查看更多>>摘要：The aim of this study is to create a fast and stable iterative technique for numerical solution of a quasi-linear elliptic pressure equation.We developed a modified version of the Anderson acceleration(AA)algorithm to fixed-point(FP)iteration method.It computes the approximation to the solutions at each iteration based on the history of vectors in extended space,which includes the vector of unknowns,the discrete form of the operator,and the equation's right-hand side.Several constraints are applied to AA algorithm,including a limitation of the time step variation during the iteration process,which allows switching to the base FP iterations to maintain convergence.Compared to the base FP algorithm,the improved version of the AA algorithm enables a reliable and rapid convergence of the iterative solution for the quasi-linear elliptic pressure equation describing the flow of particle-laden yield-stress fluids in a narrow channel during hydraulic fracturing,a key technology for stimulating hydrocarbon-bearing reservoirs.In particular,the proposed AA algorithm allows for faster computations and resolution of unyielding zones in hydraulic fractures that cannot be calculated using the FP algorithm.The quasi-linear elliptic pressure equation under consideration describes various physical processes,such as the displacement of fluids with viscoplastic theology in a narrow cylindrical annulus during well cementing,the displacement of cross-linked gel in a proppant pack filling hydraulic fractures during the early stage of well production(fracture flowback),and multiphase filtration in a rock formation.We estimate computational complexity of the developed algorithm as compared to Jacobian-based algorithms and show that the performance of the former one is higher in modelling of flows of viscoplastic fluids.We believe that the developed algorithm is a useful numerical tool that can be implemented in commercial simulators to obtain fast and converged solutions to the non-linear problems described above.
查看更多>>摘要：Tri-axial fracturing studies were carried out to understand the impact of lateral mechanical parameters on fracture propagation from multiple in-plane perforations in horizontal wells.Additionally,the dis-cussion covered the effects of geology,treatment,and perforation characteristics on the non-planar propagation behavior.According to experimental findings,two parallel transverse fractures can be successfully initiated from in-plane perforation clusters in the horizontal well because of the in-plane perforation,the guide nonuniform fishbone structure fracture propagation still can be exhibited.The emergence of transverse fractures and axial fractures combined as complex fractures under low hori-zontal principal stress difference and large pump rate conditions.The injection pressure was also investigated,and the largest breakdown pressure can be also found for samples under these conditions.The increase in perforation number or decrease in the cluster spacing could provide more chances to increase the complexity of the target stimulated zone,thus affecting the pressure fluctuation.In a contrast,the increase in fracturing fluid viscosity can reduce the multiple fracture complexity.The fracture propagation is significantly affected by the change in the rock mechanical properties.The fracture geometry in the high brittle zone seems to be complicated and tends to induce fracture reor-ientation from the weak-brittle zone.The stress shadow effect can be used to explain the fracture attraction,branch,connection,and repulsion in the multiple perforation clusters for the horizontal well.The increase in the rock heterogeneity can enhance the stress shadow effect,resulting in more complex fracture geometry.In addition,the variable density perforation and temporary plugging fracturing were also conducted,demonstrating higher likelihood for non-uniform multiple fracture propagation.Thus,to increase the perforation efficiency along the horizontal well,it is necessary to consider the lateral fracability of the horizontal well on target formation.
查看更多>>摘要：The cyclic hydraulic stimulation(CHS)has proven as a prospective technology for enhancing the permeability of unconventional formations such as coalbeds.However,the effects of CHS on the microstructure and gas sorption behavior of coal remain unclear.In this study,laboratory tests including the nuclear magnetic resonance(NMR),low-temperature nitrogen sorption(LTNS),and methane sorp-tion isotherm measurement were conducted to explore changes in the pore structure and methane sorption characteristics caused by CHS on an anthracite coal from Qinshui Basin,China.The NMR and LTNS tests show that after CHS treatment,meso-and macro-pores tend to be enlarged,whereas micro-pores with larger sizes and transition-pores may be converted into smaller-sized micro-pores.After the coal samples treated with 1,3,5 and 7 hydraulic stimulation cycles,the total specific surface area(TSSA)decreased from 0.636 to 0.538,0.516,0.505,and 0.491 m2/g,respectively.Fractal analysis based on the NMR and LTNS results show that the surface fractal dimensions increase with the increase in the number of hydraulic stimulation cycles,while the volume fractal dimensions exhibit an opposite trend to the surface fractal dimensions,indicating that the pore surface roughness and pore structure connectivity are both increased after CHS treatment.Methane sorption isothermal measurements show that both the Langmuir volume and Langmuir pressure decrease significantly with the increase in the number of hydraulic stimulation cycles.The Langmuir volume and the Langmuir pressure decrease from 33.47 cm3/g and 0.205 MPa to 24.18 cm3/g and 0.176 MPa after the coal samples treated with 7 hydraulic stimulation cycles,respectively.The increments of Langmuir volume and Langmuir pressure are positively correlated with the increment of TSSA and negatively correlated with the increments of surface fractal dimensions.
查看更多>>摘要：Multistage fracturing of horizontal wells is a critical technology for unconventional oil and gas reservoir stimulation.Ball-throwing temporary plugging fracturing is a new method for realizing uniform frac-turing along horizontal wells and plays an important role in increasing oil and gas production.However,the transportation and sealing law of temporary plugging balls(TPBs)in the perforation section of horizontal wells is still unclear.Using COMSOL computational fluid dynamics and a particle tracking module,we simulate the transportation process of TPBs in a horizontal wellbore and analyse the effects of the ball density,ball diameter,ball number,fracturing fluid injection rate,and viscosity on the plugging efficiency of TPB transportation.This study reveals that when the density of TPBs is close to that of the fracturing fluid and a moderate diameter of the TPB is used,the plugging efficiency can be sub-stantially enhanced.The plugging efficiency is greater when the TPB number is close to twice the number of perforations and is lower when the number of TPBs is three times the number of perforations.Adjusting the fracturing fluid injection rate from low to high can control the position of the TPBs,improving plugging efficiency.As the viscosity of the fracturing fluid increases,the plugging efficiency of the perforations decreases near the borehole heel and increases near the borehole toe.In contrast,the plugging efficiency of the central perforation is almost unaffected by the fracturing fluid viscosity.This study can serve as a valuable reference for establishing the parameters for temporary plugging and fracturing.
查看更多>>摘要：Fracturing operations can effectively improve the production of low-permeable reservoirs.The perfor-mance of fracturing fluids directly affects the fracturing efficiency and back flow capacity.As polymer-based fracturing fluids(such as guar gum(GG),polyacrylamide(HPAM),etc.)are high-viscosity fluids formed by viscosifiers and crosslinking agents,the degree of gel breakage after the fracturing operation directly influences the damage degree to the reservoir matrix and the mobility of oil angd gas produced from the reservoir into the wellbore.This study compared the viscosity,molecular weight,and particle size of the fracturing fluid after gel breakage prepared by GG and HPAM as viscosifiers,as well as evaluate their damage to the core.Results show that the viscosities of the gel-breaking fluid increased with the concentration of the viscosifier for both the HPAM-based and GG-based fracturing fluids.For the breaking fluid with the same viscosity,the molecular weight in the HPAM-based gel-breaking fluid was much larger than that in the GG-based system.Moreover,for the gel-breaking fluid with the same viscosity,the molecular particle size of the residual polymers in the HPAM-based system was smaller than that in the GG-based system.The damage to the core with the permeability of 1 x 10-3 pm2 caused by both the HPAM-based and GG-based gel-breaking fluids decreased with the increase in the solution viscosity.For the gel-breaking fluid systems with the same viscosity(i.e.,2-4 mPa s),the damage of HPAM-based fracturing fluid to low-permeability cores was greater than the GG-based fracturing fluid(45.6％-80.2％)since it had a smaller molecular particle size,ranging from 66.2％to 77.0％.This paper proposed that the damage caused by hydraulic fracturing in rock cores was related to the partilce size of residual polymers in gel-breaking solution,rather than its molecular weight.It was helpful for screening and optimizing viscosifiers used in hydraulic fracturing process.
查看更多>>摘要：Understanding fingering,as a challenge to stable displacement during the immiscible flow,has become a crucial phenomenon for geological carbon sequestration,enhanced oil recovery,and groundwater pro-tection.Typically governed by gravity,viscous and capillary forces,these factors lead invasive fluids to occupy pore space irregularly and incompletely.Previous studies have demonstrated capillary numbers,describing the viscous and capillary forces,to quantificationally induce evolution of invasion patterns.While the evolution mechanisms of invasive patterns have not been deeply elucidated under the con-stant capillary number and three variable parameters including velocity,viscosity,and interfacial tension.Our research employs two horizontal visualization systems and a two-phase laminar flow simulation to investigate the tendency of invasive pattern transition by various parameters at the pore scale.We showed that increasing invasive viscosity or reducing interfacial tension in a homogeneous pore space significantly enhanced sweep efficiency,under constant capillary number.Additionally,in the fingering crossover pattern,the region near the inlet was prone to capillary fingering with multi-directional in-vasion,while the viscous fingering with unidirectional invasion was more susceptible occurred in the region near the outlet.Furthermore,increasing invasive viscosity or decreasing invasive velocity and interfacial tension promoted the extension of viscous fingering from the outlet to the inlet,presenting that the subsequent invasive fluid flows toward the outlet.In the case of invasive trunk along a unidi-rectional path,the invasive flow increased exponentially closer to the outlet,resulting in a significant decrease in the width of the invasive interface.Our work holds promising applications for optimizing invasive patterns in heterogeneous porous media.
查看更多>>摘要：Shale gas,as an environmentally friendly fossil energy resource,has gained significant commercial development and shows immense potential.However,accurately predicting shale gas production faces substantial challenges due to the complex law of decline,nonlinear and non-stationary features in production data,which greatly repair the robustness of current models in predicting shale gas pro-duction time series.To address these challenges and improve accuracy in production forecasting,this paper introduces a novel and innovative approach:a hybrid proxy model that combines the bi-directional long short-term memory(BiLSTM)neural network and random forest(RF)through deep learning.The BiLSTM neural network is adept at capturing long-term dependencies,making it suitable for understanding the intricate relationships between input and output variables in shale gas production.On the other hand,RF serves a dual purpose:reducing model variance and addressing the concept drift problem that arises in non-stationary time series predictions made by BiLSTM.By integrating these two models,the hybrid approach effectively captures the inherent dependencies present in long and nonstationary production time series,thereby reducing model uncertainty.Furthermore,the combina-tion of BiLSTM and RF is optimized using the recently-proposed marine predators algorithm(MPA)to fine-tune hyperparameters and enhance the overall performance of the proxy model.The results demonstrate that the proposed BiLSTM-RF-MPA model achieves higher prediction accuracy and dem-onstrates stronger generalization capabilities by effectively handling the complex nonlinear and non-stationary characteristics of shale gas production time series.Compared to other models such as LSTM,BiLSTM,and RF,the proposed model exhibits superior fitting and prediction performance,with an average improvement in performance indicators exceeding 20％.This innovative framework provides valuable insights for forecasting the complex production performance of unconventional oil and gas reservoirs,which sheds light on the development of data-driven proxy models in the field of subsurface energy utilization.
查看更多>>摘要：The application of carbon dioxide(CO2)in enhanced oil recovery(EOR)has increased significantly,in which CO2 solubility in oil is a key parameter in predicting CO2 flooding performance.Hydrocarbons are the major constituents of oil,thus the focus of this work lies in investigating the solubility of CO2 in hydrocarbons.However,current experimental measurements are time-consuming,and equations of state can be computationally complex.To address these challenges,we developed an artificial intelligence-based model to predict the solubility of CO2 in hydrocarbons under varying conditions of temperature,pressure,molecular weight,and density.Using experimental data from previous studies,we trained and predicted the solubility using four machine learning models:support vector regression(SVR),extreme gradient boosting(XGBoost),random forest(RF),and multilayer perceptron(MLP).Among four models,the XGBoost model has the best predictive performance,with an R2 of 0.9838.Additionally,sensitivity analysis and evaluation of the relative impacts of each input parameter indicate that the prediction of CO2 solubility in hydrocarbons is most sensitive to pressure.Furthermore,our trained model was compared with existing models,demonstrating higher accuracy and applicability of our model.The developed machine learning-based model provides a more efficient and accurate approach for predicting CO2 solubility in hydrocarbons,which may contribute to the advancement of CO2-related applications in the petroleum industry.
查看更多>>摘要：In-situ thermal upgrading is used to tune the pore system in low-maturity oil shales.We introduce fractal dimension(D),form factor(ff)and stochastic entropy(H)to quantify the heating-induced evo-lution of pore morphological complexity and azimuthal disorder and develop a model to estimate the impact on seepage capacity via permeability.Experiments are conducted under recreated in-situ tem-peratures and consider anisotropic properties-both parallel and perpendicular to bedding.Results indicate that azimuthal distribution of pores in the bedding-parallel direction are dispersed,while those in the bedding-perpendicular direction are concentrated.D values indicate that higher temperatures reduce the uniformity of the pore size distribution(PSD)in the bedding-parallel direction but narrow the PSD in the bedding-perpendicular direction.The greater ff(＞0.7)values in the bedding-parallel direction account for a large proportion,while the dominated in the bedding-perpendicular direction locates within 0.2-0.7,for all temperatures.The H value of the bedding-parallel sample remains stable at-0.925 during heating,but gradually increases from 0.808 at 25 ℃ to 0.879 at 500 ℃ for the bedding-perpendicular sample.Congruent with a mechanistic model,the permeability at 500 ℃ is elevated-1.83 times(bedding-parallel)and-6.08 times(bedding-perpendicular)relative to that at 25 ℃-confirming the effectiveness of thermal treatment in potentially enhancing production from low-maturity oil shales.
查看更多>>摘要：Laboratory modeling of in-situ combustion is crucial for understanding the potential success of field trials in thermal enhanced oil recovery(EOR)and is a vital precursor to scaling the technology for field ap-plications.The high combustion temperatures,reaching up to 480 ℃,induce significant petrophysical alterations of the rock,an often overlooked aspect in thermal EOR projects.Quantifying these changes is essential for potentially repurposing thermally treated,depleted reservoirs for CO2 storage.In this study,we depart from conventional combustion experiments that use crushed core,opting instead to analyze the thermal effects on reservoir properties of carbonate rocks using consolidated samples.This technique maintains the intrinsic porosity and permeability,revealing combustion's impact on porosity and mineralogical alterations,with a comparative analysis of these properties pre-and post-combustion.We characterize porosity and pore geometry evolution using low-field nuclear magnetic resonance,X-ray micro-computed tomography,and low-temperature nitrogen adsorption.Mineral composition of the rock and grain-pore scale alterations are analyzed by scanning electron microscopy and X-ray diffraction.The analysis shows a significant increase in carbonate rocks'porosity,pore size and mineral alter-ations,and a transition from mixed-wet to a strongly water-wet state.Total porosity of rock samples increased in average for 15％-20％,and formation of new pores is registered at the scale of 1-30 μm size.High-temperature exposure results in the calcite and dolomite decomposition,calcite dissolution and formation of new minerals-anhydrite and fluorite.Increased microporosity and the shift to strongly water-wet rock state improve the prospects for capillary and residual CO2 trapping with greater capacity.Consequently,these findings highlight the importance of laboratory in-situ combustion modeling on consolidated rock over tests that use crushed core,and indicate that depleted combustion stimulated reservoirs may prove to be viable candidates for CO2 storage.
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