Prediction of Stimulation From Acid Fracturing Treatments

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.

Experience of Carbonate Acidizing in the Challenging Environment of the Volga-Urals Region of Russia

“…Acid fracturing deals with much more complex physical and chemical phenomena than matrix stimulation. The first numerical models of acid fracturing were reported in work by Nierode et al (1972). Fracture geometry, fluid-loss rate, injection rate, acid penetration distance, temperature, formation type, acid type, and other parameters must be evaluated to design acid fracturing treatments and predict post-treatment well performance.…”

Analysis of Flow Behavior for Acid Fracturing Wells in Fractured-Vuggy Carbonate Reservoirs

“…Generally, carbonate minerals are usually composed of dolomite and calcite which are easy 2 Mathematical Problems in Engineering to be dissolved by hydrochloric acid [8,9]. As one of the fundamental ways to stimulate carbonate reservoirs, acid fracturing alters the flow pattern in the reservoirs from a radial one to a linear one [10] and has dual characteristics of hydraulic fracturing and acidification. On the one hand, like hydraulic fracturing, multiple artificial fractures may occur near the wellbore when injecting fluids into a formation at a high pressure [11][12][13][14].…”

“…However, several field and laboratory observations have shown very complex fracture paths and presence of multiple fractures and multisegmented fractures near the wellbore after hydraulic fracturing [16][17][18][19][20]. On the other hand, the injected acid etches the face of the crack and these acid-etched patterns, namely, wormholes, remain to provide flow channels when hydraulic pressure is released [8,10,[21][22][23]. Therefore, wormholes, together with artificial fractures, form the main channels of the fluid flowing into the wellbore.…”

New Correlations of Acid-Fracture Conductivity at Low Closure Stress Based on the Spatial Distributions of Formation Properties

“…Generally, effective fracture length is shorter than dynamic fracture length and liveacid-penetration distance because the fracture may be closed under closure stress. In early acid-fracture designs Nierode et al 1972;van Domselaar et al 1973;Roberts and Guin 1974;Coulter et al 1974), fracture conductivity was assumed to be infinite to a distance from the wellbore where the acid is 10% of its initial concentration.…”