Design of Acid Fracturing Treatments

Abstract

A model has been developed that accurately predicts acid penetration distance; it allows the effects of fracture geometry, acid injection rate, formation temperature, acid concentration, and rock type to be included in the treatment design. Results predicted by the model can be used in modifying acid treatments to maximize the stimulation ratio. Introduction Acid fracturing is a production stimulation technique that has been widely used by the oil industry. In such a treatment, acid or a fluid used in a pad ahead of the acid, is injected down the well casing or tubing at rates greater than the rate at which 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, forming a crack (fracture). Continued fluid injection increases the fracture's length and width. Acid injected into the fracture reacts with the formation to create a flow channel that remains open when the well is put 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 that approaches linear flow. McGuire and Sikora conducted an analog simulation of the productivity of a fractured well that serves as the basis productivity of a fractured well that serves as the basis for predicting the stimulation achievable with vertical fractures. Their study indicated that the variables that determine stimulation ratio are the ratio of fracture length to drainage radius, L/re, and the ratio of fracture conductivity to formation permeability, wkf/k. To design an acid fracture treatment, therefore, it is necessary to predict the fracture geometry during the treatment, the predict the fracture geometry during the treatment, the conductive fracture length, and the fracture conductivity created by acid reaction. A number of authors have studied various aspects of acid fracturing treatment design. Methods for predicting fracture geometry were first proposed by Howard and Fast. Techniques that give improved results have recently been presented by Keel and Geertsma and de Klerk. Although presented by Keel and Geertsma and de Klerk. Although these last two calculation procedures differ somewhat in formulation, the resulting geometry predictions are in agreement. Either procedure, therefore, can be used to predict the dynamic fracture geometry in acid fracturing predict the dynamic fracture geometry in acid fracturing treatments. Acid reaction characteristics have been studied in static reaction tests by several authors and design procedures using data from these tests were proposed procedures using data from these tests were proposed by Hendrickson et al. Use of the static test to design acid fracturing treatments is of marginal value since the test models only the ratio of fracture area to acid volume. An improved design procedure was presented by Barron et al., who studied acid reaction by flowing acid through a channel between limestone plates and derived a correlation to relate acid penetration distance along a fracture to treatment variables. The usefulness of the correlation is limited, however, since the experiments were run in a smoothwalled fracture, at room temperature, and with the fracture oriented in a horizonal plane. Smith et al. studied acid reaction at high temperatures in a reaction cell where reactive plates of limestone were rotated through acid and noted the effect of velocity on acid spending time and acid penetration. JPT P. 849