Dynamic Etching Tests Aid Fracture-Acidizing Treatment Design

Abstract

Summary Satisfactory stimulation of oil- and gas-bearing formations by fracture acidizing depends on development of adequate fracture conductivity in the hydraulically induced fracture. The created fracture conductivity is a result of the reaction of the injected acid with the fracture face. This paper describes a laboratory core-testing procedure to evaluate the feasibility of an acid treatment and to allow optimization of several treatment parameters. Applications to treatment design for formations that typically are difficult to stimulate by fracture acidizing are given. Introduction The use of various fracturing methods for stimulation of wells has become common in the oil and gas industry. Fracturing treatments are performed on wells of various potentials to help increase production. Concurrent with the desire for increased production is the need to optimize treatment designs and to predict the expected increase, regardless of whether the stimulation method is hydraulic fracturing with proppants or fracture acidizing. Over the past 25 years, numerous authors have investigated the factors affecting the production increase of a fractured well. Early studies used curves developed from electrical and mathematical models to predict theoretical production increases. The advent of high-speed computers has allowed production-increase calculations to advance to the point where the theories of transient-pressure analysis are used with numerical simulators or type-curve simulators. Regardless of the method selected, all production- increase calculations indicate that three factors are critical to the success of a fracturing treatment: effective fracture length, fracture height, and fracture conductivity. In hydraulic fracturing with proppants, the ability to predict fracture geometry, and hence fracture length and height, has always been an important research topic. Information on the subject of realistic downhole propped fracture conductivities has recently increased dramatically. Fracture geometry relating to carbonate stimulation by fracture acidizing has also been researched. With regard to fracture conductivity, however, more emphasis has been placed on the volume of rock removed and the length of etched fracture than on the actual conductivity generated. Most etched-fracture-conductivity calculations are based on an ideal fracture conductivity. While recent authors have addressed the effect of variable fracture conductivity on the productivity of a fracture-acidized well, most authors have ignored the topic of acid-etched fracture conductivity as it relates to specific reservoir rock characteristics. This paper presents a laboratory technique to determine the etched conductivity resulting from acid reaction with formation cores. While the actual downhole fracture conductivity resulting from a fracture-acidizing treatment cannot be predicted with certainty, this test has been very useful in optimizing treatment designs. Of particular interest are treatment design modifications that have allowed successful stimulation of formations that historically have not responded well to conventional fracture-acidizing techniques. Factors Affecting Etched Fracture Conductivity In fracture acidizing, the created fracture conductivity is a result of the action of acid on the exposed formation face. The two primary factors influencing the resultant fracture conductivity are the quantity of rock removed and the pattern of rock removal. Kinetic parameters-such as acid type and strength, reaction temperature, reaction time, and flow regime-affect the amount of rock removed during the acidizing process. Not only is the total amount of rock removed important, but an estimate of the relative amounts removed in each element of the fracture must be made to optimize treatment designs and to predict the magnitude of production improvement that might be expected. A number of experimental and theoretical models are currently used to help predict the distance that live acid will penetrate along a hydraulically induced fracture and the amount of reaction that will occur in each segment of the fracture. Recent studies have even addressed the effects of ad-ditives and filterable solids on acid reaction rates. hile the kinetic parameters govern the depth of live acid penetration, formation characteristics dominate the conductivity, re-sulting from the acidizing process. The mineralogical composition of a formation is probably the most important factor because the etching pattern will be a direct result of the degree of heterogcneity in the fracture face. Any rock characteristic that contributes to heterogeneity in the formation will enhance differential etching. The physical and chemical composition of the formation rock will influence the reaction rate of the acid; as a result, some areas will be dissolved to a greater extent than others. For example, dolomites and iron carbonates tend to react more slowly than calcite. In a formation with uniform chemical composition, variances in crystal size may also affect the rate of reaction because the action of acid is a surface phenomenon. Differences in permeability and porosity can also affect the etching pattern because of variable acid leakoff rates. Once differential etching is achieved, formation hardness and fracture closure stress influence the resultant fracture conductivity. As with propped fractures, the conductivity of an etched fracture will decrease with increasing closure stress. The magnitude of reduction in conductivity depends on the hardness of the formation and the ratio of supporting area to etched area. No accurate mathematical model can accurately predict the fracture conductivity that will result from the etching process. One proposed empirical method uses the dissolved-rock equivalent conductivity and a correction factor that is a function of the rock embedment strength and formation closure stress, however, this method does not account for the mineralogical characteristics of a formation. Experimental Dynamic Acid-Etched-Fracture-Conductivity Tests Etching tests are performed to measure the conductivity of a created and acid-etched fracture. Test samples are circular disks, 2.25 to 4 in. [5.72 to 10.2 cm] in diameter and 0.50 in. [1.27 cm] thick. These samples are cut from the formation core so that the surface exposed to acid is in the same plane as a vertical fracture. Orientation of the samples can be varied, if necessary, depending on specific well conditions. Core disks are mounted in lead and the faces of the samples turned flat with a lathe. Other investigators used rough, broken surfaces for etching studies. A flat surface has proved better for comparative studies, however. because the starting point of a smooth surface eliminates one experimental variable. Also, initiating etching over a flat surface makes achieving, differential etching inherently more difficult because minimum turbulence exists at the fracture face during the first portion of the test. It has been shown that increased turbulence generally increases the rate of dissolution of the fracture face. After the face of the test sample is turned flat, a hole is drilled through the axis of the sample and the lead mounting. The sample is then placed in the etching test cell with the turned face of the sample touching an ampcoloy plate, which acts as the opposite fracture face. SPEPE P. 443^