查看更多>>摘要：The phase behavior of gas condensate in reservoir formations differs from that in pressure-volume-temperature(PVT)cells because it is influenced by porous media in the reservoir formations.Sandstone was used as a sample to investigate the influence of porous media on the phase behavior of the gas condensate.The pore structure was first analyzed using computed tomography(CT)scanning,digital core technology,and a pore network model.The sandstone core sample was then saturated with gas condensate for the pressure depletion experiment.After each pressure-depletion state was stable,real-time CT scanning was performed on the sample.The scanning results of the sample were reconstructed into three-dimensional grayscale images,and the gas condensate and condensate liquid were segmented based on gray value discrepancy to dynamically characterize the phase behavior of the gas condensate in porous media.Pore network models of the condensate liquid ganglia under different pressures were built to calculate the characteristic parameters,including the average radius,coordination number,and tor-tuosity,and to analyze the changing mechanism caused by the phase behavior change of the gas condensate.Four types of condensate liquid(clustered,branched,membranous,and droplet ganglia)were then classified by shape factor and Euler number to investigate their morphological changes dynamically and elaborately.The results show that the dew point pressure of the gas condensate in porous media is 12.7 MPa,which is 0.7 MPa higher than 12.0 MPa in PVT cells.The average radius,volume,and coordination number of the condensate liquid ganglia increased when the system pressure was between the dew point pressure(12.7 MPa)and the pressure for the maximum liquid dropout,Pmax(10.0 MPa),and decreased when it was below Pmax.The volume proportion of clustered ganglia was the highest,followed by branched,membranous,and droplet ganglia.This study provides crucial experi-mental evidence for the phase behavior changing process of gas condensate in porous media during the depletion production of gas condensate reservoirs.
查看更多>>摘要：To investigate the relationship between grain sizes,seepage capacity,and oil-displacement efficiency in the Liushagang Formation of the Beibuwan Basin,this study identifies the multistage pore-throat structure as a crucial factor through a comparison of oil displacement in microscopic pore-throat ex-periments.The two-phase flow evaluation method based on the Li-Horne model is utilized to effectively characterize and quantify the seepage characteristics of different reservoirs,closely relating them to the distribution of microscopic pores and throats.It is observed that conglomerate sandstones at different stages exhibit significant heterogeneity and noticeable differences in seepage capacity,highlighting the crucial role played by certain large pore throats in determining seepage capacity and oil displacement efficiency.Furthermore,it was found that the displacement effects of conglomeratic sandstones with strong heterogeneity were inferior to those of conventional homogeneous sandstone,as evidenced by multiple displacement experiments conducted on core samples with varying granularities and flooding systems.Subsequently,core-based experiments on associated gas flooding after water flooding were conducted to address the challenge of achieving satisfactory results in a single displacement mode for reservoirs with significant heterogeneity.The results indicate that the oil recovery rates for associated gas flooding after water flooding increased by 7.3％-16.4％compared with water flooding alone at a gas-oil ratio of approximately 7000 m3/m3.Therefore,considering the advantages of gas flooding in terms of seepage capacity,oil exchange ratio,and the potential for two-phase production,gas flooding is recommended as an energy supplement mode for homogeneous reservoirs in the presence of sufficient gas source and appropriate tectonic angle.On the other hand,associated gas flooding after water flooding is suggested to achieve a more favorable development effect compared to a single mode of energy supplementation for strongly heterogeneous sandstone reservoirs.
查看更多>>摘要：The analysis of interwell connectivity plays an important role in the formulation of oilfield development plans and the description of residual oil distribution.In fact,sandstone reservoirs in China's onshore oilfields generally have the characteristics of thin and many layers,so multi-layer joint production is usually adopted.It remains a challenge to ensure the accuracy of splitting and dynamic connectivity in each layer of the injection-production wells with limited field data.The three-dimensional well pattern of multi-layer reservoir and the relationship between injection-production wells can be equivalent to a directional heterogeneous graph.In this paper,an improved graph neural network is proposed to construct an interacting process mimics the real interwell flow regularity.In detail,this method is used to split injection and production rates by combining permeability,porosity and effective thickness,and to invert the dynamic connectivity in each layer of the injection-production wells by attention mechanism.Based on the material balance and physical information,the overall connectivity from the injection wells,through the water injection layers to the production layers and the output of final production wells is established.Meanwhile,the change of well pattern caused by perforation,plugging and switching of wells at different times is achieved by updated graph structure in spatial and temporal ways.The effectiveness of the method is verified by a combination of reservoir numerical simulation examples and field example.The method corresponds to the actual situation of the reservoir,has wide adaptability and low cost,has good practical value,and provides a reference for adjusting the injection-production relationship of the reservoir and the development of the remaining oil.
查看更多>>摘要：Evaluating the physical mechanisms that link hydraulic fracturing(HF)operations to induced earth-quakes and the anticipated form of the resulting events is significant in informing subsurface fluid in-jection operations.Current understanding supports the overriding role of the effective stress magnitude in triggering earthquakes,while the impact of change rate of effective stress has not been systematically addressed.In this work,a modified critical stiffness was brought up to investigate the likelihood,impact,and mitigation of induced seismicity during and after hydraulic fracturing by developing a poroelastic model based on rate-and-state fraction law and linear stability analysis.In the new criterion,the change rate of effective stress was considered a key variable to explore the evolution of this criterion and hence the likelihood of instability slip of fault.A coupled fluid flow-deformation model was used to represent the entire hydraulic fracturing process in COMSOL Multiphysics.The possibility of triggering an earth-quake throughout the entire hydraulic fracturing process,from fracturing to cessation,was investigated considering different fault locations,orientations,and positions along the fault.The competition between the effects of the magnitude and change rate of effective stress was notable at each fracturing stage.The effective stress magnitude is a significant controlling factor during fracturing events,with the change rate dominating when fracturing is suddenly started or stopped.Instability dominates when the magnitude of the effective stress increases(constant injection at each fracturing stage)and the change rate of effective stress decreases(the injection process is suddenly stopped).Fracturing with a high in-jection rate,a fault adjacent to the hydraulic fracturing location and the position of the junction between the reservoir and fault are important to reduce the Coulomb failure stress(CFS)and enhance the critical stiffness as the significant disturbance of stresses at these positions in the coupled process.Therefore,notable attention should be given to the injection rate during fracturing,fault position,and position along faults as important considerations to help reduce the potential for induced seismicity.Our model was verified and confirmed using the case of the Longmaxi Formation in the Sichuan Basin,China,in which the reported microseismic data were correlated with high critical stiffness values.This work supplies new thoughts of the seismic risk associated with HF engineering.
查看更多>>摘要：Surfactants are widely used in the fracturing fluid to enhance the imbibition and thus the oil recovery rate.However,current numerical models cannot capture the physics behind capillary imbibition during the wettability alteration by surfactants.Although the interacting capillary bundle(ICB)model shows potential in characterizing imbibition rates in different pores during wettability alteration,the existing ICB models neglect the influence of wettability and viscosity ratio on the imbibition behavior,making it difficult to accurately describe the oil-water imbibition behavior within the porous media.In this work,a new ICB mathematical model is established by introducing pressure balance without assuming the position of the leading front to comprehensively describe the imbibition behavior in a porous medium under different conditions,including gas-liquid spontaneous imbibition and oil-water imbibition.When the pore size distribution of a tight rock is known,this new model can predict the changes of water saturation during the displacement process in the tight rock,and also determine the imbibition rate in pores of different sizes.The water saturation profiles obtained from the new model are validated against the waterflooding simulation results from the CMG,while the imbibition rates calculated by the model are validated against the experimental observations of gas-liquid spontaneous imbibition.The good match above indicates the newly proposed model can show the water saturation profile at a macroscopic scale while capture the underlying physics of the multiphase flow in a porous medium at a microscopic scale.Simulation results obtained from this model indicate that both wettability and viscosity ratio can affect the sequence of fluid imbibition into pores of different sizes during the multiphase flow,where less-viscous wetting fluid is preferentially imbibed into larger pores while more-viscous wetting fluid tends to be imbibed into smaller pores.Furthermore,this model provides an avenue to calculate the imbibition rate in pores of different sizes during wettability alteration and capture the non-Darcy effect in micro-and nano-scale pores.
查看更多>>摘要：Slickwater fracturing fluids are widely used in the development of unconventional oil and gas resources due to the advantages of low cost,low formation damage and high drag reduction performance.How-ever,their performance is severely affected at high temperatures.Drag reducing agent is the key to determine the drag reducing performance of slickwater.In this work,in order to further improve the temperature resistance of slickwater,a temperature-resistant polymeric drag reducing agent(PDRA)was synthesized and used as the basis for preparing the temperature-resistant slickwater.The slickwater system was prepared with the compositions of 0.2 wt％PDRA,0.05 wt％drainage aid nonylphenol pol-yoxyethylene ether phosphate(NPEP)and 0.5 wt％anti-expansion agent polyepichlorohydrin-dimethylamine(PDM).The drag reduction ability,rheology properties,temperature and shear resis-tance ability,and core damage property of slickwater were systematically studied and evaluated.In contrast to on-site drag reducing agent(DRA)and HPAM,the temperature-resistant slickwater dem-onstrates enhanced drag reduction efficacy at 90 ℃,exhibiting superior temperature and shear resis-tance ability.Notably,the drag reduction retention rate for the slickwater achieved an impressive 90.52％after a 30-min shearing period.Additionally,the core damage is only 5.53％.We expect that this study can broaden the application of slickwater in high-temperature reservoirs and provide a theoretical basis for field applications.
查看更多>>摘要：The long-term strength retrogression of silica-enriched oil well cement poses a significant threat to wellbore integrity in deep and ultra-deep wells,which is a major obstacle for deep petroleum and geothermal energy development.Previous attempts to address this problem has been unsatisfactory because they can only reduce the strength decline rate.This study presents a new solution to this problem by incorporating fly ash to the traditional silica-cement systems.The influences of fly ash and silica on the strength retrogression behavior of oil well cement systems directly set and cured under the condition of 200 ℃ and 50 MPa are investigated.Test results indicate that the slurries containing only silica or fly ash experience severe strength retrogression from 2 to 30 d curing,while the slurries con-taining both fly ash and silica experience strength enhancement from 2 to 90 d.The strength test results are corroborated by further evidences from permeability tests as well as microstructure analysis of set cement.Composition of set cement evaluated by quantitative X-ray diffraction analyses with partial or no known crystal structure(PONKCS)method and thermogravimetry analyses revealed that the conversion of amorphous C-(A)-S-H to crystalline phases is the primary cause of long-term strength retrogression.The addition of fly ash can reduce the initial amount of C-(A)-S-H in the set cement,and its combined use with silica can prevent the crystallization of C-(A)-S-H,which is believed to be the working mechanism of this new admixture in improving long-term strength stability of oil well cement systems.
查看更多>>摘要：Real-time intelligent lithology identification while drilling is vital to realizing downhole closed-loop drilling.The complex and changeable geological environment in the drilling makes lithology identifi-cation face many challenges.This paper studies the problems of difficult feature information extraction,low precision of thin-layer identification and limited applicability of the model in intelligent lithologic identification.The author tries to improve the comprehensive performance of the lithology identification model from three aspects:data feature extraction,class balance,and model design.A new real-time intelligent lithology identification model of dynamic felling strategy weighted random forest algo-rithm(DFW-RF)is proposed.According to the feature selection results,gamma ray and 2 MHz phase resistivity are the logging while drilling(LWD)parameters that significantly influence lithology identi-fication.The comprehensive performance of the DFW-RF lithology identification model has been verified in the application of 3 wells in different areas.By comparing the prediction results of five typical li-thology identification algorithms,the DFW-RF model has a higher lithology identification accuracy rate and F1 score.This model improves the identification accuracy of thin-layer lithology and is effective and feasible in different geological environments.The DFW-RF model plays a truly efficient role in the real-time intelligent identification of lithologic information in closed-loop drilling and has greater applica-bility,which is worthy of being widely used in logging interpretation.
查看更多>>摘要：The backreaming operation plays a significant role in safe drilling for horizontal wellbores,while it may cause severe stuck pipe accidents.To lower the risk of the stuck pipe in backreaming operations,the mechanism of cuttings transport needs to be carefully investigated.In this research,a transient cuttings transport with multiple flow patterns model is developed to predict the evolution of cuttings transported in the annulus while backreaming.The established model can provide predictions of the distribution of cuttings bed along the wellbore considering the bulldozer effect caused by large-size drilling tools(LSDTs).The sensitivity analyses of the size of LSDTs,and backreaming operating parameters are con-ducted in Section 4.And a new theory is proposed to explain the mechanism of cuttings transport in the backreaming operation,in which both the bit and LSDTs have the"cleaning effect"and"plugging effect".The results demonstrate that the cuttings bed in annuli is in a state of dynamic equilibrium,but the overall trend and the distribution pattern are obvious.First,larger diameters and longer drilling tools could lead to a higher risk of the stuck pipe.Second,we find that it is not the case that the higher flow rate is always better for hole cleaning,so three flow-rate intervals are discussed separately under the given conditions.When the"dangerous flow rate"(＜33 L/s in Case 4)is employed,the cuttings bed completely blocks the borehole near the step surface and causes a stuck pipe directly.If the flow rate increases to the"low flow rate"interval(33-35 L/s in Case 4),a smaller flow rate instead facilitates borehole cleaning.If the flow rate is large enough to be in the"high flow rate"interval(＞35 L/s in Case 4),the higher the flow rate,the better the cleaning effect of cuttings beds.Third,an interval of tripping velocity called"dangerous velocity"is proposed,in which the cuttings bed accumulation near the LSDTs is more serious than those of other tripping velocities.As long as the applied tripping velocity is not within the"dangerous velocity"(0.4-0.5 m/s in Case 5)interval in the backreaming operation,the risk of the stuck pipe can be controlled validly.Finally,through the factors analyses of the annular geometry,particle properties,and fluid properties in Section 5,it can be found that the"low flow rate","high flow rate"and"dangers flow rate"tend to decrease and the"dangerous velocity"tends to increase with the conditions more favorable for hole cleaning.This study has some guiding significance for risk prediction and parameter setting of the backreaming operation.
查看更多>>摘要：A method for in-situ stress measurement via fiber optics was proposed.The method utilizes the rela-tionship between rock mass elastic parameters and in-situ stress.The approach offers the advantage of long-term stress measurements with high spatial resolution and frequency,significantly enhancing the ability to measure in-situ stress.The sensing casing,spirally wrapped with fiber optic,is cemented into the formation to establish a formation sensing nerve.Injecting fluid into the casing generates strain disturbance,establishing the relationship between rock mass properties and treatment pressure.Moreover,an optimization algorithm is established to invert the elastic parameters of formation via fiber optic strains.In the first part of this paper series,we established the theoretical basis for the inverse differential strain analysis method for in-situ stress measurement,which was subsequently verified using an analytical model.This paper is the fundamental basis for the inverse differential strain analysis method.
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