An improved viscoelastic surfactant (VES) based self-diverting acid system has been developed for matrix treatment of carbonate formations. The self-diverting acid contains a novel viscoelastic surfactant that undergoes an increase in viscosity as the fluid penetrates carbonate formations. The decrease in acid concentration as the acid reacts with the carbonate rocks promotes the transformation of spherical micellar structure into a worm-like structure that imparts high viscosity to the fluid. The highly viscous fluid acts as a temporary barrier to reduce further fluid loss into the wormholes and allows complete stimulation of all treating zones. After acid treatment, the viscous fluid is broken by either formation hydrocarbons or pre-flush fluids. The VES based self-diverting acid contains no solids, so there is no bridging when it is pumped through tubing. Rheological studies showed that the acid rapidly developed viscosity as the acid was spent with CaCO3, and the spent fluid was stable up to 300°F. Diversion tests using multiple-core flood equipment showed effective wormholing in both high permeability and low permeability cores. The effectiveness of the new system in diversion and stimulation was confirmed in more than 70 field applications, both through increased well production and /or injection and production logs. Introduction During matrix acidizing treatment of carbonate reservoirs, acids are injected into the formation below the fracturing pressure. When acids reach the formation, they will follow the path of least resistance and enter zones with the highest injectivity, which are high permeability zones or zones with the least damage. Acids dissolve carbonate minerals as they enter the formation creating highly conductive flow channels called wormholes resulting in a further increase in injectivity. Consequently, most treatment acids will flow through the same wormholes leaving other zones of interest unstimulated. Diversion, therefore, is required to ensure stimulation of the entire interval. Over the years, many chemicals including polymer gels, foams, oil soluble resins and rock salts1,2 have been developed as diverting agents. Field application of those diverting agents generally requires pumping multiple stages of alternating acid and diverting agent. A self-diverting acid is therefore highly desirable to simplify the process. Polymer based acid systems have been applied successfully in the field as self-diverting fluid3. The systems rely on in-situ increase in viscosity when the pH of the fluid rises as a result of acid spending. The in-situ increase in viscosity creates resistance to flow into the high injectivity zones allowing subsequent acids to enter zones with lower injectivity. The viscosity of the fluid then breaks down with further increase in pH as acids are completely spent with carbonates. However, there have been concerns about the potential damage to the formation as the system contains polymeric materials4,5. Recently, Chang et al reported the laboratory development of a VES based self-diverting acid for carbonate acidizing treatments6. The reduction of acid concentration as the acid reacts with the carbonate rocks promotes the transformation of spherical structure into a worm-like structure that imparts high viscosity to the fluid (Figure 1). The highly viscous fluid reduces the chance of further acid loss into the wormholes, thereby enabling complete acidizing of all zones. After acid treatment, the viscous fluid breaks down upon contact with formation hydrocarbons. The VES acid system contains no solids, so there is no bridging when it is pumped through tubing. However, this VES self-diverting acid system has upper temperature limit of 200°F.
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. AbstractEffective matrix acidizing of horizontal and multi-lateral wells is a very difficult task. Unlike vertical wells, horizontal wells can extend several thousands feet in the formation.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDrilling horizontal wells with extended reach is intended to maximize reservoir drainage and minimize water production due to water coning. However, an inherent problem with these wells is poor acid distribution during matrix acidizing, especially in reservoirs with high permeability streaks. This paper discusses an innovative approach to treat horizontal wells with extended reach. This new technique comprises mechanical diversion in the wellbore, and chemical diversion in carbonate formations.Coiled tubing has been used for years to better distribute the acid in vertical and horizontal wells. However, application of coiled tubing in long horizontal wells is a function of wellbore diameter and length. Coiled tubing cannot reach the total depth of the well if there is large washout, or if the length of the openhole is greater than what the CT can reach. The maximum length that CT can reach depends on the length of the reel, diameter of the coil and wellbore geometry. To extend this length, we have used a hydraulic tractor to pull the coiled tubing to the total depth of the well. This will ensure better acid distribution over the wellbore. To enhance acid diversion in the formation, a visco-elastic surfactant-based acid system was employed.Wells selected are horizontal that were drilled in a carbonate reservoir in the oil fields (both land and offshore) that are present in the eastern part of Saudi Arabia. The total length of the target zone for the well "A" and well "B" is 13,543 and 20,304 ft, respectively. Typical coiled tubing (1 ¾-inch) cannot reach the total depth in these wells (CT lockup length is 10,300 ft for well "A" and 13,200 ft for well "B").A special hydraulic tractor was used to pull the coiled tubing to the total depth of these two wells. A visco-elastic surfactant based-acid system was utilized to remove formation damage induced by the drilling fluid (water-based mud) and enhance the permeability of the formation in the critical wellbore area. Corrosion inhibitor and other acid additives were carefully selected to maintain the integrity of well tubulars, coiled tubing and the tractor (metallic parts, O-rings and seals of the tractor). Before attempting the stimulation of the extended reach well, a water jetting method was adopted to remove near well bore damage resulting from the drilling mud cake and mud invasion. Production logging tests were conducted after drilling the well, after the water jetting treatment, and after the matrix stimulation. The productivity index of the well decreased after the water jetting treatment. However, the acid matrix treatment delivered through the CTtractor nearly doubled the productivity index of the treated well.
Effective matrix acidizing of horizontal and multilateral wells can be a highly challenging task. Unlike vertical wells, horizontal wells can extend several thousand feet into the formation. Reservoir heterogeneity and the length of the horizontal leg can make acid placement and diversion very difficult. In addition, the low drawdown encountered in horizontal wells results in longer times to lift the spent acid from the well, especially in tight formations.To achieve better acid diversion in horizontal wells drilled in carbonate reservoirs, a viscoelastic-surfactant-based system was used. The components of this new system are HCl and a viscoelastic surfactant. The acid dissolves calcite and dolomite minerals and produces calcium and magnesium chlorides. The increase in pH forces the surfactant molecules to form rod-shaped micelles. The produced chloride salts further stabilize these structures, especially at high temperatures. The rod-shaped micelles will significantly increase the viscosity of the acid, diverting the acid into tight, unstimulated, or severely damaged zones.More than 100 wells with openhole (OH) completions were successfully stimulated in two offshore oil fields in Saudi Arabia by use of the new acid system. With a water zone 30 ft away from these OH sections in one of the fields, growth of any dominant wormhole into these sections could increase water production. The wells that used the new treating fluid produced an average of 1,600 BOPD more than conventionally treated wells, with no indication of water production. Field results [pre-and post-oil and -water production rates and flowing wellhead pressure (FWHP)] demonstrate the effectiveness of the new acid system to matrix acidize long horizontal wells with OH completions. The simplicity of the system makes it the fluid of choice, especially in offshore and sour environments. The absence of metallic crosslinkers in this system eliminates problems associated with sulfide precipitation in sour wells.
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