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Abstract The purpose of matrix treatments in carbonate reservoirs is to increase connectivity of a formation with the wellbore in the entire zone of interest. Successful matrix treatments depend on the uniform distribution of the treating fluid over the entire interval. When fluids are pumped into a well, they naturally tend to flow into the zone with the highest permeability or least damage. Field experiences showed that there is no assurance of complete zone coverage without proper diversion. Therefore, diversion is recommended in all treatments, especially in extended reach and multi-lateral wells. Diversion techniques can be classified as mechanical or chemical. Mechanical control of treating fluid placement can be accomplished by coiled tubing with an inflatable packer, or with conventional straddle packers or ball sealers. Although mechanical techniques are very effective, they are more expensive and time consuming than chemical techniques and they are often not applicable or not effective in wells with open-hole completion. More importantly, mechanical means diverts treatment fluids from the wellbore; however, there is no control once the fluid enters the formation. Chemical diversion can be achieved through placing a viscous fluid, foam or gel to lower the penetration of treatment fluid in the created wormholes and their surrounding matrix, or a particulate carrying fluid, which creates a filter cake on the surface of the wormholes. This filter cake results in temporary skin effect which alters the injection profile. Gelled and foamed acids are also being used as a means of improving acid placement by combining stimulation and diversion in one step. Diversion is a critical step to ensure the success of matrix acid treatments. Understanding how chemical diverters interact with the formation rock and fluid is the key to selecting the proper product for a specific treatment. It is the intent of this paper to provide a technical overview of mechanical and chemical diverters used in the oil industry. The various mechanisms by which these chemicals to achieve acid diversion, their application histories, and their limitations are presented. This paper provides guidelines for production engineers to optimize the fluid placement. Introduction Matrix acidizing in carbonates provides opportunity not only to remove or by pass damage in the vicinity of the wellbore, but to also improve the near-wellbore permeability by creating large flow channels (wormholes) with the acid dissolution. The chemistry of carbonate acidizing is much more straight forward than sandstone acidizing. The simplicity results form the fact that the rock is composed of calcite (CaCO3) and/or dolomite (CaMg(CO3)2). Their reaction products are soluble in the spent hydrochloric acid. However, the physics and engineering aspects of the carbonate acidizing process is much more complex. This is because the rock structure is significantly altered by the dissolution reaction, which increases the permeability contrast between the treated and the untreated zones. Unless effectively diverted, the treated region eventually becomes the sink for the acid and leaving other regions not adequately acidized. Therefore, one of the most important factors affecting the success or failure of a matrix acid treatment is the correct downhole placement of the acid for optimum zonal coverage. Though over the years there have been many products and techniques developed in the industry for acid diversion, the preferred ones generally have to possess the following characteristics: Must not cause permanent damage to the formation. The diverting agent must be compatible with the treating fluids (overflush or displacement fluids) and formation brines. Must clean up rapidly and completely when the well is put back on production. The chemical and physical properties of the diverting agent must sustain at the bottom hole treating temperature. Understanding how each acid diversion technique works is the first step towards the optimized carbonate acidizing design. Two techniques can be applied to achieve acid diversion. Mechanical diversion, including coiled tubing and utilizing rate and pressure during pumping; and chemical diversion. Combination of these techniques is often practiced for added efficiency.
- Asia > Middle East (1.00)
- North America > United States > Texas > Harris County > Houston (0.29)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.54)
Abstract Acid fracturing is the commonly applied stimulation technique in low permeability carbonate reservoirs. Achieving adequate fracture length is challenging due to the fast acid spending rates and high leakoff resulting from these treatments. The problem is exacerbated when treating high temperature formations and compounded with the difficulty of providing adequate corrosion control. In addition, the health, safety and environmental implications of acid handling at surface and shortage of hydrochloric acid in certain regions must also be considered to fully appreciate the challenges imposed by acid fracturing operations. The industry has successfully tried different methods to deal with each, or a combination, of these problems. However, none of them fully address all of the challenges discussed. This paper describes a detailed laboratory evaluation of an innovative, solid-based acid fracturing system to address the above-stated limitations of conventional systems. Extensive laboratory studies, which included acid capacity, etching patterns, conductivity measurements, solubility of reaction products, reaction kinetics, and corrosion tests were conducted at temperatures up to 300F. The studies demonstrate that the new system results in heterogeneous etching and wormholing in both limestone and dolomite rocks. In addition, this material exhibits increased fluid efficiency as compared to conventional acid fracturing systems, with the potential of achieving heterogeneously etched half-lengths that approach the length-scale achieved with traditional proppant fracturing operations. This paper will demonstrate the applicability of the novel solid-based acid fracturing treatment. Additionally, the paper will highlight some of the unique challenges of placing a solid-based acid system in the formation and the engineering steps taken to mitigate these challenges. Finally, application limitations of the system will be discussed. Introduction Hydrochloric acid (HCl) is widely used to stimulate oil and gas wells in carbonate formations, to improve the rate of hydrocarbon production. It is also used to stimulate water injection wells and disposal wells, to increase the formation uptake of the injected fluids. Oil and gas wells producing from carbonate formations are generally acidized by matrix acidizing or acid-fracturing treatments. In matrix acidizing treatments, a relatively small volume of acid is used to improve the conductivity of the area adjacent to the wellbore. In acid fracturing treatments, however, the acid is used to etch a created fracture, which penetrates deep into the formation. In such treatments the injected acid is consumed by either reacting with the fracture walls or leaking off through the walls of the fracture then reacting with the carbonate matrix.
- Asia (1.00)
- North America > United States > Texas > Harris County > Houston (0.28)
- Research Report > New Finding (0.93)
- Research Report > Experimental Study (0.66)
- North America > United States > Texas > Permian Basin > Midland Basin > Good Field (0.99)
- Asia > Middle East > Saudi Arabia > Eastern Province > Al-Ahsa Governorate > Arabian Basin > Widyan Basin > Ghawar Field > Lower Fadhili Formation (0.99)
- Asia > Middle East > Saudi Arabia > Eastern Province > Al-Ahsa Governorate > Arabian Basin > Widyan Basin > Ghawar Field > Khuff D Formation (0.99)
- (5 more...)
Using Biosurfactants Produced From Agriculture Process Waste Streams To Improve Oil Recovery in Fractured Carbonate Reservoirs
Johnson, Stephen John (U. of Kansas) | Salehi, Mehdi | Eisert, Karl Friedemann (U. of Kansas) | Liang, Jenn-Tai (U. of Kansas) | Bala, Gregory Alan (Idaho Natl. Engrg. Lab) | Fox, Sandra L. (Battelle Energy Alliance INL)
Abstract We examined the ability of surfactin, an anionic lipopeptide surfactant produced by Bacillus subtilis, to mediate wettability changes that positively affect oil recovery in fractured carbonate rock. On both a weight for weight and a molar basis, surfactin has a greater ability to alter a carbonate rock to a more water-wet state than does sodium laureth sulfate (SLS). Surfactin, was produced by growing Bacillus subtilis on high-starch medium to represent agricultural effluent, and a suitable benchmark chemical surfactant was selected for comparative study. Cores and crushed samples of Lansing-Kansas City (LKC) carbonate reservoir material were cleaned and characterized. Crushed rock was aged in crude oil to ensure that it was strongly oil-wet and rapid wettability tests were performed to assess the wettability change mediated by low concentrations of surfactin and SLS. Static adsorption of surfactants on LKC rock was assessed and dynamic adsorption in intact cores was measured. Surfactin exhibits higher specific adsorption onto crushed LKC. Introduction Conventional surfactant flooding relies on the ability of the injected high-concentration surfactants to create ultra-low interfacial tension (IFT) between oil and water, thereby mobilizing the stranded oil. Due to the cost of injecting high-concentration chemical surfactants, the economics of surfactant injection have rarely been favorable in actual field applications. To improve process economics, injection of dilute surfactant solutions has been considered. The intent in this case is to not achieve ultra-low IFT and thus to recover only part of the stranded oil. Several studies have revealed that the injection of dilute surfactants into oil reservoirs can modify the wettability of the reservoir rock. This phenomenon may be useful in fractured carbonate reservoirs where the wettability change can improve oil recovery by accelerating the spontaneous imbibition process. The overall objective of this project is to evaluate the use of low-cost biosurfactants produced from agriculture process waste streams to improve oil recovery in fractured carbonate reservoirs. Specifically, we examine the ability of surfactin, an anionic lipopeptide surfactant (Figure 1) produced by Bacillus subtilis, to mediate wettability changes that positively affect oil recovery in fractured carbonate rock by accelerating the spontaneous imbibition process during water flooding. Experimental materials are representative of the Lansing-Kansas City (LKC) formation in central Kansas, a fractured carbonate reservoir exhibiting intermediate wettability. The economic feasibility of this process, like many other enhanced oil recovery (EOR) applications, is determined by the improved oil recovery and the degree of retention of injected chemicals in the reservoir. In this work, dilute solutions of surfactin produced by bacteria grown on high-starch liquid media are assessed for their effectiveness in mediating the wettability change of LKC carbonate rocks, and compared to similar concentrations of a benchmark chemical surfactant. Adsorption of surfactants considered for EOR applications has been studied extensively. Surfactant retention is evaluated by comparing the adsorption isotherm of surfactin with that of the benchmark chemical surfactant.
- North America > United States > Kansas (0.90)
- North America > United States > Missouri > Jackson County > Kansas City (0.45)
- North America > United States > Texas > Harris County > Houston (0.28)
- Geology > Petroleum Play Type > Unconventional Play > Fractured Carbonate Reservoir Play (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock (0.86)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.68)
- Materials > Chemicals > Commodity Chemicals > Petrochemicals (0.46)
Field Cases of a Zero Damaging Stimulation and Diversion Fluid from the Carbonate Formations in North Kuwait
Al-Mutawa, Majdi (Kuwait Oil Company) | Al-Anzi, Ealian (Kuwait Oil Company) | Ravula, Chakravartha (Kuwait Oil Company) | Al Jalahmah, Faisal (Schlumberger Middle East and Asia) | Jemmali, Mohammed (Schlumberger Middle East and Asia) | Samuel, Elsamma (Schlumberger Middle East and Asia) | Samuel, Mathew (Schlumberger Middle East and Asia)
Abstract 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). 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 acids in conjunction with chemical diverters including foams to fully stimulate long, non-uniform carbo-nate formation. The most commonly used chemical diverters are polymer based, and are associated with induced formation damage. To perform stimulation of the entire zone, a new chemical diverter with a solids-free self-diverting acid was recently developed. 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 coverage. 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.
- Asia > Middle East > Kuwait (1.00)
- North America > United States > Texas > Harris County > Houston (0.28)
Abstract Oil recovery by water flooding in fractured formations is often dependent on spontaneous imbibition. However, spontaneous imbibition is usually insignificant in oil-wet, carbonate rocks. Sodium carbonate and anionic surfactant solutions are evaluated for enhancing oil recovery by spontaneous imbibition from oil-wet carbonate rocks. Crude oil samples must be free of surface-active contaminants to be representative of the reservoir. Calcite, which is normally positively charged, can be made negative with sodium carbonate. The ease of wettability alteration is a function of the aging time and temperature and the surfactant formulation. Introduction Much oil remains in fractured, carbonate oil reservoirs after waterflooding and in some cases in paleo-transitions zones, which result from the oil/water contact moving upward before discovery. The high remaining oil saturation is due to a combination of poor sweep in fractured reservoirs and the formation being preferentially oil-wet during imbibition1,2. Poor sweep is not an issue in paleo-transition zones but yet the remaining oil saturation may still be significant. There are several reasons for high remaining oil saturation in fractured, oil-wet, carbonate formations. Poor sweep was mentioned earlier. If the formation is preferentially oil-wet, the matrix will retain oil similar to an oil-wet blotter and high oil saturation transition zones will exist where the upward oil film flow path is interrupted by fractures. This is illustrated in Fig. 1, which shows the oil retained by oil-wet capillaries of different radii. The height of the capillary retained oil column is greater for the smaller pores. In oil-wet systems, oil is the phase contacting rock surfaces, and surface trapping is likely to be particularly important in rocks with highly irregular surfaces and large surface areas, Fig. 2. The objective of this investigation is to develop a process to overcome the mechanisms for oil retention illustrated by Figs. 1 & 2. Oil is retained by wettability and capillarity. Thus altering the wettability to preferentially water-wet conditions and reducing the interfacial tension to ultra-low values can overcome these mechanisms. Introducing an injected fluid into the matrix of a fractured formation is challenging because the injected fluid will flow preferentially in the fractures rather than through the matrix. Thus the process must spontaneously imbibe the injected fluid from the fracture system into the matrix, as illustrated in Fig. 3. Spontaneous capillary imbibition may no longer be important because of low interfacial tension. However, if wettability is altered to preferentially water-wet and/or capillarity is diminished through ultra-low interfacial tensions, buoyancy will tend to allow oil to flow upward and out of the matrix into the fracture system. The injected fluid will replace the displaced oil in the matrix and thus the spontaneous imbibition will continue as long as oil flows out of the matrix. Spontaneous imbibition is an important mechanism in oil recovery from fracture reservoirs. A recent survey by Morrow and Mason reviews the state-of-the-art. They state that imbibition rates with different wettability can be several orders of magnitude slower and displacement efficiencies range from barely measurable to better than very strongly water-wet. The primary driving force for imbibition in strongly water-wet conditions is the capillary pressure. Reduction of interfacial tension reduces the contribution of capillary imbibition. Buoyancy, as measured by the Bond number then becomes the dominant parameter governing the displacement, even of the wetting phase. Application of surfactant to alter wettability and thus enhance spontaneous imbibition has been investigated by Austad, et al. with chalk and dolomite cores. Chen, et al., investigated enhanced imbibition with nonionic surfactants. Spinler, et al., evaluated 46 surfactants for enhanced imbibition in chalk formations. Standes, et al. and Chen, et al. used either nonionic or cationic surfactant with a strategy to alter wettability but avoiding ultra-low tensions. The work presented here differs from the previous work in that sodium carbonate and anionic surfactants are used to alter wettability and reduce interfacial tension to ultra-low values.
- Asia > China (1.00)
- Europe > Norway > Norwegian Sea (0.64)
- North America > United States > Texas > Harris County > Houston (0.28)
- Geology > Mineral (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Dolomite (0.35)
- Energy > Oil & Gas > Upstream (1.00)
- Materials > Chemicals > Commodity Chemicals > Petrochemicals (0.46)
- North America > United States > Wyoming > Kiehl Field (0.99)
- North America > United States > California > Sacramento Basin > 4 Formation (0.99)
- Asia > China > Xinjiang Uyghur Autonomous Region > Junggar Basin > Karamay Field (0.99)
- (4 more...)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Carbonate reservoirs (1.00)
- Reservoir Description and Dynamics > Improved and Enhanced Recovery > Waterflooding (1.00)
- Reservoir Description and Dynamics > Improved and Enhanced Recovery > Chemical flooding methods (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management (1.00)
Abstract Acid-wormholing in a carbonate formation is the result of several chemical and physical processes, including acid diffusion, reaction kinetics and dynamics of fluid flow in porous media. A proper understanding of the combined effect of these processes is essential for the design of acid treatments below and above fracturing pressure. In matrix acid treatments wormholes are favourable. Depending on the interaction of the driving mechanisms, compact dissolution, a few dominating flow channels or a branched network of thin wormholes may emerge. A sound understanding of these mechanisms is the basis for treatment optimization and development of new stimulation strategies. This extended abstract summarizes some results of a study on the mechanisms behind wormholing. These results will be used to build a wormhole evolution model that can aid in the design of acid-fracturing and matrix-acid treatments in carbonate formations. Such a model will be described in a future paper. Acid Spending in a Single Wormhole Acid flow and reaction in a wormhole was studied by modelling the wormhole as a cylinder and numerically solving the mass balance equation that describes acid transport by convection and diffusion. The acid-rock reaction rate enters the equations as a boundary condition. Often the mathematical problem is simplified when an infinite acid-rock reaction rate is assumed or, similarily, a completely mass-transport controlled acid spending rate is assumed. The boundary condition is in that case: C(x,r=rw)=0. However, this assumption excludes the study of situations in which the acid spending rate is (partly) reaction-rate controlled, such as low-temperature dolomites or low-reactivity acids. Furthermore, the spending rate control mechanism is a function of pore diameter. In the smaller pores near the wormhole tip, acid spending is often limited by the reaction rate. Precisely this area is important for wormhole growth. In this work, a finite acid-rock reaction rate was assumed, given by the power relation J=kCn. The parameters that define the acid concentration profile in the wormhole (Q, D, rw, C0, krate and n) can be grouped into dimensionless numbers. These dimensionless numbers follow naturally when the diffusion-convection and reaction-rate boundary equations are expressed in terms of normalized variables for acid concentration and axial and radial position (CN=C(xN,rN)/C0,xN=x/rw, and rN=r/rw): (1) (2) The dimensionless numbers Npe and Nki are the Peclet and kinetic number respectively and are given by: (3) The Peclet number describes the relative importance of mass transport by convection and diffusion, while the kinetic number describes the relative importance of reaction rate and diffusion rate. A third dimensionless number that is sometimes used is the Damkohler number, equal to the ratio Nki/Npe. If Nki»1, the spending rate is controlled completely by mass transport processes and the acid concentration profile in the wormhole is a function of Npe only. For this situation, Levich derived an approximate solution of equation (1) in terms of a mass-transport coefficient KMT. P. 683^
- North America > United States > Texas (0.29)
- North America > United States > Louisiana (0.29)
- Europe > Norway > Norwegian Sea (0.24)
Abstract The efficiency of the matrix acidizing process in carbonates depends strongly on the wormholing phenomenon - if worm holes are formed, the effects of near wellbore damage can be overcome with relatively small volumes of acid. Numerous previous studies have shown that worm hole patterns can be placed in these general categories:compact dissolution in which most of the acid is spent near the rock face; the wormholing pattern; and uniform dissolution in which many pores are enlarged, as typically occurs in sandstone acidizing We have developed a theory of the wormholing process which predicts when the wormholing pattern is most efficiently created as a function of the acid flux and other treatment variables. By testing this theory with several independent sets of laboratory data, we can now demonstrate the important roles that surface reaction rate and fluid loss play in the wormholing process. This theory accurately predicts the optimal flux (that which leads to dominant wormholes with a minimum of branching and hence a minimum acid volume) for experiments with HCl in limestone and dolomite at several temperatures and with acetic acid in limestone. Surface reaction rate differs by several orders of magnitude in these experiments and is the only process variable that differs greatly among them. Paradoxically, though worm holes are formed because the overall reaction rate is controlled by mass transfer in the wormholes, diffusion rates play only a minor role in the wormholing process. Fluid loss through the walls of the wormholes ultimately limits the distance to which worm holes can propagate. Because of this effect, laboratory linear core floods will give optimistic predictions of worm hole penetration distances. We developed a cylindrical flow model to represent the flow field around a worm hole propagating from a wellbore which illustrates how to translate laboratory results to field conditions. We have used these theories to predict optimal acid formulations and injection rates for field conditions. In general, the lower the reaction rate (such as at low temperatures in dolomites or with weak acids in limestones), the lower the injection rate required, making it easier to propagate dominant wormholes under matrix treating conditions in the field. Introduction Numerous studies of the wormholing process in carbonate acidizing have shown that the dissolution pattern created can be characterized as being one of three types:compact dissolution in which most of the acid is spent near the rock face; the wormholing pattern; and uniform dissolution in which many pores are enlarged, as typically occurs in sandstone acidizing. These studies have also shown that the acidizing process is most efficient (defined as the process that will enhance near-wellbore permeability to the greatest depth with the smallest volume of acid) when the wormholing pattern develops. A third observation common to these studies is that the pattern created depends on acid flux, with the compact pattern created at relatively low acid flux, the worm hole pattern developing at intermediate flux, and the uniform pattern at high flux. Of course, there is not an abrupt transition from one pattern to another. As acid flux is increased, the compact pattern will change to one in which large diameter worm holes are created; further increases in flux yield narrower wormholes which propagate farther for a given volume of acid injection; and finally, as acid flux is increased more, the worm holes become more and more branched until ultimately the uniform pattern is observed. P. 775^
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.94)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (0.67)
Abstract The success of conventional matrix acidizing treatments with hydrochloric acid is often limited due to rapid acid spending at low injection rates. Previous studies have demonstrated the effectiveness of ethylenediaminetetraacetic acid (EDTA) as an alternative to HCl for stimulating carbonate formations. This work extends the study to include other chelating agents of the aminopolycarboxylic acid group. Results show that 1,2-cyclohexanediaminetetraacetic acid (CDTA) and diethylenetriaminepentaacetic acid (DTPA) effectively wormhole in limestone, even when injected at moderate pH values and at low flow rates where only face dissolution would occur with HCl. Rotating disk experiments have demonstrated that the dissolution of calcite by chelating agents is not necessarily limited by reactants transport to the surface. Therefore, we have derived a modified Damkohler number that includes the effects of reactant transport, reversible surface reactions, and products transport. The wormhole structure and permeability response depend on this modified Damkohler. In addition, there exists an optimum modified Damkohler number at which a single dominant wormhole channel is obtained and the pore volumes to breakthrough is minimized. This optimum Damkohler number occurs at approximately 0.17 for all of the fluids investigated. Introduction Matrix acidizing treatments often require low injection rates to prevent fracturing the formation rock or are required in heterogeneous formations with zones of low-conductivity (which need stimulation the most) that accept acid at low rates. It is at these low injection rates that the problem of rapid acid spending severely limits the acid penetration distance. The injection of hydrochloric acid into carbonate formations at low rates results in face dissolution, or complete dissolution of the carbonate matrix near the wellbore. This face dissolution consumes large volumes of acid and provides negligible increases in the conductivity of the formation. Various acid systems such as oil external microemulsions containing HCl and foamed acids (nitrogen gas and aqueous HCl) have been shown to stimulate carbonate formations at lower injection rates. However, strong acids such as HCl destabilize asphaltene particles in crude oil and cause the formation of asphaltic sludge and rigid film emulsions. This common problem is even more severe when ferric ions are present. A variety of acid additives (anti-sludging agents, corrosion inhibitors, and iron reducing agents) have been used to prevent the sludging problem. However, their effectiveness is limited by the need to obtain a compatible combination of additives and a lack of understanding of the complex chemistries involved in the precipitation reactions. These limitations demonstrate the need for alternative stimulation fluids that combine the ability to stimulate at low injection rates with fluid properties that are not conducive to asphaltic sludge precipitation or corrosion problems. Previous work in our laboratories has demonstrated that the chelating agent ethylenediaminetetraacetic acid (EDTA) can effectively wormhole in limestone, even when injected at moderate or non-acidic pH values (4 to 13) and at low flow rates where HCl is ineffective. The dissolution mechanism involves chelation of calcium ions and does not require conventional acid attack. This ability to stimulate under non-acidic conditions combined with the ability to chelate metal ions provide other benefits of using EDTA. It has been shown that EDTA does not induce the precipitation of asphaltic sludge from crude oil, even in the presence of 3000 ppm of ferric iron. In addition, corrosion is negligible for alkaline solutions of EDTA below 204 C (with possible exceptions when copper, tin, and aluminum are present). Therefore, EDTA provides the properties necessary for a matrix stimulation fluid (wormholes formed in carbonates at low injection rates) while not requiring additives to control corrosion or asphaltic sludge precipitation. The success of EDTA as an alternative stimulation fluid for carbonate formations has led to further investigation of chelating agents of the aminopolycarboxylic acid family. P. 23
- Geology > Mineral (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (0.57)
- Well Completion > Acidizing (1.00)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Carbonate reservoirs (1.00)
- Reservoir Description and Dynamics > Improved and Enhanced Recovery (1.00)
- Production and Well Operations > Production Chemistry, Metallurgy and Biology > Inhibition and remediation of hydrates, scale, paraffin / wax and asphaltene (1.00)