Mechanical Behavior of Small-Scale Channels in Acid-etched Fractures

Abstract

The conductivity of acid-etched fractures highly depends on spaces along the fracture created by uneven etching of the fracture walls remaining open after fracture closure. Formation heterogeneities such as variations of mineralogy and permeability result in channels that contribute significantly to the fracture conductivity. Current numerical simulators or empirical correlations do not account for this channeling characteristic because of the scale limitations. The purpose of this study is to develop new correlations for conductivity of acid-etched fracturing at the intermediate scale. The new correlations close the gap between laboratory scale measurements and macro scale acid fracture models. Beginning with acid-etched fracture width profiles and conductivity at zero closure stress obtained by the previous work, I modeled the deformation of the fracture surfaces as closure stress is applied to the fracture. At any cross-section along the fracture, I approximated the fracture shape as being a series of elliptical openings. With the assumption of elastic behavior for the rock, the numerical simulation presents how many elliptical openings remain open and their sizes as a function of the applied stress. The sections of the fracture that are closed are assigned a conductivity because of small-scale roughness features using a correlation obtained from laboratory measurements of acid fracture conductivity as a function of closure stress. The overall conductivity of the fracture is then obtained by numerically modeling the flow through this heterogeneous system. The statistical parameters of permeability distribution and the mineralogy distribution, and Young’s modulus are the primary aspects that affect the overall conductivity in acid-etched fracturing. A large number of deep, narrow channels through the entire fracture leads to high conductivity when the rock is strong enough to resist closure stress effectively. Based on extensive numerical experiments, I developed the new correlations in three categories to predict the fracture conductivity after closure. Essentially, they are the exponential functions that incorporate the influential parameters. Combined with the correlations for conductivity at zero closure stress from previous work, the new correlations are applicable to a wide range of situations.