Carbonate Matrix Acidizing Research Articles

Wormhole Geometry Modelling on Carbonate Matrix Acidizing: A Literature Review

Matrix acidizing is a critical well-stimulation technique used to enhance the permeability of carbonate reservoirs by creating channels or "wormholes" through the dissolution of the rock matrix. The efficiency of this process is significantly influenced by the geometry of the wormholes formed. This review paper provides a comprehensive analysis of current research on wormhole geometry modeling in carbonate matrix acidizing, synthesizing findings from experimental studies, analytical models, and numerical simulations. Key factors affecting wormhole geometry, such as acid concentration, injection rate, and rock properties, are discussed. The review highlights the contributions of various modeling approaches in predicting wormhole formation and propagation, emphasizing the importance of accurately capturing the coupled effects of fluid flow, chemical reactions, and rock dissolution. The implications for optimizing acidizing treatments and enhancing hydrocarbon recovery are explored, alongside recommendations for future research. These include the need for field validation, advanced modeling techniques, real-time monitoring, interdisciplinary collaboration, and sustainability considerations. This synthesis aims to provide a foundation for improving the design and execution of matrix acidizing operations in carbonate reservoirs.

A comprehensive analysis of carbonate matrix acidizing using viscoelastic diverting acid system in a gas field

This paper explores matrix acidizing, a method to enhance well productivity by injecting acid into the formation to dissolve damage or create flow channels. Focusing on gas well acidizing, it introduces a groundbreaking three-stage approach with hydrochloric acid (HCl) and viscoelastic diverting acid (VDA). Unlike recent research, which often overlooked specific VDA stages and favored VES or surfactant gelled systems, this study innovatively integrates VDA throughout laboratory experimentation, simulation modeling, and operational execution. The article showcases the effectiveness of HCl and VDA in dissolving reservoir materials, preventing issues like emulsion formation and iron precipitation, reducing corrosion and H2S emissions, enhancing penetration depth, fluid flow channels, and stimulating all reservoir layers. Utilizing a numerical model, it recommends an optimal acidizing method with five main acid injection stages and five VDA injection stages. The results demonstrate a notable increase of 100% in gas production, an 84% rise in gas pressure, and a reduction of BS&W from 7 to 3%. Aimed at industry professionals, this paper serves as a guide for optimizing well productivity and gas recovery processes.

A new insight into wormhole propagation prediction in carbonate matrix acidizing based on experimental approaches

Carbonate acidizing is a common practice for production/injection enhancement in damaged oil/gas wells by creating conductive flow tunnels called wormholes. Previous linear acid flood studies are mostly performed on core plugs limited to the Darcy scale with a strong dependency on lab results for upscaling techniques. In this study, a novel scale-free approach of wormhole propagation inside linear porous media for small to very large core diameters is introduced. The model associates the wormhole tip velocity and wall fluid loss to the injection rate using novel flow correlations in terms of core diameter and wormhole geometries (wormhole density, length and diameter). These correlations are constructed based on the results obtained from a large number of 3D-FEM (Finite Element Method) simulations of water flow utilizing the porous rock with predefined dominant cylindrical wormholes. The application of the model is easy and quick without complex or time-consuming calculations even for large rock diameters up to 28 in (71 cm) which simultaneously predicts the acid response curve by matching with the acid coreflood experiments on specific acid/rock systems. Also, the output matching parameters for each acid/rock system can be directly used for larger core diameters without extra-large scale acid flood experiments. In the experimental parts of this study, acid flooding is performed on various rock types of four Iranian carbonate reservoir layers to find their corresponding acid response curves and optimum flow conditions required to design stimulation programs. Besides, the collected data bank could help to enrich the data pool required to validate the developed model practically. The proposed model is validated by literature model results and also experimental data obtained in our laboratory and those presented in the literature and the results showed that good agreement is obtained in regression analysis with R2 = 0.953, MSE = 0.046452, RMSE = 0.215528 and MAPE = 24.11357.

Numerical studies and analysis on reaction characteristics of limestone and dolomite in carbonate matrix acidizing

Carbonate reservoirs with abundant oil and gas resources are mainly composed of limestone and dolomite. As the acid-rock reaction progresses, limestone is gradually consumed and more acid gradually reacts with the slower dolomite. However, the existing acid rock-reaction models do not consider the competition between limestone and dolomite with acid, and cannot objectively describe the degree of contribution of limestone and dolomite to wormholes at each stage of acidizing. Here, an acidizing model was developed to study and analyze the effect of the reaction characteristics of limestone and dolomite on the carbonate acidizing. In this study, the probability of limestone and dolomite contact with acid is assumed to be related to the content of limestone and dolomite, and a new acid-rock reaction model is proposed by considering the reaction rate and reaction sequence of limestone and dolomite. The acid-rock reaction model is coupled with the porous medium heat transfer model and the two-scale continuum model, and a comprehensive carbonate acidizing model is established. The model can respond to the competition between limestone and dolomite in each stage of acidizing at different temperatures and quantitatively characterize the dynamic change of different mineral contents. Based on the model, this work studies and analyzes the effects of the temperature, dolomite characteristics (uniform, blocky, and columnar) and distribution orientation on wormholes propagation and acidizing efficiency. The simulation results show that the comprehensive carbonate acidizing model can accurately reveal the higher contribution of limestone to the wormhole when the temperature is less than 338 K. As the temperature increases, the difference of reaction rate between limestone and dolomite gradually decreases, and the contribution of dolomite to the wormholes increases. When the temperature is 418 K, the dissolution of limestone and dolomite in the wormholes and the area around the wormholes is almost the same. Moreover, when the temperature is 298 K, the smaller size and density of blocky and columnar dolomite can make the wormholes tortuous and complicated, thus reducing the acidification efficiency. When the size and density are larger, the presence of dolomite can hinder the generation of branching wormholes and guide the propagation of wormholes, enhancing the acidizing efficiency. In addition, the orientation of dolomite does not change the propagation distance of wormholes, but it changes the propagation orientation of wormholes. This study privates a guide for the acidizing treatment of carbonate reservoirs with the different mineral compositions of limestone and dolomite.

CASE STUDY OF ACIDIZING MATRIX DESIGN WITH ATTENTION TO MECHANISMS FORMATION OF WORMHOLES IN CARBONATE RESERVOIRS

This paper discusses the comparison of acid calculation required on carbonate matrix acidizing between the volumetric method and considering the wormhole formation mechanism. Normally the standard estimation of the required acid volume in carbonate rocks is based on the volumetric method where the acid will be distributed evenly in all directions in the targeted formation. However, the real acid mechanism at carbonate rock is affected by the creation of a wormhole. This case study compares the amount of acid volume in both scenarios. The result shows a more optimistic result for the volumetric method compared to the wormhole influences.

Phase-field modeling of wormhole formation and growth in carbonate matrix acidizing

Acidic fluid flow in geologic formations leads to mineral dissolution and, under certain circumstances, to localized dissolution forming a dendritic pattern, known as wormhole. Such patterns of conduits and caves are often observed in karstic aquifer and deliberately engineered in oil and gas well stimulation with acid injection. Two different kinds of instability are thought to cause wormholes. One is heterogeneous property in rocks. The other is a process itself such as reaction infiltration or viscous fingering. To assess each one properly, we need to separate one from the other. While most numerical studies rely on randomly seeded material heterogeneities to induce wormholes, Daccord and Lenormand (1987a) demonstrated that even water injection into a homogeneous plaster can form wormholes. Here, we focus on the process instability driven by reaction-infiltration in homogeneous materials. We apply a phase-field approach, which diffuses a sharp interface in a continuous manner and show that it is capable of simulating wormhole without random seeds by accounting for the energy expenditure in the dissolution topology. We verified the model against the sharp interface counterpart in one-dimensional simulations. We then performed the two-dimensional simulations to qualitatively validate wormhole formation and growth patterns against acid injection experiments on carbonate rocks under radial flow conditions. Our simulation results indicate that the injected acid is rapidly consumed near the acid entry point at low injection rates while the live acid becomes available at the tip of the dissolved cavity under high rates and thus wormhole starts to grow resulting in much faster acid breakthrough.

A Study on the Preparation and Stability of Emulsified Acid for Carbonate Matrix Acidizing

본 연구에서는 탄산염암의 암체 산처리 공법을 위해 유화산을 제조하고 안정성 평가 연구를 수행하였다. 상온과 고온에서 안정성을 확보한 W/O (water-in-oil) 에멀젼(emulsion)을 제조하기 위해 양이온성 유화제와 비이온성 유화제를 친수성-친유성 균형(hydrophile-lipophile balance, HLB) 기준으로 혼합하여 사용하였고, 안정성을 평가하기 위해 액적 시험(droplet test)과 상분리 시험(separation test)을 수행하였다. 유화산 제조 결과, 상온과 고온(90°C)에서 상분리가 일어나지 않는 유체 안정성을 가진 W/O 에멀젼을 확보하였으며, 이때 양이온성 유화제와 비이온성 유화제를 단독으로 사용하는 것 보다 두 유화제를 혼합하여 사용하였을 때 안정성 측면에서 우수함을 확인하였다. 실험을 통해 도출한 결과는 향후 탄산염암 저류층 현장에서 유화산 제조 가이드라인을 제공할 수 있을 것으로 기대된다.

Evaluation of Surfactant Mixture for Supercritical Carbon Dioxide Foamed Acid in Carbonate Matrix Acidizing

The effectiveness of matrix acidizing using CO2 foamed acid is dependent on the duration of foam stability. This paper presents a supercritical CO2 foamed acid with a surfactant mixture to improve the foam stability in carbonate matrix acidizing. The experimental apparatus was developed to conduct foam-stability and wormhole-propagation tests under high-pressure and high-temperature (HPHT) conditions. The foam decay times of five types of surfactants were measured under atmospheric conditions. Trimax (blend of cocamidopropyl betaine, disodium cocoamphodiacetate, and amine oxide) and Aromox C/12W (coco bis-(2-hydroxyethyl) amine oxide) had a high foam decay time. The surfactant mixture was prepared using these two surfactants. The foam stability tests of the surfactant mixture were performed according to the HCl concentration, surfactant mixing ratio, and injection rate of HCl under HPHT conditions. As a result, the foam stability could be improved by adding an HCl concentration of 20% to the surfactant mixture. Wormhole-propagation tests were performed using Indiana and Indonesian limestones. Because of the supercritical CO2 foamed acid injection, dominant wormholes were formed in all the core samples; thus, the absolute permeabilities significantly increased. The results of the scanning electron microscopy/energy-dispersive X-ray spectroscopy and thin-section analyses revealed that the number of large pores with pore sizes of ≥0.5 mm increased by the injection of CO2 foamed acid. Therefore, the supercritical CO2 foamed acid with the surfactant mixture exhibited a high efficiency of matrix acidizing in carbonate reservoirs.

WITHDRAWN: Evaluation of Surfactant Mixture for Supercritical Carbon Dioxide Foamed Acid in Carbonate Matrix Acidizing

Abstract The effectiveness of matrix acidizing using CO2 foamed acid is dependent on the duration of foam stability. This paper presents a supercritical CO2 foamed acid with a surfactant mixture to improve the foam stability in carbonate matrix acidizing. The experimental apparatus was developed to conduct foam stability and wormhole propagation tests under high-pressure and high-temperature (HPHT) conditions. The foam decay times of five types of surfactants were measured under atmospheric conditions. Trimax (blend of cocamidopropyl betaine, disodium cocoamphodiacetate, and amine oxide) and Aromox C/12 W (coco bis-(2-hydroxyethyl) amine oxide) have a high foam decay time. The surfactant mixture was prepared using these two surfactants. The foam stability tests of the surfactant mixture were performed according to the HCl concentration, surfactant mixing ratio, and injection rate of HCl under HPHT conditions. As a result, the foam stability could be improved by adding an HCl concentration of 20% to the surfactant mixture. Wormhole propagation tests were performed using Indiana and Indonesian limestones. Because of the supercritical CO2 foamed acid injection, dominant wormholes were formed in all the core samples; thus, the absolute permeabilities significantly increased. The results of the scanning electron microscopy/energy-dispersive x-ray spectroscopy and thin-section analyses revealed that the number of large pores with pore sizes of ≥0.5 mm increased by the injection of CO2 foamed acid. Therefore, the supercritical CO2 foamed acid with the surfactant mixture exhibited a high efficiency of matrix acidizing in carbonate reservoirs.

A modified acid system to enhance carbonate matrix acid stimulation: An experimental study

AbstractIn carbonate matrix acidizing, the critical design parameters are interstitial velocity and pore volume to breakthrough for wormhole propagation (vi, opt and PVBT,opt). Hydrochloric acid (HCl), the most commonly used acid in carbonate acidizing, sometimes shows low efficiency because the injection rate needed for optimal wormhole propagation is not attainable, especially for when long completion intervals, low permeability carbonates, or coiled tubing operations are involved. It also raises safety, corrosion, and environmental concerns. When conventional retarded acid systems are used to overcome these challenges, the efficiency of acid stimulation often suffers from low reaction rate. This study presents testing results of a modified acid system that has controlled reaction rate with favourable wormhole propagation characteristics, especially at low interstitial velocities. Due to the increased activation energy barriers utilized in these modified‐acid systems, it is possible to control the reaction rate of the hydrogen proton and optimize the wormholing effect based on the completion method and formation specifications. Laboratory linear core flooding experiments and acid jetting experiments were conducted to study the wormhole efficiency with the new acid systems. The experimental results showed clear advantages of the modified acid systems. The modified acids have similar or better wormhole efficiency parameters compared with HCl having comparable dissolving power. When combined with acid jetting, further improvement in wormhole growth in low permeability limestone is achieved. In addition to minimizing the hazardous exposure levels and corrosion rates compared with HCl, the new acid system provides the positive aspects of solubilizing ability, thus improving optimal wormhole conditions.

Numerical studies and analysis on reactive flow in carbonate matrix acidizing

Matrix acidizing is a stimulation technique to improve oil recovery in carbonate reservoirs. In this work, a new two-scale continuum model is used to study acidizing process in radial flow. Naiver-Stokes_Darcy equation is adapted to quantity fluid velocity instead of Darcy's law. Furthermore, the continuity equation of fluid phase is redeveloped to consider mass exchange between fluid and solid phases. Based on this model, several numerical case studies are conducted to ascertain effect of acid and rock physical parameters. Results show that acidizing process is obviously affected by acid injection velocity and acid surface reaction rate. Ignoring the mass exchange term between fluid and solid phases results in an overestimation of pore volume to breakthrough. Darcy's law is not applicable when acid is injected into core sample at low velocity. Pore volume to breakthrough calculated by original two-scale continuum model based on the Darcy equation is much lower than that calculated by updated two-scale continuum model based on Naiver-Stokes_Darcy equation with low acid injection velocity. The simulation results are compared with available experimental data and are found to keep a consistent pattern.

Simulation study of wormhole formation and propagation during matrix acidizing of carbonate reservoirs using a novel in-situ generated hydrochloric acid

Optimum acid selection based on reservoir condition is one of the key elements in achieving a successful treatment in carbonate matrix acidizing. High reaction and corrosion rate of regular hydrochloric acid especially in tight reservoirs with high temperatures is considered as a limitation of this acid. One alternative in a high-temperature reservoir is to use in-situ generated hydrochloric. The objective of this study is to simulate acidizing treatment using both regular hydrochloric and in-situ generated hydrochloric acid. A continuum two-scale model for linear and radial flow geometry was used to simulate dissolution pattern. A finite-volume method with an implicit backward Euler scheme was used to solve the governing equations. The results of the simulation in a linear system showed that, due to low face dissolution, the in-situ generated hydrochloric at low injection rates could create longer wormholes with less pore volume to breakthrough. The results also proved that using the in-situ generated hydrochloric acid could lead to less face dissolution and deeper penetration of acid into the reservoir. This acidizing system is very suitable in situations where a low injection rate is needed and regular hydrochloric cannot be used due to higher pore volume to breakthrough and high corrosion rate. Cited as : Kiani, S., Jafari, S., Apourvari, S. N., Mehrjoo, H. Simulation study of wormhole formation and propagation during matrix acidizing of carbonate reservoirs using a novel in-situ generated hydrochloric acid. Advances in Geo-Energy Research, 2021, 5(1): 64-74, doi: 10.46690/ager.2021.01.07

Propagation characteristics of optimum wormhole in carbonate matrix acidizing using micro X-ray CT imaging

This study aims to investigate the propagation characteristics of an optimum wormhole to make an efficient acid treatment in carbonate matrix acidizing based on wormhole visualization using micro X-ray computerized tomography (CT) with high resolution. Five cases comprising 25 core-flood experiments under different experimental conditions using Indiana limestone cores were carried out at various injection rates to obtain the optimum injection rates. From the results, the optimum injection rates tended to increase with permeability but was not related to acid concentrations. The pore volume to breakthrough (PVBT), the amount of acid consumption required to breakthrough, was lower at higher permeability and acid concentration. This means that less amount of acid is consumed to generate the wormholes. Wormhole visualization with X-ray CT imaging showed that the wormhole diameters were gradually reduced along with the wormhole propagation axis in all cases. In addition, the mean of wormhole diameter tended to increase according to the higher permeability and acid concentration. The tortuosity, which is the ratio of actual wormhole length to core length, was exceeded at least 1.35 in each case. This implies that the actual flow length of the optimum wormhole is much longer than the core length. However, previous studies of wormhole propagation regarding the configuration of optimum wormhole are sparse, more intensive research would be required. Therefore, these propagation characteristics of optimum wormholes should be considered to improve the wormhole prediction model, and the wormhole prediction results would be more accurate and realistic than those of the existing models. Further study is needed to develop a new wormhole prediction model considering the propagation characteristics, such as diameter reduction and actual flow length.

Single-Stage Stimulation of Anhydrite-Rich Carbonate Rocks Using Chelating Agent: An Experimental and Modeling Investigation

SummaryCalcium sulfate (anhydrite) exists in most of the carbonate reservoirs, and its content can reach more than 20%. The high content of anhydrite affects the efficiency of the acidizing process because of the low solubility of calcium sulfate in different acids. Carbonate stimulation, matrix acidizing, or acid fracturing is carried out mostly using hydrochloric acid (HCl)-based fluids. The solubility of calcium sulfate in HCl or HCl-based fluids and organic acids is very low.In this study, a new formulation was developed to stimulate carbonate rocks with high anhydrite content. A formulation was developed to dissolve both anhydrite and carbonate at the same time. Anhydrite dissolution was achieved by converting anhydrite to calcite using the newly developed formulation. This treatment can be conducted in a single stage, and the formulation consists of a high pH chelating agent such as ethylenediaminetetraacetic acid (EDTA) in addition to potassium carbonate as a converter. Coreflooding experiments were conducted for carbonate rocks with varying anhydrite content. The effluent samples of the coreflooding experiments were analyzed for cations and anions concentration using inductively coupled (IC) and inductively coupled plasma (ICP) measurements. Computed tomography scans were conducted to show the wormholes generated in the samples. Rock mineralogical analysis using X-ray diffraction, thin section, and the QEMSCAN® automated mineralogy solution (FEI Company, Hillsboro, Oregon, USA) was conducted as well. The reaction kinetics were investigated using a rotating disk apparatus (RDA) using a disk that consists of almost 24% anhydrite and 71% dolomite. Also, a two-scale continuum model was built to simulate the reaction between the new formulation and carbonate rocks. The model could capture the experimental outcomes such as the number of pore volumes (PV) to break through and the shape of the wormholes. This provided confidence in the reaction parameters obtained because the model could reproduce the experimental outcomes.Coreflooding experiments showed that the new formulations were very effective in stimulating carbonate rocks with high anhydrite content in a single stage. The effluent analysis showed high sulfate concentration that indicates the dissolution of anhydrite compared to conventional acid treatments. Reaction kinetics results showed that the new formulation increased the reaction rate with anhydrite through a mass transfer reaction regime. The model reproduced the wormholing behavior obtained from the experiments, indicating that the branched wormhole was due to the anhydrite conversion.

Modification of two-scale continuum model and numerical studies for carbonate matrix acidizing

Matrix acidizing is one of the most effective stimulation techniques to achieve industrial development of carbonate reservoirs. In this work, a two-scale continuum model is modified to consider mass conversion between liquid and solid phases. The results show that the mass conversion between liquid and solid phases is very important and nonnegligible. Compared to modified model, original model overestimates amount of acid required for core breakthrough. A series of numerical cases based on modified model are presented for sensitivity studies on acid injection conditions, core geometries, acid, and rock properties. The modified model can well capture optimum acid injection velocity and typical dissolution patterns observed in experimental studies. Pore volume to breakthrough (PVBT) is very sensitive to acid injection concentration and acid surface reaction rate. Increasing injected acid concentration substantially reduces the amount of solute required but acid injection mass does not change significantly. Strong acid can effectively reduce least pore volume to breakthrough (PVBT) but meanwhile optimum acid injection velocity obviously increases. The simulation results are compared with available experimental and numerical data and found to attain a consistent match.

A Fundamental Model for Wormhole Formation Including Multiphase Flow

Summary The industry has been developing numerical models to simulate the wormholing phenomenon in carbonate matrix acidizing, both to save cost and time with experiments and to scale up the laboratory results to field scale. The two-scale continuum model is a fundamental model that has been successfully used for this end. Previous studies with this model only simulated single-phase flow: injection of acid into a water-saturated rock. However, significantly different behavior is observed experimentally by injecting acid into oil- or gas-saturated cores. In this work, we present a fundamental multiphase model for wormhole formation using the two-scale continuum approach, allowing the simulation of wormholing for acid injection into oil- or gas-bearing rocks, with different saturations. The two-scale continuum model represents the fluid flow and acid transport in the porous medium at the Darcy scale but calculates the acid-rock reaction with dissolution of the rock at the pore-scale. This model was implemented in transient 3D anisotropic form. Each phase occupies a volume fraction in each gridblock, defined by the porosity and fluid saturations. The fluid flow is calculated by solving the Darcy-Brinkman-Stokes equation, in which the relative permeabilities are functions of the saturations. The acid transport and reaction equations are solved, and as the acid injection proceeds and the rock is dissolved, the porosity increases in the gridblocks where dissolution occurs. The pore properties, such as permeability, pore radius, and specific surface area, are updated as porosity evolves, being scaled up from pore to Darcy scale. The simulation keeps track of the different fluid phases by calculating the saturations using the Implicit Pressure Explicit Saturation (IMPES) method. The developed model was implemented in an open-source computational fluid dynamics package and validated against experimental data. For the validation, the adjustable parameters in the model were calibrated so that the simulation results represent the different dissolution patterns and correctly reproduce the acid efficiency curves obtained experimentally. The same calibrated model was used to simulate the coreflooding experiments with water- and oil-saturated cores. The dissolution patterns (face dissolution, conical wormhole, dominant wormhole, etc.) and acid efficiency curves predicted by the new model match the experimental data. Other simulations presented include the shift in the acid efficiency curves observed for different oil viscosities, residual oil saturations, and different water saturations. To our knowledge, this is the first 3D two-scale continuum model to simulate wormhole propagation including multiphase flow. With adequate history match, it was shown to accurately predict the acidizing results for different fluid saturations, as observed in experiments.

New Insights into Carbonate Matrix Acidizing Treatments: A Mathematical and Experimental Study

Summary The design process of carbonate matrix acidizing treatments requires coring and conducting linear, radial coreflood experiments. With the current environment revolving around cutting costs, it has become increasingly important to accurately design cost-effective acidizing treatments. This work aims to introduce a novel approach to predicting the performance of acid treatments in the field using log data only. A radial reactive flow simulator, using porosity distributed from logs, is used to provide accurate predictions without the need for experiments. Coreflood acidizing experiments at 150 and 200°F with two acid concentrations were studied. A reactive flow simulator was built using porosity distribution derived from computed-tomography (CT) scans and tuned to match experimental data. A new radial simulation model of 3.25-ft radius was used to study acid propagation under field conditions. For accurate predictions, porosity was distributed using values derived from cores’ CT scans. Simulation results were compared with traditional 1D models. Different porosity distributions, including gamma distributions, were used in the radial model. The reactive flow simulator was able to accurately capture wormhole propagation inside the linear core. A greater than 90% match between the experimental and the simulated acid pore volume (PV) to breakthrough (PVBT) was obtained using two acid concentrations’ different temperatures. The simulation results from the radial field-scale model show that the optimal velocity can be higher or lower than those predicted from laboratory experiments. Accordingly, caution must be taken when linear coreflood data are used to predict acid propagation in the field. The simulations showed that traditional upscaling models overpredict acid volumes; the predicted volumes are double at moderate to high injection rates. Models using statistically distributed porosity can provide accurate acid-propagation predictions, with a relative percentage error of less than 25% at extremely high injection rates. This work introduces an accurate model using porosity directly from logs to predict acid performance while avoiding expensive designs. The simulation results reveal that traditional designs overpredict acid volumes required for field treatments. The statistically distributed porosity can be used as a substitute for CT-scan-derived porosity with a low effect on model predictability. The reactive flow simulator can accurately match experimental data.

A novel acidizing technology in carbonate reservoir: In-Situ formation of CO2 foamed acid and its self-diversion

The serious fingering and nonuniform injectivity profile of acid liquid result from the features of heterogeneity in most carbonate reservoir, leading to a poor effect on the acidizing process. In the present contribution, for the first time, we report a novel diversion acidizing strategy that uses acid-rock reaction between the acidizing fluid itself and carbonate core to produce CO2 at supercritical state. Under the action of a mixture of foaming agent and stabilizer contained in this working fluid, a CO2 foamed acid fluid is in-situ formed in the acidizing layer, which would play a diverting role in carbonate matrix acidizing operations. Experiments on in-situ formation of CO2 microfoams (emulsions) were performed utilizing a visible acid-rock reaction simulation device and a self-made high pressure and temperature foam generator. A series of divided-flow experiments were undertaken by the core displacement setup.Results based on these experiments indicate that with 0.5% SDBS as a foaming agent and 0.15% CMC as a stabilizing agent, the supercritical CO2 microfoams (emulsions) could be created in-situ in the process of acidizing through the carbonate reservoirs. The foamed acid has the accumulating and plugging effects, with divided-flow selectivity not only in the parallel cores of oil saturation and water saturation, but also in the parallel cores of different permeability. This foamed acid fluid could provide better diversion acidizing operations for heterogeneous carbonate formation. Finally, the mechanism of this novel diversion acidizing technology was uncovered in detail. It is suggested that the whole working process is actually an interface effect in the dynamic system of gas/liquid/solid multiphase fluid. Such findings are potentially important for a better understanding on the mechanisms of in-situ formation of CO2 foamed acid and its self-diversion. Looking out to the future, our report attempts to provide a new and efficient technology for the improvement of acid stimulation in the carbonate reservoirs.

Damköhler number-based research on dividing dissolution patterns in carbonate acidizing

In general, the main dissolution patterns in carbonate acidizing include face dissolution, wormhole dissolution and uniform dissolution. Wormholes can also be divided into single, dominant wormhole, ramified wormhole and so on. The wormholes can establish advantageous channels for oil and gas in the reservoirs, increase the permeability of formations and achieve the ultimate goal of carbonate matrix acidizing. Considering the factors that affect carbonate acidizing, the flow rate of acid plays the key role in determining dissolution patterns. Many scholars have carried out the researches by means of numerical simulation or physical simulation experiment individually, but the reports which combine the two methods and define the flow rate boundary between different dissolution patterns in carbonate acidizing are rare. In this paper, the flow rate of acid was dimensionless on the basis of Damköhler number (Da), and the boundary of acid injection rate was confirmed by combining numerical simulation with physical simulation experiments. Among the various types of dissolution models, the two-scale continuum model comprehensively considers the convection, diffusion and acid-rock interfacial reaction and is capable of dynamically monitoring the acidizing region at different time steps. This paper, using the finite difference method, solved the typical two-scale continuum model. In order to extend the calculation of the range of Damköhler number, Steffensen's acceleration method was applied to the porosity variation equation. The previous conclusion that at higher injection rate the breakthrough pore volume of acid is in direct proportion to the one-third power of the average pore velocity was validated. Furthermore, the typical pressure fall-off curve was analyzed. Results show that the typical pressure drop curve changes from the power-law curve to the straight line, as the injection rate decreases. Based on the analysis of porosity field and concentration field, combined with physical simulation experiment, the change law of dissolution pattern with Da was analyzed, which clarify the injection rates boundary of dissolution pattern. The results show when Da is between 50 and 200, the core appears wormholes, when Da is between 1 and 50, the dissolution pattern is the transition stage from wormhole to uniform dissolution and when Da is between 200 and 500, the core dissolution pattern is the transition stage from wormhole to face dissolution. The Damköhler number and injection rates of different dissolution patterns can be used to guide the design of acidizing construction.

Investigating the effects of rock and fluid properties in Iranian carbonate matrix acidizing during pre-flush stage

Acidizing of carbonate oil-wet rocks saturated with oil and saline formation water is subjected failure in some cases due to acid-induced damage such as sludge and emulsion formations. This condition may also lead to mineral precipitation, oil film barrier between acid and rock and diversion chemical malfunctions. Therefore, pre-flush process has been proposed as one of the most efficient stage for oil-wells matrix acidizing to reduce these challenges significantly. Besides, the pre-flush stage would result in more clean rock as the reservoir fluids are pushed back from the near wellbore regions, restoring rock wettability to more water wet state, preventing direct acid-oil contact and cooling rock surface to control acid-rock reaction at high temperature reservoir.This laboratory work is designed to study the effectiveness of three different pre-flush fluids and assess their ability in reducing interfacial tension (IFT) to displace original reservoir fluids around the wellbore as well as changing the rock wettability to increase rock dissolution rate. The results showed that a 0.5%wt of C16TAB aqueous solution (cationic surfactant additive) has passed successfully the pre-flush optimization criteria which can be utilized as pre-flush fluid of the carbonate oil-wet acidizing. It was shown that the wettability and IFT were changed quickly as it is requested for the pre-flush stage. The contact angle was decreased from 172° to 33° in two hours period and the IFT was reduced from 8.9 to 0.62 (dyne/cm) at reservoir condition (P = 4000 psia, T = 85 °C).In addition, the dissolution rate of dolomite rock from an Iranian oil reservoir at three different states of oil-wet, water-wet and pre-flush treated with 15% HCl was measured and compared. The measured dissolution rate of initially water-wet rock was measured to be 1 × 10−4 mol/cm2.s which is the same as the dissolution rate measured for C16TAB treated oil wet rock. This confirms the suitable wettability alteration through C16TAB pre-flush.