基于等温吸附曲线的煤储层产气潜力定量评价——以黔北地区长岗矿区为例

含气饱和度、临储比等指标在用于煤层气选区选层评价时,未考虑煤层气解吸能力以及解吸过程中储层压力对气体解吸的影响,因而难以全面反映煤储层的产气潜力。为此,以煤样等温吸附实验为基础,提取临储压差、临废压差、有效解吸量、解吸效率等指标,建立了煤层气产出潜力的定量评价方法,并基于黔北地区长岗矿区煤层气井排采历史进行了分析验证。研究结果表明:(1)长岗矿区7号煤层的临储压差为2.35 MPa,0.2~1.0 MPa废弃压力下的临废压差介于2.06~2.86 MPa,煤层气有效解吸量介于9.32~18.9m3/t,具备较高的产气潜力;(2)研究区煤层气解吸过程只经历敏感解吸阶段,解吸效率高,煤层吸附时间短,见气后短时间内可获得较高产的气流;(3) FX2井煤层气产出潜力定量评价及排采历史验证了该区的煤储层具有煤层气开发产气潜力。结论认为:(1)研究区煤层气井排采初期应缓慢排采,尽可能减小降压速度、扩大降压漏斗波及范围和有效解吸半径;(2)优选相对高渗区及开展高质量的压裂,以扩大有效渗流半径,充分释放煤层气产能。

Quantitative evaluation on the gas production potential of coal reservoirs based on isothermal adsorption curves: A case study of the Changgang Field,northern Guizhou,China

When gas saturation and critical desorption pressure/reservoir pressure ratio are applied to coalbed methane (CBM) area/layer selection, they can hardly reflect the gas production potential of coal reservoirs fully, for CBM desorption capacity and the effect of reservoir pressure on gas desorption in the process of desorption are not considered. In this paper, critical desorption pressure-reservoir pressure difference, critical desorption pressure-abandonment pressure difference, effective desorption quantity and desorption efficiency were extracted based on isothermal adsorption experiment of coal samples. Then, a quantitative evaluation method for evaluating CBM production potential was established. Finally, it was analyzed and verified based on the production history of CBM wells in the Changgang Field, northern Guizhou. And the following research results were obtained. First, the critical desorption pressure–reservoir pressure difference of No.7 coal seam is 2.35 MPa, the critical desorption pressure–abandonment pressure difference under the abandonment pressure of 0.2–1.0 MPa ranges from 2.06 to 2.86 MPa and the CBM effective desorption quantity is between 9.32 and 18.9 m3/t, indicating a favorable gas production potential. Second, the desorption process of CBM in the study area only undergoes the sensitive desorption stage. The coal reservoirs have a higher desorption efficiency and a short adsorption time, so high-yield gas flow is produced shortly after initial gas production. Third, the quantitative CBM potential evaluation and production history of Well FX2 verify that coal reservoirs in this area have the potential of CBM development and production. In conclusion, it is necessary to slow down the production of CBM wells in the early stage, decrease the pressure decline rate as much as possible and increase the sweeping range of pressure drop funnel and the effective desorption radius. In addition, it is necessary to select the relative high permeability areas and carry out high quality fracturing to enlarge the effective seepage radius and realize the maximum release of CBM productivity.