SPE 31098 A Systematic Experimental Study of Acid Fracture Conductivity Mirza S. Beg, SPE, A. Oguz Kunak, SPE, Ming Gong, SPE, Ding Zhu, SPE, and A. Daniel Hill, SPE, U. of Texas at Austin Copyright 1996, Society of Petroleum Engineers, Inc.
Acid fracturing is a stimulation technique in which acid is injected at pressures above the parting pressure of the formation so that a hydraulic fracture is created. Usually, a viscous pad fluid is injected ahead of the acid to initiate the fracture, then plain acid, gelled acid, foamed acid, or an emulsion containing acid is injected. Fracture conductivity is created by the acid differentially etching the walls of the fracture; i.e., the acid reacts nonuniformly with the fracture walls so that after closure, the fracture props itself open, with the relatively undissolved regions acting as pillars that leave more dissolved regions as open channels. Thus, acid fracturing is an alternative to the use of proppants to create fracture conductivity after closure. The primary issues to be addressed in designing an acid fracturing treatment are the penetration distance of live acid down the fracture, the conductivity created by the acid (and its distribution along the fracture), and the resulting productivity of an acid fractured well. Since acid fracturing should be viewed as an alternative means of creating fracture conductivity in a carbonate formation, sufficient fracture conductivity must be created with the acid compared with the conductivity that can be obtained with proppants.
The conductivity (kfw) of an acid fracture is difficult to predict because it inherently depends on a stochastic process; if the walls of the fracture are not etched heterogeneously, very little fracture conductivity will result after closure. Thus, the approach taken to predict acid fracture conductivity has been an empirical one. Most predictions are made with the Nierode-Kruk correlation, which was developed based on a series of laboratory measurements of acid fracture conductivity. In these experiments, acid was flowed through a vertical fracture created by breaking core samples in tension, but there was no fluid loss through the rock samples. Other than this work, very few studies of acid fracture conductivity have been reported.
In this paper, we present the results from a series of acid fracture conductivity tests performed with a special acid fracture conductivity cell that allows for acid flow through a vertical fracture with fluid loss. We have studied the effects of rock type, acid contact time, and acid fluid loss on the resulting fracture conductivity and compared these results with the predictions of the Nierode-Kruk correlation. We find that the Nierode-Kruk correlation accurately predicts the effect of rock embedment strength and closure stress on acid fracture conductivity, fluid loss affects fracture conductivity, and the Nierode-Kruk correlation gives better predictions if the acid that flows into the formation through fluid loss is not included in the prediction. Contrary to most predictions, we find that acid fracture conductivity is sometimes lower with longer acid contact times than shorter ones. We also observed a strong dependence on the etching pattern; i.e., when the acid tended to form a channel along the core faces, the conductivity was higher than when less heterogeneous etching occurred.
Experimental Apparatus and Procedures In our studies, the acid fracture conductivity cell designed by Malik and Hill was used with some modification to make the assembly of the cell easier and the measurement more reliable. Figure 1 is a top view of the cross section of the acid fracture conductivity cell as it would appear during acidizing with two core samples mounted inside. P. 283