Effect of Foams Used During Carbonate Acidizing

Abstract

Summary Although acidization has been used successfully for many years to increase the productivity of petroleum wells in carbonate formations, demands on the performance and application of the acidizing process are increasing. This study investigated a method of in-situ foam generation that allows deeper wormhole penetration yet uses less acid than conventional methods. The dissolution patterns were imaged with neutron radiography, which provided an in-depth understanding of the effects of foam and other critical parameters. Results show that foam is effective in promoting efficient stimulation, even at low acid injection rates. promoting efficient stimulation, even at low acid injection rates. Introduction The acidizing technique was patented in 1895. The first successful acid job was performed in 1932 on a limestone formation in Michigan. Since then, acidizing has remained an important part of petroleum engineering. The acidizing process involves injection of acid into a wellbore to dissolve some of the surrounding formation rock. This dissolution allows better inflow of formation fluids, easier injection of completion fluids, or easier injection during secondary recovery. Most carbonate acidizations today are performed with HCl plus a mixture of corrosion inhibitors, penetration fluids, and other chemical additives. HCl is a strong acid penetration fluids, and other chemical additives. HCl is a strong acid that is mass-transfer-limited in its reaction with limestone at temperatures above 32F. Consequently, the rate of spending is a function of the rate of injection, and at small injection rates, the acid penetrates only a limited depth before consumption. This in turn causes excessive dissolution near the wellbore and prevents deep stimulation. The most obvious solution to this problem might appear to be the injection of acid at high rates. Pressure limitations, however, sometimes prevent high injection rates. More important, natural heterogeneities cause some formation zones to accept acid at very slow rates. These low-conductivity zones need stimulation the most. Various solutions to this problem have been proposed, most of which incorporate some method to slow the acid's reaction with the rock. Acetic and formic acid react with limestone at a slower rate than HCl because of lower H + concentration. Chemical inhibitors also have been formulated to slow the rate of HCl consumption. Hoefner and Fogler developed a stable acid microemulsion that retarded acid diffusion and thus allowed deeper penetration of live acid. Coreflood experiments with the microemulsion penetration of live acid. Coreflood experiments with the microemulsion produced a breakthrough after injection of about 1 acid PV. While this produced a breakthrough after injection of about 1 acid PV. While this technique demonstrated the ability to stimulate carbonates at low injection rates, more cost-effective methods are needed. This paper describes a method of acidizing in the presence of foam that allows deep stimulation yet uses less acid than previous techniques. The process begins with injection of an aqueous surfactant into a core sample. process begins with injection of an aqueous surfactant into a core sample. Foam is created during acidizing by injecting commingled acid (i.e., nitrogen and aqueous HCl injected simultaneously). Wormholes are formed by the same phenomena as in conventional acidizing, but the presence of foam prevents acid from spending outside the primary dissolution channel. prevents acid from spending outside the primary dissolution channel. Results show the formation of a conductive wormhole rather than dead-end branches. In addition, significant core-face erosion is not a problem, even at very low flow rates. Neutron radiographs were used to study the structure of the wormholes generated by this and other methods. The wormholes created with foam consistently show uniform thickness with very little branching from the primary channel. The long, thin channels indicate the efficiency of this primary channel. The long, thin channels indicate the efficiency of this method, with some experiments producing a channel breakthrough after injection of less than 0.2 PV of 3 N HCl. Background To understand the process of foamed acid stimulation, we must combine knowledge from two complex subjects: the transport of fluids through foam in porous media and the stochastic process of wormholing. The behavior of foam in porous media has been studied extensively. Research in carbonate acidizing has been more limited, although a fairly good understanding of the process has been gained in recent years. Few papers on the use of foams in stimulation contain results from actual acidizing experiments, and none of these papers demonstrates that foam can enhance the wormholing process. Some of the theories on these topics are discussed below. Foams in Porous Media. Foamed fluids have been used in the field for more than 2 decades. Included in the wide range of applications are leakoff control in fracture acidizing, mobility control in waterflooding, flow diversion, and profile modification. The ability of a foam to provide mobility control and to prevent leakoff has prompted many theoretical studies into the structure, mechanism of formation, and transport properties of foams in porous media. Of these topics, the transport of properties of foams in porous media. Of these topics, the transport of liquid through foam is of the most concern to this work. Foam exists in a porous medium as a two-phase system of gas and liquid. Liquid is generally the wetting phase and thus resides as a series of lamellae bridging across pore throats and as thin films on the rock's surface. Gas is a discontinuous phase, residing in the larger void spaces of the medium. The addition of a surfactant allows the foam to maintain a stable two-phase configuration in which the lamellae can break and reform during dynamic events. The texture of a foam refers to the number density of gas-phase bubbles. The quality of a foam is the volume percentage of the pore space occupied by the gas phase. Bernard et al. performed the initial work relating to liquid (water) transport through a foam and found that the liquid permeability of a porous medium does not depend on the foam structure but rather on only the fluid saturation. Thus, fluid leakoff could be prevented simply by the introduction of a second phase (e.g., gas). However, foams are generally considered to be especially effective in reducing liquid permeability because they provide a stable method of maintaining a low liquid saturation, even during the flow of surfactant-free water. Holm later showed that liquid flows through foam by means of continuous films and lamellae. The implication of this flow mechanism is that the smaller pores containing no foam (i.e., no gas) will carry the majority of the liquid flow. This concept contradicts single-phase models that proportion the amount of fluid flow to the diameter of channels in a porous medium. Carbonate Acidizing. The stimulation of a carbonate is very different from the process that occurs in sandstone, primarily because the entire matrix of a carbonate rock is reactive. As a result, carbonate acidizing causes the formation of large flow channels (relative to the pore size) in some portions of the rock, while other portions are unaffected. This type of portions of the rock, while other portions are unaffected. This type of dissolution is extremely heterogeneous. Because of their macroscopic size, these flow channels are highly conductive to fluid, thus increasing the effective permeability of the medium. In contrast, the stimulation of sandstone causes pore-scale dissolution throughout the matrix so that the permeability is increased more homogeneously. permeability is increased more homogeneously. SPEPE p. 350