A rotating-disk instrument was used to measure the dissolution rates of both calcite and dolomite rock samples in HCl solutions. The results of more than 60 experiments are reported in this paper. The effect of common acidizing additives on the rock dissolution rate is measured for different acids containing quaternary amines, polymer, surfactant, mutual solvent, iron-chelating additive, and dissolved iron. Measurements are made at 23 and 50°C for calcite and dolomite marble samples. Marble samples from Turkey, Greece, and Italy were analyzed to find suitable reference materials. Marble composed of 100% calcite (calcite marble) as well as 91% dolomite (dolomite marble) was used and compared very well with previously published results. Results of rock dissolution rates with common acidizing additives showed significant differences. • 1.5 vol% cationic acrylamide copolymer decreased the calcite and dolomite dissolution rates significantly. At 1,000 rpm, the calcite dissolution rate with 1.5 vol% polymer and 0.1 M (0.36 wt%) HCl had a value that was 11.4% of the value measured with 0.1 M of HCl alone. • Polymer changed the acid/rock reaction from mass-transferlimited to surface-reaction-limited with both calcite and dolomite. This surface effect is possibly caused by polymer adsorption. • 10 vol% mutual solvent increased the acid dissolution rate by 9% for calcite and by up to 29% for dolomite. • 5000 mg/L iron (III) resulted in surface deposition of iron (III) hydroxide for both calcite and dolomite. At low rotational speeds, this surface layer had an inhibiting effect on the dissolution rate. • 2 vol% corrosion inhibitor decreased the calcite dissolution rate by approximately 9%. • Citric acid at 12 g/L decreased the calcite dissolution rate by an average of 9%, possibly because of the formation of calcium citrate at the surface. • 0.2 vol% nonionic surfactant had no significant effect on the acid dissolution rate of calcite.

Summary This paper examines a new class of viscoelastic surfactants (amphoteric) that are used to enhance sweep efficiency during matrix acid treatments. It appears that surfactant molecules align themselves and form rod-shaped micelles once the acid is spent. These micelles might cause the viscosity to significantly increase, and induce viscoelastic properties to the spent acid. The enhancement in these properties depends on the micelle shape and magnitude of entanglement. The effects of acid additives and contaminants [mainly iron (III)] on the rheological properties of these systems were examined over a wide range of parameters. Viscosity measurements were conducted using specially designed viscometers to handle very corrosive fluids. Measurements were made between 25 and 100°C, and at 300 psi at various shear rates from 58 to 1,740 s-1. Acid additives included corrosion inhibitors, inhibitor aids, an iron control agent, a hydrogen sulfide scavenger, an anti-sludge agent, and a nonionic surfactant. Effects of mutual solvents and methanol on the apparent viscosity were also investigated. It is observed that temperature, pH, shear conditions, and acid additives have a profound influence on the apparent viscosity of the surfactant-acid system. The viscosity and related properties are very different from what were observed with both natural and synthetic polymers. The differences in these properties were characterized and correlated with the type and nature of the additives used. Optimum conditions for better fluid performance in the field were derived. Introduction Previous studies (Thomas et al. 1998) highlighted the need for proper diversion during matrix acidizing treatments of carbonate reservoirs. Various systems were introduced to enhance diversion by increasing the viscosity of the injected acid. Depending on the viscosifiying agent, these systems can be divided into two main categories: polymer-based acids and surfactant-based acids. Acid-soluble polymers have been used to increase the viscosity of HCl, and to improve its performance (Pabley et al. 1982; Crowe et al. 1989). As the viscosity of the acid increases, the rate of acid spending decreases and, as a result, deeper acid penetration into the formation can be achieved (Deysarkar et al. 1984). Addition of suitable synthetic or natural polymers to HCl improved acid penetration; however, acid placement did not significantly improve (Yeager and Shuchart 1997). Crosslinked acids were introduced in the mid-70s, as cited by Metcalf et al. (2000). These acids have much higher viscosity than regular acids or acids containing uncross-linked polymers. Two types of crosslinked acids are available The first type consists of a polymer, a crosslinker, and other acid additives [e.g., corrosion inhibitors and iron control agents (Johnson et al. 1988)]. The acid in this case is crosslinked on the surface and reaches the formation already crosslinked. The second type of crosslinked acid consists of a polymer, a crosslinker, a buffer, a breaker, and other acid additives. The acid in this case reaches the formation uncrosslinked, and the crosslinking reaction occurs in the formation (Yeager and Shuchart 1997; Saxon et al. 2000). In-situ gelled acids were the subject of several lab and field studies. In general, lab and field results were positive; however, there were several concerns raised about these acids. Taylor and Nasr-El-Din (2002, 2003) noted that in-situ gelled acids caused loss of core permeability in tight carbonate cores. Permeability loss was attributed to polymer retention in the core and on the injection face of the core. A similar observation was noted by Chang et al. (2001). Lynn and Nasr-El-Din (2001) noted precipitation of the crosslinker (iron) when in-situ gelled acids were used to enhance the permeability of tight cores at high temperatures. Nasr-El-Din et al. (2002) showed that the crosslinker (Fe(III)) may precipitate in sour environments. Mohamed et al. (1999) reported poor field results when large volumes of polymer-based acids were used to stimulate seawater injectors with tight carbonate zones.

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Surfactants have been used over the past few years as a diverting material when mixed with acid solutions. Several papers are aimed to understand the overall process and the diversion capability. However, most of the coreflood experiments, if not all, used relatively short cores usually ranging between 2 to 6 inches in length and 1 to 1.5 inches in diameter. In this paper and for the first time, 20 inches long cores were used in conducting the acidizing experiments. The lithology of the rock type used was calcite, which has permeability of 50 to 120 md. In addition to the appropriate surfactant concentration, 15 wt% HCl was used. A key parameter such as flow rate, affects the behavior of the gelling material significantly. Therefore to capture that effect, flow rates chosen to be representative ranged between 5 and 15 cm 3 /min. A further analysis included is the effluent concentration for the mineral composition mainly Ca ++ . In addition, the acid concentration in the effluent was measured and validated with an analytical model to address the acid profile along the wormhole, which becomes an important aspect in understanding the flow of VES fluids in carbonate cores.Part of this study is to characterize the wormhole propagation expected from the acidizing process. To accomplish this task, computerized tomography (CT) was used to generate 3-D images to describe the shape of the wormhole. There is a major difference in the shape of the wormhole when using the diverting material compared to conventional acids. The wormhole path observed in the first case tended to have several changes in direction as the acid tried to find its way to propagate further and avoid any possible blockage caused by the diverting material. This phenomenon would not be captured with short cores that were used in the previous studies.

Summary The purpose of matrix stimulation in carbonate reservoirs is to bypass damaged areas and increase the effective wellbore area. This can be achieved by creating highly conductive flow channels known as wormholes. A further injection of the acid will follow a wormhole path where the permeability has increased significantly, leaving substantial intervals untreated. This problem can be more significant as the contrast in permeability increases within the target zones. Diverting materials, such as viscoelastic-surfactant (VES) -based acids, play an important role in mitigating this problem. The acid-injection rate was found to be a critical parameter to maximize the efficiency of the use of VES-based acids as a diverting chemical in addition to creating wormholes. It was found that the maximum apparent viscosity, which developed during VES-based-acids injection, occurred over a narrow window of acid-injection rates. Higher injection rates were not effective in enhancing the acidizing process, and the use of diverting material became similar in effect to that of regular acids. The use of VES-based acid was also found to be constrained by the scale of the initial permeability ratio. For initial permeability ratios greater than approximately 10, the diversion was insufficient. The results were obtained by conducting a large set of acidizing experiments by use of 20-in.-long cores. Both single- and parallel-coreflood experiments were performed in this study. Carbonate cores were used with initial permeabilities of 4–150 md, and the flow rate was varied from 1.5 to 50 cm3/min. The initial ratio of permeability between the two cores ranged from 2 to 15. To characterize the wormholes, computerized tomography (CT) was used to generate a 3D view of the wormholes in each core. By use of the results obtained from single cores, the acid-injection rate was found to be a critical parameter in maximizing the efficiency of the use of VES as a diverting agent during matrix-acidizing treatments. Higher injection rates were not effective in enhancing the acidizing process, and the use of diverting material produced results similar to those of regular hydrochloric acid (HCl). Parallel-coreflood experiments indicated that the use was found to be constrained by the scale of the initial permeability ratio. For initial permeability ratios greater than approximately 10, diversion was insufficient in 20-in. coreflood tests. For permeability ratios greater than 10, the acid-placement treatment needs to be designed more carefully.

A rotating disk instrument was used to measure acid reaction rates, reaction order, and activation energy of reservoir rock from a deep dolomitic gas reservoir in Saudi Arabia. These values are required to optimize acidizing simulations. Results of more than 50 experiments are reported in this paper. Measurements were made from room temperature to 85 ° C at rotational speeds of 100 to 1,000 r/min and acid concentrations of 0.05 to 5 M HCl (0.2 to 17 wt%). The results show how acid dissolution rates change as the reservoir rock varied from 3 to 100 wt% dolomite. Factors affecting the measured parameters are discussed in detail. It was found that the reactivity of the rock varied from values expected for pure calcite marble to those expected for pure dolomite marble. At grain densities near 2.72 kg/dm3 (expected for pure calcite), rock dissolution rates varied by more than an order of magnitude, due to rock mineralogy. At grain densities near 2.83 kg/dm3 (expected for pure dolomite), rock dissolution rates were igher than that observed with pure dolomitic marble. Reaction rates depended on mineralogy and the presence of trace components such as clays. Introduction An accurate knowledge of acid reaction rates of deep gas reservoirs can contribute to the success of matrix and acid fracture treatments. These parameters are used in simulation models to estimate the optimum acid concentration, pumping rate, and shut-in time of acid treatments. Many studies of acid stimulation treatments of formation K, a deep, dolomitic gas reservoir in Saudi Arabia, have been published(1–3). This is the first study of acid reaction rates and reaction coefficients of this important formation. The rotating disk instrument is widely used in the petroleum industry for kinetic studies of the reaction of acidic fluids and chelating agents with reactive rock(4–10). This system allows the determination of rock dissolution rate, reaction rate constants, reaction order, and diffusion coefficients(4,11). In the rotating disk instrument, a rock disk 3.81 cm (1.5 in.) in diameter is mounted on a spindle using heat-shrink Teflon ® tubing. The rock disk can be spun at rates up to 1,000 r/min in a Hastelloy? B2 reaction vessel. Acid solution is preheated in a Hastelloy? B2 acid reservoir and transferred under pressure into the reaction vessel(4). Small volumes (8 mL) are removed from the reaction vessel every 2 minutes for 20 minutes. Concentrations of calcium and magnesium in the samples are determined by inductively coupled argon plasma emission spectroscopy. From the calcium and magnesium concentrations, and accounting for the loss of acid volume from the reactor, the amount of acid that reacted with the rock can be calculated for each sample(8). A plot of calculated acid concentrationvs. time shows the rate of acid consumption with time. From the slope of this line, and the surface area of the rock disk, the dissolution rate (moles/s cm2) can be calculated(6, 8). Test pressure was set at 6.9 MPa so that carbon dioxide would remain in solution.

This work uses a rotating disk instrument to measure dissolution rates of both calcite and dolomite rock samples. Results of more than 60 experiments are reported in this paper. The effect of common acidizing additives on the acid dissolution rate is measured for different acids containing quaternary amines, polymer, surfactant, mutual solvent, iron chelating additive, and dissolved iron. Measurements are made at 23 and 50 ºC for calcite marble and dolomite marble samples. Marble samples from Turkey, Greece and Italy were analyzed to find suitable reference materials. Marble composed of 100% calcite (calcite marble) as well as 91% dolomite (dolomite marble) was used and compared very well with previously published results. Results of acid dissolution rates with common acidizing additives showed significant differences.·1.5 vol% cationic acrylamide polymer decreased the calcite and dolomite dissolution rates significantly. At 1000 rpm the calcite dissolution rate with 1.5 vol% polymer and 0.1N HCl had a value that was 11.4% of the value measured with 0.1N HCl alone.·Polymer changed the acid-rock reaction from mass transfer limited to surface reaction limited with both calcite and dolomite. This is a surface effect possibly due to polymer adsorption.·10 vol% mutual solvent increased the acid dissolution rate by 9% for calcite and by up to 29% for dolomite.·5,000 mg/L iron (III) resulted in surface deposition of iron (III) hydroxide. At low rotational speeds, this surface layer had an inhibiting effect on dissolution rate for both calcite and dolomite.·2 vol% corrosion inhibitor decreased the calcite dissolution rate by approximately 9%.·Citric acid at 12 g/L decreased the dissolution rate of calcite by an average of 9%, possibly due to the formation of calcium citrate at the surface.·0.2 vol% nonionic surfactant had no significant effect on acid dissolution rate of calcite. Introduction The rotating disk instrument has been widely used in the petroleum industry for kinetic studies of the reaction of acidic fluids and chelating agents with reactive rock. This system allows the determination of rock dissolution rate, reaction rate constants, reaction order, and diffusion coefficients. Lund et al. studied the dissolution of both calcite and dolomite with the rotating disk instrument. Their work showed that at 25 ºC the dissolution of calcite is mass transfer limited even at high disk rotational speeds, while at –15.6 ºC both mass transfer and surface reaction rates limit the dissolution rate.In contrast, Lund et al. showed that the dissolution of dolomite was surface reaction rate limited at 25 ºC even at low disk rotational speeds. As the temperature was increased to 100 ºC, the dissolution process approached diffusion limitation even at relatively high rotational speeds.

The purpose of matrix stimulation in carbonate reservoirs is to bypass the damaged zones and increase the effective wellbore area. This can be achieved by creating highly conductive flow channels known as wormholes. A further injection of acid will follow the wormhole path where the permeability has increased significantly, leaving substantial intervals untreated. This problem can be significant as the contrast in permeability increases within the target zones. Diverting materials such as surfactant-based acids play an important role in mitigating this problem. Several papers published in the literature aimed to verify the diversion capability and the performance of the self-diverting acids. However, most of the parallel-coreflood experiments, if not all, used relatively short cores (2 to 6 inches in length).In this paper, acidizing experiments were conducted using two 20-inch long cores with different permeabilities. Carbonate cores were used with a permeability of 5 to 150 md and the total flow rate was varied from 3 to 20 cm 3 /min. The initial contrasts in permeability between the two cores ranged from 2 to 15 fold. To characterize the wormholes generated, a computerized tomography (CT) was used to generate 3-D images to describe the shape of the wormholes in both cores.Several periods were identified from the shape of the flow rate distribution entering each core. Acid injection rate was found to influence the efficiency of surfactant to divert acid. Acid diversion was noted to be most efficient at low flow rates (3 cm 3 /min). No significant diversion was noted at higher flow rates (> 6 cm 3 /min). Also, no significant diversion was noted at high initial permeability ratios at least for the given core length. In conjunction with the experimental study, an analytical model was developed to verify the experimental results obtained with regular acids. IntroductionThe stimulation process starts by injecting an acid into the formation below the fracture pressure, which will result in dissolving the carbonate minerals. The dissolution patterns created are known as wormholes. A relatively close permeability ratio is essential to ensure proper placement of acid to stimulate the target zones. When such condition is not part of the characteristic of the candidate formation, diversion is necessary for better acid placement. In this case, diverting materials play an important role to equalize the flow, allowing untreated zones to be stimulated and thus benefit the overall productivity of the treated well. Several techniques have been implemented with the aim of diverting the stimulation fluids to the target zones. These procedures can be classified into two main categories: mechanical and chemical.Extensive laboratory testing has been conducted to evaluate the diversion of a viscoelastic surfactant-based acid systems. Chang et al. (2001) conducted multi-core flood testing, incorporating a post acidizing computed tomography (CT) scan that showed the self-diverting acid successfully diverted acid from the high permeabili...