Analysis of Stress Sensitivity and Its Influence on Oil Production From Tight Reservoirs

Abstract

Abstract This paper presents a study of the relationship between permeability and effective stress in tight petroleum reservoir formations. Specifically, a quantitative method is developed to describe the correlation between permeability and effective stress, a method based on the original in situ reservoir effective stress rather than on decreased effective stress during development. The experimental results show that the relationship between intrinsic permeability and effective stress in reservoirs in general follows a quadratic polynomial functional form, found to best capture how effective stress influences formation permeability. In addition, this experimental study reveals that changes in formation permeability, caused by both elastic and plastic deformation, are permanent and irreversible. Related pore-deformation tests using electronic microscope scanning and constant-rate mercury injection techniques show that while stress variation generally has small impact on rock porosity, the size and shape of pore throats have a significant impact on permeability-stress sensitivity. Based on the test results and theoretical analyses, we believe that there exists a cone of pressure depression in the area near production within such stress-sensitive tight reservoirs, leading to a low-permeability zone, and that well production will decrease under the influence of stress sensitivity. Introduction Within the petroleum literature, there are many studies on the sensitivity of permeability to stress fields in tight reservoirs [1-8]. However, most of these studies are carried out in conditions under the low range of effective stress (e.g., generally no more than 7 MPa) as reference stress. Therefore, the extent of "damage" caused by stress or stress sensitivity is found to be very high from such studies. As a result, these studies indicate that low-permeability tight oil reservoirs are inadvisable to be developed under large pressure gradients, because of the formation's high sensitivity to change in effective stress. In fact, during well drilling and core sampling the state of stress within core samples will vary from the initial in situ state of stress, to a mud-hydrostatic-pressure state inside wellbores and to atmospheric conditions on the surface with stress release. If laboratory experimental conditions are not set approximately to actual in situ stress level of reservoirs, experimental results often show substantial changes in core pore-throat structures with changes in effective stress. The resulting stress sensitivity or formation deformation results cannot in general reflect the actual situation in formations. It has been shown in many experiments [9-11] that studies using stress fields lower than those for reservoir conditions overestimate the effects of stress on formation deformation (e.g., the results from laboratory experiments using conventional cores under low effective stress conditions fail to predict realistic changes in pore throats and structures). This paper presents results and analyses of our recent laboratory experiments, conducted under reservoir stress conditions, to study tight oil reservoir stress sensitivity. The specific objective of this work is to investigate the mechanisms by which effective stress affects rock deformation, formation permeability, and porosity, under relevant reservoir conditions. Experimental Method The properties of five core samples used for the experiments are given in Table 1. These core samples are utilized after washing out any oil in the sample and then drying. Dry nitrogen is used as an experimental gas source, a soap-bubble flowmeter is used to measure low-rate gas flow, and a floating-type flowmeter is used to measure high-rate gas flow. Confining pressure is controlled and regulated using a hand pump. The experiments are conducted according to the Reference Standard of China petroleum and natural gas industry, SY/T5358–2002. Minimum effective stress is set at 2 MPa, the original reservoir effective stress is set at 15 MPa, and the maximum effective stress is set at 25 MPa.