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Summary Hydrochloric (HCl) acid is used to stimulate carbonate formations in both matrix and fracturing treatments. However, the reaction rate of the acid with calcite is fast. In addition, the viscosity of regular HCl solutions is relatively low. Acid-soluble polymers are usually added to the acid to increase its viscosity, which is needed to enhance acid diversion during matrix acidizing and reduce acid leakoff rate during acid fracturing. Gelled acids are extensively used in matrix and acid-fracturing treatments performed in carbonate formations. However, a few studies examined the impact of these polymers on the reaction of HCl acids with calcite. This paper uses a rotating disk instrument to measure the dissolution rate of calcite by use of gelled acids. Measurements were conducted over a temperature range of 25 to 65°C, a pressure of 1,000 psi, and rotational speeds of 100 to 1,000 rpm. Acid formulations that are typically used in the field were examined. Polymer concentration was varied from 0.5 to 2 wt%. The apparent viscosity of the gelled acid was measured with a Brookfield viscometer. Measurements were done for the same solutions tested with the rotating disk instrument. The temperature was varied from 25 to 100°C, while the pressure was maintained at 300 psi. The shear rate was varied from 57 to 1,700 s-1. Evidence of reverse and toroidal flows was noted for the first time by examining the etching patterns of the reacted disks. The etching pattern on the surface of the disk depended, among other factors, on the disk rotational speed and polymer concentration. There was a significant increase in the apparent viscosity of gelled acids and a major decrease in the dissolution rate as the polymer concentration was increased from 0.5 to 1.5 wt%. The reaction of gelled acids with calcite was controlled by a surface reaction at 25°C, and by mass transfer at 65°C. Temperature increased the dissolution rate of calcite at all conditions examined. It did also reduce the viscosity of the gelled acid, which affected the way the acid reacted with calcite. Introduction Carbonate reservoirs are heterogeneous, with large variations in rock permeability. Stimulation fluids, in matrix acidizing, will flow through the path of the least resistance where the permeability is high or the damage (skin) is low. There is a need for proper fluid diversion to enhance the outcome of matrix acid treatments. One way to enhance diversion is to increase the viscosity of the acid (Woo et al. 1999). High viscosity is also needed in acid-fracturing treatments to achieve deep acid penetration and longer fractures (Deysarkar et al. 1984). Gelled (Pabley et al. 1982; Johnson et al. 1988; Crowe et al. 1989; Nasr-El-Din et al. 2002a) and in-situ gelled acids (Mukherjee and Cudney 1993; Magee et al. 1997; Yeager and Schuchart 1997; Buijse et al. 2000; Saxon et al. 2000; Taylor and Nasr-El-Din 2003) have been used to increase the viscosity of the acid on the surface or in the formation. An acid-soluble polymer is typically added to the injected acid to increase its viscosity on the surface. A suitable polymer, a crosslinker, and a breaker are added to the acid to form a gel in the formation over a certain pH range. To overcome some of the concerns raised about polymer-based acids, visco-elastic surfactants (Nasr-El-Din et al. 2006) were introduced to replace high-molecular-weight polymers, which are thought to cause formation damage (Lynn and Nasr-El-Din 2001). Similar to other acid additives, polymers can affect the way the acid reacts with the rock. Several authors reported that the addition of polymers to the acid decreased the dissolution rate of rock by the acid (Taylor et al. 2004a) and the diffusivity of H+ (Hansford and Litt 1968; Mishra and Singh 1978; de Rozieres et al. 1994). There are several ways that polymers can affect the reaction of the acid with the rock. The polymer will increase the viscosity of the acid, which will reduce the diffusion rate of H+ from the bulk solution to the surface of the rock. Polymer molecules can adsorb on the rock surface and form a barrier that reduces acid reaction with the rock. Finally, polymers can change the flow pattern close to surface of the rock, and therefore, affect the way the acid reacts with the rock. The present study uses the rotating-disk instrument to examine the reaction of gelled acids with calcite. This instrument has been extensively used to investigate the reaction of acids and chelating agents (Newtonian fluids) with carbonate rocks (Boomer et al. 1972; Lund et al. 1975; Anderson 1991; Fredd and Fogler 1998a, 1998b, 1998c; Conway et al. 1999; Gautelier et al. 1999; Alkattan et al. 1998, 2002; Frenier and Hill 2002; Taylor et al. 2004b, 2006; Lungwitz et al. 2007). It has been also used to study mass and heat transfer into non-Newtonian fluids (Hansford and Litt 1968; Mishra and Singh 1978; de Rozieres et al. 1994). The reaction between acid and rock is a three-step process that involves the following:Transport of the H+ from the bulk solution to the rock surfaceReaction at the surfaceTransfer of the reaction products away from the surface The slowest step controls the overall reaction rate (de Rozieres et al. 1994). The objectives of the present study are to (1) examine the effect of polymer concentration and disk rotational speed on the etching pattern on the surface of the rock; (2) assess the effect of polymer concentration, temperature, and disk rotational speed on the dissolution rate of calcite by use of gelled acids; and (3) determine the relationship between the apparent viscosity of gelled acids and the dissolution rate of calcite rock.

Summary Unlike other acid systems, such as gelled and viscoelastic surfactant-based (VES) acids, where the mobility of hydrogen ion controls the overall rate of the reaction, emulsified acid/calcite reaction involves the transport of acid droplets in the diesel phase to the rock surface, breaking of acid droplets, and then the actual reaction on the surface. A limited number of papers have been published on the reaction kinetics of emulsified acid. However, none of the published work considered the effect of acid droplet size on the reaction of emulsified acid. The objective of this work is to examine the effect of the acid droplet size on the reaction rate of emulsified acid with calcite. The acid was 15 wt% HCl emulsified in diesel with 70 to 30 acid-to-diesel volume ratio. The emulsifier concentration was varied from 1 to 10 gpt. All emulsions were characterized by measuring the droplet size distribution, viscosity, and thermal stability. Diffusivities were measured using the rotating disk device. The experiments were carried out at 25, 50, and 85°C, under 1,000 psi pressure, and disk rotational speeds from 100 to 1,000 rev/min. Samples of the reacting acid were collected and analyzed to measure calcium concentration in the reactor. The effect of the acid droplet size on the overall reaction rate was significant. The diffusion rate of acid droplets to the surface of the disk was found to decrease with increasing emulsifier concentration because of higher viscosities and smaller droplet sizes. The effective diffusion coefficient of emulsified acid was found to increase linearly with the average droplet size. Emulsions with low emulsifier concentrations (1 gpt) had average droplet sizes of nearly 13 µm. These emulsions were found to have high effective diffusion coefficients (5.093×10-9 cm2/s) and low retardation. On the other hand, emulsions with high emulsifier concentrations (10 gpt) had smaller average droplet sizes (nearly 6 µm) and found to have low effective diffusion coefficients (4.905×10-11 cm2/s) and high retardations. The new sets of data can be used to determine the optimum emulsified acid formulation to yield deeper acid penetration in the formation. It is suggested that droplet size can be adjusted to produce the desired diffusion coefficients for acid fracturing treatments.

Manganese tetraoxide (Mn3O4) has been recently used as a weighting material for water-based drilling fluids (Al-Yami et al., 2007). Mn3O4 particles are spherical, 1–2 µm in diameter, and have a specific gravity of 4.8 g/cm3. A mud (102 ± 5 pcf) was developed to drill deep gas wells. The filter cake formed by this fluid contained polymers (starch, XC-polymer, and polyanionic cellulose polymers), Mn3O4 and a small amount of CaCO3. Unlike CaCO3, Mn3O4 is a strong oxidizer and, as a result, HCl not recommended to be used to remove the filter cake. The objective of this work is to develop a new cleaning fluid to effectively and safely remove the filter cake that contains large amounts of Mn3O4. Various organic acids, chelating agents, enzymes, and a combination of these chemicals were tested up to 300 °F. Characterization of filter cake before and after soaking in several cleaning fluids were conducted using XRD/XRF/SEM techniques. Solubility of Mn3O4 was conducted using various acids and chelating agents at different temperatures up to 284 °F and 200 psi. The concentration of manganese in spent chemicals was measured using Inductivity Coupled Plasma (ICP). Extensive lab testing indicated that Mn3O4 particles in the filter cake were coated by polymeric materials that acted as a barrier. The coating material reduced the ability of cleaning fluids to remove filter cake. The efficiency of cleaning fluid was improved by soaking filter cake in a starch specific enzyme, and then applying the cleaning fluids. HCl, citric, and in-situ lactic acid were found to be the most effective fluids in dissolving Mn3O4 particles. This paper will discuss the characteristics of filter cake and the effectiveness of various cleaning fluids. New cleaning fluids were designed to remove Mn3O4 filter cake, while maintaining the integrity of the formation and well tubulars. Introduction Drilling horizontal/multilateral wells is utilized to enhance both hydrocarbon recovery and total well productivity from many types of reservoirs (Yildiz 2005 and Tronvoll et al., 2001). Drilling, workover and production operations may result in near-wellbore formation damage that in most cases cannot be prevented e.g. Pore plugging by calcium carbonate particles from drilling fluid, drilled solid particles, or particles from the formation (Ismail et al., 1994). Manganese tetraoxide was introduced to potassium formate drilling fluid back in 1995 to overcome the main drawback of potassium formate, which is the production of brine of density 1.7 g/cm3 (106 lb/ft3). Due to the partial solubility of barite in concentrated formate brines and the decision not to acidize prior well completion of the well, CaCO3 and barite were excluded as options to increase the density of the fluid. The most effective breaker fluid of the filter cake formed by this drilling fluid found to be 10 wt% citric acid with formate brine. (Sevendsen et al., 1995) Mn3O4 was introduced as a weighting material to oil-based drilling fluid due to the achievement of the very low plastic viscosity at the fluid density requirement and the ability to suspend the solid particles, Mn3O4, at lower fluid viscosity (Franks et al., 2004). In 2007, a water-based drilling fluid weighted with manganese tetraoxide and small amount of CaCO3 were developed. CaCO3 were added to control the filtration properties of the drilling fluid. The needs for using a drilling fluid with high rheological properties were achieved using manganese tetraoxide particles. (Al-Yami et al., 2007) Current approaches introduced to remove filter cake include the use of live acids, strongly buffered organic acids (Ali et al., 2000), chelating agents, oxidizing agents (Brady et al., 2000), enzymes (Butler et al., 2000, Al-Otaibi et al., 2000, and 2005), in-situ organic acids (Al Moajil et al., 2007), or combinations of these chemicals.

Fracture acidizing has been a dominant practice in the industry to enhance well productivity in low-permeability carbonate reservoirs. Many acid systems have been developed to improve this stimulation process. The most desirable characteristics for an acid system to be suitable for fracture acidizing are leakoff control and retarded reaction rate. These characteristics are required for deep acid penetration, so that when the fracture closes, long flow channels are etched on the fracture surfaces. Leakoff control can be achieved by pumping a pad containing a viscosifying agent or solid bridging agents to plug wormholes generated by acid dissolution. Reaction retardation is attempted usually by lowering the effective diffusivity of the hydrogen ion.It is well known that during an acid-fracturing operation, the overall reaction rate of hydrochloric acid (HCl) with limestone is mass-transfer-limited. Designing the treatment requires knowing the effective diffusivity of the hydrogen ion in the acid system, which, to the best of the authors' knowledge, has not been determined before. Because of their combined leakoff-control and retardation capabilities, surfactant-based acids have been used in acid-fracturing treatments. Because more carbonate reservoirs are treated by use of this acid system, it is important to obtain the effective diffusivity of H + .The rotating-disk device has been used to investigate the reaction kinetics between a reactive solution and carbonate rocks because the thickness of the boundary layer is uniform throughout the disk surface. This paper discusses the reaction-rate data generated recently for surfactant-based acid by use of a rotating-disk apparatus and presents the methodology used to determine the effective diffusivity from the measurements.The results obtained indicated that the viscoelastic surfactant examined (carboxybetaine-type) reduced the dissolution rate of calcite with HCl acid. The surfactant reduced the diffusion coefficient for H + . The effect of temperature on the diffusion coefficient did not follow the Arrhenius law.