低渗透储层应力敏感特征探讨

学术界对低渗透储层是否存在强应力敏感具有较大争议，给油气生产决策带来困扰。争论的焦点在于，渗透率应力敏感实验过程中岩芯是否发生塑性变形，岩芯与封套之间是否存在微间隙及其对实验结果是否具有重要影响。针对传统实验无法证明岩芯是否发生塑性变形的问题，改进实验方法，在渗透率测试前、后增加岩芯力学测试。根据弹性力学理论和有效应力理论，推导出应力敏感评价的理论公式，并进行了定量计算。实验测试和理论计算结果表明，应力敏感实验过程中，岩芯所受的有效应力小于其弹性极限，岩芯不会产生塑性变形；岩芯与封套之间存在微间隙，对渗透率测试结果及变化规律具有重要影响；低渗透岩芯在低有效应力条件下测得的渗透率具有较大误差；微间隙的存在导致岩芯的应力敏感程度被高估；实验过程中岩芯的应变极小，渗透率变化极其微弱；低渗透储层不存在强应力敏感，一般来说岩石渗透率越低应力敏感性越弱。

Stress Sensitivity Characteristics of Low Permeability Reservoirs

Researchers disagree on the existence of high stress sensitivity in low permeability reservoirs,which has caused hindrance in the policy making in oil and gas production.The focus of these disagreement is to determine if the core undergoes plastic deformation during permeability stress sensitivity tests,if a micro-gap exists between the core and envelope,and if such a gap exhibits significant influences on the experimental results.In view of the incapability of traditional experiments to verify the plastic deformation of the core,the current experimental method is improved by performing mechanical tests on the core before and after conducting the permeability tests.Based on the theories of elastic mechanics and effective stress,the theoretical formula of stress sensitivity evaluation is deduced and quantitatively calculated.Experimental and theoretical results showed that during the stress sensitivity tests,the effective stress on the core is below the elastic limit,indicating that the core do not undergo plastic deformation.A micro-gap is found between the core and envelope,which has a significant effect on the permeability test results and their variations.The permeability of a low permeability core measured under a low effective stress incur considerable measurement errors.The existence of the micro-gap leads to an overestimation of stress sensitivity of the core.During the experiments,the core strain is diminutive,and the resulting change in permeability is extremely small.Therefore,it is concluded that there is no strong stress sensitivity present in the low permeability reservoirs,and in general,the lower is the permeability of the rock,the lower is the stress sensitivity.