Limits of Acid Fracture Conductivity Correlation Improvement Through Acid Fracture Surface Characterization

Abstract

Abstract The classical theoretical framework of fracture conductivity for rough acid-etched surfaces is the Nierode and Kruk correlation, given by an exponential equation of two parts: the conductivity under zero closure stress and the rate of conductivity change with closure stress. The first part has been related to fracture width and the second one, to rock embedment strength. This study, based on experiments that used samples from whole cores of an oil producer limestone, analyzed the fitness of laboratory-measured conductivities to the exponential conductivity correlation model. By means of a comprehensive 3D acid-etched surface characterization, this study showed that the use of surface parameters, measured at zero closure stress, can improve only the first part of the exponential correlation. The rate of conductivity change with closure stress is determined by the fracture width reduction under closure stress, while the effects of asperities in contact and related tortuosity are small. This conclusion is based on a physical hypothesis, on correlations of the surface parameters with the mentioned parts or coefficients, and on the relation between contact area ratio and fracture width reduction. The best acid fracture conductivity correlation developed has its first part based on a surface characterization parameter and the second one based on the rock embedment strength. This correlation showed superior accuracy compared with the Nierode and Kruk correlation and represents the maximum possible improvement through the use of surface parameters for the studied limestone. There is still significant room for improvement in the rate of conductivity change with closure stress. This requires a new model of fracture width deformation under closure stress or at least the replacement of the rock embedment strength for a more representative rock strength parameter.