Acid fracturing is performed to improve well productivity in acid-soluble formations such as limestone, dolomite, and chalk. Hydrochloric acid is generally used to create an etched fracture, which is the main mechanism for maintaining the fracture open during the life of a well. Proppant fracturing is an alternative option that has been applied in carbonate formations. In certain areas, proppant fracturing has been used as a standard stimulation method for carbonate formations. There is no quantitative method to provide an answer of whether acid fracturing or proppant fracturing is an appropriate stimulation method for a given carbonate formation. In proppant fracturing, proppant is used to sustain the effect of the minimum horizontal stress from closing the fracture. In acid fracturing the etched, non-smooth, surface with sufficient roughness should leave open channels upon closing. The effect of elastic, plastic, and creeping deformations in acid fracturing and the proppant crushing and embedment in proppant facturing, on reducing fracture permeability is investigated. The viscous effect, creeping, is a slow displacement that incurred over a long period of time. The creeping effect on fracture closure following an acid fracturing treatment is demonstrated in this paper. Laboratory experiments have been performed to simulate acid and proppant fracturing treatments. The effect of elastic, plastic and viscoelastic rock behavior on fracture conductivity was studied for acid and proppant fracturing treatments, using full core samples. Comparison of acid vs. proppant fracturing conductivity in carbonate formation is also presented. Introduction Hydraulic fracturing (acid or proppant) is used to create a conductive fracture in the formation to enhance well productivity. The induced fracture will tend to close due to the effect of the minimum horizontal stress. Fracture closure is controlled by elastic, plastic, and viscous rock properties. In acid fracturing the etched, non-smooth, fracture surfaces would leave open pathways upon closing in addition to wormholes and channels created from the fracture into the formation. Fracture conductivity is generated by the quantity of rock removed and the pattern of rock removal. Depending on the pattern of natural fracture system, acid solubility of the formation, magnitude of the minimum horizontal stress, and reservoir temperature, acid fracturing vs. proppant fracturing should be evaluated to select the most effective stimulation treatment for a given formation. Interesting observations relavant to stimulation of carbonate reservoirs, have been reported in the literature. Fracture conductivity does not increase with increasing amounts of dissolved rock1. After successful application of proppant fracturing in a chalk formation, it was concluded that proppant fracturing yielded sustained production rate and became the standard stimulation treatment2. Chalk formations are usually soft with Brinell hardness less than 10 Kg/mm2 and therefore creeping is pronounced. The effect of increased effective stress, due to reservoir depletion, on fracture and matrix permeabilities, was reported3. Proppant's importance in sustaining fracture conductivity in carbonate formation was demonstrated4.

An oil field in central Saudi Arabia produces super-light crude from a sandstone reservoir. The formation contains up to 2 wt. percent authigenic clays dominated by kaolinite and illite/montmorillonite mixed layer clays. To improve well productivity, a well stimulation treatment has been conducted on the producing wells in this field. The treatment introduces several fluids which might invade the formation and induce formation damage. An experimental study was conducted to determine potential formation damage due to fines migration and clay swelling and design an effective clay stabilizer treatment. The work included performing coreflood experiments using reservoir cores at reservoir conditions (180 F and overburden pressure of 1500 psi). The critical salt concentration (defined as the salt concentration below which there is loss of permeability) was first determined. Several commercial clay stabilizers, cationic polymers, were evaluated. The effects of stabilizer concentration (0 to 2 vol. percent.), soaking time, and acids (15 wt. percent HCl) on core permeability were investigated in detail. The experimental results indicated that the critical salt concentration (KCl brine) was nearly 5 wt. percent. Severe loss of permeability was observed when brines of lower salt concentrations were injected into reservoir cores. The effectiveness of clay stabilizer was found to be a function of chemical type and concentration. Some of the chemicals tested were not effective at concentrations less than 1.5 vol. percent. Others were effective clay stabilizers, but caused loss of injectivity at higher concentrations (2 vol. percent). After conducting a thorough investigation, it was found that these chemicals did not readily dissolve in water and formed fish- eyes. When these chemicals were injected into reservoir cores, they formed an external filter cake which caused loss of permeability. This problem was solved by good mixing of the stabilizer and proper filtration. Hydrochloric acid did improve the performance of at least of the clay stabilizers examined. Based on lab testing, a cost effective clay stabilizer was tested in the field at a concentration of 2 vol. percent. Field results indicated that the chemical did not cause loss of injectivity, and minimized formation damage due to fines migration and clay swelling. P. 613

An oil field in central Saudi Arabia produces super-light crude from a sandstone reservoir. The formation contains up to 7.0 wt% authigenic clays dominated by kaolinite and illite/montmorillonite mixed layer clays. To mitigate sand production and improve well productivity, a frac pack stimulation treatment was conducted on most producing wells in this field. The treatment introduced several fluids, which might invade the formation and induce formation damage.An experimental study was conducted to determine potential formation damage due to fines migration and clay swelling, and to design an effective clay stabilizer treatment. The work included performing coreflood experiments using reservoir cores under reservoir conditions. The critical salt concentration ͑defined, as the salt concentration below which there is loss of permeability͒ was first determined. Several commercial clay stabilizers, cationic polymers, were evaluated. The effects of stabilizer type, concentration and acids on core permeability were investigated in detail.The experimental results indicated that the critical salt concentration ͑KCl brine͒ was nearly 5 wt%. Severe loss of permeability was observed when brines of lower salt concentrations were injected into reservoir cores. The effectiveness of clay stabilizers was found to be a function of chemical type and concentration. Some of the chemicals caused loss of injectivity at higher concentrations. It was found that these chemicals did not readily dissolve in water and formed fish eyes. When these chemicals were injected into reservoir cores, they formed an external filter cake, which caused loss of permeability. Good mixing of the stabilizer and proper filtration solved this problem. Hydrochloric acid at 15 wt% did improve the performance of at least one of the clay stabilizers examined.Based on lab testing, a cost effective clay stabilizer was tested in the field. Field results indicated that the chemical did not cause loss of injectivity, and minimized formation damage due to fines migration and clay swelling. cating the presence of quaternary ammonium chloride. CS-B contained a small amount of the chloride ion, most likely from an inorganic source ͑NaCl͒. CS-C did not contain a significant

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High power laser research has been advancing in targeting several areas for downhole applications such as perforation, clay treatment and wellbore flow enhancement among others. This paper presents an overview of the laser research in well stimulation; past, current and future applications. The focus will be on the use of laser to support fracturing of horizontal wells by creating a vertical notch to help initiate the fracture in the point of interest along the horizontal section. Hydraulic fracturing is the process used by the petroleum industry to enhance productivity from low producing wells. Unconventional reservoirs such as tight sands and shale are associated with productivity challenges due to extremely low formation permeability. The process entails high pressure injection of fluids, mostly water based, in the stimulated well such that the pressure applied on the well sand face exceeds the formation fracturing gradient. Although the process of hydraulic fracturing is mostly used to enhance productivity, there are other applications of this process such as sand production control and bypassing of formation damage caused by drilling fluid. The outcome of hydraulic fracturing is a maximized surface area that connects the reservoir to the producing well. This process also leads to activation of natural fractures thus making the stimulated reservoir volume at its maximum value.