关键词: 深部煤层气, 耦合控制, 等温吸附, 微构造, 开发单元, 鄂尔多斯盆地, 解吸阶段

Abstract: Deep coalbed methane (CBM) with a buried depth of greater than 2 000 m is an important field of CBM exploration and development. Different from the middle-shallow CBM that mainly consists of adsorbed gas, the occurrence state of deep CBM is characterized by the coexistence of free gas and adsorbed gas. At present, the desorption law of deep CBM, the opportunity for conversion between free gas and desorbed gas and the reasons for the difference in development effect are still unclear. This paper is a case study of Daning-Jixian block in the eastern margin of Ordos Basin. Since the distribution law of its key geological parameters is consistent with the tectonic trend, the block can be divided into four development areas based on the microstructural morphology, including negative microtectonic area, gentle tectonic area, positive microtectonic area and tectonic uplift area, of which the geological characteristics have been determined. The key factors that impact the development effect of deep CBM can be summarized as "five essential conditions" and "one degree". Moreover, an analysis is performed on the coupling control mechanism of these key factors in deep CBM development. The "five essential conditions" include the preservation condition, resource condition, desorption condition, seepage condition, and reservoir stimulation condition, which are the geological foundation; the "one degree" refers to the degree of fracturing reformation, which needs to be differentially adjusted according to the "five essential conditions" in the hydrocarbon development process. Based on the "five essential conditions" (25 geological parameters), the paper systematically summarizes the evaluation results, comprehensive production characteristics, and typical production curves of coalbed methane development in 4 development areas, and further proposes corresponding engineering countermeasures. Based on the desorption condition that has been neglected in the evaluation of CBM, the main controlling factors affecting the isothermal adsorption characteristics of deep CBM were identified through isothermal adsorption experiments. The adsorption capacity of deep coal rock is weakened with the increase of temperature, ash content, and moisture content, and increased with the increasing level of the thermal evolution of organic matter. Meanwhile, Langmuir pressure keeps increasing with the increase of moisture content; when the ash content in coal rock varies greatly, the adsorption capacity of coal rock is not significantly correlated with the thermal evolution level of coal rock, and the ash content becomes the main controlling factor for desorption; from the gentle tectonic area to positive microtectonic area, the isothermal adsorption curve of coal rock is varied from steep to gentle, the Langmuir volume is decreased by 10.7%, and the Langmuir pressure is increased by 36.8%. In combination with the experimental results from basic theoretical methods, the paper further determines the desorption laws of deep CBM in different tectonic development areas, and compares and analyzes the differences in desorption characteristics. The production process of deep CBM can be divided into four stages:(1) low efficiency desorption stage + free gas dominant stage (Stage Ⅰ), (2) slow desorption stage (Stage Ⅱ), (3) high efficiency desorption stage (Stage Ⅲ), and (4) sensitive desorption stage (Stage Ⅳ). The starting pressure for the desorption of deep CBM in Daning-Jixian block is 9.05 MPa to 9.30 MPa, the desorption turning pressure is about 6.00 MPa, and the desorption sensitive pressure is 2.30 MPa to 2.70 MPa. In the production of deep CBM wells, the transition from dominant free gas production to dominant adsorbed gas production is a process that mainly depends on the duration of the slow desorption stage, the size of key desorption pressure points (desorption starting pressure and turning pressure), pressure drop, and the characteristics of desorption curve. Finally, based on geological laws and relevant understandings, the reasons for the difference in the development effect of CBM wells in the gentle tectonic area and positive microtectonic area have been analyzed in detail, which aims to provide scientific guidance for the fine geological evaluation and units division, prediction of development laws, optimization of gas production equipment, and formulation of drainage and production system in promoting the efficient development of deep CBM.

Key words: deep coalbed methane, coupling control, isothermal adsorption, microtectonics, development unit, Ordos Basin, desorption stage