无烟煤储层煤层水的相变传质过程及其煤层气开发的工程启示

Phase change and mass transfer process of coalbed water in anthracite reservoirand its engineering inspiration for coalbed methane development

科学认识煤层孔裂隙中流体流动与传质机理是实现煤层气高效产出的治本之策。煤层气开发过程中,煤层水的赋存和运移产出特征不仅决定了煤层气井疏水降压效果,同时显著影响了CH4解吸、运移、产出过程,对煤层气井高效排采管控至关重要。以沁水盆地南部樊庄区块为例,基于煤层水赋存形式、产出过程和流动形态的认识,开展了煤层水相变传质机制的研究,并量化了工程背景下无烟煤储层煤层水相变传质的阈值孔径,探讨了其对煤层气开发的工程启示。结果表明:煤层水的赋存状态可动态转变,流体压差是发生动态转变的主要机制。水的单相流动阶段,可动水的运移主要受表面张力作用;工程背景下,大孔(孔径>50 nm)尺度孔裂隙中可动水的运移产出是煤层压力整体释放的前提,并奠定了煤层气井高产稳产的基础。气水两相流条件下,束缚水(毛管水)向可动水的转变受煤岩润湿性以及气水之间的界面性质与相互作用的重要影响,并因毛管效应发生而压力条件要求增加。其中,泡状流和气团流为主的阶段,CH4的解吸产出促进了部分介孔(2~50 nm)尺度孔裂隙中束缚水(毛管水)向可动水的转变,并决定了煤层压力的整体释放效果;指示了见套压后,煤层压力的整体释放是气水两相共同作用的结果,排水阶段向降压采气阶段缓慢过渡对煤层气井高产是必需,持续憋压可能造成介孔尺度孔裂隙中过早形成段塞流,不利于煤层气井高产稳产。随煤层压力降低,等效孔径>40 nm的孔裂隙中的液态水可汽化脱离形成气态水自由水,与吸附态CH4形成竞争吸附/解吸,而等效孔径<40 nm的孔裂隙中的煤层水则以液态形式保存或进行润湿迁移,对气水两相流阶段束缚水(毛管水)向可动水的转变影响较小。

The scientific understanding of the fluid flow and mass transfer mechanism in pores and fractures of coal seam are the fundamental strategy to achieve the efficient production of coalbed methane(CBM).During CBM development,the occurrence,migration,and production characteristics of coalbed water not only determine the drainage and depressur-ization effect of CBM well,but also significantly affect the desorption,migration,and production process of CH4,which is crucial to the efficient drainage and production control of CBM well.Taking the Fanzhuang block in the southern Qinshui Basin as an example,based on the occurrence form,production process,and flow form of coalbed water,the research on the mechanism of phase change and mass transfer process of coalbed water has been carried out.The threshold pore dia-meter of phase change and the mass transfer process of coalbed water in anthracite reservoir under the engineering back-ground have been quantified,and its engineering implications for CBM production have been discussed.The results show that the occurrence state of coalbed water can be changed dynamically,and the fluid pressure difference is the main mech-anism of its dynamic change.In the single-phase flow stage of water,the migration of movable water is mainly affected by the surface tension.Under the engineering background,the migration and production of movable water in pores and frac-tures with macropore(pore diameter>50 nm)is the precondition of the overall release of coal seam pressure,and lays the foundation for a high and stable production of CBM well.In the gas-water two-phase flow stage,the transformation of bound water(capillary water)to movable water is greatly affected by the wettability of coal,and the interface property and interaction between gas and water.The pressure condition requirements of the transformation of bound water(capillary water)to movable water increase due to the capillary effect.In the bubbly flow and plug flow stage,CH4 desorption pro-motes the transformation of bound water(capillary water)to movable water in some pores and fractures with mesopore(2-50 nm)scale,which determines the overall release of coal seam pressure.This also indicates that after the CBM well has casing pressure,the overall release of the coal seam pressure is the result of the joint action of the gas and water,and the slow transition from the water production stage to the depressurization and gas production stage is necessary for the high production of the CBM well.The continuous casing pressure holding may lead to early formation of slug flow in pores and fractures with mesopore scale,which is not conducive to the high and stable production of CBM well.Moreover,with the decrease of coal seam pressure,the liquid water in pores and fractures with the equivalent pore size>40 nm can be vaporized and separated to the gaseous free water,which forms competitive adsorption/desorption with the adsorbed CH4.However the water in pores and fractures with the equivalent pore size<40 nm is preserve with liquid or transported by wetting migration,and has little influence on the transformation of bound water(capillary water)to mov-able water in the gas-water two-phase flow stage.