Flow of Emulsified Acid in Carbonate Rocks

Abstract

We conducted tracer experiments on core samples from six different carbonates to study the flow of acid fluid through the porous media of carbonate rocks and correlate the carbonate pore classes to the acid response. Pore structures were characterized by section analysis, high pressure mercury injection tests, and nuclear magnetic resonance measurements; mineralogy was determined by X-ray diffraction. Tracer concentration profiles were analyzed to quantify the carbonate pore-scale heterogeneity. The heterogeneity is expressed using a parameter fthe available fraction of pore structure contributing to the flow. From the results, different characterization methods could be connected to the acid fluid flow through the porous media of carbonate rocks. It was also found that the response of the acid depends on the carbonate pore classes. Applications include the design of matrix acid treatments in carbonate rocks. ■ INTRODUCTIONThe goal of acid-stimulation treatments in carbonate reservoirs is to remove or bypass formation damage by creating highconductivity channels known as wormholes. To propagate deeper wormholes in the formation for better productivity enhancement, retarded acids should be used. Emulsified acids are one of the most widely used retarded-acid systems. HCl acid is commonly used as the internal phase of the emulsion. For the external phase, oil and diesel are the most commonly used hydrocarbons. The external hydrocarbon phase acts as a diffusion barrier to reduce the diffusion of the acid droplets to the surface of the rock, 1,2 which helps in the creation of deep wormholes. Several studies 3−5 examined the reaction of emulsified acid with carbonate. Navarrette et al. 4 indicated that the reaction rate of 28 wt % HCl emulsified acid with limestone was 8.5 times less than that of 28 wt % HCl with limestone. Al-Anazi et al. 6 studied the application of the emulsified acid in tight carbonate rocks using coreflood experiments, and the results showed that the emulsified acid can form deep wormholes in tight carbonate cores. Buijse and van Domelen 7 studied the effect of the injection rate, viscosity, and acid volume fraction on the emulsified acid pore volume to breakthrough in carbonate rocks. Sayed et al. 8 studied the performance of the emulsified acid in low and high permeability carbonate formations using coreflood experiments. It was found that the high permeability carbonate cores consumed more acid volume to breakthrough than did the low permeability cores. Also, an optimum injection rate was observed for the highpermeability cores whereas the acid pore volumes to breakthrough decreased with increases in the injection rate for lowpermeability formations.Hoefner and Fogler 9 studied the pore evolution during the flow and the reaction of the acid in porous media. The initiation of wormholes occurs when the live acid penetrates into pores present in carbonate rocks, and then, pore structure evolution occurs. The process is more efficient when only the large pores in the matrix lead to wormholes. 10 C...

Digital Rock Technology for Quantitative Prediction of Acid Stimulation Efficiency in Carbonates

“…Therefore, scale-up using this methodology would still require significant amount of rock material for the formation of interest. Zakaria et al proposed a flow based technique using non-reactive tracer tests to predict PVBT for plain and emulsified acids [1,2] (Table 1). These tracer tests are non-destructive as the injected fluid is non-reactive.…”

“…The present study explores alternative methods for estimating flowing fraction which require even smaller rock samples than those used by Zakaria et al [1,2]. This is desirable because for many formations only small cores (such as side wall cores) maybe available which maybe too small for method of Zakaria et al One alternative is to correlate flowing fraction with other rock properties that are more routinely measured and use these correlations to estimate the flowing fraction.…”

“…The normalized tracer concentration in the outflow solution was plotted versus total pore volume injected. In the method of Zakaria et al the flowing fraction is defined as the cumulative pore volume injected corresponding to the normalized tracer concentration of C/C 0 ϭ0.5 [1,2]. Table 3 Results of DHD tracer flow simulations were used to determine flow heterogeneity of the rock types and describe the flow and storage geometry of flow paths.…”

“…For example, for Edwards White, 35% of the PV contributes to about 70% of the flow-an indication of low pore volume contacted by the tracer. [1,2] are also plotted for reference. The simulations were carried out for 5 out of 6 outcrop samples.…”

“…Recently, it was shown that nondestructive tracers can be used to predict pore volume of acid to breakthrough (PVBT) for both plain and emulsified acids [1,2]. The present study explores the use of digital rock technology to simulate tracer injection in carbonate rocks.…”