查看更多>>摘要：During the production,the fluid in the vicinity of the directional well enters the wellbore with different rates,leading to non-uniform flux distribution along the directional well.However,in all existing studies,it is oversimplified to a uniform flux distribution,which can result in inaccurate results for field appli-cations.Therefore,this paper proposes a semi-analytical model of a directional well based on the assumption of non-uniform flux distribution.Specifically,the direction well is discretized into a carefully chosen series of linear sources,such that the complex well trajectory can be captured and the non-uniform flux distribution along the wellbore can be considered to model the three-dimensional flow behavior.By using the finite difference method,we can obtain the numerical solutions of the transient flow within the wellbore.With the aid of Green's function method,we can obtain the analytical solutions of the transient flow from the matrix to the wellbore.The complete flow behavior of a directional well is perfectly represented by coupling the above two types of transient flow.Subsequently,on the basis of the proposed model,we conduct a comprehensive analysis of the pressure transient behavior of a directional well.The computation results show that the flux variation along the direction well has a significant effect on pressure responses.In addition,the directional well in an infinite reservoir may exhibit the following flow regimes:wellbore afterflow,transition flow,inclined radial flow,elliptical flow,horizontal linear flow,and horizontal radial flow.The horizontal linear flow can be observed only if the formation thickness is much smaller than the well length.Furthermore,a dip region that appears on the pressure derivative curve indicates the three-dimensional flow behavior near the wellbore.
查看更多>>摘要：Research on reservoir rock stress sensitivity has traditionally focused on unary granular structures,neglecting the binary nature of real reservoirs,especially tight reservoirs.Understanding the stress-sensitive behavior and mathematical characterization of binary granular media remains a challenging task.In this study,we conducted online-NMR experiments to investigate the permeability and porosity evolution as well as stress-sensitive control mechanisms in tight sandy conglomerate samples.The re-sults revealed stress sensitivity coefficients between 0.042 and 0.098 and permeability damage rates ranging from 65.6％to 90.9％,with an average pore compression coefficient of 0.0168-0.0208 MPa-1.Pore-scale compression occurred in three stages:filling,compression,and compaction,with matrix pores playing a dominant role in pore compression.The stress sensitivity of binary granular media was found to be influenced by the support structure and particle properties.High stress sensitivity was associated with small fine particle size,high fines content,high uniformity coefficient of particle size,high plastic deformation,and low Young's modulus.Matrix-supported samples exhibited a high irre-versible permeability damage rate(average=74.2％)and stress sensitivity coefficients(average=0.089),with pore spaces more slit-like.In contrast,grain-supported samples showed low stress sensitivity co-efficients(average=0.021)at high stress stages.Based on the experiments,we developed a mathe-matical model for stress sensitivity in binary granular media,considering binary granular properties and nested interactions using Hertz contact deformation and Poiseuille theory.By describing the change in activity content of fines under stress,we characterized the non-stationary state of compressive defor-mation in the binary granular structure and classified the reservoir into three categories.The model was applied for production prediction using actual data from the Mahu reservoir in China,showing that the energy retention rates of support-dominated,fill-dominated,and matrix-controlled reservoirs should be higher than 70.1％,88％,and 90.2％,respectively.
查看更多>>摘要：Deep shale gas reservoirs have geological characteristics of high temperature,high pressure,high stress,and inferior ability to pass through fluids.The multi-stage fractured horizontal well is the key to exploiting the deep shale gas reservoir.However,during the production process,the effectiveness of the hydraulic fracture network decreases with the closure of fractures,which accelerates the decline of shale gas production.In this paper,we addressed the problems of unclear fracture closure mechanisms and low accuracy of shale gas production prediction during deep shale gas production.Then we established the fluid-solid-heat coupled model coupling the deformation and fluid flow among the fracture surface,proppant and the shale matrix.When the fluid-solid-heat coupled model was applied to the fracture network,it was well solved by our numerical method named discontinuous discrete fracture method.Compared with the conventional discrete fracture method,the discontinuous discrete fracture method can describe the three-dimensional morphology of the fracture while considering the effect of the change of fracture surface permeation coefficient on the coupled fracture-matrix flow and describing the displacement discontinuity across the fracture.Numerical simulations revealed that the degree of fracture closure increases as the production time proceeds,and the degree of closure of the secondary fractures is higher than that of the primary fractures.Shale creep and proppant embedment both in-crease the degree of fracture closure.The reduction in fracture surface permeability due to proppant embedment reduces the rate of fluid transfer between matrix and fracture,which has often been overlooked in the past.However,it significantly impacts shale gas production,with calculations showing a 24.7％cumulative three-year yield reduction.This study is helpful to understand the mechanism of hydraulic fracture closure.Therefore,it provides the theoretical guidance for maintaining the long-term effectiveness of hydraulic fractures.
查看更多>>摘要：Under the policy background and advocacy of carbon capture,utilization,and storage(CCUS),CO2-EOR has become a promising direction in the shale oil reservoir industry.The multi-scale pore structure distribution and fracture structure lead to complex multiphase flow,comprehensively considering multiple mechanisms is crucial for development and CO2 storage in fractured shale reservoirs.In this paper,a multi-mechanism coupled model is developed by MATIAB.Compared to the traditional Eclipse 300 and MATIAB Reservoir Simulation Toolbox(MRST),this model considers the impact of pore structure on fluid phase behavior by the modified Peng-Robinson equation of state(PR-EOS),and the effect simultaneously radiate to Maxwell-Stefan(M-S)diffusion,stress sensitivity,the nano-confinement(N-C)effect.Moreover,a modified embedded discrete fracture model(EDFM)is used to model the complex fractures,which optimizes connection types and half-transmissibility calculation approaches between non-neighboring connections(NNCs).The full implicit equation adopts the finite volume method(FVM)and Newton-Raphson iteration for discretization and solution.The model verification with the Eclipse 300 and MRST is satisfactory.The results show that the interaction between the mechanisms signifi-cantly affects the production performance and storage characteristics.The effect of molecular diffusion may be overestimated in oil-dominated(liquid-dominated)shale reservoirs.The well spacing and in-jection gas rate are the most crucial factors affecting the production by sensitivity analysis.Moreover,the potential gas invasion risk is mentioned.This model provides a reliable theoretical basis for CO2-EOR and sequestration in shale oil reservoirs.
查看更多>>摘要：Natural fractures(NFs)are common in shale and tight reservoirs,where staged multi-cluster fracturing of horizontal wells is a prevalent technique for reservoir stimulation.While NFs and stress interference are recognized as significant factors affecting hydraulic fracture(HF)propagation,the combined influ-ence of these factors remains poorly understood.To address this knowledge gap,a novel coupled hydro-mechanical-damage(HMD)model based on the phase field method is developed to investigate the propagation of multi-cluster HFs in fractured reservoirs.The comprehensive energy functional and control functions are established,while incorporating dynamic fluid distribution between multiple perforation clusters and refined changes in rock mechanical parameters during hydraulic fracturing.The HMD coupled multi-cluster HF propagation model investigates various scenarios,including single HF and single NF,reservoir heterogeneity,single HF and NF clusters,and multi-cluster HFs with NF clusters.The results show that the HMD coupling model can accurately capture the impact of approach angle(θ),stress difference and cementation strength on the interaction of HF and NF.The criterion of the open and cross zones is not fixed.The NF angle(α)is not a decisive parameter to discriminate the interaction.According to the relationship between approach angle(θ)and NF angle(α),the contact relationship of HF can be divided into three categories(θ=α,θ＜α,and θ＞α).The connected NF can increase the complexity of HF by inducing it to form branch fracture,resulting in a fractal dimension of HF as high as 2.1280 at angles of±45°.Inter-fracture interference from the heel to the toe of HF shows the phe-nomenon of no,strong and weak interference.Interestingly,under the influence of NFs,distant HFs from the injection can become dominant fractures.However,as α gradually increases,inter-fracture stress interference becomes the primary factor influencing HF propagation,gradually superseding the domi-nance of NF induced fractures.
查看更多>>摘要：Proppant transport within fractures is one of the most critical tasks in oil,gas and geothermal reservoir stimulation,as it largely determines the ultimate performance of the operating well.Proppant transport in rough fracture networks is still a relatively new area of research and the associated transport mechanisms are still unclear.In this study,representative parameters of rough fracture surfaces formed by supercritical CO2 fracturing were used to generate a rough fracture network model based on a spectral synthesis method.Computational fluid dynamics(CFD)coupled with the discrete element method(DEM)was used to study proppant transport in this rough fracture network.To reveal the turning transport mechanism of proppants into branching fractures at the intersections of rough fracture net-works,a comparison was made with the behavior within smooth fracture networks,and the effect of key pumping parameters on the proppant placement in a secondary fracture was analyzed.The results show that the transport behavior of proppant in rough fracture networks is very different from that of the one in the smooth fracture networks.The turning transport mechanisms of proppant into secondary fractures in rough fracture networks are gravity-driven sliding,high velocity fluid suspension,and fracture structure induction.Under the same injection conditions,supercritical CO2 with high flow Reynolds number still has a weaker ability to transport proppant into secondary fractures than water.Thickening of the supercritical CO2 needs to be increased beyond a certain value to have a significant effect on proppant carrying,and under the temperature and pressure conditions of this paper,it needs to be increased more than 20 times(about 0.94 mPa s).Increasing the injection velocity and decreasing the proppant concentration facilitates the entry of proppant into the branching fractures,which in turn results in a larger stimulated reservoir volume.The results help to understand the proppant transport and placement process in rough fracture networks formed by reservoir stimulation,and provide a theoretical reference for the optimization of proppant pumping parameters in hydraulic fracturing.
查看更多>>摘要：Viscoelastic surfactants(VES)are often used as viscous diverters in acidizing stimulation to prolong the acid consumption time and maximize zonal coverage of the acid for improving well productivity.However,the ceiling temperature of commercial VES cannot exceed 120 ℃ in practical use because of the poor thermal stability and fragile molecular structure,hindering their implementation in high-temperature oil reservoirs,i.e.,≥150 ℃.Here we synthesized a novel C22-tailed diamine,N-erucami-nopropyl-N,N-dimethylamine(EDPA),and examined comparatively its rheological behavior,assemblies morphology and molecular stability in 20 wt％HCl with a commercial VES,erucyl dimethyl amidopropyl betaine(EDAB).The feasibility of EDPA for acidizing stimulation was assessed by acid etching of car-bonate rock with its HCl solution at 150 ℃.Rheological results showed that the 2.5 wt％EDPA-20 wt％HCl solution maintains stable viscosity of 90 mPa s at 150 ℃ for 60 min,while that of 2.0 wt％EDAB HCl solution is just 1 mPa s under identical conditions.1H NMR spectra and cryo-TEM observations revealed that the chemical structure and self-assembled architectures of EDPA remained intact in such context,but the EDAB suffered from degradation due to the hydrolysis of the amide group,accounting for the poor heat-resistance and acid-tolerance.The reaction rate of 2.5 wt％EDPA HCl solution with carbonate rock was one order of magnitude lower than that of 20 wt％HCl solution at 150 ℃,underpinning the potential of EDPA to be used in the high-temperature reservoirs acidizing.This work improved the thermal tolerance of VES in highly concentrated HCl solution,paving a feasible way for the acidization of high-temperature reservoir environments(-150 ℃).
查看更多>>摘要：A gel based on polyacrylamide,exhibiting delayed crosslinking characteristics,emerges as the preferred solution for mitigating degradation under conditions of high temperature and extended shear in ultra-long wellbores.High viscosity/viscoelasticity of the fracturing fluid was required to maintain excellent proppant suspension properties before gelling.Taking into account both the cost and the potential damage to reservoirs,polymers with lower concentrations and molecular weights are generally preferred.In this work,the supramolecular action was integrated into the polymer,resulting in signif-icant increases in the viscosity and viscoelasticity of the synthesized supramolecular polymer system.The double network gel,which is formed by the combination of the supramolecular polymer system and a small quantity of Zr-crosslinker,effectively resists temperature while minimizing permeability damage to the reservoir.The results indicate that the supramolecular polymer system with a molecular weight of(268-380) × 104 g/mol can achieve the same viscosity and viscoelasticity at 0.4 wt％due to the supra-molecular interaction between polymers,compared to the 0.6 wt％traditional polymer(hydrolyzed polyacrylamide,molecular weight of 1078 × 104 g/mol).The supramolecular polymer system possessed excellent proppant suspension properties with a 0.55 cm/min sedimentation rate at 0.4 wt％,whereas the 0.6 wt％traditional polymer had a rate of 0.57 cm/min.In comparison to the traditional gel with a Zr-crosslinker concentration of 0.6 wt％and an elastic modulus of 7.77 Pa,the double network gel with a higher elastic modulus(9.00 Pa)could be formed only at 0.1 wt％Zr-crosslinker,which greatly reduced the amount of residue of the fluid after gel-breaking.The viscosity of the double network gel was 66 mPa s after 2 h shearing,whereas the traditional gel only reached 27 mPa s.
查看更多>>摘要：Two allyldimethylalkyl quaternary ammonium salt(AQAS)monomers,N,N-dimethylallylphenylpropy-lammonium bromide(AQAS1)and N,N-dimethylallylnonylammonium bromide(AQAS2),were synthe-sized and used to prepare modified polyacrylamide materials.Two new drag reducers were synthesized from acrylamide(AM),sodium acrylate(NaAA)and a cationic modified monomer(AQAS1 or AQAS2)via aqueous solution polymerization,and the copolymers were named P(AM/NaAA/AQAS1)and P(AM/NaAA/AQAS2),respectively.The structures of the drag reduction agents were confirmed by IR and 1H NMR spectroscopies.The molecular weight(Mw)of P(AM/NaAA/AQAS1)was 1.79 × 106 g/mol.When the copolymer concentration was 1000 mg/L and the flow rate was 45 L/min,in fresh water the highest drag reduction rate was 75.8％,in 10,000 mg/L NaCl solution the drag reduction rate decreased to 72.9％.The molecular weight of P(AM/NaAA/AQAS2)was 3.17 × 106 g/mol.When the copolymer concentration was 500 mg/L and the flow rate was 45 L/min,the drag reduction rate reached 75.2％,and in 10,000 mg/L NaCl solution the drag reduction rate was 73.3％,decreased by approximately 1.9％.The drag reduction rate for partially hydrolyzed polyacrylamide(HPAM)was also investigated,and the results showed that the drag reduction rates for 500 and 1000 mg/L HPAM solutions were merely 43.2％and 49.0％in brine,respec-tively.Compared with HPAM,both of the above copolymers presented better drag reduction capacities.
查看更多>>摘要：At high cycles of steam huff & puff,oil distribution in reservoirs becomes stronger heterogeneity due to steam channeling.Thermal solidification agent can be used to solve this problem.Its solution is a low-viscosity liquid at normal temperature,but it can be solidified above 80 ℃.The plugging degree is up to 99％at 250 ℃.The sweep efficiency reaches 59.2％,which is 7.3％higher than pure steam injection.In addition,simultaneous injection of viscosity reducer and/or nitrogen foams can further enhance oil recovery.The mechanism of this technology depends on its strong plugging ability,which changes the flowing pattern of steam to effectively mobilize remaining oil.Viscosity reducer and nitrogen foams further expand the sweep range and extends the effective period.Therefore,thermal solidification agent can plug steam channeling paths and adjust steam flowing direction to significantly enhance oil recovery at high cycles of steam huff & puff.
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