STRESSED PERMEABILITY IN SHALES: EFFECTS OF MATRIX COMPRESSIBILITY AND FRACTURES-A STEP TOWARDS MEASURING MATRIX PERMEABILITY IN FRACTURED SHALE SAMPLES

摘要

To assess how fractures affect the fluid flow in shale plugs,we conducted stressdependent permeability measurements on both intact and fractured shale samples. We characterized the degree of fracturing with the help of micro X-ray CT images. As expected,permeabilities decreased significantly during the initial effective stress increase. During the subsequent effective pressure decrease,the permeability remained relatively unchanged. The degree of hysteresis depended on the sample integrity,i.e. fracture density,type,and distribution. We recorded an average hysteresis loss of 35% for intact samples,55% for samples with a low density of hairline or discontinuous midsized fractures,and over 75% for samples with thick fractures,high fracture density,or continuous mid-sized cracks. Micro X-ray CT images acquired of fractures subjected to increasing confining stress showed that both,hairline and mid-sized fractures closed up completely at sufficiently high confining pressure. While hairline fractures tended to close at confining pressures as low as 1000 psi,thicker fractures required confining pressures as high as 4000 psi. The results of this study suggest that,as the effect of compressibility of the matrix compared to fractures is relatively small,successful matrix permeability measurements can be conducted even on fractured samples provided a sufficiently high confining pressure is being applied.