Experimental assessment of the stress-sensitivity of combined elastic and electrical anisotropy in shallow reservoir sandstones
Experimental assessment of the stress-sensitivity of combined elastic and electrical anisotropy in shallow reservoir sandstones
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DOI：
10.1190/geo2019-0612.1
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发表时间：
2020-09
期刊：
影响因子：
3.3
通讯作者：
I. Falcon‐Suarez;L. North;B. Callow;G. Bayrakci;J. Bull;A. Best
中科院分区：
文献类型：
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作者：
I. Falcon‐Suarez;L. North;B. Callow;G. Bayrakci;J. Bull;A. Best
地震和电磁特性通常是各向异性的，取决于微观岩石组构和宏观应力场。结合超声波（0.6 MHz P波速度，[公式：见正文]和衰减[公式：见正文]）和电阻率测量，我们通过实验评估了固结不良（孔隙度约为0.35）砂岩（广泛代表浅层储层）的应力相关各向异性。我们使用了从露头砂岩样品中提取的三个岩心，相对于可见的地质层面呈0°、45°和90°角，并使它们经受具有不同围压（20-35 MPa）和孔隙压力（2-17 MPa）的卸载/加载循环。结果表明，应力场方向、加载历史、岩石组构和测量尺度都对弹性和电性各向异性产生影响。[公式：见正文]、[公式：见正文]和三个方向上的电阻率之间的强线性相关性（[公式：见正文]）表明，应力方向同样会影响固结不良、高孔隙度（浅）砂岩储层的弹性和电性质。然而，电阻率对孔隙压力变化（有效应力系数[公式：见正文]）更敏感，而P波特性提供了有关围压（来自[公式：见正文]，n略小于1）和孔隙压力（来自[公式：见正文]，n略大于1）变化的同时信息。我们发现n也是各向异性的三个测量属性，因为更激烈和快速的晶粒重排发生时，应力场的变化，从斜应力方向相对于岩石分层的结果。总之，我们的研究结果强调了联合弹性-电应力相关各向异性评估的潜力，以提高在生产或注入活动期间对储层的地质力学解释。
Seismic and electromagnetic properties generally are anisotropic, depending on the microscale rock fabric and the macroscale stress field. We have assessed the stress-dependent anisotropy of poorly consolidated (porosity of approximately 0.35) sandstones (broadly representative of shallow reservoirs) experimentally, combining ultrasonic (0.6 MHz P-wave velocity, [Formula: see text], and attenuation [Formula: see text]) and electrical resistivity measurements. We used three cores from an outcrop sandstone sample extracted at 0°, 45°, and 90° angles with respect to the visible geologic bedding plane and subjected them to unloading/loading cycles with variations of the confining (20–35 MPa) and pore (2–17 MPa) pressures. Our results indicate that stress field orientation, loading history, rock fabric, and the measurement scale all affect the elastic and electrical anisotropies. Strong linear correlations ([Formula: see text]) between [Formula: see text], [Formula: see text], and resistivity in the three considered directions suggest that the stress orientation similarly affects the elastic and electrical properties of poorly consolidated, high-porosity (shallow) sandstone reservoirs. However, resistivity is more sensitive to pore-pressure changes (effective stress coefficients [Formula: see text]), whereas P-wave properties provide simultaneous information about the confining (from [Formula: see text], with n slightly less than 1) and pore pressure (from [Formula: see text], with n slightly greater than 1) variations. We found n is also anisotropic for the three measured properties because a more intense and rapid grain rearrangement occurs when the stress field changes result from oblique stress orientations with respect to rock layering. Altogether, our results highlighted the potential of joint elastic-electrical stress-dependent anisotropy assessments to enhance the geomechanical interpretation of reservoirs during production or injection activities.