Acid fracturing is a stimulation process in which the dissolution of formation rock leaves unevenly etched fracture faces that creates lasting conductivity after fracture closure. The success of acid fracturing depends on the conductivity created and retained under overburden stress in addition to the length of conductive fracture. In order to have sufficient conductivity after fracture closure, the fracture face must be non-uniformly etched by the acid while the strength of the rock is still maintained at high levels to withstand the closure stress.
A systematic experimental study to investigate the effect of etching pattern and rock strength on the resulting fracture conductivity was conducted in a laboratory facility designed to perform acid fracture conductivity characterization. Detailed etched surface characterization and rock strength measurement were performed on acid etched fracture faces in order to correlate to resulting conductivity.
Resulting fracture conductivities clearly show the importance of characterizing acid etching patterns. In evaluating the measured fracture conductivities, we distinguished between those tests in which a prominent channel was created along the middle of the rock samples from those tests in which more uniformly distributed roughness features were created on the fracture faces. The channels developed in small scale laboratory tests are likely artifacts of the apparatus and cannot be expected to scale to field conditions.
For the roughly etched fracture faces, statistical parameters describing the surface roughness in addition to average fracture width were needed to accurately predict measured conductivities. Both etched pattern characterization and resulting conductivity measurements suggested the need to model conductivity of limestone and dolomite samples separately. New conductivity correlations were developed both for limestone and dolomite formations which matched the experimental results better than previous correlations.
In an acid fracturing process, fracture conductivity is created by acid dissolution along the face of the hydraulically induced fracture. While success of the acid fracturing process depends highly on the resulting fracture conductivity, the resulting conductivity is very difficult to predict as it inherently depends on a stochastic process and is affected by a wide range of parameters. There have been many experimental and theoretical studies to gain understanding of acid fracture performance (Nierode and Kruk 1973; Beg et al. 1996; Gong et al. 1998; Navarrete et al. 1998; Navarrete et al. 2000). Today the commonly used correlation for acid fracture conductivity is still the one developed by Nierode and Kruk (1973). Even though the effect of etching pattern on conductivity has been shown to be significant, most studies on fracture conductivity have placed more emphasis on volume of rock removed than etching pattern generated. There has been little effort to characterize the etching pattern in order to distinguish effect of etching profile and fracture width on resulting conductivity. While conductivity of a channeled etched fracture depends mainly on the etched width of channel, surface profile characterization of a rough acid etched fracture controls created conductivity.
In this study, experimental conditions were scaled to give the same magnitude of acid transport along the fracture, acid leakoff, and acid reaction at the fracture face as occurs in field treatments. The experiments were conducted at an injection rate of 1 L/min to properly represent the hydrodynamic effects, while the core samples were 3 in. in the direction of leakoff to enable better control of leakoff rate. The apparatus was designed for temperatures up to 275°F.