深部煤层气地质条件特殊性与储层工程响应

Geological particularity and reservoir engineering response of deep coalbed methane

鄂尔多斯盆地东缘、新疆等地区中深部/深部煤层气的试采成功大大推进了中国深部煤层气的开发进程。为进一步推动深部煤层气的规模性高产,有必要依托当前钻探成果和开发实践动态,系统梳理深部地质条件的特殊性及其储层工程响应。研究结果表明,煤层气成藏关键参数的深度效应主要在于3方面:①埋深增大,温度场、压力场和应力场等地层环境参数的大小趋于增高、梯度趋于收敛,深部应力场类型发生转换、水平应力的各向异性减弱;②深部多为高饱和或超饱和气藏,游离气的工业开发价值大幅提升,但当前开采深度的高阶煤储层仍以吸附气为主;③深部高应力环境下煤岩自身组构因素被弱化,原位渗透率及力学性质趋于均质收敛,流体产出严重依赖于人工渗流通道。地层环境下,深部煤层水力裂缝扩展的水平定向性弱,变形破坏韧性增强,易形成由高而短的裂缝构成的复杂缝网,低排量施工面临裂缝生长受限、支撑性差等问题,压裂应保证在近井地带最大化地实现全方位体积改造,由"压得开"转向"压得碎、撑得住";深部高饱和—超饱和的含气特点可保证投产初期的强地层能量和高渗透率优势得以充分利用,但只有降至一定压力节点后吸附气方可形成产能接替效应。研究认为,深部地质—工程一体化攻关研究应聚焦煤层气成藏运聚过程和不同相态气体富集的地质配置,探索地层环境下储渗系统优化改造技术和优化原理,明确排采诱导的煤储层相态再平衡过程、物性变化模式和产气规律,谋求地质条件、增产措施、排采管控和储量动用的最佳耦合。

The successful pilot production of medium-deep and deep coalbed methane(CBM)in the eastern margin of Ordos Basin and Xinjiang has greatly boosted the progress of deep CBM exploration and development in China.To further promote the large-scale high production of deep CMB,it is necessary to systematically summarize and analyze the geological particularity and reservoir engineering response in deep layers by use of the current drilling results and development practices.Research results show that the depth effect on the critical parameters for CBM accumulation lies primarily in three aspects.(1)With the increase of burial depth,the stratigraphic environment parameters such as temperature field,pressure field and stress field tend to increase,the gradients tend to converge,the type of deep stress field changes,and the horizontal stress anisotropy weakens.(2)Highly saturated or oversaturated gas is generally developed in deep seams,thus greatly enhancing the industrial development value of free gas,but the higher-rank coal reservoirs are still dominated by adsorbed gas according to current mining depth.(3)Under deep condition of high stress,the coal structures are weakened,in-situ permeability and mechanical properties tend to converge,and fluid production depends heavily on artificial seepage channels.Under the stratigraphic condition,the decrease in horizontal hydraulic fracture extension and the increase in ductile deformation easily lead to the formation of a complex network with high-angle and short fractures in deep coal seam.Low-output construction faces challenges in the limited fracture growth and poor sand-support so that fracturing operations should effort to maximize the full range of reservoir volume stimulation in the near-wellbore area,focusing on the change from being highly compressed to being crushed and supported.The gas-bearing characteristics of high to oversaturated conditions in deep seams can ensure the full utilization of strong reservoir energy and high permeability during initial production.However,the production capacity replacement by adsorbed gases can be only achieved when the reservoir pressure drops to a certain pressure.Research suggests that the integrated geological and engineering study on deep seams focus on CBM accumulation,migration and accumulation process as well as geological configuration for gas enrichment with different phases,explore optimization techniques and principles for reservoir stimulation,clarify the drainage and production-induced phase re-equilibrium process of coal reservoirs,property change patterns and gas production regularity,and seek for the best coupling of geological conditions,production enhancement measures,drainage and production control as well as reserve utilization.