Prediction of Stimulation From Acid Fracturing Treatments

Abstract

Abstract A procedure for predicting the stimulation ratio that will result from an acid fracturing treatment is presented. This procedure combines a theoretical model for acid reaction during flow along the fracture and experimentally determined rates of acid transfer to the fracture wall to predict the distance that reactive acid can move along the fracture. This distance, called the acid penetration distance, combined with the fracture conductivity allows the stimulation ratio to be predicted. Stimulation ratios predicted using this procedure are compared to results of acid fracturing treatments in limestone and dolomite formations. Included are treatments in Imperial's Boundary Lake and Quirk Creek fields. The predicted stimulation ratio is in general agreement with observed field results, thereby validating the procedure. Introduction ACID FRACTURING is a production stimulation technique that has been widely used by the oil industry. In an acid fracturing treatment, acid, or a fluid used in a pad prior to the acid, is injected down the well casing or tubing at rates greater than the fluid can flow into the reservoir matrix. This injection produces a buildup in wellbore pressure sufficient to overcome compressive earth stresses and the formation's tensile strength. Failure then occurs and a crack (fracture) is formed. Continued fluid injection increases the fracture's length and width. Acid injected into the fracture reacts with the formation to create a flow channel which remains open when the well is placed back on production. To achieve reservoir stimulation, an acid fracturing treatment must produce a conductive flow channel long enough to alter the flow pattern in the reservoir from a radial pattern to one which approaches linear flow. McGuire and Sikora(1) conducted an analog simulation of the productivity of a fractured well which serves as the basis for predicting the stimulation achievable with vertical fractures. Their study indicated that the variables which determine stimulation ratio are the ratio of fracture length to drainage radius, L/r and the ratio of fracture conductivity to formation permeability, wkr/k. To design an acid fracture treatment, therefore, it is necessary to predict the fracture geometry during the treatment, the fracture conductivity created by acid reaction and the conductive fracture length. A number of authors have studied various portions of the over-all problem of acid fracturing treatment design. Techniques for predicting fracture geometry were first proposed by Howard and Fast(2). Techniques which give improved results have been presented by Kiel(3) and Geertsma and deKlerk(4). Although the formulation of these last two calculation procedures is somewhat different, the resulting geometry predictions are in agreement. Either procedure can therefore be used to predict the dynamic fracture geometry in acid fracturing treatments. Broaddus and Knox(15,23) have carried out experiments to determine the conductivity resulting from acid reaction with different formations and have shown that conductivity is a function of formation type, acid concentration, and contact time between acid and rock. The conductivity which will result from an acid treatment, however, cannot be predicted with certainty.