Sensitivity Study of Foam Diversion Processes for Matrix Acidization

Abstract

Factors key to the success of foam diversion processes for matrix acidization were measured in Berea cores. Foam mobility was low in high-permeability (847-md) Berea and higher in low-permeability (92 md) Berea, suggesting effective foam diversion. Injection of surfactant solution, emulating foam-compatible acid injected following foam, trapped some of the foam in place. but imputed diversion of acid was not as complete as diversion of foam itself. After a period of injection of liquid, more gas was displaced, starting from the core inlet, and mobility rose further. Using these coreflood results, fractional-flow methods predict effective acid diversion between layers differing in permeability in field application. A sensitivity study indicates that the size of the preflush and the propagation rates of foam within rock are secondary factors in the success of foam diversion in the field. Foam strength and especially gas trapping following foam injection are the keys to successful application. In a process in which gas trapping following foam injection is ineffective, or is less effective in high-permeability layers, a continuous-injection foamed-acid process would outperform a process of alternating slugs of foam and acid. Further data, especially in field cores, are needed to confirm these conclusions. Introduction Foams are widely used to divert acid to desired intervals in matrix acidization treatments. The goal of such a diversion process is to reduce the injectivity of acid into layers where less is needed and thereby divert it into layers more in need of stimulation. Foams do not directly alter the mobility of an aqueous phase such as acid in rock; relative permeability krw is the same function of water saturation Sw in the presence of foam as in its absence. The individual liquid films, or lamellae, that aerate gas bubbles in a foam do restrict the flow of gas, however. By driving down gas mobility, foam indirectly forces down Sw and thereby krw(Sw), accomplishing the process goal of lower acid injectivity into the given layer. Moreover, foams are stronger, reducing liquid mobility more, in higher-permeability layers, diverting acid into lower-permeability intervals that otherwise would not receive much acid. Whether foams are stronger in more-heavily-damaged rock than less-damaged rock is less clear. Many foams collapse in the presence of oil, which could also help divert acid into productive, oil-bearing intervals. We do not address the effects of oil or of formation damage on foam further here, however. The key to foam effectiveness in acid diversion is the ability of acid following foam to maintain low water saturation and low krw(Sw) during acid injection following foam. To accomplish this the acid slug must contain surfactant and be formulated for compatibility with foam. There are no published data on this property for acid slugs, but there are data for surfactant slugs without acid. Persoff et al. found that surfactant injected without acid or gas following foam maintained for some time the same low Sw and krw(Sw) created by the foam, evidently by trapping all the gas in the foam in place. This effect is highly desirable in foam diversion. Bernard et al. found less complete trapping of gas by surfactant solution injected after foam. Zerhboub et al. found that diversion of a surfactant slug following foam can be increased by adding a brief shut-in period following foam injection. Exactly how the shut-in period works is not yet clear. There are various injection strategies for a foam diversion process: injecting foam with or without a surfactant preflush; foaming the acid itself or alternating acid injection with foam; designing an acid formulation either to destroy or maintain foam; incorporating a shut-in period between foam and acid injection. There are few data in the literature to guide choices among these alternatives, although there is a wide body of literature on foams for diverting gas flow in enhanced oil recovery (EOR). Building on this literature, Zhou and Rossen developed a model for the foam diversion process based on fractional-flow methods. They conclude that the best foam process is one in which an optimally-sized preflush precedes foam injection and the acid slug is compatible with the foam. The preflush satisfies surfactant adsorption in the near- well region and greatly accelerates foam propagation there. Since preflush injection precedes any diversion or damage removal, most reflush by far enters the high-permeability or least-damaged layers. Therefore the preflush accelerates foam propagation most in the layer that is to be blocked, because that layer receives the most preflush. It is important to design the acid slug for compatibility with foam in order that the acid slug not immediately destroy the diversion brought about by foam. Zhou and Rossen conclude that both foamed acid and schemes of alternate injection of acid and foam can be effective in diversion. P. 347^

Mechanistic Studies of Improved Foam Eor Processes

“…As a result, pressure gradient increased in these experiments during liquid injection ( Fig. 2.8); all our previous experiments (Kibodeaux et al, 1994;Zeilinger et al, 1995;Rossen and Wang, 1999) had seen a decrease in pressure gradient during liquid injection.…”

“…The early modeling work of Kibodeaux et al (1994) concluded that the decline in pressure gradient seen shortly after liquid injection begins reduces the ability of foam to overcome the effects of permeability and divert the flow of liquid in acid-diversion processes.…”

Development of More-Efficient Gas Flooding Applicable to Shallow Reservoirs

“…In the otherwise-successful SAG foam field trial in the Snorre field (Martinssen and Vassenden, 1999), low liquid injectivity caused fracturing of the injection well and loss of most of the injected surfactant in the second and third liquid slugs. Low liquid mobility after foam injection is also crucial to the success of foam-acid diversion processes (Kibodeaux et al, 1994).…”

“…Previous experiments on liquid injection after foam had been conducted in conjunction with foam-acid diversion for well stimulation (Kibodeaux et al, 1994;Rossen and Wang, 1999). New experiments were conducted in parallel with a simulation study (Cheng et al, 2002).…”

Gas Trapping During Foamed Flow in Porous Media

“…The insight gained into the FDL process is mainly due to co-workers [1994, 2001], who conducted systematic studies using two-stage experiments of foam placement followed by liquid injection. Rossen et al [1994Rossen et al [ , 2001 found that the pressure gradient declines in two steps. The pressure gradient first falls rapidly, but levels off to a value that remains relatively high.…”