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Success rates of carbonate acidizing, when upscaling from single, aqueous-phase laboratory analyses to field scale levels have been poor. Analyzing matrix acidizing performances in carbonate reservoirs with two-phase environments. i.e., oil and water, has picked up importance currently in the stimulation industry. A lack of understanding of the mechanisms taking place in this complex subsurface process still exists, particularly with carbon dioxide (CO2) evolution from limestone dissolution, and the presence of a secondary fluid phase in the medium.
A detailed investigation via experimental core studies and fluid modeling / characterization has been performed for oil/water environments in the porous media. Moderate permeability carbonate cores, with an average of 14% porosity were used for this study. Six acidizing experiments with 15 wt% hydrochloric (HCl) acid were performed using outcrop Indiana limestone cores, at temperatures of 150°F and initial pore pressures of 600 and 1,200 psi. The presence of a light crude oil in the cores at residual conditions was tested for acid efficiencies in un-aged and aged conditions. Rock wettability measurements via contact angle experiments, and oil/water interfacial tension,
Cores with residual oil at water-wet conditions yielded the least acid pore volumes to breakthrough (PVbt), at all tested pore pressures. An increased oil-wetness of the rock resulted in greater acid PVbt's, when compared to water-wet systems with both residual oil and fully water saturated cores. CO2's capability to alter rock-wettability did not show any benefits toward improving acid efficiencies in cores at residual oil saturations, Sor. The acid efficiencies in porous media containing oil and water largely depend on the access of sufficient rock surface to the acid species, for which rock-wettabilities are a governing factor.
Panuganti, Sai Ravindra (Petroliam Nasional Berhad PETRONAS) | Misra, Sanjay (Petroliam Nasional Berhad PETRONAS) | Salleh, Intan Khalida (Petroliam Nasional Berhad PETRONAS) | M. Ibrahim, Jamal Mohamad (Petroliam Nasional Berhad PETRONAS) | Rodzali, Mohamad Azmeer (Petroliam Nasional Berhad PETRONAS)
The process of matrix acidizing, despite being one of the oldest operations in the petroleum industry, is still a challenge for tight carbonate reservoirs. This project considers the development of a multi-functional environment friendly chemical, for reservoir stimulation and formation damage remediation.
A microemulsion solution of biodegradable chelating agent is formulated, which is effective for tight carbonate reservoir stimulation even at high temperature. Together with the chelate based inorganic dissolver, aromatic naphtha as an organic dissolver is the other main active ingredient in the proposed microemulsion formulation. For this reason, the microemulsion solution can also be used to treat inorganic and organic mixed deposits which can involve in formation damage. The developed multi-application chemical is later tested for compatibility with reservoir fluids and production chemicals encountered during well flow back.
Formulations with low reactivity are required when the injection of stimulant is not possible at high rate. By making microemulsion with chelate, the reactivity and diffusivity of the chelating agent can be controlled further. Core flooding experiments on core samples from high pressure, high temperature and tight carbonate formation, are conducted to demonstrate wormhole formation during the matrix acidizing treatment with the formulated microemulsion. The synthesized stable microemulsion chemical is also subjected for detailed dissolution study on deep wellbore deposits of different composition from different fields. These inorganic and organic mixed deposits are otherwise hard to be remediated by aqueous or organic solvent alone.
The novelty of this article is in developing a chelate based microemulsion as the main stimulation fluid. Another uniqueness of the microemulsion solution is in the treatment of formation damage causing deposit species which are mixed in nature, without the need of any additives.
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.
This study demonstrates the application of an alternative numerical-simulation approach to effectively describe the flow field in a two-scale carbonate-matrix-acidizing model. The modified model accurately captures the dissolution regimes that occur during carbonate-matrix acidizing. Sensitivity tests were performed on the model to compare the output with experimental observations and previous two-scale models in the literature. A nonlinear reaction-kinetics model for alternative acidizing fluids is also introduced.
In this work, the fluid-field flow is described by the Navier-Stokes momentum approach instead of Darcy’s law or the Darcy-Brinkman approach used in previous two-scale models. The present model is implemented by means of a commercial computational-fluid- dynamics (CFD) package to solve the momentum, mass-conservation, and species-transport equations in Darcy scale. The software is combined with functions and routines written in the C programming language to solve the porosity-evolution equation, update the pore-scale parameters at every timestep in the simulation, and couple the Darcy and pore scales.
The output from the model simulations is consistent with experimental observations, and the results from the sensitivity tests performed are in agreement with previously developed two-scale models with the Darcy approach. The simulations at very-high injection rates with this model require less computational time than models developed with the Darcy approach. The results from this model show that the optimal injection rate obtained in laboratory coreflood experiments cannot be directly translated for field applications because of the effect of flow geometry and medium dimensions on the wormholing process. The influence of the reaction order on the optimal injection rate and pore volumes (PVs) of acid required to reach breakthrough is also demonstrated by simulations run to test the applicability of the model for acids with nonlinear kinetics in reaction with calcite.
The new model is computationally less expensive than previous models with the Darcy-Brinkman approach, and simulations at very-high injection rates with this model require less computational time than Darcy-based models. Furthermore, the possibility of extending the two-scale model for acid/calcite reactions with more-complex chemistry is shown by means of the introduction of nonlinear kinetics in the reaction equation.
Matrix acidizing is a stimulation technique aiming at improving formation permeability or bypassing damaged zones. In this process, acid is injected through the well into the wellbore vicinity to dissolve the rock. For either production or injection wells, the formation may contain multiple phases (oil and water) near the wellbore region when acid treatment begins. In this paper, a two-phase two-scale continuum model is developed to simulate wormhole propagation under radial coordinates. The model describes the mechanisms of convection, dispersion, and reaction in two-phase flow during matrix acidizing. We have validated the simulation model with two methods: one is to compare with the previous simulation results; the other is to compare with the analytical solution. We have investigated conditions that will affect the wormhole-propagation process, including rock wettability, oil viscosity, and initial oil saturation. It is found that the water/oil mobility ratio is a key factor that affects acidizing efficiency. In addition, we have proposed a new criterion for acid breakthrough because the pressure response is affected not only by reaction, but also by overall mobility change in the formation. The traditional criterion for the single-phase model is no longer applicable to the current two-phase model. The results show that adverse water/oil mobility ratio leads to a higher efficiency for wormhole breakthrough. In carbonate reservoirs with heterogeneity, water/oil displacement and wormhole propagation contribute to narrower, less-branched channels. For the first time, it is possible to simulate formations with multiple phases during carbonate acidizing. The presented model improves our understanding in the optimization of carbonate acidizing.
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.
Carbonate rock holds 60% of the global oil and gas reserves, but they are becoming more and more expensive and difficult to develop. With large reservoirs maturing, operators are forced to explore and produce from deeper resources, which are tight, highly stressed, and under high temperature. In today’s economic environment of USD 50/bbl, the cost of extracting hydrocarbon from these reservoirs needs to be scrutinized to maximize profitability. This means increasing drainage of wells using effective stimulation and optimizing production profile along the well. Generically, carbonate matrix stimulation means pumping acids, retarded or unretarded with various functional additives, through coiled tubing or by bullheading, followed with diverting agents.
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.