization. The results show that hard-brittle shale has a notable capillary effect, by which spontaneous imbibition of micropores among particles of rock matrix will result in a strong hydration that promotes the generation, expansion and connectivity of secondary micropores. The continuous development of micropores into cracks till penetrated fracturing through rocks will bring about macroscopic damage to rocks. Therefore, one of the major causes that lead to the instability of brittle shale formations is the damage of secondary micropores generated from the hydration by spontaneous imbibition. Thus, intensifying plugging, controlling filter loss and reducing the surface tension of filtrate are effective measures to prevent hard-brittle shale formations from the hydration damage by spontaneous imbibition.

In order to study the fluid flow behavior in hydrocyclone, the whole flow field of fluids in hydrocyclone with a bitangential inlet and twin cones was measured with the laser particle image velocimetry (PIV), then the Tecplot was utilized to display the flow field, extract the tagential axial and radial velocities, calculate the vorticity, and plot the locus of zero vertical velocity (LZVV). Distributional patterns of tangential and radial velocities in the swirl chamber of hydrocyclone as well as the axial velocity, local vorticity and locus of zero vertical velocity (LZVV) of fluids within the upper cone section under various flow conditions were investigated. The results show that in the swirl chamber, the tangential velocity of fluid is distributed in a shape of centrally symmetrical concave parabola, and its rotary momentum is mainly concentrated on the wall and at gas column, while the radial velocity shows an asymmetric “double-M-shaped” distribution with a velocity at the gas column bigger than that on the wall. In the upper cone section, the axial velocity of fluid has an irregular “M-shaped” distribution, inconsistent with the flow rate but fluctuating with its increase, the local vorticity of fluid shows an irregular “double-M-shaped” distribution with a absolute value of the negative vorticity getting larger near to the swirl chamber wall, the LZVV is somehow approximately like an irregular conical surface, and both the shape and the distributional locality of the LZVV are affected by the fluid flow rate within the bitangential inlet and by viscosity of the medium used.

Most of the existing hydrate prediction models can not describe the formation process of hydrate slug in gas pipelines. We proposed a physical model for the hydrate formation state in gas pipelines based on distributional characteristics of gas and liquid in pipelines, established a mathematical model to predict the formation of hydrate slug in gas pipelines by means of the heat-transfer, multiphase-flow and phase-equilibrium theories, deduced definite conditions to solve the related problems, and finally described the formation process of hydrate slug by numerical simulation. The results show that the model can forecast the formation position, configuration and changing process of hydrate slug successfully. The gas flow rate, and the thickness and the thermal conductivity of thermal-insulating materials can significantly affect the initial formation region of hydrate. With the increase of gas flow rates, the liquid-carrying capacity of gas cores will get better and the thickness of liquid film will reduce. Furthermore, a series of changes in parameters will take place during the formation of gas hydrate slug, such as the increase of pipeline pressure differences, the decrease of temperatures, the enhancement of liquid-carrying capacity of gas cores and the decrease of liquid film thickness, all of these may, in return, react on the further formation of hydrate slug.

On the basis of the extremum statistical principle, a method to evaluate the overhaul interval of crude oil tanks was put forward, which includes statistical analyses of corrosion inspection data, the determination of corrosion rates and the assessment of serviceable life of on-line maintenance. A statistical analysis of corrosion inspection data for 18 in-service tanks of a crude storage was then made using this proposed evaluation method. Corrosion rates of tank floors and walls were statistically determined and their serviceable life was assessed. The results show that the serviceable life of the most tanks exceeds 8 years, and thickening the floor of tanks or taking other measures can extend their serviceable life. Based on these evaluation results, we recommended that the overhaul interval should be extended, especially the second and subsequent overhaul interval should be extended from 5~7 years specified in Standard SY/T 5921 to 8~10 years.

A damage caused by alternating current (AC) corrosion on buried metal pipelines can not be ignored because alternating current may not only result in corrosion on metal pipelines through stray current but also generate an alternating electric current within metal pipelines by induction, which creates alternating current interference. Moreover, risks of AC corrosion or overprotection on buried metal pipelines are increasing with the increase of laying coated pipelines in proximity to AC powered rail transit systems or high voltage AC power transmission lines. Here we made a systematic review on AC corrosion researches of recent years in terms of the experimental study, theoretical analysis and corrosion mechanism, respectively. It is generally believed that a higher alternating current could lead to a higher risk of AC corrosion. The results of laboratory experiments on the influence of AC interference show that an alternating electric current can not only accelerate metal corrosion, destroy the passive film and induce pitting corrosion, but also interfere with the normal operation of a cathodic protection system due to a sharp increase of the cathodic protection current, a severe disturbance of the protection potential and even an anodic polarity reversal. Mechanisms of AC corrosion are very complex and people have made great attempts to explain these complicated processes through theoretical analyses and experimental simulations. There are only a few typical hypotheses on mechanisms of AC corrosion proposed so far and many difficult problems remain to be unsolved though studies on AC corrosion may date back to the late 19th century. In addition, we discussed some key problems of AC corrosion in details, predicted the research prospect and development trend of this frontier, which may be a help to researchers involved in the related domains.

It is an inevitable trend for oil and gas industry to transform exploration & development domain from conventional hydrocarbon accumulations to unconventional hydrocarbon ones, which are obviously different in types, geological features and genesis. Conventional petroleum focuses on accumulation mechanism, and the key answer is whether the trap contains petroleum, otherwise, unconventional petroleum focuses on the reservoir space, and the key answer is how much the reservoirs capture petroleum. Unconventional hydrocarbon resources are mainly characterized by continuous distribution and no natural oil and gas production from per well. Currently, unconventional problems occur in the exploration and development of conventional hydrocarbon resources, thus it is necessary to transform unconventional hydrocarbon resources into new “conventional” hydrocarbon resources. With technology development, unconventional hydrocarbons can be transformed into conventional ones. Generally, conventional hydrocarbon deposits consist of structural and litho-stratigraphic hydrocarbon reservoirs, where oil and gas are distributed in an isolated structure or a larger structure group with clear trap boundaries and pore-throat systems in millimeter-micrometer scale. Oil and gas in this case accumulate by buoyancy to form hydrocarbon pools. However, unconventional hydrocarbon accumulations, including tight sandstone oil and gas, tight carbonate oil and gas, shale oil and gas, coal-bed methane, oil shale, oil sand, hydrate, etc., are distributed continuously or quasi-continuously in basin's slopes or centers. Commonly, they are characterized by source-reservoir paragenesis and have no distinct trap boundaries. Pore-throat systems in nanometer scale are well-developed in unconventional reservoir rocks, and related hydrocarbons are mainly detained in situ or migrate for a short distance into reservoirs that are close to source rocks because buoyancy is limited. The present paper systematically analyzed geological characteristics and exploration potential of tight hydrocarbons in some typical basins of China, where pores in nanometer scale with partial micrometer-millimeter pores dominate the reservoir space of unconventional hydrocarbons, the diameter of reservoir pores is 5~200 nm in gas shale, 40~500 nm in tight oil limestone, 50~900 nm in tight oil sandstone and 40~700 nm in tight gas sand. In terms of the rapid development of globe petroleum industry and nano-technology, a concept of nano-hydrocarbons is proposed in this paper that indicates that “nano-hydrocarbon” is the development direction of oil and gas industry in the future, urgently requiring developing vicarious technologies, such as nano-hydrocarbon perspective viewing mirror, nano-hydrocarbon displacement agent and nano-hydrocarbon exploitation robots. Petroleum intellectualization times will come in following.

A sand injectite phenomenon is recognized in the Zhujiangkou Basin of the South China Sea through an analytical comparison of core, well logging and seismic data with those reported abroad. These sand injectites are characterized by mudstone debris in different scale and various structures including liquidized deformation, dewatering, folding and filling of sandstones. Their Gamma curves are almost box-shaped and the dipmeter logs are unorderly, casually with a high-and-steep dip. Thus, this phenomenon in seismic profiles indicates a winglike or coniform intrusion, which, however, exclusively suggests sand injectites on a relatively large scale. As for the recognition of small-scale dikes and sills from deposited parent sandstones, seismic profiles with a higher resolution are needed.

The present paper studied characteristics of the fault development and activities in the Beibuwan Basin and discussed control factors of the fault formation and evolution on hydrocarbon reservoirs. The study showed that four types of faults developed in the Beibuwan Basin, including the early rift fault, middle strike-slip extension fault, late extension fault and long-term active fault. In Cenozoic, the Beibuwan Basin experienced both evolutionary stages, i.e. the rifting stage in Paleogene and the subsidence stage in Neogene. There were three major periods of fault activities in the extensional rifting stage. There formed NE-and NEE-direction extension faults during Paleocene and Eocene. In Oligocene, NEE-direction strike-slip extension faults and approximately EW-direction sub-faults were formed under the influence of strike-slip extension. The formation and evolution of faults controlled the distribution of hydrocarbon generative depressions, accelerated the thermal evolution of source rocks to generate various traps of hydrocarbons and provided major pathways for hydrocarbon migration. Zones with long-term active faults adjacent to hydrocarbon generative depressions and central uplift zones in depressions are the most favorable area for hydrocarbon migration and accumulation.

Buried hills in the Shaleitian bulge of the western Bohai offshore are composed of Archaeozoic granite, which is immediately covered by Neogene fluvial clasolites. In this area there is a well-developed NE dextral strike-slip fault that stretches for about 20km and slantingly cuts through the whole Shaleitian bulge. The angle between the NE and NW strike-slip faults within the buried hills is about 100°. The major NE strike-slip fault keeps unceasingly active, while NW faults being no longer active in Neogene extend discontinuously with partial segments displayed as channels. Strike-slip faults in this area commonly show a minus flower structure and the Neogene tectonic movement induced numerous approximately-EW dextral extensional minor faults near the major strike-slip fault, which cut the major strike-slip fault occasionally. Based on detailed interpretations of high-resolution seismic data, we divided the evolution of faults in this area into three stages: the formation of base strike-slip faults in pre-Neogene, the formation of feebly-en-echelon faults in cover strata in early Neogene and the formation of tenso-shear and strongly-en-echelon faults in cover strata in late Neogene. Multiperiodic activities of strike-slip faults in the uplifted area not only formed faulted-anticline traps but also controlled Paleogene deposits on both sides of the faults, exerting an influence on the accumulation and redistribution of hydrocarbons.

Reconstruction of the tectonic uplift and thermal history since Mesozoic in the Dangyang synclinorium of the central Yangtze area was made by means of apatite and zircon fission track thermochronology, vitrinite reflectance and basin modeling. Analyses of the zircon and apatite fission track data(107～126 Ma and 9.8～54 Ma) suggested that twice enormous tectonic uplift and cooling events occurred in the study area, one in the Early Cretaceous Yanshan stage and the other during the Eocene to Miocene Himalayan stage. The abruption of vitrinite reflectance values around Jurassic-Cretaceous and Paleogene-Neogene unconformities and abnormally high vitrinite reflectance values of the present land surface also gave evidence for the occurrence of these two events. Study results show that the Upper Triassic-Lower Jurassic formations in wells Dangshen-3 and Jianyang-1 had a maximum paleotemperature of 190~210 ℃ at the beginning of Early Cretaceous(~137 Ma), resulting in a zircon fission track annealing; the formations then cooled down to a near surface temperature of 20~40 ℃ due to the uplift and erosion of strata at the end of Early Cretaceous and the beginning of Late Cretaceous(~97 Ma); during Late Cretaceous to Paleogene(around 45 Ma), the strata had been deeply reburied with paleogeothermal values up to 140~165 ℃, leading to a complete apatite fission track annealing. Finally, the strata uplifted and were eroded again in the Himalayan stage, with paleotemperatures reduced to the present-day values of 70~90 ℃. Therefore, the maximum geotemperature that Paleogene strata in the study area have ever experienced was 100~120 ℃ and their present-day geothermal values range between 40~50 ℃. The heat flow of the study area was relatively steady in Mesozoic, with a lower value around 53.5 mW/m², it began to increase from Late Cretaceous and had the highest value of 59 mW/m² in Paleogene. The geothermal gradient for Mesozoic and Cenozoic strata in the study area is 36.3~43.0 ℃/km and 30.2~37.8 ℃/km, respectively, while the present-day geothermal gradient decreases to 28.9 ℃/km. The model indicates that the maximum erosion thickness of Early Cretaceous and Eocene to Miocene strata in the study area is about 4800 m and 2400 m, respectively. Consequently, the Dangyang synclinorium must have received enormous Jurassic sediments, and the maximum thickness of the Jurassic formation is likely to reach 5000 m.

In Early Paleozoic, the Tarim Basin was a craton basin with diverse deposits, where tectonic movements and eustatic sea-level changes dominated the development of Cambrian and Ordovician sequence formations. Integrated with outcrop, core, well logging and seismic data, we identified 15 sequence interfaces in the Cambrian and Ordovician of the Tarim Basin and divided the Cambrian into 6 sequence formations and the Ordovician into 8 sequence formations. Based on characteristics of these sequence formations and in view of the importance of faults and unconformity interfaces in hydrocarbon migration and accumulation, petroleum systems of Cambrian and Ordovician sequence frameworks in the Tarim Basin were divided into three different types, the continuous, discontinuous and complex. In practice, the exploration of petroleum systems includes continuous or discontinuous source-reservoir-cap rock combinations with lateral changes and the exploration of potential source-reservoir-cap rock combinations encompasses also continuous or complex source-reservoir-cap rock combinations characterized by an upper source-middle reservoir-upper seal play. It is worth exploring continuous or enclosed-continuous source-reservoir-cap rock combinations characterized by a lower source-middle reservoir-upper seal play.

Natural gamma ray measurement was applied to the Saergan shale and Yingan shale from the Middle-Upper Ordovician system in the Dawangou section of the Keping area. On the basis of data obtained from this measurement, variation curves of high-frequency sea-level changes for these two suites of shales were established. Meanwhile, analyses of pyrolysis, organic carbon content, X-diffraction of clay minerals and contents, and mudstone density were performed on 54 mud shales sampled from the Saergan and Yingan formations. A comparison of the analytical results with sea-level changes of the Saergan shale and Yingan shale indicated that: ① The Saergan shale with a thickness of 13.4 m remained with a third-order sequence of sea-level changes, 4 times and 3 times of high-frequency sea-level changes occurred in the transgressive systems tract (TST) and the highland systems tract (HST),respectively. Similarly, the Yingan shale with a thickness of 34m also fell into the third-order sequence of sea-level changes, but there occurred 2 times and 6 times of high-frequency sea-level changes in TST and HST, respectively. ② Both the Saergan and Yingan shales showed an increasing content of clay minerals as sea level rising, which was accompanied with an elevation of organic matter abundance (an increase of TOC). Compared with the Yingan shale, the Saergan shale had a higher concentration of clay minerals and more abundant organic matter but a lower content of carbonates. ③ Clay minerals in the Saergan shale and Yingan shale were composed mainly of a mixture of illite and smectite. Since the illitization of smectite into illite supplied the main catalyst for thermal maturation of source rocks, intervals of the Saergan and Yingan shales that were rich in the mixture of illite and smectite would be the best source rock in the study area. ④ Both the Saergan and Yingan shales were characterized by an increase of organic matter abundance and a decrease of mudstone density as sea level rising, but a decrease of organic matter abundance and an increase of mudstone density as sea level falling.

Characteristics of the lower part of the Xiaganchaigou Formation in the Mabei-Ⅷ structure in the northern margin of the Qaidam Basin were investigated based on analytical data of cores, logging, scanning electron microscope, X-ray diffraction, physical property, common thin sections, casting thin sections, and mercury injection test. The results show that the reservoir pore space includes mainly primary residual intergranular pores with partial secondary intergranular pores, indicating a good reservoir property, and the reservoir has low displacement and saturation median-point pressures, big pore-throat radius, good pore-throat sorting and rough skewness, the parameters of pore structure are wholly better. Generally, the quality of a reservoir is affected jointly by depositional facies and diagenesis, among which the riverbed and the beach-bar of a braided river are the most favorable zones for reservoir development, in addition, weaker compaction, medium cementation in early diagenesis and universal corrosion are also favorable for the protection of primary pores as well as the development of partial secondary pores and throats in a sandstone reservoir. According to petrophysical properties and pore structure parameters of reservoirs, the lower part of the Xiaganchaigou Formation in the Mabei-Ⅷ structure could be divided into typeⅠ, type Ⅱ and type Ⅲ pools, among which the type Ⅰ and type Ⅱ pools were proposed as preferred target zones for increasing reserves under current technical conditions.

The scanning electron microscope (SEM) image analysis of clay rocks collected from the field demonstrates that pores of clay minerals are distributed among grains and at natural fractures of intragranular plates, where the former has an irregular shape and the latter shows a wedge-shaped or plane gap. Connected nano-pores develop in layers of the smectite that has interlayer structures, micropores are most frequently found in smectite clay and secondly in illite and smectite mixed layers. In clay minerals, the size of intergranular pores and plane gaps ranges from 20 to 100 nm and connected pores within layers are less than 50 nm in size. Kaolinite clay mainly has intergranular pores with a size of 20-100 nm. Illite and chlorite are of larger crystal particles and dominated by micropores. The methane adsorption capacity of various clay minerals determined by experiments is consistent with the degree of micropore development measured by SEM, indicating that dominant nano-pores in shales, to a certain extent, determine the surface to volume ratio of shales as well as the storage capacity of shale gas. The porosity development of clay rocks is related to the genesis and diagenetic evolution of shales, while the pore size is controlled mainly by the particle size of clay minerals.

A point-bar reservoir readily forms dominant flow channels after long-term waterflooding development, resulting in a decrease of the waterflooding sweep efficiency and substantially affecting the distribution of remaining oils. Taking the S17-19 Block of the Fuyu Oilfield as an example, we studied the distributional pattern of dominant flow channels within point bars and its control on the distribution of remaining oils. Data of core analyses indicate that the water cut of dominant flow channels within point bars is greater than 95% and the corresponding oil displacement efficiency is greater than 35%. Reservoirs with either high heterogeneity or homogeneity and permeability greater than 80 mD are more easily to form dominant flow channels. Under the current well-pattern condition, dominant flow channels, with an average thickness of 2.2 m and width between 2 m and 4 m, commonly occur in the middle and lower parts of point bars. Controlled by main flow lines between injection and production wells, relatively high-permeability zones and high-permeability intervals in the lower part of point bars, dominant flow channels have built a complex network in space. Waterflooding is more serious in dominant flow channels than in non-dominant flow channels, but the latter is rich in remaining oils. Therefore, measures, such as rebuilding well-patterns, drilling horizontal wells, carrying out profile control and water plugging, adjusting development ways and implementing cyclic waterflooding, can break the current network of dominant flow channels and expand volumes of the waterflooding sweep efficiency to improve the displacement efficiency of point-bar reservoirs.

Based on a capillary model and integrated with the boundary layer theory, we proposed a new mathematical model for seepage in low-permeability reservoirs by introducing two parameters c₁ and c₂ that could characterize seepage of low-permeability reservoirs. This model well explained origins of a start-up pressure gradient and nonlinear seepage. According to the new model, a well-testing model considering the influence of moving boundaries was established and solved with the finite difference method, an impact of dimensionless characteristic c_1D and c_2D parameters on well-testing curves was investigated as well. The results show that pressure and pressure derivative curves for both the new model and a pseudo pressure gradient model are upward compared with the Darcy model though the upward degree for the new model is gentle. And c_1D and c_2D are characteristic parameters that determine the shape of well-testing curves. The upward degree increases with an increase of c_1D or c_2D when the sum of c_1D and c_2D is not a constant, while the upward degree increases with an increase of c_1D but a decease of c_2D when the sum of c_1D and c_2D is a constant. On well-testing curves, a pressure response to the new model is in agreement with that to the Darcy model both for closed boundaries and for constant pressure boundaries.

Based on the generalized Darcy’s law that considers a comprehensive influence of the threshold pressure gradient and the pressure-sensitive effect, we proposed transient seepage theories on infinite- and finite-conductivity vertically fractured wells respectively, which follow the elliptic seepage theory and average mass conservation law. A production decline model for fractured horizontal wells with arbitrary numbers of transverse hydraulic fractures was then established on the basis of the superposition principle, and accordingly production decline laws of fractured horizontal wells at unsteady-state periods were investigated. The results show that the initial production of multi-fractured horizontal wells(MFHW) is relatively high but soon followed by a sharp decline. Then, the production keeps in a relatively stable state and declines slowly. The greater the threshold pressure gradient and the deformation coefficient, the lower the per-well production of MFHW will be. Furthermore, the threshold pressure gradient will influence the production and decline laws of MFHW remarkably when it exceeds 0.01MPa/m. The more the hydraulic fracture numbers the longer the fracture length and the bigger the flow conductivity, and then the higher the production of MFHW and the faster the decline rate will be. However, the increase amplitude will be getting smaller and smaller along with the increase of hydraulic fracture numbers, fracture length and flow conductivity.

The minimum miscibility pressure (MMP) of the CO2-crude oil system is one of the most critical parameters for the CO₂ flooding design in reservoir development. In order to improve prediction accuracy, an eight-parameter prediction model for the MMP of the CO₂-crude oil system, which utilized 46 experimental MMP data, was established through the statistics and regression of 8 parameters, such as reservoir temperature, volatile components, intermediate components and C₅₊ in oils, and methane, ethane-butane, N₂ and H₂S contents in the injected CO₂. A comparison was made between MMP of the CO₂-crude oil system for 46 oil samples predicted by the presented model and the calculated results of these samples derived from the other 9 published MMP prediction methods. The result shows that the accuracy of MMP predicted by the presented model is greatly improved. Moreover, this model was applied to the prediction of MMP for oil samples from 5 wells in the CO₂ flooding trial zone of the Jilin Oilfield, and the comparison between the predicted MMP with the present model and the experimental results showed that their relative errors were in a range of 0.05%~3.39%, which confirmed the reliability of the presented model.

Reservoir pressure of condensate gas reservoir is an indispensable parameter for calculation of gas reserves, evaluation of gas-well productivity, dynamic analysis and assessment of retrograde condensation. The development of water-drive condensate gas reservoir is ranked among the most complex types of gas reservoir development. During the process of development, an accurate calculation and prediction of reservoir pressure is particularly difficult but important because reservoir pressure declines ceaselessly, condensate oil drops out continuously with an increase of the water-vapor content in condensate gas, and the edge water invades constantly. We tested the water-vapor content in condensate gas under different pressures by calculating the retrograde gas condensate, which is influenced by adsorption of porous media and capillary pressure. According to material balance principles, a novel material-balance equation for water-drive condensate gas reservoir was established in consideration of porous-medium absorption, capillary pressure and water vapor as well, and finally reservoir pressure at any time during production could be computed by iteration. Application results show that the reservoir pressure calculated with this method gets much closer to actual reservoir pressure, which virtually reduce the workload in field testing.

The Wei-8 fault block is characterized by a low-permeability and heavy-oil reservoir with gas-cap and bottom-water. In view of its real conditions including gas coning, water coning and hydraulic fracturing, we studied influencing factors in waterflooding development of this reservoir by integrating the dynamic analysis of reservoir production with numerical simulation. First, a numerical simulation model for the whole oil reservoir was built based on fine reservoir description, and then it was applied to simulating the development and production history of the reservoir by analyzing dynamic features. Then, using the typical well-patterns of this model, the effect of development layer series combination, horizontal-vertical well combination pattern and injection conditions were studied during the development by dynamic adjusting permeability, oil-viscosity, effective thickness, well-row data, penetration ratio, injection temperature, injection rate and injection-production well spacing. A number of optimum values for these parameters were derived with this model and they could direct an effective and scientific development of the Wei-8 fault block.

The present paper deals with geological characteristics of sandstone reservoirs in China and influential factors of water cut variations. Based on correlation curves normalized by water cut versus percent of recoverable reserves, we obtained varying curves of water cut and their theoretical formulas that represent the current level of waterflooding development in China by means of applying development data of 15 typical large waterflooding sandstone reservoirs in China, which have been at a high water cut stage. The results of quantitative analysis on the degree of reserve recovery at different water cut stages indicate that variations of water cut in waterflooding reservoirs in China are wholly characterized by a convex curve, suggesting nearly 70% of recoverable reserves would be recovered after reservoirs enter into a high water cut stage(water cut >60%), and the high water cut stage(water cut >60% and <90%) is the most important period to enhance oil recovery, during which about 50% of recoverable reserves would be recovered. Besides a help to fully understand regulations of water cut variations of waterflooding sandstone reservoirs in China and to analyze them quantitatively, the present study result can also be applied to the prediction of development indexes and the evaluation of both development potential and development effects.