二氧化碳注入过程中咸水层孔隙压的演变规律及预测方法

Carbon dioxide geological storage becomes a hot and novel issue of geology engineering around the world,confronted to lots of challenges,of which is the evolution of pore pressure in saline aquifers during carbon dioxide injection.Pore pressure evolution is associated with carbon dioxide migration underground, and early leakage detection of carbon dioxide from specific reservoirs,and also as a typical problem of compressible fluid and solid interaction in a porous medium.In this research proposal,by integrity of theoretical analysis with X-ray CT scan based laboratory measurements of carbon dioxide flooding in sandstone rock sample,we tentatively establish the controlling mechanism model for propagation and dispersion of pore pressure in saline aquifers,with the four dominant factors taken into account. Especially,we make the pilot project of carbon dioxide geological storage in the saline aquifer of Nagaoka in Japan as benchmark problem.Furthermore,the dispersion relationship is derived analytically for pore pressure waves.The dispersion process of pore pressure is mathematically analyzed and simulated numerically,through the method of space and time conservation element and solution element (CE/SE) proposed for the Euler hyperbolic equations.Therefore,the principle for propagation and dispersion of pore pressure is elucidated.Particularly,the evolution disciplinarian is preliminarily found out.The numerical prediction method is finally proposed for pore pressure propagation and dispersion in saline aquifers during carbon dioxide injection. Relevant studies can not only provide theorical base for seismic wave survery for monitoring carbon dioxide migration and distribution in reservoirs as well as the safety assessment of carbon dioxide geological storage,and potentially promote studies on the fluid and solid interaction in porous media.

二氧化碳地质封存是一个新型的国际热点地质工程问题，仍然面临着诸多挑战，其中之一是二氧化碳注入过程中咸水层孔隙压力的演变规律问题，这涉及到二氧化碳的迁移规律与早期渗漏监测，也是多孔介质中可压缩流体和固体相互作用的典型问题。本项目拟以日本长岡二氧化碳咸水层地质封存示范工程为典型实例，结合理论分析、X射线CT扫描和岩石物性测试室内实验，重点考虑影响孔隙压力传播和耗散的四个机理性因素，建立二氧化碳注入过程中咸水层孔隙压力的传播理论模型。把孔隙压力波的扩散方程的解析方法，与三维欧拉（Euler）方程的高精度时空守恒元与解元（CE/SE算法）求解相结合，分析和模拟孔隙压力的耗散过程，阐明孔隙压力的耗散与波动机理，揭示二氧化碳注入过程中咸水层孔隙压的演变规律，提出预测孔隙压耗散的数值方法。有关成果为二氧化碳咸水层地质封存现场地震波监测和安全封存提供理论依据，还有望推动多孔介质流固耦合机制的研究。

本项目《二氧化碳注入过程中咸水层孔隙压的演变规律及预测方法》采用理论分析和数值仿真、岩石压裂渗流微震室内实验和场地监测相结合的方法，初步建立了二氧化碳注入过程中咸水层孔隙压传播的理论模型，基本阐明了孔隙压波的演变规律，提出了预测孔隙压波传播的高精度CE/SE数值模拟算法。具体进展如下。（1）理论分析：在经典孔隙弹性Levy模型基础上，将流体和固体之间的微观界面作为物质界面，重点引入了流体和固体之间的动量和能量交换机制（Forchheimer效应），将孔隙度梯度从源（汇）项中分离，直接引入流通量，初步改进了流-固耦合孔隙弹性介质理论模型。采用傅里叶分析方法，初步给出孔隙压波的传播和衰减关系。提出了孔隙介质中二维黎曼问题，并揭示了孔隙压力波存在接触间断、激波、膨胀波、压缩波等复杂的结构特征。在此基础上初步建立了纳米流体地质封存理论模式，克服地层非均质性对流体运移和孔隙压演化的影响。建立了求解流固耦合模型的三维高精度时空守恒元/解元算法。根据前人的激波管试验结果，对改进的理论模型和CE/SE算法进行了验证。（2）室内试验：开展了两个砂岩试样的压裂渗流微震试验，采用声发射、超声波方法对岩石压裂过程中的微地震、地震波数据进行记录，并监测试样出口和注入端流体压力，初步得出破裂面的滑脱效应和孔隙压梯度对微震起控制作用，出口端孔隙压表现出滞后效应和波动特征。（3）现场示范：结合日本长罓和吉林油田实际地质模型，对上述理论模型和算法进行检验，结果显示，预测的井底压力和二氧化碳突破时间基本一致，但理论计算结果不能有效预测孔隙压的滞后效应。今后需要进一步研究扩散波机制对孔隙压传播的影响机理。