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Further discussion of CO_(2) huff-n-puff mechanisms in tight oil reservoirs based on NMR monitored fluids spatial distributions

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Petroleum Science 2023年第1期20卷 350-361页

作者： Wei-Yu Tang james j.sheng Ting-Xue jiang Unconventional Petroleum Research Institute China University of PetroleumBeijing102249China Bob L.Herd Department of Petroleum Engineering Texas Tech UniversityLubbockTX79409USA Sinopec Research Institute of Petroleum Engineering Beijing100101China

Due to the poor physical properties of tight reservoirs,CO_(2) huff-n-puff(HNP)is considered a potential enhanced oil recovery(EOR)method after primary depletion.Optimization plays a critical role in the effective implementation of CO_(2) huff-n-puff.But the optimization requires a good understanding of the EOR mechanisms.In this work,the spatial distribution of oil saturation under different experimental conditions was analyzed by the NMR method to further discuss the HNP mechanisms.According to the variation of 1D frequency signal amplitude,we divided the core into the hardly movable area and movable area,the region with the obvious signal decline was defined as the movable area,and the hardly movable area was the region with limited signal decline.Based on that the recovery characteristics of different scenarios were evaluated.Firstly,the necessity of the soaking stage was studied,where three scenarios with different soaking times were carried out.Secondly,the injection pressure was adjusted to investigate the effect of the pressure gradient.The T_(2) spectra show that soaking has significantly improved the production of crude oil in small pores,and higher oil recovery in a single cycle is observed,but it is lower when the elapsed time(total operation time)is the same.31.03% of oil can be recovered after 3 cycles HNP,which increases to 33.8% and 37.06% for the 4 cycles and 6 cycles cases.As the pressure gradient increases,more oil is removed out of the matrix,and the oil in the deep part of the reservoir can be effectively recovered.During the CO_(2) huff-n-puff process,the oil distributions are similar to the solution gas drive,the residual oil is distributed at the close end of the core and the range that the oil can be efficiently recovered is limited.

关键词： Tight reservoirs Low-field nuclear magnetic resonance (NMR) CO_(2)huff-n-puff Enhanced oil recovery

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Statistical distribution of geomechanical properties and‘Sweet Spots’identification in part of the upper Bakken

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Petroleum Research 2023年第3期8卷 301-308页

作者： Nelson R.K.Tatsipie james j.sheng Texas Tech University United States

Completions and Reservoir Quality are two key attributes that are used to characterize nonconventional hydrocarbon assets.This is because,for optimum exploitation of these unconventional assets,horizontal wells need to be drilled in“Sweet Spots”(i.e.,regions where Completions and Reservoir Quality are both superior).One way to quantify these qualities is to use reservoir and geomechanical properties.These properties can be estimated on a location basis from well logs,and then mapped over terrain using geostatistical modeling.This study presents a‘Sweet Spots’identification workflow based on three performance indexes(Storage Potential Index,Brittleness Index,and Horizontal Stress Index)that can be used to quantify CQ and RQ.The performance indexes are computed from petrophysical property volumes(of Young's Modulus,Bulk Modulus,Shear Modulus,Poisson's Ratio,Minimum Horizontal Stress,Volume of Shale,Total Organic Carbon,Thickness,and Porosity)which are in turn computed from well logs and geostatistical simulation.In the end,the study offers a method to compare the predicted“Sweet Spots”against available production data via their correlation coefficient.The resulting reasonable formation property maps,the successful identification of‘Sweet Spots’,and a correlation coefficient of 0.88(between the predicted“Sweet Spots”and well production data)point to the potential of the proposed effort.

关键词： Sweet spots Sequential Gaussian simulation Storage potential index Brittleness index Horizontal stress index

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Experimental study on the degree and damage-control mechanisms of fuzzy-ball-induced damage in single and multi-layer commingled tight reservoirs

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Petroleum Science 2023年第6期20卷 3598-3609页

作者： Chinedu j.Okere james j.sheng Li-Kun Fan Xiao-Wei Huang Li-Hui Zheng Pan-Feng Wei Bob L.Herd Department of Petroleum Engineering Texas Tech UniversityTXUSA College of Petroleum Engineering China University of Petroleum(Beijing)Beijing102249China China Changqing Oilfield Company CNPCXi'an710018ShaanxiChina Beijing LihuiLab Energy Technology Co. Ltd.Beijing102200China

Fuzzy-ball working fluids(FBWFs)have been successfully applied in different development phases of tight reservoirs.Field reports revealed that FBWFs satisfactorily met all the operational and reservoir damage control requirements during their application.However,the damage-control mechanisms and degree of formation damage caused by fuzzy-ball fluids have not been investigated in lab-scale studies so far.In this study,the degree of fuzzy-ball-induced damage in single-and double-layer reservoirs was evaluated through core flooding experiments that were based on permeability and flow rate indexes.Additionally,its damage mechanisms were observed via scanning electron microscope and energy-dispersive spectroscopy tests.The results show that:(1)For single-layer reservoirs,the FBWF induced weak damage on coals and medium-to-weak damage on sandstones,and the difference of the damage in permeability or flow rate index on coals and sandstones is below 1%.Moreover,the minimum permeability recovery rate was above 66%.(2)For double-layer commingled reservoirs,the flow rate index revealed weak damage and the overall damage in double-layer was lower than the single-layer reservoirs.(3)There is no significant alteration in the microscopic structure of fuzzy-ball saturated cores with no evidence of fines migration.The dissolution of lead and sulfur occurred in coal samples,while tellurium in sandstone,aluminum,and magnesium in carbonate.However,the precipitation of aluminum,magnesium,and sodium occurred in sandstone but no precipitates found in coal and carbonate.The temporal plugging and dispersion characteristics of the FBWFs enable the generation of reservoir protection layers that will minimize formation damage due to solid and fluid invasion.

关键词： Drilling Fracture Fuzzy-ball fluids Formation damage analysis Muti-layer tight reservoirs Permeability damage index Flow rate damage index

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Comparative study of well soaking timing(pre vs.post flowback)for water blockage removal from matrix-fracture interface

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Petroleum 2020年第3期6卷 286-292页

作者： Nur Wijaya james j.sheng Bob L.Herd Department of Petroleum Engineering Texas Tech UniversityTexasUSA

Water blockage after hydraulic fracturing is one of the major challenges in shale oil recovery which affects the optimal production from the reservoir.The water blockage represents a higher water saturation near the matrixfracture interface,which decreases the hydrocarbon relative permeability.The removal of water blockage in the field is typically carried out by soaking the well(i.e.,shut-in)after hydraulic fracturing operation is finished.This soaking period allows water redistribution,which decreases the water saturation near the matrix-fracture interface.However,previous field reports show that there is not a strong consensus on whether shut-in is beneficial in terms of production rate or ultimate oil recovery.Due to the large number of parameters involved in hydraulic fracturing and tight formations,it is challenging to select which parameter plays the dominant role in determining the shut-in performance.Furthermore,literature on field case studies does not frequently report the parameters which are of researchers’interest.In other words,the challenge of evaluating shut-in performance not only lies on the complexity of parameters and effects involved within the reservoir,but also the limited number of field case studies which report a comprehensive list of fracturing and reservoir parameters.This paper aims to investigate the effect of well soaking timing on shut-in performance.This idea to investigate the shut-in timing effect is motivated by the fact that in the field,shut-in can take place either immediately after hydraulic fracturing but before the first flowback(i.e.,pre-flowback)or sometime after the first flowback(i.e.,post-flowback).The timing of shut-in is believed to influence the production performance,because it dictates how much water is allowed to imbibe from the fractures to the matrix before the extended production.A numerical model is built and validated by a successful history match with numerous data from core-flood experiments.Our previous study shows that shut-in performance depends heav

关键词： Water blockage Formation damage Flowback Shut-in Desiccation

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Status of surfactant EOR technology

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Petroleum 2015年第2期1卷 97-105页

作者： james j.sheng Texas Tech University P.O.Box 43111LubbockTX 79409United States

Surfactant enhanced oil recovery(EOR)includes surfactant flooding and surfactant stimulation.The main functions of surfactants are to reduce interfacial tension and wettability alteration.This paper is to review the EOR technology related to surfactant injection.The reviewed topics include the following:Surfactant EOR mechanismsFactors affecting interfacial tension Trapping numberScreening criteriaLaboratory work Numerical simulation work Summary of pilot and large-scale applications Surfactants used Salinity gradientSurfactant EOR in carbonate reservoirs Surfactant EOR in shale reservoirs.

关键词： Surfactant flooding Surfactant EOR Review Enhanced oil recovery Chemical EOR

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Optimization of huff-n-puff gas injection in shale oil reservoirs

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Petroleum 2017年第4期3卷 431-437页

作者： james j.sheng Texas Tech University P.O.Box 43111LubbockTX 79409USA Southwest Petroleum University Chengdu 610500China

Earlier studies show that huff-n-puff injection is preferred to continuous gas flooding to improve liquid oil production in shale oil reservoirs.Compared to gas flooding,huff-n-puff has more operational parameters to optimize so that liquid oil production can be maximized.This paper is to discuss the optimum huff-n-puff times,number of cycles and soaking time under practical operational and reservoir conditions.The operational and reservoir conditions dictate the maximum injection and production rates,and the maximum injection pressure and minimum production pressure.The numerical simulation results and discussions show that the optimum huff time is so long that the pressure near the wellbore reaches the set maximum injection pressure during the huff period;and the optimum puff time is the time required for the pressure near the wellbore to reach the set minimum production pressure during the puff period.The benefits of soaking may not compensate the loss in injection and production due to the time lost in the soaking period.Therefore,soaking may not be necessary during the huff-n-puff gas injection in shale oil reservoirs.The number of huff-n-puff cycles is determined when an economic rate cut-off is reached.

关键词： shale reservoirs injection

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Simulation study of huff-n-puff air injection for enhanced oil recovery in shale oil reservoirs

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Petroleum 2018年第1期4卷 7-14页

作者： Hu jia james j.sheng State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation Southwest Petroleum UniversityChengduSichuan 610500China Bob L.Herd Department of Petroleum Engineering Texas Tech UniversityLubbockTX 79409USA

This paper is the first attempt to evaluate huff-n-puff air injection in a shale oil reservoir using a simulation approach.Recovery mechanisms and physical processes of huff-n-puff air injection in a shale oil reservoir are investigated through investigating production performance,thermal behavior,reservoir pressure and fluid saturation features.Air flooding is used as the basic case for a comparative study.The simulation study suggests that thermal drive is the main recovery mechanism for huff-n-puff air injection in the shale oil reservoir,but not for simple air flooding.The synergic recovery mechanism of air flooding in conventional light oil reservoirs can be replicated in shale oil reservoirs by using air huff-npuff injection strategy.Reducing huff-n-puff time is better for performing the synergic recovery mechanism of air injection.O2 diffusion plays an important role in huff-n-puff air injection in shale oil reservoirs.Pressure transmissibility as well as reservoir pressure maintenance ability in huff-n-puff air injection is more pronounced than the simple air flooding after primary depletion stage.No obvious gas override is exhibited in both air flooding and air huff-n-puff injection scenarios in shale reservoirs.Huffn-puff air injection has great potential to develop shale oil reservoirs.The results from this work may stimulate further investigations.

关键词： Air injection huff-n-puff Shale reservoirs Unconventional Air flooding Enhanced oil recovery

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Discussion of the feasibility of air injection for enhanced oil recovery in shale oil reservoirs

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Petroleum 2017年第2期3卷 249-257页

作者： Hu jia james j.sheng State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation Southwest Petroleum UniversityChengduSichuan610500China Bob L.Herd Department of Petroleum Engineering Texas Tech UniversityLubbockTX 79409USA

Air injection in light oil reservoirs has received considerable attention as an effective,improved oil recovery process,based primarily on the success of several projects within the Williston Basin in the United States.The main mechanism of air injection is the oxidation behavior between oxygen and crude oil in the reservoir.Air injection is a good option because of its wide availability and low cost.Whether air injection can be applied to shale is an interesting topic from both economic and technical perspectives.This paper initiates a comprehensive discussion on the feasibility and potential of air injection in shale oil reservoirs based on state-of-the-art literature review.Favorable and unfavorable effects of using air injection are discussed in an analogy analysis on geology,reservoir features,temperature,pressure,and petrophysical,mineral and crude oil properties of shale oil reservoirs.The available data comparison of the historically successful air injection projects with typical shale oil reservoirs in the U.S.is summarized in this paper.Some operation methods to improve air injection performance are recommended.This paper provides an avenue for us to make use of many of the favorable conditions of shale oil reservoirs for implementing air injection,or air huff‘n’puff injection,and the low cost of air has the potential to improve oil recovery in shale oil reservoirs.This analysis may stimulate further investigation.

关键词： shale reservoirs reservoir

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Discussion of liquid threshold pressure gradient

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Petroleum 2017年第2期3卷 232-236页

作者： Xiukun Wang james j.sheng Texas Tech University USA Southwest Petroleum University China

Some authors believe that a minimum pressure gradient(called threshold pressure gradient(TPG))is required before a liquid starts to flow in a porous medium.In a tight or shale oil formation,this TPG phenomenon becomes more important,as it is more difficult for a fluid to flow.In this paper,experimental data on TPG published in the literature are carefully reviewed.What we found is that a very low flow velocity corresponding to a very low pressure gradient cannot be measured in the experiments.Experiments can only be done above some measurable flow velocities.If these flow velocities and their corresponding pressure gradients are plotted in an XY plot and extrapolated to zero velocity,a non-zero pressure gradient corresponds to this zero velocity.This non-zero pressure gradient is called threshold pressure gradient in the literature.However,in the regime of very low velocity and very low pressure gradient,the data gradually approach to the origin of the plot,demonstrating a non-linear relationship between the pressure gradient and the velocity.But the data do not approach to a point of zero velocity and a threshold pressure gradient.Therefore,the concept of threshold pressure gradient is a result of data misinterpretation of available experimental data.The correct interpretation is that there are two flow regimes:nonlinear flow regime(non-Darcy flow regime)when the pressure gradients are low,and linear flow regime(Darcy flow regime)when the pressure gradient is intermediate or high.The nonlinear flow regime starts from the origin point.As the pressure gradient is increased,the curve becomes a straight line demonstrating the linear flow regime.We have verified our views by first analyzing the causes of non-Darcy flow,and then systematically analyzed typical experimental data and correlations in the literature.We conclude that TPG does not exist.We also use several counter examples to support our conclusion.

关键词： Low velocity non-Darcy flow Threshold pressure gradient Shale oil reservoirs Tight oil reservoirs

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Effects of rock pore sizes on the PVT properties of oil and gas-condensates in shale and tight reservoirs

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Petroleum 2018年第2期4卷 148-157页

作者： Arash Kamari Lei Li james j.sheng Bob L.Herd Department of Petroleum Engineering Texas Tech UniversityP.O.Box 43111LubbockTX 79409United States Southwest Petroleum University ChengduChina

Shales make up a large proportion of the rocks with extremely low permeability representing many challenges which can be complex in many cases.A careful study of rock and fluid properties(i.e.PVT of shales)of such resources is needed for long-term success,determining reservoirs quality,and increased recovery factor in unique unconventional plays.In this communication,a comprehensive thermodynamic modelling is undertaken in which capillary pressure is coupled with the phase equilibrium equations.To this end,the data associated with both shale oil and gascondensates of Eagle Ford shale reservoir located in South Texas,U.S.,is used.Different properties,including bubble and dew point pressures,capillary pressure,interfacial tension,liquid and gas densities,and liquid and gas viscosities,are predicted observing the effects of rock pore sizes by the thermodynamic modelling performed in this study.The results demonstrate that the thermodynamic model developed in this study is capable of simulating the PVT properties of oil and gascondensates in shale and tight reservoirs.For a binary mixture 25:75 C1/C6,the bubble point pressure at different reservoir temperature is increased by increasing the pore sizes from 10 to 50 nm.Furthermore,an increase in pore sizes from 10 to 50 nm can increase the dew point pressure for a studied binary mixture 75:25 C1/C6.

关键词： Shale Pore size Oil Gas-condensate Thermodynamic modelling

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