毛细作用对部分饱和孔隙岩石弹性性质的影响规律研究

Capillarity is a crucial petrophysical parameter which is extensively used by petroleum engineer for evaluating the reservoir quality and performance. Capillary force controls the two-phase flow behavior and is inherently coupled with fluid distribution. In low porosity and low permeability rocks, capillarity becomes more dominant in controlling fluid flow due to the presence of nano-scale pore channels. In conventional rock physics model building, the capillary force effect has been largely overlooked. In this proposal, we plan to investigate the influence of capillarity on the elastic properties of the partially saturated porous rocks. The effect of the microscopic capillary force in response to dynamic wave propagation can be represented by a macroscopic boundary condition in poroelasticity. By solving the corresponding boundary value problem, we investigate the effect of capillarity on the redistribution of the wave-induced fluid pressure gradient and the resulting dissipation due to viscous fluid flow. We further incorporate the capillary effect in the rock physics models, i.e., we generalize the Gassmann model of fluid substitution and the continuous random patchy saturation model for modeling velocity dispersion and attenuation, respectively. Combining ultrasonic core measurement and nuclear magnetic resonance imaging, we investigate the effect of capillarity on the velocity- and the attenuation- saturation relations. We achieve different fluid distribution and surface tension by varying the injection rate and by adding the surfactant in the injected fluid, respectively. Through processing the imaging results, we establish the relationship between the saturation and the morphology parameters, such as the autocovariance function of the fluid distribution, the gas-water correlation length, the specific surface area of the fluids. By modeling the laboratory data using the new model, we investigate the effect of pore structure (i.e., characteristic pore diameter, tortuosity), porosity, permeability, surface tension and fluid distribution on the velocity- and attenuation- saturation relations. We also compare the results with the predictions from the conventional model to examine the applicability of the new theory.

在致密储层岩石中，纳米级微细孔隙吼道发育，毛细作用将显得更加突出。这导致孔隙流体与孔隙介质表面形成更为复杂的流固耦合系统而不完全符合经典渗流方程，并伴随出现非均匀的流体空间分布特征。传统的岩石物理模型大多忽视了毛细力对弹性波速度和衰减的影响。本项目就毛细力对部分饱和孔隙岩石弹性性质的影响规律展开研究。基于能够反映流体表面张力、岩石渗透特征以及微观孔隙结构特征的宏观孔弹性动力学边界条件，从流固耦合力学模型入手，系统研究毛细力与波致流联合作用下的孔压平衡过程与粘性耗散机理。基于等效介质理论，对经典Gassmann方程以及斑块饱和模型进行推广，建立纳入毛细作用的新岩石物理模型。通过岩心驱替过程中的超声测量和核磁共振成像实验，研究低渗透储层岩石的弹性波速度及衰减随流体饱和度的变化关系。比较毛细模型与传统模型对实验结果的预测，考察新理论适用性。研究将为非匀质致密储层的地震流体检测提供理论及实验依据。

本项目开展了毛细作用对部分饱和孔隙岩石弹性性质的影响规律研究。探讨了孔隙-裂隙-流体之间的相互作用机理及其引起的频散衰减特征，研究了毛细力对于含不同流体孔隙介质分界面处地震波反射系数的影响规律，同时形成了一套基于弹性波衰减的低孔渗储层含气饱和度评价方法流程。本项目主要研究成果如下：（1）提出了具有复杂流体分布特征的岩石物理新模型;（2）建立了毛细力作用下孔隙介质地震波反透射系数的解析表达式；（3）基于毛细力作用下的弹性波频散和衰减特征，提出了一种新的低孔渗储层岩石的饱和度计算方法。实际应用结果表明，该方法能够对页岩及碳酸盐岩等致密储层的含气性进行有效评价。