低渗储层启动压力特征及动用半径的确定

Threshold pressure gradient characteristics and drainage radius determination of low-permeability reservoirs

田巍 李中超 余传谋 韩宏彦 王开成 郭立强 王坤 阳潇

Wei TIAN Zhongchao LI Chuanmou YU Hongyan HAN Kaicheng WANG Liqiang GUO Kun WANG Xiao YANG

针对当前低渗储层启动压力梯度认识不足及动用半径确定方法不完善的问题，采用实验与数值模拟相结合的手段，详细分析了启动压力梯度变化特征和变化机制，并结合实际储层应力条件提出了低渗储层动用半径的计算方法。研究结果表明：①储层岩石启动压力梯度并不是一个定值，而是随着净围压的改变而发生变化，两者呈现良好的正相关关系。②启动压力梯度与净围压的关系曲线呈三段式特征。第一阶段为塑性形变阶段，主要为大裂缝闭合和大孔道急剧压缩，对渗流能力影响较大，启动压力梯度数值提高5倍以上；第二阶段为拟塑性形变阶段，主要为大孔道和微裂缝压缩，启动压力梯度数值提高40 %以上；第三阶段为弹性形变阶段，主要为小孔道的压缩，对渗流能力影响小，启动压力梯度数值变化幅度较小，仅提高了17 %。实际低渗储层中岩石的应力状态大多处于第三阶段。③以启动压力梯度动态变化为基础，以实际储层存在的压降漏斗为依据确定净上覆压力变化情况，并对储层中某一段渗流距离上的启动压力进行积分计算。在该启动压力与驱动压差数值相等的情况下反算求得渗流距离（即驱动动用半径），并通过实例应用验证了该方法的可行性。本研究深化了对低渗储层特征的认识，为储层评价及开发方案优化提供了重要的理论依据和技术支撑。

In view of the limited insights into the threshold pressure gradients （TPGs） of low-permeability reservoirs and inadequate methods for determining their drainage radii， we present a detailed analysis of the variation characteristics and mechanisms of the TPG in combining experiments with numerical simulations， and propose a method for calculating the drainage radii of low-permeability reservoirs which considering the stress conditions of actual reservoirs. The results indicate that the TPG of a reservoir is not a fixed value. Instead， it changes with the net confining pressure， exhibiting a strong positive correlation. The curve showing the relationship between the TPG and the net confining pressure follows a three-stage pattern. In the first stage， plastic deformations occur， primarily involving the closure of microfractures and the sharp contraction of large pore throats. This stage has a significant impact on the seepage capacity of reservoirs， with the TPG increasing by more than five times. The second stage is characterized by pseudoplastic deformations， principally reflected in the contraction of large pore throats and microfractures， with the TPG increasing by over 40 %. The third stage shows elastic deformations， characterized primarily by the contraction of small pore throats. This stage has a minimal impact on the seepage capacity， with the TPG increasing by only 17 %. Notably， the stress state of rocks in most actual low-permeability reservoirs corresponds to this elastic deformation stage. Based on the dynamic changes in the TPG and the presence of pressure drop funnels in actual reservoirs， we determine the changes in the net overburden pressure and then calculate the threshold pressure along a certain seepage distance in reservoirs through integration. Assuming that the threshold pressure equals the driving pressure difference， we derive the seepage distance （i.e.， the driving drainage radius） through inversion. Finally， the feasibility of the proposed method for determining the drainage radius is verified through practical application. This study enhances the understanding of the characteristics of low-permeability reservoirs while also providing a significant theoretical basis and technical support for the assessment and exploitation scheme optimization of low-permeability reservoirs.

10.11743/ogg20250317