A new acid-fracture conductivity model based on the spatial distributions of formation properties
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The success of acid fracturing depends on uneven etching along the fracture surfaces caused by heterogeneities such as variations in local mineralogy and variations in leakoff behavior. Acid fracture conductivity, created only if less dissolved parts act like pillars to keep the fracture open under closure stress, depends on the spatial distribution of fracture roughness, which is determined by the spatial distribution of formation properties such as permeability and mineralogy distributions. Existing laboratory experiment-based acid fracture conductivity correlations are for random roughness distributions, that is, they do not consider the effect of channels on conductivity. Channels provide greater conductivity at low closure stress and lasting conductivity after fracture closure, and occur on a scale that is neither used in laboratory measurements of acid fracture conductivity which use core samples that are too small to observe such a feature, nor in typical acid fracture simulations, in which the grid block size is much larger than the scale of local heterogeneities. In this study, an extensive numerical study was done using an intermediate-scale acid fracture model with total dimensions comparable to a grid block size in acid fracturing simulators and grid sizes comparable to core sample sizes in laboratory acid fracture conductivity experiments. An acid fracture conductivity correlation consists of two parts: conductivity at zero closure stress and the rate of conductivity change with closure stress. This study focuses on the first part. With the model, by analyzing the relationship among fracture surface etching patterns, conductivities, and the spatial distributions of permeability and mineralogy, we developed acid fracture conductivity correlations as functions of statistical parameters of mineralogy and permeability distributions on the fracture surfaces. The new correlations consider the contribution of channels, which can be created by acids if the formation is heterogeneous in either permeability or mineralogy, or both. In these correlations, the acid fracture conductivities depend strongly on statistical properties of the permeability and mineralogy distributions, namely the standard deviations, and the horizontal and vertical correlation lengths of these distributions. Copyright 2010, Society of Petroleum Engineers.
