An experimental study on horizontal well waterflooding in the Cretaceous porous carbonate reservoir of Oman

Porous carbonate reservoirs,prevalent in the Middle East,are lithologically dominated by bioclastic limestones,exhibiting high porosity,low permeability,intricate pore structure,and strong heterogeneity.Waterflooding through horizontal wells is commonly used for exploiting these reservoirs.However,challenges persist,such as significant uncertainty and complex operational procedures regarding adjustment effects during the exploitation.The USH reservoir of the Cretaceous D oilfield,Oman exemplifies typical porous carbonate reservoirs.It initially underwent depletion drive using vertical wells,followed by horizontal well waterflooding in the late stage.Currently,the oilfield is confronted with substantial developmental challenges,involving the understanding of residual oil distribution,effective water cut control,and sustaining oil production since it has entered the late development stage.Employing a microscopic visualization displacement system equipped with electrodes,this study elucidated the waterflooding mechanisms and residual oil distribution in the late-stage development of the USH reservoir.The results reveal that the reservoir's vertical stacking patterns act as the main factor affecting the horizontal well waterflooding efficacy.Distinct water flow channels emerge under varying reservoir stacking patterns,with post-waterflooding residual oil predominantly distributed at the reservoir's top and bottom.The oil recovery can be enhanced by adjusting the waterflooding's flow line and intensity.The findings of this study will provide theoretical insights of waterflooding mechanisms and injection-production adjustments for exploiting other porous carbonate reservoirs in the Middle East through horizontal wells.

Separate-layer balanced waterflooding development technology for thick and complex carbonate reservoirs in the Middle East

Based on the waterflooding development in carbonate reservoirs in the Middle East,in order to solve the problem of the poor development effects caused by commingled injection and production,taking the thick bioclastic limestone reservoirs of Cretaceous in Iran-Iraq as an example,this paper proposes a balanced waterflooding development technology for thick and complex carbonate reservoirs.This technology includes the fine division of development units by concealed baffles and barriers,the combination of multi well type and multi well pattern,and the construction of balanced water injection and recovery system.Thick carbonate reservoirs in Iran-Iraq are characterized by extremely vertical heterogeneity,development of multi-genesis ultra-high permeability zones,and highly concealed baffles and barriers.Based on the technologies of identification,characterization,and sealing evaluation for concealed baffles and barriers,the balanced waterflooding development technology is proposed,and three types of balanced waterflooding development modes/techniques are formed,namely,conventional stratigraphic framework,fine stratigraphic framework,and deepened stratigraphic framework.Numerical simulations show that this technology is able to realize a fine and efficient waterflooding development to recover,in a balanced manner,the reserves of thick and complex carbonate reservoirs in Iran and Iraq.The proposed technology provides a reference for the development optimization of similar reservoirs.

Impacts of interactions with low-mineralized water on permeability and pore behavior of carbonate reservoirs

Laboratory filtration experiments are employed to investigate effective well killing while minimizing its impacts on surrounding rocks.The novelty of this experimental study lies in the prolonged exposure of rock samples to the killing fluid for seven days,corresponding to the average duration of well workovers in the oilfields in Perm Krai,Russia.Our findings indicate that critical factors influencing the interactions between rocks and the killing fluid include the chemical composition of the killing fluid,the mineralogical composition of the carbonate rocks,reservoir pressure and temperature,and the contact time.Petrophysical analyses using multi-scale X-ray computed tomography,field emission scanning electron microscopy,and X-ray diffraction were conducted on samples both before and after the well killing simulation.The experiments were performed using real samples of cores,crude oil,and the killing fluid.The results from this study indicate that low-mineralized water(practically fresh water)is a carbonate rock solvent.Such water causes the dissolution of rock components,the formation of new calcite crystals and amoeba-like secretions,and the migration of small particles(clay,quartz,and carbonates).The formation of deep channels was also recorded.The assessment reveals that the change in the pH of the killing fluid indicates that the observed mineral reactions were caused by carbonate dissolution.These combined phenomena led to a decrease in the total number of voids in the core samples,which was 25%on average,predominantly among voids measuring between 45 and 70μm in size.The change in the pore distribution in the bulk of the samples resulted in decreases in porosity of 1.8%and permeability of 67.0%in the studied core samples.The results from this study indicate the unsuitability of low-mineralized water as a well killing fluid in carbonate reservoirs.The composition of the killing fluid should be optimized,for example,in terms of the ionic composition of water,which we intend to investigate in future research.

Fracture identification of carbonate reservoirs by deep forest model:An example from the D oilfield in Zagros Basin

Identifying fractures along a well trajectory is of immense significance in determining the subsurface fracture network distribution.Typically,conventional logs exhibit responses in fracture zones,and almost all wells have such logs.However,detecting fractures through logging responses can be challenging since the log response intensity is weak and complex.To address this problem,we propose a deep learning model for fracture identification using deep forest,which is based on a cascade structure comprising multi-layer random forests.Deep forest can extract complex nonlinear features of fractures in conventional logs through ensemble learning and deep learning.The proposed approach is tested using a dataset from the Oligocene to Miocene tight carbonate reservoirs in D oilfield,Zagros Basin,Middle East,and eight logs are selected to construct the fracture identification model based on sensitivity analysis of logging curves against fractures.The log package includes the gamma-ray,caliper,density,compensated neutron,acoustic transit time,and shallow,deep,and flushed zone resistivity logs.Experiments have shown that the deep forest obtains high recall and accuracy(>92%).In a blind well test,results from the deep forest learning model have a good correlation with fracture observation from cores.Compared to the random forest method,a widely used ensemble learning method,the proposed deep forest model improves accuracy by approximately 4.6%.

Theoretical exploration of water injection gravity flooding oil in ultra-deep fault-controlled fractured-cavity carbonate reservoirs

Based on the analysis of geological characteristics of ultra-deep fault-controlled fracture-cavity carbonate reservoirs and division of reservoir units, two physical models were made, and physical simulations of oil displacement by water injection were carried out to find out water flooding mechanism in the fault-controlled fracture-cavity carbonate reservoir under complex flow state. On this basis, a mathematical model of fault-controlled carbonate reservoir with coexisting seepage and free flow has been established. Pilot water injection tests have been carried out to evaluate the effects of enhancing oil recovery by water injection. The results show that: fault-controlled fracture-cavity carbonate reservoir units can be divided into three types:the strong natural energy connected type, the weak natural energy connected type and the weak natural energy isolated type;the fault-fracture activity index of the fault-controlled fractured-cavity body can effectively characterize the connectivity of the reservoir and predict the effective direction of water injection;the mathematical model of fault-controlled carbonate reservoir with coexisting seepage and free flows can quantitatively describe the fluid flow law in the fracture-cavity body;the water injected into the fault-controlled fracture-cavity body is weakly affected by the capillary force of the lithologic body, and the oil-water movement is mainly dominated by gravity. The development modes of single well water injection, unit water injection,and single well high pressure water injection proposed based on the connection structure of fracture-cavity space and well storage space configuration are confirmed effective by pilot tests, with obvious water injection gravity flooding effect.

A review of development methods and EOR technologies for carbonate reservoirs

Carbonate reservoirs worldwide are complex in structure,diverse in form,and highly heterogeneous.Based on these characteristics,the reservoir stimulation technologies and fluid flow characteristics of carbonate reservoirs are briefly described in this study.The development methods and EOR technologies of carbonate reservoirs are systematically summarized,the relevant mechanisms are analyzed,and the application status of oil fields is catalogued.The challenges in the development of carbonate reservoirs are discussed,and future research directions are explored.In the current development processes of carbonate reservoirs,water flooding and gas flooding remain the primary means but are often prone to channeling problems.Chemical flooding is an effective method of tertiary oil recovery,but the harsh formation conditions require high-performance chemical agents.The application of emerging technologies can enhance the oil recovery efficiency and environmental friendliness to a certain extent,which is welcome in hard-to-recover areas such as heavy oil reservoirs,but the economic cost is often high.In future research on EOR technologies,flow field control and flow channel plugging will be the potential directions of traditional development methods,and the application of nanoparticles will revolutionize the chemical EOR methods.On the basis of diversified reservoir stimulation,combined with a variety of modern data processing schemes,multichannel EOR technologies are being developed to realize the systematic,intelligent,and cost-effective development of carbonate reservoirs.

The influence of pore structure on P-& S-wave velocities in complex carbonate reservoirs with secondary storage space

Secondary storage spaces with very complex geometries are well developed in Ordovician carbonate reservoirs in the Tarim Basin,which is taken as a study case in this paper.It is still not clear how the secondary storage space shape influences the P-＆ S-wave velocities （or elastic properties） in complex carbonate reservoirs.In this paper,three classical rock physics models （Wyllie timeaverage equation,Gassmann equation and the Kuster-Toks z model） are comparably analyzed for their construction principles and actual velocity prediction results,aiming at determining the most favourable rock physics model to consider the influence of secondary storage space shape.Then relationships between the P-＆ S-wave velocities in carbonate reservoirs and geometric shapes of secondary storage spaces are discussed from different aspects based on actual well data by employing the favourable rock physics model.To explain the influence of secondary storage space shape on V P-V S relationship,it is analyzed for the differences of S-wave velocities between derived from common empirical relationships （including Castagna＇s mud rock line and Greenberg-Castagna V P-V S relationship） and predicted by the rock physics model.We advocate that V P-V S relationship for complex carbonate reservoirs should be built for different storage space types.For the carbonate reservoirs in the Tarim Basin,the V P-V S relationships for fractured,fractured-cavernous,and fractured-hole-vuggy reservoirs are respectively built on the basis of velocity prediction and secondary storage space type determination.Through the discussion above,it is expected that the velocity prediction and the V P-V S relationships for complex carbonate reservoirs should fully consider the influence of secondary storage space shape,thus providing more reasonable constraints for prestack inversion,further building a foundation for realizing carbonate reservoir prediction and fluid prediction.

Pre-stack inversion for caved carbonate reservoir prediction:A case study from Tarim Basin,China

The major storage space types in the carbonate reservoir in the Ordovician in the TZ45 area are secondary dissolution caves.For the prediction of caved carbonate reservoir,post-stack methods are commonly used in the oilfield at present since pre-stack inversion is always limited by poor seismic data quality and insufficient logging data.In this paper,based on amplitude preserved seismic data processing and rock-physics analysis,pre-stack inversion is employed to predict the caved carbonate reservoir in TZ45 area by seriously controlling the quality of inversion procedures.These procedures mainly include angle-gather conversion,partial stack,wavelet estimation,low-frequency model building and inversion residual analysis.The amplitude-preserved data processing method can achieve high quality data based on the principle that they are very consistent with the synthetics.Besides,the foundation of pre-stack inversion and reservoir prediction criterion can be established by the connection between reservoir property and seismic reflection through rock-physics analysis.Finally,the inversion result is consistent with drilling wells in most cases.It is concluded that integrated with amplitude-preserved processing and rock-physics,pre-stack inversion can be effectively applied in the caved carbonate reservoir prediction.

A new method of formation evaluation for fractured and caved carbonate reservoirs:A case study from the Lundong area,Tarim Basin,China

The carbonate reservoirs in the Tarim Basin are characterized by low matrix-porosity,heterogeneity and anisotropy,which make it difficult to predict and evaluate these reservoirs.The reservoir formations in Lundong area experienced a series of diagenesis and tectonic evolution stages.And secondary storage spaces such as fractures and dissolution caves were developed while nearly all the primary pores have disappeared.Based on a summary of different types of storage spaces and their responses in conventional logs,FMI and full waveform sonic logs which are sensitive to different reservoirs,the comprehensive probability index （CPI） method is applied to evaluating the reservoirs and a standard of reservoir classification is established.By comparing the evaluation results with actual welllogging results,the method has proven to be practical for formation evaluation of carbonate reservoirs,especially for the fractured carbonate reservoirs.In reservoir fluid identification,the multivariate stepwise discriminant analysis （MSDA） method is introduced.Combining the CPI method and MSDA method,comprehensive formation evaluation has been performed for fractured and caved carbonate reservoirs in the Tarim Basin.Additionally,on the basis of secondary pore inversion results,another new method of formation evaluation is also proposed in the discussion part of this paper.Through detailed application result analysis,the method shows a promising capability for formation evaluation of complex carbonate reservoirs dominated by various secondary pores such as holes,caves,and cracks.

A two-step method to apply Xu–Payne multi-porosity model to estimate pore type from seismic data for carbonate reservoirs

Carbonate reservoirs exhibit strong heterogeneity in the distribution of pore types that can be quantitatively characterized by applying Xu–Payne multi-porosity model.However,there are some prerequisites to this model the porosity and saturation need to be provided.In general,these application conditions are difficult to satisfy for seismic data.In order to overcome this problem,we present a two-step method to estimate the porosity and saturation and pore type of carbonate reservoirs from seismic data.In step one,the pore space of the carbonate reservoir is equivalent to a single-porosity system with an effective pore aspect ratio;then,a 3D rock-physics template(RPT)is established through the Gassmann’s equations and effective medium models;and then,the effective aspect ratio of pore,porosity and fluid saturation are simultaneously estimated from the seismic data based on 3D RPT.In step two,the pore space of the carbonate reservoir is equivalent to a triple-porosity system.Combined with the inverted porosity and saturation in the first step,the porosities of three pore types can be inverted from the seismic elastic properties.The application results indicate that our method can obtain accurate physical properties consistent with logging data and ensure the reliability of characterization of pore type.

The application of amplitude-preserved processing and migration for carbonate reservoir prediction in the Tarim Basin,China

Conventional seismic exploration method based on post-stack data usually fails to identify the distribution of fractured and caved carbonate reservoirs in the Tarim Basin,so the rich pre-stack information should be applied to the prediction of carbonate reservoirs.Amplitude-preserved seismic data processing is the foundation.In this paper,according to the feature of desert seismic data （including weak reflection,fast attenuation of high frequency components,strong coherent noises,low S/N and resolution）,a set of amplitude-preserved processing techniques is applied and a reasonable processing flow is formed to obtain the high quality data.After implementing a set of pre-stack amplitude-preserved processing,we test and define the kernel parameters of amplitude-preserved Kirchhoff PSTM （pre-stack time migration） and subsequent gathers processing,in order to obtain the amplitude-preserved gathers used to the isotropic pre-stack inversion for the identification of caved reservoirs.The AVO characteristics of obtained gathers fit well with the synthetic gathers from logging data,and it proves that the processing above is amplitudepreserved.The azimuthal processing techniques,including azimuth division and binning enlargement,are implemented for amplitude-preserved azimuthal gathers with the uniform fold.They can be used in the anisotropic inversion to detect effective fractures.The processing techniques and flows are applied to the field seismic data,and are proved available for providing the amplitude-preserved gathers for carbonate reservoir prediction in the Tarim Basin.

Using a Modified GOI Index(Effective Grid Containing Oil Inclusions)to Indicate Oil Zones in Carbonate Reservoirs

The GOI（grains containing oil inclusions） index is used to distinguish oil zones,oil-water zones and water zones in sandstone oil reservoirs.However,this method cannot be directly applied to carbonate rocks that may not have clear granular textures.In this paper we propose the Effective Grid Containing Oil Inclusions（EGOI） method for carbonate reservoirs.A microscopic view under10× ocular and 10 x objective is divided into 10×10 grids,each with an area of 0.0625 mm×0.0625 mm.An effective grid is defined as one that is cut（touched） by a stylolite,a healed fracture,a vein,or a pore-filling material.EGOI is defined as the number of effective grids containing oil inclusions divided by the total number of effective grids multiplied by 100%.Based on data from the Tarim Basin,the EGOI values indicative of the paleo-oil zones,oil-water zones,and water zones are 〉5%,1%-5%,and 〈1%,respectively.However,the oil zones in young reservoirs（charged in the Himalayan） generally have lower EGOI values,typically 3%-5%.

Origin, hydrocarbon accumulation and oil-gas enrichment of fault-karst carbonate reservoirs: A case study of Ordovician carbonate reservoirs in South Tahe area of Halahatang oilfield, Tarim Basin

Based on comprehensive analysis of tectonic and fault evolution, core, well logging, seismic, drilling, and production data, the reservoir space characteristic, distribution, origin of fault-karst carbonate reservoir in Yueman block of South Tahe area, Halahatang oilfield, Tarim Basin, were studied systematically. And the regular pattern of hydrocarbon accumulation and enrichment was analyzed systematically based on development practice of the reservoirs. The results show that fault-karst carbonate reservoirs are distributed in the form of "body by body" discontinuously, heterogeneously and irregularly, which are controlled by the development of faults. Three formation models of fault-karst carbonate reservoirs, namely, the models controlled by the main deep-large fault, the secondary fault and the secondary internal fault, are built. The hydrocarbon accumulation and enrichment of fault-karst carbonate reservoirs is controlled by the spatiotemporal matching relation between hydrocarbon generation period and fault activity, and the size and segmentation of fault. The study results can effectively guide the well deployment and help the efficient development of fault-karst carbonate reservoirs of South Tahe area, Halahatang oilfield.

Numerical simulation of fracture propagation in Russia carbonate reservoirs during refracturing

Refracturing treatment is often performed on Russian carbonate reservoirs because of the quick production decline of reservoirs.The traditional refracturing model assumes that a refracture initiates in the normal direction relative to the initial hydro-fracture.This assumption is inconsistent with oilfield measurements of refracture propagation trajectories.Indeed,the existing model is not based on an indepth understanding of initiation and propagation mechanisms of the second hydraulic fractures during refracturing.In this study,we use the extended finite element method to investigate refracture propagation paths at different initiation angles.Both the enriched function approach and phantom mode technique are incorporated into the refracturing model,thereby ensuring that the refracture can freely extend on the structured mesh without any refinement near the crack tips.Key factors including production time,stress anisotropy and initiation angle,and the propped mechanical effect are analyzed in detail.This study provides new insight into the mechanism of refracture propagation in unconventional reservoirs.

An intelligent prediction method of fractures in tight carbonate reservoirs

An intelligent prediction method for fractures in tight carbonate reservoir has been established by upgrading single-well fracture identification and interwell fracture trend prediction with artificial intelligence,modifying construction of interwell fracture density model,and modeling fracture network and making fracture property equivalence.This method deeply mines fracture information in multi-source isomerous data of different scales to reduce uncertainties of fracture prediction.Based on conventional fracture indicating parameter method,a prediction method of single-well fractures has been worked out by using 3 kinds of artificial intelligence methods to improve fracture identification accuracy from 3 aspects,small sample classification,multi-scale nonlinear feature extraction,and decreasing variance of the prediction model.Fracture prediction by artificial intelligence using seismic attributes provides many details of inter-well fractures.It is combined with fault-related fracture information predicted by numerical simulation of reservoir geomechanics to improve inter-well fracture trend prediction.An interwell fracture density model for fracture network modeling is built by coupling single-well fracture identification and interwell fracture trend through co-sequential simulation.By taking the tight carbonate reservoir of Oligocene-Miocene AS Formation of A Oilfield in Zagros Basin of the Middle East as an example,the proposed prediction method was applied and verified.The single-well fracture identification improves over 15%compared with the conventional fracture indication parameter method in accuracy rate,and the inter-well fracture prediction improves over 25%compared with the composite seismic attribute prediction.The established fracture network model is well consistent with the fluid production index.

Review and Significance of Microbial Carbonate Reservoirs in China

1 Introduction Numerous studies on the Meso-Neoproterozoic life evolution show that the cyanobacteria which thrived and dominated the biological world in the Proterozoic Era is closely related to the genesis of microbial carbonate rocks.Considerable oil and gas resources can be found in microbial carbonate rocks and many related oil and gas

Pore-throat structure characteristics and its impact on the porosity and permeability relationship of Carboniferous carbonate reservoirs in eastern edge of Pre-Caspian Basin

Carboniferous carbonate reservoirs at the eastern edge of the Pre-Caspian Basin have undergone complex sedimentation,diagenesis and tectonism processes,and developed various reservoir space types of pores,cavities and fractures with complicated combination patterns which create intricate pore-throats structure.The complex pore-throat structure leads to the complex porosity-permeability relationship,bringing great challenges for classification and evaluation of reservoirs and efficient development.Based on the comprehensive analysis on cores,thin sections,SEM,mercury intrusion,routine core analysis and various tests,this paper systematically investigated the features and main controlling factors of pore-throats structure and its impact on the porosity-permeability relationship of the four reservoir types which were pore-cavity-fracture,pore-cavity,pore-fracture and pore,and three progresses are made.(1)A set of classification and descriptive approach for pore-throat structure of Carboniferous carbonate reservoirs applied to the eastern edge of the Pre-Caspian Basin was established.Four types of pore-throat structures were developed which were wide multimodal mode,wide bimodal mode,centralized unimodal mode and asymmetry bimodal mode,respectively.The discriminant index of pore-throat structure was proposed,realizing the quantitative characterization of pore-throat structure types.(2)The microscopic heterogeneity of pore reservoir was the strongest and four types of pore-throat structures were all developed.The pore-fracture and pore-cavity-fracture reservoirs took the second place,and the microscopic heterogeneity of pore-cavity reservoir was the weakest.It was revealed that the main controlling factor of pore-throat structure was the combination patterns of reservoir space types formed by sedimentation,diagenesis and tectonism.(3)It was revealed that the development of various pore-throat structure types was the important factor affecting poroperm relationship of reservoirs.The calculation accuracy of permeability of reservoirs can be improved remarkably by subdividing the pore-throat structure types.This study deepens the understanding of pore-throat structure of complicated carbonate reservoirs,and is conducive to classification and evaluation,establishment of precise porosity-permeability relationship and highly efficient development of carbonate reservoirs.

Inter-well interferences and their influencing factors during water flooding in fractured-vuggy carbonate reservoirs

Based on the characteristics of injection-production units in fractured-vuggy carbonate reservoirs,nine groups of experiments were designed and performed to analyze the interference characteristics and their influencing factors during water flooding.Based on percolation theory,an inversion model for simulating waterflooding interferences was proposed to study the influence laws of different factors on interference characteristics.The results show that well spacing,permeability ratio,cave size,and cave location all affect the interference characteristics of water flooding.When the cave is located in high permeability fractures,or in the small well spacing direction,or close to the producer in an injection-production unit,the effects of water flooding are much better.When the large cave is located in the high-permeability or small well spacing direction,the well in the direction with lower permeability or smaller well spacing will see water breakthrough earlier.When the cave is in the higher permeability direction and the reserves between the water injector and producer differ greatly,the conductivity differences in different injection-production directions are favorable for water flooding.When the injection-production well pattern is constructed or recombined,it’s better to make the reserves of caves in different injection-production directions proportional to permeability,and inversely proportional to the well spacing.The well close to the cave should be a producer,and the well far from the cave should be an injector.Different ratios of cave reserves to fracture reserves correspond to different optimal well spacings and optimal permeability ratios.Moreover,both optimal well spacing and optimal permeability ratio increase as the ratio of cave reserves to fracture reserves increases.

Fracture characterization of Asmari Formation carbonate reservoirs in G Oilfield, Zagros Basin, Middle East

The Asmari Formation in the G oilfield on the Iran-Iraq border is a fractured-porous multi-lithology mixed reservoir, for which fracture is an important factor affecting oil productivity and water cut. The characterization and modeling of fractures in the carbonate reservoir of G oilfield are challenging due to weak conventional well log responses of fractures and a lack of specific logs, such as image logs. This study proposes an integrated approach for characterizing and modeling fractures in the carbonate reservoir. The features, formation mechanism, influencing factors, and prediction methods of fractures in the Asmari Formation carbonate reservoirs of G oilfield were studied using core observation, thin section, image log, cross-dipole acoustic log (CDAL), geomechanics numerical simulation (GNS), and production data. According to CDAL-based fracture density interpretation, GNS-based fracture intensity prediction between wells, and DFN-based rock fracture properties modeling, the quantitative fracture characterization for G oilfield was realized. This research shows that the fractures in the Asamri Formation are mainly medium-to high-angle shear fractures. The substantial compression stress during the Miocene played a major role in the formation of the prominent fractures and determined their trend in the region, with primary trends of NNW-SSE and NNE-SSW. The fracture distribution has regularity, and the fractures in zone A dolomites are more highly developed than that in zone B limestones vertically. Horizontally, fractures intensity is mainly controlled by faults and structural location. The results of this study may benefit the optimization of well design during field development. From 2019 to 2021, three horizontal wells pilot tests were deployed in the fractures belt in zone A, and these fractures prominently increased the permeability of tight dolomite reservoirs. The initial production of the wells is four to five times the average production of other wells in the area, showing a good development effect. Meanwhile, the updated numerical simulation validates that the history match accuracy of water cut based on the dual-porosity model is significantly improved, proving the fracture evaluation and prediction results to be relatively reliable and applicable.

Polymer efficiency and sulfate concentration for hybrid EOR application to an acidic carbonate reservoir

Polymers play an important role in hybrid enhanced oil recovery (EOR), which involves both a polymer and low-salinity water. Because the polymer commonly used for low-salinity polymer flooding (LSPF) is strongly sensitive to brine pH, its efficiency can deteriorate in carbonate reservoirs containing highly acidic formation water. In this study, polymer efficiency in an acidic carbonate reservoir was investigated experimentally for different salinity levels and SO42− concentrations. Results indicated that lowering salinity improved polymer stability, resulting in less polymer adsorption, greater wettability alteration, and ultimately, higher oil recovery. However, low salinity may not be desirable for LSPF if the injected fluid does not contain a sufficient number of sulfate (SO42−) ions. Analysis of polymer efficiency showed that more oil can be produced with the same polymer concentration by adjusting the SO42− content. Therefore, when river water, which is relatively easily available in onshore fields, is designed to be injected into an acidic carbonate reservoir, the LSPF method proposed in this study can be a reliable and environmentally friendly method with addition of a sufficient number of SO42− ions to river water.