Carbonate reservoir matrix acidizing is commonly conducted with HCl. In these treatments, HCl acid is used to create conductive channels (wormholes) to enhance well productivity/injectivity. However, its use has been limited due to associated rapid tubulars corrosion and formation face dissolution, especially in deep hot reservoirs. Emulsified acid was used as an effective alternative to HCl, but it is associated with drawbacks such high friction losses and emulsion stability. In this paper, an aqueous single-phase retarded HCl alternative system was evaluated as an alternative to straight and emulsified acid fluids.
Coreflood experiments were conducted using Indiana limestone core plugs at 180 and 270°F. Computerized Tomography (CT) scan analysis was conducted on the core plugs before/after coreflood testing. Compatibility testing was conducted on prepared retarder acid recipes. ESEM, TGA, and ICP analysis was used to analyze prepared retarder acid recipes and associated solids. Turbiscan LAB was used to assess the stability of the retarded acid recipes.
The low pore volume to breakthrough (PVBT) values (i.e., 0.9-1.6) obtained from coreflood testing at 180 and 270°F, confirmed the retarded HCl acid recipes were effective to stimulate carbonate reservoirs. Compatibility testing showed presence of significant white precipitate. ESEM analysis showed the precipitates were rod-like crystals composed of mainly of Cl and high C with small amounts of N, O, Al and Mg. TGA results showed the major constituent of precipitate were organic-based materials. The precipitate was mainly H4EDTA and chloride. Despite presence of white precipitate at the core inlet, the effect on the performance of the retarded acid system was insignificant. CT scanning analysis of the plug samples before/after the coreflooding experiments showed that wormholes along the plug length with multiple branches were formed in all cases indicating the compatibility of the selected acid recipe.
Aidagulov, Gallyam (Schlumberger Dhahran Carbonate Research Center) | Gwaba, Devon (Schlumberger Dhahran Carbonate Research Center) | Kayumov, Rifat (Schlumberger Middle East S.A.) | Sultan, Abdullah (King Fahd University of Petroleum and Minerals) | Aly, Moustafa (King Fahd University of Petroleum and Minerals) | Qiu, Xiangdong (Schlumberger Dhahran Carbonate Research Center, Now with Branch of Sinopec International Petroleum Service Corporation) | Almajed, Haidar (King Fahd University of Petroleum and Minerals) | Abbad, Mustapha (Schlumberger Dhahran Carbonate Research Center)
Carbonate reservoirs host a significant amount of hydrocarbon reserves in the Middle East and worldwide. In matrix acidizing stimulation, hydrochloric acid (HCl) is commonly injected into the well at pressures less than fracturing pressure to dissolve the carbonate rock and create high-conductivity channels, known as wormholes. Wormholes propagate through the damaged near-wellbore zone connecting the well with the reservoir. In this work, we aim to study the effects of pre-existing fractures on wormhole development.
Matrix acidizing processes were reproduced in controlled laboratory experiments where a 15% HCl solution was injected into a borehole drilled in a carbonate block sample containing pre-existing fractures, allowing the acid to penetrate radially into the rock sample. The experiment was conducted inside a polyaxial load frame to accommodate large block samples (20×16×16 in.). Prior to acid injection, the block was fully saturated with water and taken to 2,000-psi pore pressure and 4,000-psi confining stress to simulate downhole conditions. To evaluate the created wormholes, the tested block was cut open along the fractures followed by X-ray CT scanning of selected zones.
Here we report experimental results for matrix stimulation of one Indiana limestone block containing a series of parallel pre-existing fractures. Acid was injected at a constant rate through the 1-in. diameter borehole containing an 8-in.-long openhole section in the center of the block. Although the acid injection pressure was maintained below the pressure required to open the fractures, acid breakthrough was found to be governed by the pre-existing fractures. Indeed, unlike similar radial acidizing experiments in intact blocks, there were no indications of wormholes exiting the outer faces of the block. Moreover, the post-test evaluation of the central fracture along the openhole section clearly revealed the wormholes that etched the fracture faces. However, a closer look into the stimulated openhole section showed that the wormholes initiated in other directions inside the matrix as well. An X-ray CT scan of a 4-in. diameter cored borehole regions allowed us to compare the density and characteristics of the wormhole growth along the fracture and into the matrix.
Although radial acidizing experiments describe more closely real conditions of matrix acidizing, few cases have been published, particularly for large-block experiments. The large-scale block experiments presented in this study provide new insights on the impact of pre-existing fractures on wormholing mechanisms.
Scaling precipitation in the form of calcium or iron sulfate (CaSO4 – FeSO4) appears to be a common problem in most of the wells completed in the Yeso formation in the Permian Basin. Accumulation of these forms of scale in the perforations and the downhole equipment can create severe production losses and therefore they require constant, some times costly, treatments. Additionally, their presence in the wellbore hinders the efficiency of the acidizing procedures, performed by the operators to further increase the productivity of their wells.
The objective of this paper is to present an innovative workflow, which significantly reduces the scaling effects and allows the acidizing of these carbonate reservoirs to be more efficient. Results are avialble for more than 20 wells, from Occidental Petroleum’s assets in the North New Mexico region. Details regarding the evolution of the procedure, the types and amounts of the chemicals are extensively discussed in this paper.
The procedure starts with the candidate well selection, which preferably are located in the highest OOIP areas to maximize the efficiency. The second step involves the water sampling and analysis to determine the types of scaling and consequently the required chemical treatment. The innovation in this step has to do with the time that converters are left in the wellbore and the extra chemicals that have been added compared to the previous procedures. The last step involves the deployment of the acid downhole, where three different methodologies have been tested and evaluated based on their efficiency and well returns.
The proposed approach has been successfully applied to more than 20 wells and the results are encouraging showing an average incremental oil production of ~600% while the execution cost remains very low.
Golenkin, M. Y. (LUKOIL-Nizhnevolzhskneft) | Khaliullov, I. R. (LUKOIL-Nizhnevolzhskneft) | Byakov, A. P. (LUKOIL-Nizhnevolzhskneft) | Charushin, A. B. (Schlumberger) | Burdin, K. V. (Schlumberger) | Vereschagin, S. A. (Schlumberger) | Olennikova, O. V. (Schlumberger) | Borisenko, A. A. (Schlumberger) | Lobov, M. A. (Schlumberger) | Kobets, V. (Schlumberger)
In 2016, the first application in Russia of a diversion technology with multimodal granules was performed during matrix treatment of a carbonate reservoir in a water-absorbing well in an offshore field in the northern Caspian Sea. The operator's main objectives were the recovery of water-absorbing well injectivity while simultaneously straightening the profile by a temporary isolation of high-absorbing intervals. To achieve the objectives, two operations needed to be performed: large-volume acidizing of J3V Volgian regional stage and acid spotting in the interval of the Neocomian superstage.
The PDF file of this paper is in Russian.
Maintaining sustainable oil production is possible, largely, due to successful application of technologies for wellbore zone treatment, in particular, selective acidizing of carbonate reservoirs. Reservoir heterogeneity results in poor conformance, uneven distribution of displacement agent, and, consequently, forming of flushed zones with low flow coefficients. This leads to increased water production and higher water cut of produced oil, while, low-permeability layers remain underproduced, or even bypassed. Selective acidizing of productive layers might be a good solution in this case. The mechanism of selective acidizing is based on temporary blocking of flushed (fractured) intervals with some high-viscosity system (a kind of “liquid plug”) inert to acid and solved with oil while production. High-concentration hydrophobic water-in-oil emulsions meet these requirements to the point. They are characterized by a wide range of viscosity control, from few dozens of centipoise to dozens of thousands of centipoise, up to non-flowing, as well as by controllable physical properties, including yield point (static and dynamic shear stress), thixotropic properties (structure strengthening with time), non-Newtonian rheology (pseudoplastic and/or viscoelastic flow). This paper presents results of studies aimed at development of special formulations of high-concentration hydrophobic emulsions to be used in challenging conditions of fractured-cavernous reservoirs, where conventional hydrophobic emulsions fail to block permeable zones and direct acid systems to target intervals. The designed formulation of high-concentration hydrophobic emulsion provides for enhanced blocking properties needed to redirect the injected acid systems from the fractured (and, as a rule, water-saturated) zones to oil-saturated matrix zones, high-performance killing of wells during workover operations, and enhanced water shutoff properties, resulting in overall cost saving.
Поддержание текущих темпов добычи нефти во многом обеспечивается эффективным применением различных методов и технологий обработки прискважинной зоны пласта, в частности, направленных (селективных) кислотных воздействий на карбонатные коллекторы. Неоднородность продуктивного пласта приводит к неравномерному продвижению вытесняющего агента и образованию промытых зон с низким фильтрационным сопротивлением, что способствует обводнению добывающих скважин, при этом, как правило, менее проницаемые (низкопроницаемые) прослои вырабатываются более медленными темпами или вовсе не вовлекаются в разработку. Селективность воздействия кислот на нефтенасыщенные интервалы решается временным блокированием дренированной (трещинной) части пласта какой-либо высоковязкой системой (наподобие «жидкой пробки»), не реагирующей с кислотой, но растворяемой нефтью при освоении. Такой системой являются высококонцентрированные гидрофобные (обратные) эмульсии (ВГЭ), имеющие широкий диапазон регулирования вязкости, вплоть до нетекучего состояния, а также регулируемые структурно-механические свойства, важнейшими из которых являются статическое и динамическое напряжения сдвига и тиксотропия, неньютоновская реология (псевдопластическое и/или вязкоупругое течение). В статье приведены результаты исследований по разработке специальных рецептур ВГЭ для применения в наиболее осложненных трещинно-кавернозных карбонатных коллекторах, в которых, как показывает практика, применение традиционных составов не обеспечивает эффективного блокирования трещинных интервалов и отклонения кислот. Техническими результатами использования предложенной рецептуры ВГЭ являются повышение тампонирующих, блокирующих свойств гидрофобной эмульсии при отклонении поступления кислот в трещинно-кавернозные, как правило, водонасыщенные интервалы пласта и их перенаправление в нефтенасыщенные матричные зоны; обеспечение качественного глушения скважин с аномально высокой приемистостью пластов и повышение водоизоляционных свойств с одновременным снижением материальных затрат.
The optimal injection rate for wormhole propagation and face dissolution at low injection rates during carbonate matrix acidizing is well-established. However, little research is documented on the subject of how the presence of oil affects this process. This study demonstrates the impact of oil saturation on wormhole characteristics while acidizing reservoir and outcrop cores under reservoir conditions (200°F).
Coreflood experiments at flow rates ranging from 0.5 to 20 cm3/min were performed to determine the optimal acid-injection rate for wormhole propagation when acidizing homogeneous limestone reservoir cores, low-permeability Indiana limestone cores, and homogeneous dolomite cores with dimensions of a 3- and 6-in. length and a 1.5-in. diameter. The experimental work involved acidizing cores saturated with water, oil, and waterflood residual oil by use of 15-wt% regular hydrochloric acid (HCl). The viscosity of the crude oil used was 3.8 cp at 200°F. Computed-chromatography (CT) scans enabled the characterization of wormholes through the cores. The concentrations of the calcium and magnesium ions in core effluent samples were measured with inductively coupled plasma optical emission spectroscopy (ICP-OES), and the effluent samples were titrated to determine the concentration of the acid.
At injection rates of 0.5 to 20 cm3/min, 15-wt% HCl was effective in creating wormholes with minimal branches for cores with residual oil saturation (ROS). Compared with brine- and oil-saturated cores, those at ROS took less acid volume to breakthrough. In addition, the efficiency of regular acid improved with increased acid-injection rates in the presence of residual oil. A decrease in the acid pore volume (PV) to breakthrough for oil-saturated cores was observed at high acid-injection rates, which could be attributed to viscous fingering of acid through oil. Unlike brine-saturated and oil-saturated cores, cores at ROS showed no face dissolution at low acid-injection rates. The conclusions of this work highlight the impact of oil saturation on matrix characteristics while acidizing carbonate rocks.
Successful matrix acidizing of carbonate reservoirs depends on the selection of optimal stimulation fluids. Because of the rapid reaction rate and corrosive nature of HCl in downhole conditions, other alternatives are much in demand. Organic acids, particularly methanesulfonic acid (MSA), offer a viable alternative to HCl in terms of being less reactive as well as less corrosive and environmentally benign. However, MSA is expensive. To reduce the cost, this study proposes to use blend of HCl and MSA for carbonate stimulation, while enhancing the properties of HCl. Coreflood studies were performed and the results were compared to those obtained by equivalent concentrations of the individual acids.
Three different ratios of HCl and MSA were used to conduct coreflood experiments on 6-in. long Indiana Limestone cores at 250°F. The volume of acid required to reach breakthrough was recorded, and the cores were analyzed using CT scans. Wormhole structures were identified, and their tortuousities were determined. The effluent samples were analyzed for pH, calcium concentration, and unconsumed acid concentration.
Coreflood studies indicated that 5:5 wt% HCl:MSA blend was the most suitable candidate for matrix acidizing among the three blends tested (2.5:7.5 and 7.5:2.5 wt% HCl:MSA being the other two blends investigated). At the optimum injection rate of 7.5 cm3/min, both 2.5:7.5 and 5:5 wt% HCl:MSA mixture required lesser pore volumes (PVs) of acid to reach breakthrough, compared to their individual acid controls. A single, straight, and dominant wormhole was observed with no branching and less tortuousity in both the cases. The control experiments with equivalent concentrations of HCl and MSA required higher PVs of acid to reach breakthrough with branching during wormhole propagation. Calcium ion dissolution was least for the 5:5 wt% mixture among the three blends tested. Higher unconsumed acid concentration was noted in case of 5:5 wt% compared to 2.5:7.5 wt% blend, thus promising greater penetration depth with the same PV of acid. On the other hand, the wormhole formed by the acid blend of 7.5:2.5 wt% HCl:MSA required almost the same PV of acid to reach breakthrough as its corresponding HCl control, and it was more enlarged and tortuous than its corresponding MSA control. 5:5 wt% HCl:MSA blend creates deeper wormholes and retards the HCl reaction with the rock matrix.
Major advantages rendered by the new acid mixture include: (1) deeper wormholes that will ultimately result in enhanced well productivity, and (2) cost effectiveness in carbonate stimulation compared to standard systems currently used in the market.
Al-dahlan, Mohammed N (Saudi Aramco PE&D) | Al-Obied, Marwa Ahmad (Saudi Aramco PE&D) | MARSHAD, KHALID Mohammad (Saudi Aramco PE&D) | Sahman, Faisal M (Saudi Aramco) | Al-Yami, Ibrahim Saleh (Saudi Aramco PE&D) | AlHajri, Abdullah (Saudi Aramco PE&D)
Description of the material
This paper presents the results of the study conducted on HCl-Replacement-Acid (HRA), a synthetic HCl replacement chemical, with health hazard rating of one and dissolving power similar to HCl. An extensive experimental scheme including: thermal stability, dissolving power, acidity, compatibility, corrosion rate & inhibition and coreflooding on carbonate formation core plugs was conducted.
Acid treatments of carbonate formations are usually carried out using mineral acid (HCl), organic acids (formic and acetic), mixed acids (HCl-formic, HCl-acetic), and retarded acids. The major challenges when using these acids are their high corrosion rate, fast reaction rate and health hazard. The improvement in corrosion inhibitors makes the use of strong acid as high as 28 wt% HCl possible. The acid reaction rate can be controlled by decreasing diffusion rate of hydronium ions (H+) to the rock surface where reactions take place by increasing acid viscosity using gelling agent or emulsifying acid droplet in a hydrocarbons liquid, acid-in-diesel emulsion. While the issues of stimulation acids reaction and corrosion rates are relatively controlled, these acids health hazard rating of 3 by the National Fire Protection Association (NFPA) is major concern. A health hazard rating of 3 is defined as an extreme danger where short exposure could cause serious injury
Results, Observations, and Conclusions
Based on this study results, the HRA was found to be thermally stable with similar dissolving power to 15 wt% HCl and lower corrosion rate. In addition, the HRA developed a breakthrough on core plugs with average pore volume (PV) of 2.7 and approximately 3 folds increase in permeability.
Significance of subject matter
An acid replacement chemical that has no or minimum health hazard rating while still has the ability to dissolve carbonate rock would be a major forward step in stimulation technology.
Carbonate acidizing is one of the main techniques for improving the production and injectivity in oil and gas fields. Various studies and field stimulation results were analyzed to develop a fit-for-purpose acid stimulation treatment design placed with coiled tubing in which diversion is achieved by using in-situ gelation, emulsified diverting acid, degradable fibers, or combinations of these methods.
The lack of downhole fluid placement control during the pumping of stimulation treatments may cause these jobs to not achieve the maximum stimulation effect or even fail, which may eventually call for more costly solutions.
An innovative stimulation approach was applied on a sour gas injector well in a carbonate oil field in the Caspian region. The field is characterized as a naturally fractured, thick, and prolific carbonate formation with high H2S content. To dispose of H2S and improve oil recovery, the produced sour gas is injected back into the reservoir through injector wells. An innovative method using fiber optic technology for acquiring distributed temperature survey (DTS) measurements and a real-time downhole sensor tool providing pressure and temperature measurements and casing collar location were used in this well to improve its injection potential.
DTS technology was utilized to better understand the movement of stimulation fluids into the reservoir through real-time monitoring, thus providing the capability to optimize the acid injection along the target zone. The DTS analysis during the post-acid injection stage identified crossflow and provided good correlation between acid reaction with carbonates and proportional warm-back trends along the formation.
The adoption of the technique enabled increased overall confidence in decision making during treatment execution, which allowed an improved placement strategy, resulting in increased stimulation effectiveness. This technology has the potential to become the next important step in the evolution of acid stimulation strategies in the Caspian region.
The existence of an optimum injection rate for wormhole propagation and face dissolution at low injection rates during matrix acidizing are well established. However, little has been documented that describes how the presence of residual oil affects carbonate acidizing. This study demonstrates the impact of oil saturation on wormholing characteristics while acidizing field and outcrop cores under reservoir conditions (200°F). Knowledge of the effect of different saturation conditions on acid performance will contribute towards the design of more effective acid treatments.
Coreflood experiments at flow rates ranging from 0.5 to 20 cm3/min were performed to determine the optimum injection rate for wormhole propagation when acidizing homogeneous limestone reservoir cores and low permeability Indiana limestone cores of dimensions 3 and 6 in. length and 1.5 in. diameter. The absolute permeability of the cores ranged from 1 to 78 md. The study involved acidizing cores saturated with water, oil, and waterflood residual oil using 15 wt% HCl. The viscosity of the crude oil used was 3.8 cp at 200°F. CT scans were used to characterize wormholes through the cores. The concentrations of the dissolved calcium and magnesium ions were measured using Inductively Coupled Plasma–Optical Emission Spectroscopy (ICP-OES) and the effluent samples were titrated to determine the concentration of the acid.
At injection rates 0.5 to 20 cm3/min, 15 wt% HCl was effective in creating wormholes with minimal branches for cores with residual oil (Sor=0.4–0.5). Compared to brine and oil saturated cores, waterflood residual oil cores took less acid volume to break through. Additionally, the wormholing efficiency of regular acid improved with increased acid injection rates in the presence of residual oil. A decrease in acid pore volumes to breakthrough for oil saturated cores was noted at high acid injection rates, which could be attributed to viscous fingering of acid through oil. Unlike brine saturated and oil saturated cores, waterflood residual oil cores showed no face dissolution at low acid injection rates. Conclusions from this work will aid in the design of better acid jobs by highlighting the impact of oil saturation on wormholing characteristics of acid while acidizing carbonate rocks.