Abstract: The shale in Member 7 of Triassic Yanchang Formation (Chang-7) in Ordos Basin has a giant potential to produce oil and gas by in-situ heating, which is attributed to its low maturity, high organic carbon content and good type of organic matter. To explore the impact of pressure during in-situ heating on the characteristics of oil and gas from shale reservoirs and provide a reliable basis for the development of Chang-7 shale oil and gas by in-situ heating, a study was performed on the third submember of Member 7 of Yanchang Formation, and in-situ heating simulation experiments were conducted on typical samples at the heating rate of 20℃/d and different pressures of 0 MPa, 2 MPa, 4 MPa and 8 MPa, respectively. The experimental results show that under the same heating conditions, pressure changes have an effect on the maximum oil production from Chang-7 shale and also the oil characteristics, and a greater effect on the composition of the gas produced. Under four different pressures, the maximum production of Chang-7 shale by in-situ heating is 36.97-41.47 kg/t, and the oil production decreases with the increasing of pressure. As pressure increases, the light components of the produced oil increase, and the oil quality becomes better, which is attributed to a high-degree cracking of heavy oil generated by organic matters under high pressure. When heated up to 450℃, the total gas production is 14.28-21.05 m³/t, and increases with pressure. Moreover, the gas-oil ratios of shale oil and gas generated in four experimental sequences ranges from 289.70 m³/m³ to 427.04 m³/m³, and increase with pressure. However, there is a great difference in the production of various gas components that increase with pressure, i.e., the total production of hydrocarbon gases and CO₂ increases with pressure, while H₂ production obviously declines and H₂S production changes little with pressure. The production of different gas components presents different changing laws with the increasing pressure, which has relation with many factors, such as the formation mechanism and shale adsorption capacity of different gaseous components, and the secondary reaction among various phase products within the certain range of heating temperature and pressure. According to the oil production, quality of produced oil, yields of different gas components, current oil-gas prices as well as the processing costs on harmful products, it is suggested that a relatively low bottom-hole pressure in production well should be hold during shale oil-gas development in Chang-7 by in-situ heating, which is of great reference significance for determining the development plan for shale oil and gas from Chang-7 shale by in-situ heating.

Key words: Ordos Basin, Yanchang Formation, shale, in-situ heating, pressure, shale oil and gas yield

摘要： 鄂尔多斯盆地三叠系延长组7段(长7段)页岩成熟度低、有机碳含量高、有机质类型较好,原位加热开发页岩油气的潜力巨大。为了探讨原位加热过程中压力对页岩产出油气特征的影响,为原位加热开发长7段页岩油气方案提供可靠依据,以长7段3亚段页岩为研究对象,对其典型样品以20 ℃/d的升温速率进行了4组不同压力(0 MPa、2 MPa、4 MPa、8 MPa)的原位加热模拟实验。实验结果表明:在相同加热条件下,压力变化对长7段页岩的最大产油量和产出油性质均有影响,对产出气体组成的影响更大。在4组不同压力下,长7段页岩原位加热的最大产油量为36.97~41.47 kg/t,产油量随压力增加有一定减少。随着压力增加,产出油中轻质组分增加,油品变好,其原因在于高压下有机质生成的重质油的裂解程度相对较高;当加热到450 ℃时,总产气量为14.28~21.05 m³/t,总产气量随压力增加而增加。4组实验序列产出页岩油气的气油比(289.70~427.04 m³/m³)随着压力增加而增大;不同气体组分随着压力增加产量变化差异较大,其中,烃类气体与CO₂的总产气量随压力增加而增大,H₂产量随压力增加而明显减少,H₂S产量随压力增加变化较小。不同气体组分的产量随压力增加呈现出不同的变化规律,这与加热温压范围内不同气体的形成机理、页岩对不同气体组分吸附能力的差异和一些次级反应等因素有关。根据不同压力下页岩原位加热的产油量、油品、不同气体组分的产率,结合考虑油气价格与油气处理成本,提出在实施原位加热开发长7段页岩油气时应尽量保持低的生产井井底流压的建议,这一认识对长7段原位加热页岩油气开发方案的确定具有重要的参考意义。

关键词: 鄂尔多斯盆地, 延长组, 页岩, 原位加热, 压力, 页岩油气产率