Acid Fracturing Treatment Research Articles

Design of Acid Fracturing Treatments

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

Characteristics of Acid Reaction in Limestone Formations

Abstract A kinetic model for the reaction of Hydrochloric acid with limestone bas been determined. Reaction order and rate constant for this model were calculated from experiments where acid reacted with a single calcium carbonate plate. Experiments were performed so that acid flow past the plate and mass transfer rate to the rock surface could be calculated theoretically. The resulting model, therefore, accurately represents the acid reaction process at the rock surface and is independent of mass transfer rate. Combination of this model with existing theory allows prediction of acid reaction during acid fracturing operations. A model for acid reaction in wormholes created during matrix acidization treatments is presented along with data for reaction of hydrochloric, formic and acetic acids in a wormhole. Factors limiting stimulation in acid fracturing or matrix acidizing treatments are then discussed. Introduction To predict the stimulation ratio resulting from acid fracturing or matrix acidizing treatments it is necessary to know the rate of acid reaction under field conditions. In acid fracturing treatments, for example, reaction occurs as acid flows through a narrow fracture. Reaction in a matrix treatment occurs during flow through wormholes (channels of roughly circular cross-section) created by acid reaction. In both treatments, a large amount of mixing occurs during flow through the fracture or channel as a result of tortuosity and wall roughness. Reaction rate can be obtained from experiments, or predicted by theoretical calculations that accurately model field conditions. In general a theoretical procedure is preferred since it can be used without recourse to laboratory testing. Acid-reaction-rate data have been reported from a number of experiments intended to simulate acid reaction in field treatments. Tests most often used are: (1) the static reaction rate test, in which a cube of limestone is contacted with unstirred acid at a known ratio of rock surface area to acid volume; (2) flow experiments, where acid is forced to flow between parallel plates of limestone; and (3) dynamic tests, whine limestone specimens are rotated through an agitated acid solution. In general, these tests model some aspects of the reaction process, such as area to volume ratio, or acid flow velocity, but do not accurately model all field conditions. To obtain an accurate mathematical model for field treatments, assuming fracture or wormhole geometry is known, it is necessary to characterize acid reaction kinetics at the limestone surface, rate of acid transfer to the surface, and rate of fluid loss from the fracture or wormhole. (Each of these processes is shown schematically in Fig. 1.) processes is shown schematically in Fig. 1.) Reaction kinetics are independent of the geometry in which reaction occurs; therefore, once kinetics have been determined for a given acid-rock system field treatments can be simulated by prediction of the rate of acid transfer to the surface and fluid loss to the formation. Unfortunately, experiments reported to dare do not allow determination of a kinetic model.

Stimulation of Thick Mississippian Carbonate Sections In Foothill Wells

Abstract This paper will review the development of the Quirk Creek gas field and the techniques used to drill, complete and stimulate the comparatively thick heterogeneous Mississippian sections found in the earlier wells. It will discuss the disappointing results obtained from these conventional stimulation treatments, and describe in detail the systematic efforts subsequently made to improve the deliverabilities of individual wells by selective and largeyo1ume acid-fracturing treatments. Introduction IN JUNE 1967, the first Quirk Creek well was spudded by Columbian Northland on a farm-in from Imperial Oil. In August 1967, the Missippian reef was penetrated and proved to be gas-bearing. Seven of the next eight wells also produced gas. This paper describes the original completions of the first four wells, and the remedial stimulation treatments subsequently used to increase their deliverabilities. Description Of The Field Quirk Creek is located 25 miles southwest of Calgary, near the north end of the Turner Valley Pool (Figure 1). The current pool size is illustrated in Figure 2, which shows the eight potential gas wells and the one dry hole. The field boundaries are defined by a fault to the northeast and a gas/water contact to the southwest. This forms a banana-shaped structural trap, the ends of which are limited by both a gas/water contact and a tightening of the formation. The better wells are located near the center of the field, where the gas plant will be built. The Mississippian reef is 600 feet thick and consists of massive limestone beds interspersed with dolomites. The porosity, ranging from that caused by very small vugs to minute inter crystalline porosity, is generally found in the dolomite. Some vertical fracturing has been noted in the cores. The net pay varies in thickness between 125 and 175 feet, and is scattered throughout the reservoir in isolated stringers as shown in Figure S. These stringers range in thickness from 2 to 50 feet, with porosities of up to 19 per cent and an over-all average of 8 per cent over the productive intervals. Low permeabilities characterize the reservoir. The average is 1.25 md, although individual streaks may reach as high as 100 md, The bottom-hole temperature at 120 °F is abnormally low the bottom-hole pressure is 2,200 psi. The average depth of the wells is 6,550 feet. The gas is moderately rich in condensate, ranging from 9 to 11 barrels per MMcf of gas. The hydrogen sulpbide content ranges from 8 to 10 percent. Original Completion And Stimulation Techniques The completion techniques used in the first four wells – 6–5, 11–33, 10–28 and 11–11 – followed recognized gas-well completion practices and are summarized on Table 1. In most cases, the well bore was evacuated with nitrogen, then perforated with one or two holes in each permeable internal as determined from micrologs and core analyses. The total number of perforations per well varied from 36 to 88, depending on the total thickness of the pay and the number of porous stringers.

The Effect of Flow on Acid Reactivity in a Carbonate Fracture

Abstract A definite relationship has been found between the reactivity of flowing hydrochloric acid and its shear rate in a carbonate fracture. Both flow velocity and fracture width affect the acid reaction rate. Laboratory studies were conducted on acid reactivity at different flow velocities through horizontal-linear fractures, using 15 per cent hydrochloric acid at 80F and approximately 1,100 psi. Fracture width varied from 0.02 to 0.20 in. These data provide a new basis from which the spending time and penetration of the acid can be estimated. Equations were derived expressing the relationship between injection rate, fracture width, acid concentration, time and fracture height, for linear and radial fracture systems. Because the penetration of the acid before spending is closely related to the extent of productivity increase resulting from an acidizing treatment, these data provide a valuable insight into some of the controlling factors that must be taken into consideration during treatment preplanning. Introduction Acidizing of carbonate reservoirs to improve production characteristics has been widely practiced since 1932. Originally, it was assumed that the acid uniformly penetrated natural formation pores and flow channels, enlarging them and thereby increasing their flow capacity. Little consideration was given to the reaction rate of the acid, or how far it would penetrate away from the wellbore into the formation, before spending. It has been shown recently that, unless fractures are present in the rock, very little penetration is attained before spending, and the benefits of the acidizing treatment are largely confined to the immediate vicinity of the wellbore. Therefore, acid treatments may be classified into two categories:matrix acidizing, in which the acid flows through multiple formation pores; andfracture acidizing, in which the bulk of the acid travels through fractures in the rock, whether natural or induced. When acidizing treatments are conducted at pressures of sufficient magnitude to open and extend such fractures, it is often desirable to inject a propping agent to hold the fracture open after the treating pressure has been released, thus providing a highly conductive flow channel through the rock. In some cases where acid attack produces surface irregularities, the resulting flow passages can be sufficient to provide high conductivity without use of a propping agent. During injection, the acid dissolves the carbonate rock with which it comes in contact until it is spent. Deeper penetration of the spent acid into the formation produces no appreciable benefit (if no propping agent is used) because the unetched fracture faces will rejoin when treating pressures are released and the fracture will "heal", with negligible resultant conductivity. The spending time of the acid thus becomes important in determining how far from the wellbore the improved- conductivity zone extends. The spending time of acid after injection into carbonate rock depends on the rate at which the acid reacts with the rock. This in turn is controlled by a number of factors, as previously reported. These include temperature, pressure, acid concentration, rock composition, injection rate and the area-volume relationship between the acid solution volume and the surface area of the formation flow channels through which it penetrates. Thus, in matrix acidizing, where an extremely large area is exposed to the acid, spending is rapid. In contrast, spending time is prolonged in open fractures, where the area-volume ratio is much lower. Thus, greatest penetration of the acid before spending will be achieved in acid-fracturing treatments where fractures are held open by hydraulic pressure. Perkins and Kern have presented equations for fracture-width determinations which, for hard formations such as limestone or dolomite, indicate that high injection rates and viscous fluids are required to create wide fractures. Increasing the injection rate will, in itself, produce faster reaction rates. Because this type of reaction can be considered to be first-order diffusion-controlled, the rate of movement of fluid past the rock surface affects the thickness of the diffusion layer, in turn affecting the reaction rate. Staudt, et al, report the results of tests in which a rotating marble cylinder was immersed in acid, and the weight loss at different speeds determined. Such test conditions, however, are not truly indicative of the situation in a formation during an acid-fracturing treatment. JPT P. 409^