Interpretation and characterization of rate of penetration intelligent prediction model

Accurate prediction of the rate of penetration(ROP)is significant for drilling optimization.While the intelligent ROP prediction model based on fully connected neural networks(FNN)outperforms traditional ROP equations and machine learning algorithms,its lack of interpretability undermines its credibility.This study proposes a novel interpretation and characterization method for the FNN ROP prediction model using the Rectified Linear Unit(ReLU)activation function.By leveraging the derivative of the ReLU function,the FNN function calculation process is transformed into vector operations.The FNN model is linearly characterized through further simplification,enabling its interpretation and analysis.The proposed method is applied in ROP prediction scenarios using drilling data from three vertical wells in the Tarim Oilfield.The results demonstrate that the FNN ROP prediction model with ReLU as the activation function performs exceptionally well.The relative activation frequency curve of hidden layer neurons aids in analyzing the overfitting of the FNN ROP model and determining drilling data similarity.In the well sections with similar drilling data,averaging the weight parameters enables linear characterization of the FNN ROP prediction model,leading to the establishment of a corresponding linear representation equation.Furthermore,the quantitative analysis of each feature's influence on ROP facilitates the proposal of drilling parameter optimization schemes for the current well section.The established linear characterization equation exhibits high precision,strong stability,and adaptability through the application and validation across multiple well sections.

Fracture geometry and breakdown pressure of radial borehole fracturing in multiple layers

Radial borehole fracturing that combines radial boreholes with hydraulic fracturing is anticipated to improve the output of tight oil and gas reservoirs.This paper aims to investigate fracture propagation and pressure characteristics of radial borehole fracturing in multiple layers.A series of laboratory experiments with artificial rock samples(395 mm×395 mm×395 mm)was conducted using a true triaxial fracturing device.Three crucial factors corresponding to the vertical distance of adjacent radial borehole layers(vertical distance),the azimuth and diameter of the radial borehole are examined.Experimental results show that radial borehole fracturing in multiple layers generates diverse fracture geometries.Four types of fractures are identified based on the connectivity between hydraulic fractures and radial boreholes.The vertical distance significantly influences fracture propagation perpendicular to the radial borehole axis.An increase in the vertical distance impedes fracture connection across multiple radial borehole layers and reduces the fracture propagation distance along the radial borehole axis.The azimuth also influences fracture propagation along the radial borehole axis.Increasing the azimuth reduces the guiding ability of radial boreholes,which makes the fracture quickly curve to the maximum horizontal stress direction.The breakdown pressure correlates with diverse fracture geometries observed.When the fractures connect multi-layer radial boreholes,increasing the vertical distance decreases the breakdown pressure.Decreasing the azimuth and increasing the diameter also decrease the breakdown pressure.The extrusion force exists between the adjacent fractures generated in radial boreholes in multiple rows,which plays a crucial role in enhancing the guiding ability of radial boreholes and results in higher breakdown pressure.The research provides valuable theoretical insights for the field application of radial borehole fracturing technology in tight oil and gas reservoirs.

Structure optimization of the organ-pipe cavitating nozzle and its erosion ability test on hydrate-bearing sediments

Cavitating jet is a promising drilling rate improvement technology in both the marine natural gas hydrate (NGH) fluidization exploitation method and the integrated radial jet drilling and completion method. In present study, we aim to improve the efficiency of jet erosion and extracting NGH. With a computational fluid dynamics (CFD) method, the pressure, velocity and cavitation field characteristics of organ-pipe cavitating jet (OPCJ) are analysed. The divergent angle, throat length, and divergent length of OPCJ nozzle are preferred to obtain stronger jet cavitation erosion effect. Laboratory experiments of gas hydrate-bearing sediments (GHBS) erosion by OPCJ and conical jet (CJ) are conducted to compare and validate the jet erosion performance. The impinging models of OPCJ and CJ are constructed to study the impact characteristics. Results show that the preferred values of divergent angle, throat length, and divergent length are 15°, 1d, and 3d, respectively, in present simulation conditions. For GHBS, the OPCJ possesses the advantages of high efficiency and low energy consumption. Moreover, the OPCJ has higher penetration efficiency, while showing equivalent penetration ability compared to CJ. During the impinging process, the OPCJ can induce stronger impact pressure and turbulence effect, and also shows stronger chambering effect and bottom cleaning ability compared to CJ. This study presents the erosion performance of OPCJ and CJ on GHBS, and provides preliminary insights on the potential field applications in NGH exploitation.

Pore structure characterization and its effect on methane adsorption in shale kerogen

Pore structure characterization and its effect on methane adsorption on shale kerogen are crucial to understanding the fundamental mechanisms of gas storage,transport,and reserves evaluation.In this study,we use 3D scanning confocal microscopy,scanning electron microscopy(SEM),X-ray nano-computed tomography(nano-CT),and low-pressure N2 adsorption analysis to analyze the pore structures of the shale.Additionally,the adsorption behavior of methane on shales with different pore structures is investigated by molecular simulations.The results show that the SEM image of the shale sample obviously displays four different pore shapes,including slit pore,square pore,triangle pore,and circle pore.The average coordination number is 4.21 and the distribution of coordination numbers demonstrates that pores in the shale have high connectivity.Compared with the adsorption capacity of methane on triangle pores,the adsorption capacity on slit pore,square pore,and circle pore are reduced by 9.86%,8.55%,and 6.12%,respectively.With increasing pressure,these acute wedges fill in a manner different from the right or obtuse angles in the other pores.This study offers a quantitative understanding of the effect of pore structure on methane adsorption in the shale and provides better insight into the evaluation of gas storage in geologic shale reservoirs.

CFD analysis and field observation of tool erosion caused by abrasive waterjet fracturing

Abrasive waterjet(AWJ)fracturing has become an accepted horizontal multistage stimulation technique due to its flexibility and high efficiency of extensive fracture placement.The downhole tool failure of AWJ fracturing becomes an issue in the massive hydraulic fracturing because of high velocity and proppant erosion.This paper proposed a 3D computational fluid dynamics(CFD)-based erosion model by considering high-velocity waterjet impact,proppant shear erosion,and specific inner structure of hydra-jet tool body.The discrete phase approach was used to track the proppant transport and its concentration distribution.Field observation provides strong evidence of erosion patterns and mechanisms obtained from CFD simulation.The results show that the erosion rate has a space dependence in the inner wall of the tool body.The severe erosion areas are primarily located at the entries of the nozzle.Evident erosion patterns are found including a‘Rabbit’s ear’erosion at the upper-layer nozzles and a half bottom loop erosion at the lower-layer nozzles.Erosion mechanisms attribute to high flow velocity at the entry of nozzles and the inertia force of proppant.Sensitivity analysis demonstrates that the pumping rate is a primary factor contributing to erosion intensity.

Effect of dilute acid treatment on adhesion properties of Longmaxi black shale

Properties of shale in an acid environment are important when acid or CO2 is injected into geologic formations as a working fluid for enhanced oil and gas recovery,hydraulic fracturing and reduced fracture initiation pressure.It has previously been shown that acid fluids can enhance the formation conductivity and decrease the hardness of shale.However,less is known about the effect of dilute acid on the adhesion properties of shale.In the study,shale samples are characterized in detail with advanced analysis.Adhesion properties of shale via dilute acid treatment were revealed by atomic force microscopy(AFM)for the first time.Results indicate that acid treatment can greatly enhance adhesion forces of the shale surface.After acid treatment,the average adhesion forces show a platform-like growth with an increase in loading force.Through analysis of results from AFM,scanning electron microscopy,and X-ray diffraction,we affirm that the enhanced adhesion forces are mainly from increased specific surface area and reduced elastic modulus.The results presented in this work help understand the adhesion properties of shale oil/gas present in an acidic environment,which have great significance in unconventional resources development.

Influence of formation in-situ stress on mechanical heterogeneity of shale through grid nanoindentation

Mechanical heterogeneity is a major characteristic of the organic-rich shale.The relation between mechanical heterogeneity and formation in-situ stress has been seldomly addressed but important to understand hydraulic fracture propagation,wellbore stability,and hydrocarbon flow.In this paper,the grid nanoindentation technique was used to characterize the heterogeneity of the mechanical properties of Longmaxi organic-rich shales from various burial depths and in-situ stress.The measured elastic modulus and hardness of each sample are deconvolved into three phases including soft phase,medium stiff phase and stiff phase according to mineral category.As the burial depth and corresponding in-situ stress increase,the overall elastic modulus and hardness of the sample enhance.Simultaneously,the percentage of soft minerals decreases,and the probability distribution tends to concentrate through 95%confidence interval evaluation which demonstrates weakened heterogeneity.Furthermore,SEM images provide evidence that extended cracking,initiated cracking,crushing and ductile deforming always occur around indentation imprints.This confirms that even under deep buried depth and high in-situ stress,brittle fracture and ductile deformation can exist synchronously.This paper demonstrates the influence of in-situ stress on the heterogeneity of shale micromechanics.

Hydrophobic epoxy resin coated proppants with ultra-high self-suspension ability and enhanced liquid conductivity

Proppant is a key material for enhancing unconventional oil and gas production which requires a long distance of migration and efficient liquid conductivity paths within the hydraulic fracture.However,it is difficult to find a proppant with both high self-suspension ability and liquid conductivity.Here,a simple method is developed to coat epoxy resin onto the ceramic proppant and fabricate a novel coated proppant with high hydrophobicity,self-suspension,and liquid conductivity performance.Compared with uncoated ceramic proppants,the epoxy resin coated(ERC) proppant has a high self-suspension ability nearly 16 times that of the uncoated proppants.Besides,the hydrophobic property and the liquid conductivity of the ERC proppant increased by 83.8% and 16.71%,respectively,compared with the uncoated proppants.In summary,this novel ERC proppant provides new insights into the design of functional proppants,which are expected to be applied to oil and gas production.

Determination of the shedding frequency of cavitation cloud in a submerged cavitation jet based on high-speed photography images

To accurately determine the shedding frequency of the cavitation cloud in a submerged cavitation jet,the spectral analysis and the proper orthogonal decomposition(POD)for high-speed photography images are performed.The spectrums of 6 different kinds of image signals(the area-averaged gray level,the line-averaged gray level,the point gray level,the cavitation length,width,and area)are calculated and compared.The line-averaged gray level is found to be optimal in determining the shedding frequency but an accurate frequency can only be obtained in the stable-frequency zone where the cavitation cloud sheds.In repeated experiments,the plateau-shape distribution of the main frequency is established with a deviation of 10.8%.A revised Reynolds number Re'is defined and the shedding frequency can be correlated to Re'by a power law when the cavitation number is less than 0.02.This relationship is validated by the experimental data in literature.The first mode of the POD characterizes the ensemble-average of the cavitation cloud while the second mode is the major part of the cavitation cloud transient components.The modes 2-5 are organized in pairs,which confirms the periodic feature of the cavitation cloud in the submerged cavitation jet.Near the nozzle exit,the modes 2-5 are symmetrically distributed in the jet shear layer.The shedding frequency of the cloud cavitation can also be precisely determined by performing the spectral analysis of the weighting coefficients of the mode 2.This paper shows that the two parameters,namely,the line-averaged gray level and the weighting coefficients of the mode 2,can be confidently used to calculate the shedding frequency of the cavitation cloud in a submerged cavitation jet.