流固耦合约束下的煤储层速敏效应及其煤岩学控因

The skeleton of coal reservoir is mainly composed of organic matter, which determines its stronger sensitivity to external factors due to this intrinsic property. The fluid velocity sensitivity of coal reservoir is one of the important reasons to the low gas production rate and fast gas production decline. This effect is subject to the petrological factors and its control mechanism under the fluid-solid coupling constraint, and the knowledge on the fluid velocity sensitivity still needs to be deepen from the perspectives of the reservoir protection, drainage and reservoir optimization. Around above scientific problem, this project will aim at the middle-high rank coal in the Hancheng Block, eastern margin of the Ordos Basin, combine with subject theories and methods of coal petrology, coal chemistry, reservoir physics and seepage mechanics, and carry out following contents. (i) Characterizing the relationship between the coal macrolithotypes and coal material composition, spatial structure, surface properties, as well as mechanical properties. (ii) Simulating the permeability changes of coals with different compositions under the various fluid velocities to reveal the control laws of microscopic components, mineral types, particle sizes, etc. on the pulverized coal production and movement. (iii) Discussing the internal control mechanism of the fluid velocity sensitivity from the perspectives of coal petrology, coal petrography, and coal seepage mechanics to provide scientific basis for the coalbed methane well drainage optimization.

煤储层骨架主要由有机质构成，这一内在属性决定了它对外在因素的敏感性更为强烈。煤储层速敏效应是气井产量低、衰减快的重要原因之一，严重受制于流固耦合约束下的煤岩学因素及其控制机理，有待于从煤层气储层保护、排采制度优化、产层优选层面上予以深化研究和认识。本项目拟围绕这一科学问题，以鄂尔多斯盆地东缘韩城区块中－高阶煤储层为研究对象，综合运用煤岩学、煤化学、油层物理学、渗流力学等理论与方法，表征宏观煤岩类型与煤物质组成、空间结构、表面性质、力学性质的关系，模拟变流速条件下煤岩成分及其组合的渗透率变化特征，揭示显微煤岩组分、矿物类型、粒度等对煤粉产出和运动的控制规律，从煤岩学、煤相学、煤渗流力学等视角探讨煤储层速敏效应的内在控制机理，为优化煤层气井排采制度提供科学依据。

在煤层气开发过程中，煤储层常会发生敏感性伤害而影响气井产气效果，流固耦合作用下的速敏效应对煤储层渗透性的伤害是造成我国煤层气单井产量总体偏低的主要原因。本项目针对流固耦合约束下的速敏效应及其煤岩学控制机理展开系统研究，研究系统揭示了不同煤岩类型孔裂隙发育的差异特征，研究表明在井下剖面尺度上（mm~km），韩城矿区以半暗煤和暗淡煤为主，含少量半亮煤和光亮煤；从光亮煤到暗淡煤，煤岩密度逐渐升高，镜质组含量逐渐降低，灰分产率逐渐升高；在裂隙尺度上（μm~mm），光亮型煤较暗淡型煤显微裂隙更为发育（光亮型煤微裂隙密度平均339条/9cm2，暗淡型煤为272条/9cm2），多呈现共轭剪裂隙或张裂隙且裂隙连通性好，分形维数大；在孔隙尺度上（nm~μm），光亮煤主要以大孔和微裂隙为主，发育较差中孔与过渡孔导致微孔和大孔之间的连通性较差，孔隙以一端封闭的渗流型孔为主，孔隙形状为板状、筒状或楔状。本次研究合理选择了不同煤岩类型代表性样品，系统探索了原煤储层敏感性实验方法，揭示了煤岩不同敏感性伤害机理，评价了煤岩不同敏感性的损害程度。煤层气开发过程中，高速流体易导致煤储层中的煤粉等微粒分散运移，堵塞裂隙系统，导致储层渗透率下降。沁南煤岩样品最大损害率为0~53.92%，平均25.14%，速敏损害程度介于弱速敏—中等偏弱。同时研究揭示了储层速敏对渗透率伤害的微观作用机理，阐明了速敏效应对煤粉产出的内在机制。煤粉运移对于煤层孔隙度和渗透率有着正负效应，考虑煤粉产出时，煤层孔隙度和渗透率都有所上升，说明煤粉产出增大了煤层孔隙度，抵消了有效应力的作用，使煤层渗透率增大；然而，煤粉滞留却会堵塞煤层有效渗流通道，降低孔隙度和渗透率。