Low Permeability Oilfield Research Articles

Mechanism recognition and application effect analysis of air injection development in ultra-low permeability oil reservoirs

Abstract There are few application examples of air drive development technology in ultra-low permeability oil reservoirs in China. The article relies on the air drive development block of H oilfield, and based on the results of core start-up pressure gradient experiment, relative permeability experiment, gas composition experiment, and gas swelling experiment, conducts research and analysis on the seepage mechanism and low-temperature oxidation mechanism of air drive development, demonstrating the advantages of air drive development in oil displacement mechanism of ultra-low permeability oilfield. Based on the basic parameters of oil fields developed by air flooding both domestically and internationally, the specific blocks suitable for air flooding development in H oilfield are selected. The feasibility of conducting air flooding development in low permeability oil fields is demonstrated from three aspects: indoor real core experimental data, safety, and the maturity of existing equipment and technology. At the same time, an analysis is conducted on the development status of the air drive block in H oilfield. By comparing the development effects with adjacent water injection blocks, it is demonstrated that the air drive development effect in ultra-low permeability oilfield is superior to water injection development.

Experimental study on oxidation characteristics of low temperature oxidation stage of thin oil in P layer in eastern Daqing Placanticline

Abstract The eastern oilfield of Daqing Placanticline belongs to a medium low permeability oilfield as a whole, with some blocks developed earlier and in the stage of high water content and high recovery. The main oil layer is severely flooded, and the remaining oil distribution is scattered. It is difficult to improve the development effect through conventional water flooding adjustments. In order to clarify the feasibility and key parameters of thin oil fire flooding development in the eastern P reservoir of Daqing Placanticline, experimental research on low-temperature oxidation stage oxidation characteristics was conducted using typical block rock cuttings and crude oil. Complete 20 sets of experiments on the oxidation characteristics of crude oil under different temperature and pressure, 4 sets of experiments on the oxidation characteristics of crude oil under different combustion systems, and 12 sets of tests on the composition of crude oil and exhaust gas. Clarify the low-temperature oxidation characteristics of crude oil under different temperatures and pressures, determine the oxidation characteristics of crude oil under different catalytic systems, and divide the temperature range for crude oil to reach ignition, providing theoretical support for improving reservoir fuel for thin oil flooding.

Development and Performance Evaluation of a New Conformance Control Agent Gel.

How to effectively plug the multi-scale fractured water channeling has always been the key to achieving efficient water flooding of fractured low-permeability oil reservoirs. In this paper, a new type of supramolecular-polymer composite gel is developed, which is suitable for plugging multi-scale fractured water channeling. The supramolecular-polymer composite gel is composed of a polymer (such as polyacrylamide), cross-linking agent (such as polyethyleneimine), supramolecular gel factor (such as cyclodextrin) and polarity regulator (such as ethyl alcohol). The mass fraction of polyacrylamide, polyethyleneimine, cyclodextrin and ethyl alcohol are 0.15%, 0.2%, 1% and 0.2%, respectively. At the initial state, the viscosity of the composite gelant system is less than 20 mPa·s. It has good injection performance in micro-scale fractures and can enter the deep part of a fractured reservoir. At 40 °C, the composite gelant system can form a gel with a double network structure after gelation. One of the networks is formed by the covalent interaction between polyacrylamide and polyethyleneimine, the other network is formed by the self-assembly of cyclodextrins under the action of the ethyl alcohol. The comprehensive performance of the composite gel is greatly improved. The strength of the composite gel is >5 × 104 mPa·s, and it has good plugging strength in large-scale fractures. The composite gel can be used as a conformance control agent for fractured low-permeability oilfields.

Design, synthesis, and characterization of novel copolymer gel particles for water-plugging applications

Abstract Multi-stage plugging represents a promising strategy for enhancing production and injection rates in medium-and low-permeability oilfields. Despite its potential, the efficacy of current plugging agents, particularly hydrophobically bonded water-soluble polyacrylamide-based gel microspheres, is hindered by notable drawbacks such as low stability and inadequate compressive strength. Therefore, a comprehensive understanding of water plugging mechanisms coupled with the optimization of gel microsphere properties is essential for advancing the development of gel-based plugging agents with superior characteristics or intelligent regulatory capabilities. The P(AA-AM-[PrSO3]Vim) ionic liquid copolymerized gel particles were designed and synthesized by using microfluidic technique and titration gel method with AM, acrylic acid (AA), 1-vinylimidazole (Vim) and 1,3-propylsulfonyl lactone (PrSO3) as the raw materials. The morphology and structure of the copolymer gel particles were characterized, and the effects of [PrSO3]Vim and cross-linker content on the water-absorbing properties and strength of the gel particles were investigated. When the amount of [PrSO3]Vim was 12%, the concentration of crosslinker was 1.5%, and the temperature was 40°C, the water absorption capacity reached the maximum value of 163 g·g−1. The strength of the P(AA–AM–[PrSO3]Vim) spherical gel particles was maximized at a [PrSO3]Vim content of 4%. Furthermore, the chemical and physical roles of the P(AA–AM–[PrSO3]Vim) spherical gel particles were studied in a typical water-plugging environment. This study provides experimental data and a theoretical basis for the application of functional spherical gel-plugging particles in current oilfield environments.

Theoretical investigation on the host−guest interactions in enhanced oil recovery

Supramolecular polymers have great application potential in oilfield production. Therefore, according to the characteristics and production requirements of low-permeability oilfields, developing new supramolecular functional units will be a research trend in the future. In this work, a cyclodextrin-modified Trimellitic acid (TA-β-CD) molecule was designed as the new supramolecular functional units. Then, molecular dynamics (MD) simulations and quantum chemistry (QC) calculations were employed to investigate the TA-βCD and poly acrylamide (pAM-Ad) assembly mechanism, intermolecular interaction and dynamic characteristics. It is found that polyacrylamide supramolecular polymer exists in the form of molecular coils in water. Through the molecular recognition and molecular docking of β-CD and Ad molecules, different molecular coils were linked together by Ad-β-CD supramolecular assembly. Finally, three-dimensional network structure was formed, which greatly increased the viscosity of the system. During this mechanism, the TA-βCD behaves as a "two-way connector", becoming the connection point of the three-dimensional space network. Based on our results, the general rules of supramolecular flooding can be summarized at the molecular level, which helps screen out efficient supramolecular polymers units.

Study on the Influencing Factors on the Effectiveness of Liquid Nitrogen Fracturing Technology

Liquid nitrogen fracturing technology is a new fracturing and permeability enhancement technology for reservoirs unsuitable for hydraulic fracturing, and its ultralow temperature, high compression ratio, high freezing and expansion force, and low viscosity characteristics are expected to provide a program for fracturing and permeability enhancement in low-permeability reservoirs. The paper systematically analyzes the influencing factors restricting the effect of liquid nitrogen fracturing, aiming to provide theoretical basis and technical support for the efficient and environmentally friendly exploitation of low-permeability oil and gas fields.

Study on the Mechanism of Coal Fracturing by Liquid Nitrogen

As a fracturing medium with excellent properties, liquid nitrogen has multiple fracturing effects on coal, and its potential application in coalbed methane mining has become a hot spot in current research. In this paper, the key scientific problems of fracturing and penetration enhancement by liquid nitrogen in low-permeability coals are discussed in depth, and the pore fracturing process and its fracturing mechanism of low-temperature fracturing coal are analyzed. It is found that the ultra-low-temperature fluid fracturing process has the effect of high-energy fluid fracturing to break the rock, and also has the coupled fracturing effect of low-temperature fracturing and phase-change fracturing to increase the energy-driven replacement. It aims to provide theoretical basis and technical support for the efficient and environmentally friendly exploitation of low-permeability oil and gas fields.

Reservoir characteristics analysis and favorable area prediction of Zhuhai Formation, Wenchang A oilfield, South China Sea

Wenchang A oilfield is a recently discovered low-permeability oilfield in the western South China Sea. The exploration target is the Zhuhai Formation, which reservoir lithology changes significantly, and the distribution of favorable reservoirs is unclear. In this study, the reservoir characteristics of the Zhuhai Formation in the Wenchang A oilfield, South China Sea, were analyzed through core identification, thin section identification, physical property testing, pre-stack geostatistical inversion, and frequency-based AVO inversion. Pre-stack geostatistical inversion is based on geostatistics and combines the seismic inversion algorithm with the stochastic sequential simulation algorithm. While the frequency-varying AVO inversion method is an extension of the conventional AVO inversion method, with its core technology being spectrum decomposition. After frequency division, multiple data sets directly participate in the calculation, which enhances the stability and accuracy of inversion.We also predict the favorable reservoir areas of six small layers in the Zhuhai Formation. The results show that 1) the reservoir lithology of the Zhuhai Formation mainly consists of feldspar quartz sandstone and feldspar quartz sandstone, indicating high compositional maturity. The porosity of the reservoir ranges from 14.0% to 19.0%, with an average value of 16.3%. The permeability of the reservoir ranges from 3.1 mD to 126.1 mD, with an average value of 22.4 mD, indicating a medium porosity and low permeability reservoir. The supporting structure of a sandstone reservoir is particle-supported, with the main contact being the “point-line” contact between particles. The main types of pores in sandstone are primary intergranular pores and secondary intergranular dissolved pores. Some pores are feldspar-dissolved pores, and occasionally there are hetero-based micropore. 2) The diagenesis of the Zhuhai Formation reservoir mainly includes compaction, cementation, and dissolution. Cementation and dissolution have minimal impact on the physical properties of reservoirs. The porosity loss of the reservoir after compaction ranges between 16.0% and 27.9%, and the compaction rate ranges between 40.0% and 69.8%, indicating a medium compaction diagenetic facies. 3) The favorable areas of each layer of the Zhuhai Formation are mainly concentrated in the southwest of the study area. The upper layer of ZH1I, the upper layer of ZH1II and the lower layer of ZH1II exhibit the best physical properties, the thickest favorable sand body, the strongest oil and gas display, the widest range of favorable areas, and the greatest exploration potential. The favorable exploration potential of layer 1 in the lower part of ZH1I and layer 2 in the upper part of ZH1II is moderate. The second layer in the lower part of ZH1II has the poorest properties and lower exploration potential. The main factors affecting the favorable area include physical properties, oil and gas display, sand body thickness, etc.

Numerical Simulation of Proppant Transportation and Placement Law In Fractured Coal Reservoirs

Hydraulic fracturing technology has been widely used in the development of low-permeability oil fields, gas fields, and coalbed methane. The morphology of the sand dike formed by proppant deposition in the fracture determines the effective shape of the fracture, the effective volume, the flow conductivity, and the validity period of the fracture, and the research on the proppant transport law in the fracture is of great significance in optimizing the fracturing construction parameters and improving the fracturing production increase effect. In this paper, a systematic study on the proppant transport law in hydraulic fracturing fracture is carried out by numerical simulation, which reveals the mechanism of proppant settling and transport, clarifies the proppant transport and placement law under different working conditions, and puts forward the optimization strategy of hydraulic fracturing process on the basis of this study.

Calculating the number of radial cracks around a wellbore fractured by liquid CO2 phase transition blasting technology

Integrating liquid CO2 phase transition blasting (LCPTB) technology with hydraulic fracturing (HF) methods can help reduce wellbore damage, create multiple radial fractures, and establish a complex fracture network. This approach significantly increases the recovery efficiency of low-permeability oil and gas fields. Accurately calculating the number of fractures caused by LCPTB is necessary to predict production enhancement effects and optimize subsequent HF designs. However, few studies are reported on large-scale physical model experiments in terms of a method for calculating the number of fractures. This study analyzed the initiation and propagation of cracks under LCPTB, derived a calculation formula for crack propagation radius under stress waves, and then proposed a new, fast, and accurate method for calculating the number of fractures using the principle of mass conservation. Through ten rock-breaking tests using LCPTB, the study confirmed the effectiveness of the proposed calculation approach and elucidated the variation rule of explosion pressure, rock-breaking scenario, and the impact of varying parameters on number of fractures. The results show that the new calculation method is suitable for fracturing technologies with high pressure rates. Recommendations include enlarging the diameter of the fracturing tube and increasing the liquid CO2 mass in the tube to enhance fracture effectiveness. Moreover, the method can be applied to other fracturing technologies, such as explosive fracturing (EF) within HF formations, indicating its broader applicability and potential impact on optimizing unconventional resource extraction technologies.

Interpretable knowledge-guided framework for modeling reservoir water-sensitivity damage based on Light Gradient Boosting Machine using Bayesian optimization and hybrid feature mining

Reservoir water sensitivity damage significantly contributes to production declines in low-permeability oil and gas fields. An accurate and rapid assessment of water sensitivity is essential for effective mitigation or prevention strategies. Facing the intricate challenge of predicting high-dimensional water sensitivity damage, this study leverages trends in intelligent drilling and completion technologies. It adopts a Knowledge-guided, Bayesian Optimization-enhanced Light Gradient Boosting Machine (KBO-LightGBM) for modeling, augmented by Multiple Imputation by Chained Equations and Synthetic Minority Over-sampling Technique (MICE-SMOTE) to address missing and unbalanced data issues in oil and gas fields, thereby enhancing the scientific efficacy of data processing. The framework's precision and practicality were confirmed using data from 270 natural core samples across 15 oil fields. Findings include a correlation coefficient of 0.9679 on the test set, a root mean square error of 3.4797, and a mean absolute percentage error of 4.0936%. Interpretability analysis identified formation water mineralization, burial depth, feldspar content, and initial porosity as the predominant factors affecting water sensitivity. This research distinguishes itself with a broader dataset, covering 15 parameters of formation fluids and rock components. Weighting factor αDK and scale βDK were designed to integrate empirical correlations into LightGBM's loss function, theoretically mitigating model overfitting. Hence, the intelligent framework proposed herein accurately predicts reservoir water sensitivity damage, aiding in the development of reservoir damage control strategies.

Design and Analysis of Harmonic Drive Reduction Pump Jack for Dynamic Performance in Oil Extraction

The purpose of this study is to provide a high gear ratio reducing pump jack system, utilizing the characteristics of high transmission ratio and precision of harmonic drives, in order to improve the efficiency of low-permeability oil field extraction. A comparison was made between the new system and the conventional walking beam pump jack. The results showed that at a stroke length of 1.5 m and pumping frequencies of 4.5 min-1, 3.5 min-1, 2.5 min-1, 1.5 min-1, and 0.5 min-1, the maximum acceleration of the suspended load in the harmonic reducing pump jack was reduced by 20.79% compared to the conventional walking beam pump jack.

New Method for Capacity Evaluation of Offshore Low-Permeability Reservoirs with Natural Fractures

In recent years, the development of two offshore low-permeability oil fields has revealed unexpected challenges. The actual productivity of these fields significantly deviates from the designed capacity. Some wells even outperform the expectations for low-permeability limestone fields. This discrepancy primarily stems from a lack of accurate understanding of natural fractures before and after drilling, resulting in substantial errors in capacity assessment. This paper addresses these challenges by proposing a new production capacity model and evaluation method for both vertical and horizontal wells in low-permeability limestone reservoirs. The method leverages logging curve data, incorporating vertical gradation and fractal analysis to effectively represent the fracture’s complexity and connectivity. It uniquely considers factors such as fracture fractal dimensions, threshold pressure, and stress sensitivity, significantly enhancing prediction accuracy. Furthermore, by analyzing the longitudinal gradient in logging curves, the method effectively identifies strong heterogeneity, leading to more accurate capacity evaluations in actual fields. The results demonstrate that our model reduces the average prediction error to less than 15%, markedly outperforming traditional methods. Calculation results of the newly developed capacity formula align closely with actual production data and tracer test results, showcasing its practical applicability and potential for widespread use. This study notably advances the evaluation of reasonable production capacity in similar offshore reservoirs.

Formula system of reduce pressure and increase injection working fluid in Tamtsag oilfield

Tamtsag Oilfield is a low-permeability oilfield. In the actual production process, there will be “high water injection pressure and high injection difficulty”. In order to improve the waterflood development effect of low permeability reservoir, the selection of working fluid plays a key role. By measuring the interfacial tension between the working fluid and crude oil, combined with the depressurization and injection performance evaluation test, the anion-non-ionic surfactant working fluid system suitable for the characteristics of the Tamtsag reservoir was prepared. Under the condition that the formation water pH of oil formation in the Block 21 of Tamtsag oilfield is 7.5 and the salinity is 3000 mg/L, the injection increasing formula selected in this paper is: 0.9 % coconut oil diethanolamide+0.7 % sodium dodecylbenzene sulfonate+0.5 % anhydrous ethanol+3 % clay stabilizer. For cores with permeability lower than 80 mD, the depressurization rate can reach more than 25 %; For cores with permeability less than 15.9 mD, the depressurization rate can reach more than 30 %. The working fluid system can effectively improve the effect of increasing injection in Tamtsag Oilfield.

Efficient Demulsification Performance of Emulsified Condensate Oil by Hyperbranched Low-Temperature Demulsifiers.

Focusing on the problem of poor demulsification performance of light crude oil emulsions in low-permeability oilfields at low temperatures, the composition of the emulsion samples, clay particle size distribution, and the viscosity-temperature relationship curve of samples were analyzed. Based on the results of emulsion composition analysis and characteristics, the bottle test method was used to analyze the demulsifying effect of different commercial types of demulsifiers, revealing the demulsification mechanism. The field tests confirm the demulsification capabilities of Polyoxyethylene polyoxypropylene quaternized polyoxyolefins surfactants (PR demulsifiers). The results reveal that PR demulsifiers combine the features of decreasing the interfacial tension between oil and water and adsorbing SiO2, allowing for quick demulsification and flocculation at low temperatures. This research serves as a theoretical and practical foundation for the study and advancement of low-temperature demulsification technology in oilfields.

Study on Fracturing Parameters Optimization of Horizontal Wells in Low-Permeability Reservoirs in South China Sea

The oil and gas resources in the deep Paleogene system of the South China Sea are abundant. However, due to its poor reservoir physical properties and strong heterogeneity, the deep Paleogene system needs to be commercially exploited by hydraulic fracturing technology. In view of the challenges of offshore low-permeability reservoirs, large-scale fracturing is not allowed because of the limited operation sites and complex string structure. Taking the H oilfield in the South China Sea as the target, based on the concept of the integration of geologic and engineering techniques, parameters such as the number of fracturing stages and the fracture length were optimized by a numerical simulation, and a study on the slurry rate and fracturing scale was carried out based on the type of fracturing and the pipe string structure. The results show that multistage fracturing technology is available in low-permeability offshore oil fields. It is suggested to adopt networking fracturing technology with a “slick water + high slurry rate” framework. A higher rate is recommended, and the fracturing scale of each stage should be 50 m3 of the sands and 700 m3 of the fluids. This research provides a new model for offshore low-permeability oilfield development.

Development and Application of Polymer Plugging Remover for Polymer Injection Well in Changqing Oilfield

Water injection is a conventional method to maintain reservoirs pressure and improve production in low permeability oilfield. To decrease high water-cut and further enhance oil recovery, Polymers, such as polymer microsphere, preformed polymer gel, cross-linked polyacrylamide, are extensively used to control injection profile in Changqing oilfield. However, due to the low permeability and narrow pore throats of tight reservoirs, pressure in some water flooding wells increases rapidly after the polymer was injected, negatively affecting the energy supplement of the well cluster. A polymer plugging remover, composed of oxidizer, acid and chelating agent to relieve blockage, was developed with viscosity reduction and structure decomposition. The release period is more than 48 hours, which is conducive to deep-seated plugging removal. After treatment, the average diameter of polymer residue is 39-254nm and the dissolution rate is 15.76-53.42%. The permeability of the core sample plugged with polymer has increased significantly before and after treatment by polymer plugging remover, especially, it recovered by 28.67% for microsphere polymer. Five field tests have been completed successfully in the Jiyuan oilfield of Changqing. Before treatment, conventional acidification was performed in the pilot wells for two times at least, which is proved invalid. However, after polymer degraded by the plugging remover, the injection rate increased by 16.2m3/d, the pressure dropped by 3.4MPa and the validity period has exceeded 130 days on average. The polymer plugging remover provides an alternative way to increase injectability when dealing with high pressure and under-injection wells caused by polymer flooding.

Wygd Study on prediction method of drilling pressure in low permeability oilfield

During the high water cut development stage of low permeability in M block, the overall recovery degree is low, there has 100 infilling wells been drilled since 2020 to improve the development effect. Before drilling, in order to avoid the risk of pressure safety, it is necessary to shut down the peripheral water injection wells to relieve the pressure, at the same time.Since the M Block reservoir is a low permeability reservoir, the pressure relief time after drilling off is long, which increases the impact on the production of developing wells and increases the fluid transportation and the related production costs, while there is no uniform standard for the time, pressure and distance of drilling off. In order to control the production loss of developing wells and save the shut-in cost. This paper applies numerical simulation and reservoir engineering method, by establishing fine geological model, optimizing simulation technology, considering the influence of start-up pressure gradient, establishing theoretical formula and mathematical model of drilling pressure, clarifying the law of drilling-off pressure relief and pressure drop, calculating the formation pressure of single well in the whole area, forming the drilling-off limit and rapid pressure prediction method, through comparison and verification, the overall deviation is 3.8%, and the method is accurate and reliable.

Study of Supercritical State Characteristics of Miscible CO2 Used in the Flooding Process

Carbon dioxide flooding is a strategic replacement technology for greatly enhancing oil recovery in low-permeability oilfields, which includes social benefits resulting from carbon emission reduction and economic benefits owing to the improvement of oil recovery. Therefore, it is of great significance to develop and apply the technology of CO2 flooding and storage in the petroleum industry. In reservoir conditions, CO2 is usually under a supercritical state, presenting both low viscosity and high diffusivity of a gaseous state and high density of a liquid state. The special phase behavior of CO2 directly affects its extraction capacity, resulting in the change of miscible behavior between CO2 and crude oil. In this paper, the ultra-high-pressure–high-temperature pressure–volume–temperature (PVT) system was used to evaluate the phase characteristics of CO2 during the process of reservoir development. The phase behaviors of the CO2/CH4/N2 crude oil system were compared and analyzed. Moreover, the matching mechanism between supercritical CO2 characteristics and oil–gas system miscibility was investigated and defined. This work deepened the understanding of the phase characteristics of CO2 in the process of miscible flooding, providing both theoretical guidance for the application of CO2 injection on oilfields and the essential scientific basis for the implementation of CCUS-EOR technology.

Oily wastewater treatment and reuse technology in low permeability oilfield

This paper takes Ordos Huaqing ultra-low permeability oilfield as the research object, analyzes the composition of the produced oily wastewater, and uses the treatment process of multiphase air flotation to shell filtration to quartz sand filtration to multi-effect evaporation to reuse the oily wastewater in the oilfield, analyzes the index changes of the purified oily wastewater. The results show that it has high oil content, salinity, sulfide, SS, sulfide and iron and manganese content. Through experimental combination and analysis of effluent quality, it is concluded that the process of multiphase air flotation→shell filtration→sand filtration→low temperature multi-effect evaporation is better, The average salinity of sewage in Huaqing oilfield used in the experiment is 2757 mg/L. and the dosage of polyaluminum chloride is 230 mg/L, polyacrylamide is 6 mg/L, the dissolved gas pressure of multiphase air flotation machine is 0.5 MPa, the reflux ratio of multiphase air flotation machine is 0.3, the dosage of defoamer is 5 mg/L, and the pH value is set at 5.5. The best treatment process of oilfield sewage and the best concentration of polyaluminum chloride and polyacrylamide are obtained. The research results provide theoretical data support for improving the treatment and reuse of oily wastewater in low permeability oilfield.