考虑孔隙变形的致密油储层基质-裂缝系统渗流机理研究

Due to the particularity of the reservoir characteristics and the way of development, the seepage law of tight oil reservoir is different from conventional reservoir. One reason is that as the pore throat is small, there exists microscale effect, the flow does not meet the Darcy's law. The second reason is that for consists of kerogen nanopores in the organic pore, deformation will easily happened under high pressure and partial kerogen cracking will occurred at high temperatures, and that will cause pore structure changes and affect seepage. The third reason is that with the development mode of one-off fracturing and repeated fracturing with horizontal well, the key is how to make the matrix fluid into the fracture and the seepage space is the matrix - fracture system. This project will combine the method of tomography (CT), scanning electron microscopy, mercury intrusion and gas adsorption, and research the changing rule of reservoir matrix and fracture structure of dense oil with the deformation of organic hole and the decomposition of kerogen, determine the description parameter of reservoir pore-fracture system, realize the reconstruction of 3D pore-joint network, and use the coupling method of molecular dynamics simulation and smooth particle dynamics to btain the relationship between pressure difference and flow rate , which is introduced into pore network simulation model to establish the fluid seepage model in Matrix - fracture system of dense oil reservoir with deformation medium. By combining the experiments of micro in-pipe displacement with different dimensions with actual core displacement experiment results, the theoretical model can be revised, and the percolation mechanism of tight oil reservoir considering pore deformation and the microscale effect can be revealed. This research has important theoretical significance in guiding the development of tight oil reservoir.

致密油藏由于储层特征和开发方式特殊性，与常规储层渗流规律不同。一是孔喉细小，具有微尺度效应，流动不满足达西定律；二是以干酪根纳米孔为主的有机孔在高压下易变形、在高温下发生部分裂解，导致孔隙结构变化影响渗流；三是水平井一次压裂和重复压裂开发，如何使基质流体进入裂缝是关键，渗流空间为基质-裂缝系统。本课题将采用CT、扫描电镜、恒速压汞和气体等温吸附等方法，研究基质与裂缝结构与有机孔变形和干酪根裂解的关系，明确孔隙-裂缝系统描述参数，实现三维孔隙-缝网重构，进而采用分子动力学与光滑粒子动力学耦合方法，确定孔隙-裂缝系统变形前后流量与压差关系，并引入到孔隙网络模拟模型，建立考虑孔隙变形的致密油储层基质-裂缝系统内气油和水油两相渗流模型，并应用不同尺寸微观实验和岩芯实验结果，对所建模型进行修正，最终揭示微尺度效应下变形介质致密油储层两相渗流机理。本课题成果对于指导致密油储层的开发具有重要理论意义。

该课题采用物理模拟实验、理论研究和数值模拟相结合的方法，开展了致密油储层微观孔隙-裂缝结构和微观渗流规律研究，给出了致密油储层水平井多段压裂开发方式下产能预测方法，课题完成了项目计划内容。取得的主要研究成果如下：. 1、重构了致密油储层三维孔隙-缝网结构. 通过二维切片统计信息，模拟计算三维孔隙介质，进行了三维孔隙结构重建。采用CT扫描、恒速压汞和气体等温吸附结果等对致密油储层物性参数进行了研究，结果表明：本研究方法能有效模拟致密油储层微观孔隙结构。. 2、建立了致密油储层渗透率与围压关系. 通过室内实验，以羟丙基瓜胶、香豆粉方作为工作液，对致密油储层进行了渗透率测定。研究发现，浸泡时间和浸泡压力相同时，渗透率降低幅度随围压增大趋于平缓；对同一种压裂液，相同浸泡时间下，致密油储层渗透率降低幅度随围压增加而增大。. 3、研究了致密油储层岩石-原油相互作用下流体渗流规律. 在不考虑孔隙变形前提下，考虑致密油储层岩石-原油相互作用，建立并求解了控制方程和涡流方程，对比发现：所建立的数值渗流模型能够满足密闭纳米孔内流体流动精度和要求，靠近壁面的相互作用效应更强，流体粘度系数在靠近壁面处变化幅度很大。. 4、基于LBM研究了致密油储层孔隙壁弹性变化对流体流动规律的影响. 将孔隙离散化处理为极限纵向强约束弹性管，从管壁物性方程出发，研究了孔隙壁在流体压力、弹力和弯曲力作用下的径向运动。采用LBM方法研究了孔隙壁的弹性变化对孔隙内流体流动规律的影响。经研究发现，孔隙弹性系数存在一个临界值0.002，大于该值流体流量对孔隙弹性不敏感，小于该值流体流量随弹性系数减小而显著增加。. 5、研究了致密油储层水平井基质-裂缝系统的渗流规律. 建立了无限大油藏、矩形油藏、含断层油藏、直角边界油藏等水平井多裂缝系统下渗流模型和产能评价模型，研究了致密油储层水平井基质-裂缝系统的渗流规律，得到了不同流压下产量递减曲线及不同生产时间下IPR曲线。