An Experimental Study on Stress Sensitivity of Tight Sandstones with Different Microfractures

Xinli Zhao,Shengchun Xiong,Yutian Luo,Wei Lin,Debin Xia,Zhongkun Niu,Zhiyuan Wang,Zhengming Yang

doi:10.1155/2020/1865464

Abstract

Aiming at the stress sensitivity problem of tight reservoirs with different microfractures, the cores of H oilfield and J oilfield with different microfractures were obtained through the fractures experiment, so as to study the change of gas permeability in tight sandstone core plug during the change of confining pressure. Besides, we use the nuclear magnetic resonance (NMR) spectra of the core before and after saturation to verify whether the core has been successfully fractured. Based on Terzaghi’s effective stress principle, the permeability damage rate (D) and the stress sensitivity coefficient (Ss) are used to evaluate the stress sensitivity of the core, which show consistency in evaluating the stress sensitivity. At the same time, we have studied the petrological characteristics of tight sandstone in detail using thin section (TS) and scanning electron microscope (SEM). The results show that the existence of microfractures is the main factor for the high stress sensitivity of tight sandstone. In addition, because of the small throat of the tight reservoir core, the throat closes when the overlying stress increases. As a result, the tight sandstone pore size is greatly reduced and the permeability is gradually reduced. Therefore, in the development of tight reservoirs, we should not only consider the complex fracture network produced by fracturing, but also pay attention to the permanent damage of reservoirs caused by stress sensitivity.

Highlights

Aiming at the stress sensitivity problem of tight reservoirs with different microfractures, the cores of H oilfield and J oilfield with different microfractures were obtained through the fractures experiment, so as to study the change of gas permeability in tight sandstone core plug during the change of confining pressure
We have studied the petrological characteristics of tight sandstone in detail using thin section (TS) and scanning electron microscope (SEM). e results show that the existence of microfractures is the main factor for the high stress sensitivity of tight sandstone
When the core permeability is more than 1 × 10−3 μm2, the sensitivity coefficient (Ss) method shows that the core has medium and weak sensitivity

Summary

Introduction

Aiming at the stress sensitivity problem of tight reservoirs with different microfractures, the cores of H oilfield and J oilfield with different microfractures were obtained through the fractures experiment, so as to study the change of gas permeability in tight sandstone core plug during the change of confining pressure. E results show that the existence of microfractures is the main factor for the high stress sensitivity of tight sandstone. Erefore, it is of great theoretical and practical significance to investigate the stress sensitivity of fractured tight reservoirs. It has been nearly 70 years since Fatt and Davis first observed permeability stress sensitivity, and stress sensitivity theory has been further developed and widely applied in the field of underground engineering [9]. We employ the triaxial compression system to fabricate microfractures in tight sandstone core plugs, mimicking the hydraulic fracturing reservoirs. We analyzed the effects of reservoir petrological characteristics on stress sensitivity. is work contributes to the calculation and the evaluation of stress sensitivity of fractured tight sandstone reservoirs

Methods

Results

Discussion

Conclusion