深部煤层孔隙结构与流体差异赋存特征研究

Pore structure and differential fluid occurrence of deep coal seams

【目的】鄂尔多斯盆地东缘已打破深部煤层气勘探禁区,多个区块呈现“单点突破、区域差异开发”特征,深部煤储层孔裂隙结构作为流体赋存与产出的物质空间,对于深部煤层气区块差异开发至关重要。【方法】系统采集鄂尔多斯盆地东缘神府区块深部煤样品,基于常规孔渗物性测试、CO_(2)吸附、低温N2吸附、压汞和核磁共振等测试,以神府区块8+9号煤为例,系统总结深煤层孔隙结构及流体赋存模式。【结果和结论】结果表明:(1)深部煤孔隙结构差异较大,神府煤介孔−宏孔均发育,多为墨水瓶型和开放型孔。综合认为中阶煤孔隙结构的跨尺度效应稍有减弱,相对有利扩散、渗流。(2)中阶煤吸附能力降低,等温吸附曲线高压段曲线平缓,初期解吸效率低;含水饱和度增高,其介孔−宏孔的束缚水含量较高导致可动水孔隙率减小,降低了游离气的储集空间。(3)研究区存在“宏孔−微裂缝主控的游离气−自由水赋存”型、“微孔−介孔−宏孔主控的吸附气−束缚水赋存”型两类气水赋存模式,导致煤层气排采差异明显,其中“宏孔−微裂缝主控的游离气−自由水赋存”型是深部煤层气快速高产模式,该模式具有“见气时间短、中高产气、低产水”的生产特征,且由于应力对中大孔−微裂缝的伤害较强,建议该模式下的气井排采需适当控制排采速度以减小储层伤害,防止产量陡降;“微孔−介孔−宏孔主控的吸附气束缚水赋存”型模式具有“短期排水、缓慢见气”生产特征,排采仍需遵循“缓慢、连续”原则保证气井稳产;此外,由于孔隙以微孔、介孔为主,束缚水含量高,气井短期难获高产,需进一步探索加大压裂改造规模提高该类气藏产量。

[Objective]Breakthroughs have been achieved in deep coalbed methane(CBM)exploration along the eastern margin of the Ordos Basin,exhibiting the characteristics of local breakthroughs but regional differential CBM production in multiple blocks.The pores and fractures in deep coal reservoirs,serving as the material spaces for fluid occurrence and occurrence,are critical to the differential development across deep CBM blocks.[Methods]In this study,coal samples were collected systematically from the deep part of the Shenfu block located along the eastern margin of the Ordos Basin.Using conventional tests on porosity,permeability,and physical properties,as well as CO_(2) adsorption,lowtemperature N2 adsorption,mercury injection capillary pressure(MICP),and nuclear magnetic resonance(NMR),this study explored the Nos.8 and 9 coal seams as examples to systematically summarize the pore structures and fluid occurrence patterns of deep coal seams.[Results and Conclusions]Key findings are as follows:(1)Deep coal seams exhibit greatly different pore structures.Coals from the Shenfu block contain both mesopores and macropores,most of which display ink-bottle and open morphologies.It is comprehensively suggested that the medium-rank coals feature pore structures with a slightly weakened cross-scale effect,which is relatively conducive to diffusion and seepage.(2)The medium-rank coals have a reduced adsorption capacity,with isothermal adsorption curves being gentle in the high-pres-sure part and exhibiting a low desorption efficiency initially.These coals demonstrate elevated water saturation.The high bound water content in mesopores and macropores leads to a decrease in the movable fluid porosity,reducing the storage spaces for free gas.(3)Two gas and water occurrence patterns are observed in the study area:free gas-free water occurrence primarily governed by macropores and microfractures and adsorbed gas-bound water occurrence primarily governed by micropores,mesopores,and macropores.The presence of both occurrence patterns leads to significantly differential CBM production.Specifically,the former is identified as a pattern of rapid and high-yield deep CBM pro-duction with characteristics of early gas production,medium to high gas yield,and low water yield.Given the severe damage of stress to medium and large pores and microfractures,it is recommended that appropriate CBM production rates be adopted for gas wells under this pattern to minimize damage to reservoirs and prevent a sharp decline in yield.The second pattern demonstrates the production characteristics of short-term drainage and slow gas production.Slow and continuous CBM production is preferred under this pattern to ensure stable production of gas wells.Additionally,due to a predominance of micropores and mesopores in pores and a high bound water content under this pattern,it is dif-ficult to achieve a high CBM yield of gas wells in the short term.Therefore,it is necessary to explore the possibility of further expanding the fracturing scale to improve the CBM yield of such gas reservoirs.