Summary

The equilibrium-acid-fracturing technique was developed to stimulate wellsin the Wasson San Andres Denver Production Unit. This new treatment techniquemaximizes acid contact time with the fracture faces while allowing control ofthe created fracture dimensions. Maximum acid contact time is essential tocreate highly conductive etched pathways on the fracture faces of cool dolomiteformations that react slowly with acid. Control of fracture dimensions isimportant in the San Andres Denver Unit because fractures tend to grow uncontained in at least one vertical direction and the oil column is bounded by permeable gas-bearing intervals above and permeable water-bearing intervals below. With this technique, a fracture of desired dimensions is created by injection of acid at fracturing rates. The volume of acid required to create the desired fracture dimensions is determined by a 2D fracture-geometry program with design parameters determined from fracture field testing and laboratory testing. Injection is then continued at reduced rates that maintain equilibrium with the fluid leakoff rate from the created fracture faces. Maintaining equilibrium between injection and leakoff allows the created fracture to beheld open without significant further fracture extension. Equilibrium is achieved in the field by maintaining the injection pressure below the fracture extension pressure but above the fracture closure pressure determined byfracture field testing. This paper presents the background and theory of this technique along with design procedures, field examples, results, and conclusions. Results of the equilibrium-acid-fracture treatments and other acid stimulations performed in the Denver Unit are also compared. performed in the Denver Unit are also compared.

Introduction

The Denver Unit is one of several production units in the Wasson San Andresfield in the west Texas counties of Gaines and Yoakum (Fig. 1). The targetinterval of the San Andres formation, a Permian dolomite, is at about 5,000 ft. The Wasson San Andres Permian dolomite, is at about 5,000 ft. The Wasson SanAndres field was discovered in 1936. Water flooding in the Denver Unit began in 1964, when the unit was formed; CO2 flooding began in 1984, and expansion isongoing today. The average permeability in the Denver Unit is about 5 md. Thepay-quality rock is split up into two major divisions. The first-porosityzones, in the upper part of the reservoir, are underlain by the main pay zones (see Fig. 2 for a type log). The reservoir temperature is about 105 degrees F. An original gas cap and an inactive aquifer exist. The oil column is boundedbelow by pay-quality water-bearing rock in all parts of the unit and bounded above by pay-quality gas-bearing rock in most of the unit. An aggressive workover program made it possible to continue efforts to improve theeffectiveness of well stimulations. The equilibrium-acid-fracturing technique was developed to improve the stimulation results achieved with othertechniques. The typical acid formulation used for most acid stimulations in the Denver Unit, including equilibrium acid fractures, is 28% HCl. Many acidfracturing and acidizing techniques, ranging from matrix acid treatments toviscous fingering of acid through a gelled fluid, have been used throughout theindustry. The equilibrium-acid-fracturing technique is significantly different from the other fracture acidizing techniques because it maximizes acid contacttime with the fracture faces while allowing for control of the created fracture dimensions. A fracture of desired dimensions is created by injection of acid at fracturing rates. The volume of acid required to create the desired fracturedimensions is determined by the fracture-geometry program ENERFRAC with designparameters obtained from fracture field testing. After the fracture is created, the acid injection rate is reduced until it matches the fluid leakoff rate fromthe fracture. When these rates match, an equilibrium is established and thecreated fracture can be held open without significant further extension. Inpractice, equilibrium is obtained by adjusting the injection rate to practice, equilibrium is obtained by adjusting the injection rate to maintain theinjection pressure below the fracture extension pressure but above the fractureclosure pressure (minimum in-situ stress) pressure but above the fractureclosure pressure (minimum in-situ stress) determined from fracture fieldtesting. Equilibrium acid fracturing is used to obtain maximum oil stimulationwithout stimulating the adjacent water or gas zones outside the oil column. This is particularly important in carbonate formations where such properties asparticularly important in carbonate formations where such properties as Young'smodulus, Poisson's ratio, minimum in-situ stress, and propagation pressure arefairly uniform and few barriers to vertical propagation pressure are fairlyuniform and few barriers to vertical fracture extension exist. The extendedacid contact time is desirable in the relatively cool [105 degrees F bottomholetemperature (BHT)] Wasson San Andres dolomite. The equilibrium-acid-facturingtechnique was used successfully to stimulate wells in the Wasson San Andres Denver Production Unit. The significant aspect of this technique is thecontinued etching of the fracture faces for extended periods of time while thefracture is open without further fracture growth after the initial fracturedimensions are created. Other fracture acidizing techniques usually consist ofhigh-rate continuous injection of either acid alone or alternating stages ofacid and various gelled fluids. Often the total fluid volumes for these othermethods are quite high and the stimulations may or may not be designed withregard to the ultimate created fracture dimensions. When fracture growth isuncontained in at least one vertical direction, as in the Wasson San Andresfield, the ultimate created fracture dimensions become important. The createdfracture dimensions become extremely critical to the overall success of thestimulation when the oil column is bounded by productive gas zones above andwater zones below. Stimulation of pay-quality zones outside the oil columnusually result in excessive water and/or gas production, both of whichnegatively affect stimulation. Out-of-zone stimulation can also havedetrimental effects if the field has secondary or tertiary recovery potential. Another fracture acidizing technique consists of creating a fracture with acidand/or other fluids, etching the fracture with acid, allowing the fracture toclose, and finally injecting acid into the closed fracture at pressures belowthe closure pressure. The concept of injecting acid into a closed fracture isalmost opposite that of equilibrium acid fracturing. The equilibrium techniqueholds the fracture open while acid continues to etch its faces withoutsignificant further fracture extension and allows live acid to reach thefracture tip in cool dolomite formations. Injection into a closed fracturetends to concentrate the stimulation effects very near the wellbore because ofthe slow rates required to maintain a closed fracture. This paper does notpresent detailed data and discussion on the reactivity of the San Andresdolomite formation with acid because that topic is thoroughly coveredelsewhere. The effectiveness of the equilibrium-acid-fracturing technique wasproved by field application in a heterogeneous, layered carbonate proved byfield application in a heterogeneous, layered carbonate formation. Largerproduction increases at lower costs were obtained with the new technique thanwith the other stimulations in the same field.

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