The high permeability contrast seen in the producing zones of the oil wells of North Kuwait Mauddud formation makes the uniform stimulation of this carbonate formation a challenge. To achieve diversion, polymer based fluids were used earlier, but with limited success. Recently, a non-polymeric system containing a Visco-Elastic Surfactant based Self Diverting Acid (VES-SDA) was used to divert and effectively stimulate these pay zones. Production logs run before and after the stimulation treatments indicates stimulation of the entire perforated intervals. Stimulation using this new system on the first 17 wells resulted in a production increase of about 30,000 BOPD, much higher than that expected from conventional treatments. The wells that were not producing earlier after several conventional treatments are now producing naturally after treatments using this non-damaging system. Introduction North Kuwait Mauddud formation consists of six main lithology sections with permeability ranging from 3 to 400 mD (Figures 1, 2, Table 1)1. This high permeability contrast in conjunction with thick reservoir layers makes the uniform stimulation of this carbonate formation difficult when using conventional matrix stimulation fluids. Effective diversion is the key for the success of stimulation treatments to achieve uniform production from all pay zones. Without diversion, acid tends to seek the path of least resistance and enters only a small portion of the interval being treated. Chemical diverting agents temporarily block the more permeable section of the interval, forcing the acid into damaged and/ or less permeable areas. In multi-layered reservoir containing zones with different injectivities due to different permeabilities and severity of damage, stimulation fluid diversion is highly recommended. Conventional stimulation treatments use regular acid or retarded acids2,3 in conjunction with chemical diverters including foams4 to fully stimulate long, non-uniform carbo-nate formation. The most commonly used chemical diverters are polymer based5, and are associated with induced formation damage6. To perform stimulation of the entire zone, a new chemical diverter with a solids-free self-diverting acid was recently developed7. The base fluid for the system is HCl, and it stimulates and diverts automatically based on in situ viscosification. The fluid is non-damaging and on breaking, it leaves no residue in the formation. The VES-SDA provides a solution for the heterogeneous carbonate reservoirs, and eliminates the concern of ineffective stimulation. Less friction pressure experienced while pumping this new acid system compared to other diversion systems provides more pumping rates and better treatment effectiveness. The simplicity of the fluid together with the flexibility in designing the treatment, make the execution easier and less cumbersome. Fewer tanks and equipment are needed and hence, fewer footprints are required for space short offshore operations. Stimulation by coiled tubing was shown to be the best tool for acid placement and to get maximum coverage8,9. The producing zones in the Mauddud formation are completed on the short string of dual completed wells. The difficulty of using coiled tubing to acidize short strings makes bullheading of chemical diverter the only choice to perform these treatments. Because of the non-damaging nature and effective-ness as a diverting agent, this VES fluid was chosen to stimulate the oil wells of North Kuwait Mauddud formation. This paper presents the first application worldwide on the use of VES self diverting acid technology in stimulating carbonate formations. Background Maximizing oil and gas recovery is one of the most complicated, but interesting tasks in the oilfield industry today. A chemical solution for uniform production is important in managing the recovery efficiency, allowing an efficient sweep of the hydrocarbons to increase the hydrocarbon recovery.

TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractDifferent acid systems have been used to stimulate production and injection wells for more than a century. Several chemicals are added to the acid to enhance the efficiency of the acidizing treatments, including various types of surfactants.Surfactants play a key role in many well treatments because of their unique properties. Surfactants are used to reduce surface tension, change wettability, mobilize residual oil, and disperse corrosion inhibitors. They are also used as anti-sludge agents, emulsifiers in acid-oil emulsions, and demulsifiers. This wide spectrum of applications is due to the ability of surfactants to adsorb on various surfaces and associate in the bulk of solutions and form micellar structures. The properties of surfactants depend on the size of the head, tail length, and configuration of the tail, surface charges, ionic strength and temperature.Recent advances in surfactant technology have introduced a new class of surfactants with special characteristics useful in oilfield applications. This new class of surfactant has the ability to form viscoelastic systems, which can be used to increase viscosity of the treating fluids, without causing formation damage. These surfactants have been used in Saudi Arabian fields to enhance diversion during matrix acid stimulation, reduce leak-off during acid fracturing and to minimize losses (well control) during workover. The system proved successful to provide fracture extensions in acid fracturing applications of low temperature seawater injectors and high temperature gas wells. These surfactant systems are non-damaging and do not require sized solids to control losses, which eliminates the need for remedial acid treatments.

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Excessive loss of high-density brines into the formation has always been a major concern during completion operations, since it leads to formation damage and well control issues. The problem becomes more complex at high temperatures and when the treatment involves running gravel-pack assemblies and downhole sand screens. Typically, the fluidloss-control pills are composed of very high concentrations of crosslinked polymers with or without bridging particulates. The sealing mechanism of these pills is a combination of viscosity, solids bridging, and polymer filter-cake buildup on the porous rock. Due to the instability of polymers at high bottomhole temperatures, incompatibility with some divalent brines, and the necessity to cleanup with acid, a new solids-free lost-circulation pill that is stable for prolonged periods at high temperatures was developed. This paper introduces the development and the first field application of a new solids-free non-damaging viscoelastic surfactant-based fluid-loss pill (VES-PILL). The single-additive system forms a "gel" with most completion brines currently used in well operations. Laboratory data demonstrate that this pill could be used up to 375°F. The "gel" structure of this system sustains viscosity high enough to effectively control or stop brine-loss, while maintaining a safe differential pressure into the formation. Several "frac and pack" completions were performed for the first time in Saudi Arabia in the Pre-Khuff sweet-gas zones. The VES-PILL was used to prevent losses after perforating, fracturing and gravel-pack operations. The pill was used in the field up to 310°F, and brine-loss was effectively controlled for more than 3 days. The effectiveness of the pill was also demonstrated by a five-fold increase in surface pumping pressure during placement. The wells were produced at rates exceeding expectations without further remediation to cleanup the fluid-loss pill. Background Fluid-loss control is very important in successful well completion operations. Loss of completion and workover fluids is unacceptable due to economic (expensive heavy brines), technical or safety reasons (formation damage, hole collapse and well control issues).1 Loss of dense brines into the productive zones is highly damaging, especially to high permeability formations. It is very difficult to unload heavy brines once losses have occurred. Because of the high-density of brines used, stratification tends to further inhibit its removal.2 Calcium and zinc bromide brines can form stable acid insoluble complexes when reacted with some formation brines.3 Hence, the most effective means of preventing the formation damage is to limit completion brine losses into the formation by either chemical or mechanical means. It is best to avoid the use of fluid-loss control pills by incorporating mechanical fluid-loss control devices into the completion string whenever possible.4 However, in the absence or failure of such devices, or in situations where they cannot be used, chemical fluid-loss pills are required. The use of a pill is normally required before and after sand control treatments and after perforating. In these treatments, the pill is spotted into the perforations or against the sand control screens. In addition, fluid-loss control pills are required in several workover operations that need temporary zonal isolation. There are several reviews on the use of different types of fluid-loss pills5–7 and guidelines on the selection of the pill.4,8 A variety of fluid-loss control pills have been used in the industry, such as foams3, oil-soluble resins3, fibers,5,9 acid soluble particulates,3 graded salt slurries,10 high concentrated linear11 and crosslinked1,6,1,13 non-biopolymers14 and bio-polymers.10,15 The polymer systems are very effective in fluid-loss control as long as the temperature limit of the specific polymer is not exceeded. One of the important features of any fluid-loss pill is its ability to maintain viscosity under bottom-hole conditions (especially high temperature). The viscosity reduction of gel at high temperatures is either due to the degradation of polymer or reduced molecular interations.16 The viscosity will not be regained on cooling if there is molecular degradation at high temperature.

The high permeability contrast seen in the producing zones of the oil wells of the north Kuwait Mauddud formation makes the uniform stimulation of this carbonate formation a challenge. To achieve diversion, polymer-based fluids were used earlier but with limited success. Recently, a nonpolymeric system containing a viscoelastic-surfactant-based (VES) self-diverting acid (SDA) was used to divert and effectively stimulate entire pay zones with great success.Production logs run before and after the stimulation treatments indicate stimulation of the entire perforated intervals. Stimulation using this new system on the first 17 wells resulted in a production increase of approximately 30,000 BOPD-much higher than that expected from conventional treatments. The wells that were not producing earlier after several conventional treatments are now producing naturally after treatments using this nondamaging system. Nearly 100 wells in the same reservoir were treated with this system after successful introduction of the technology. The use of this technology is now extended to matrix stimulation of gas producers, water injectors, and disposal wells.