Optimization of integrated geological-engineering design of volume fracturing with fan-shaped well pattern

According to the variable toe-to-heel well spacing, combined with the dislocation theory, discrete lattice method, and finite-element-method(FEM) based fluid-solid coupling, an integrated geological-engineering method of volume fracturing for fan-shaped well pattern is proposed considering the geomechanical modeling, induced stress calculation, hydraulic fracturing simulation, and post-frac productivity evaluation. Besides, we propose the differential fracturing design for the conventional productivity-area and the potential production area for fan-shaped horizontal wells. After the fracturing of the conventional production area for H1 fan-shaped well platform, the research shows that the maximum reduction of the horizontal principal stress difference in the potential productivity-area is 0.2 MPa, which cannot cause the stress reversal, but this reduction is still conducive to the lateral propagation of hydraulic fractures. According to the optimized fracturing design, in zone-Ⅰ of the potential production area, only Well 2 is fractured, with a cluster spacing of 30 m and an injection rate of 12 m^(3)/min per stage;in zone-Ⅱ, Well 2 is fractured before Well 3, with a cluster spacing of 30 m and an injection rate of 12 m^(3)/min per stage. The swept area of the pore pressure drop in the potential production area is small, showing that the reservoir is not well developed. The hydraulic fracturing in the toe area can be improved by, for example, properly densifying the fractures and adjusting the fracture distribution, in order to enhance the swept volume and increase the reservoir utilization.

Experimental investigation on frictional properties of stressed basalt fractures

The frictional strength and sliding stability of faults are crucial in interpreting earthquake mechanisms and cycles.Herein,we report friction experiments on basalt fractures,using a self-designed triaxial apparatus that allows direct shear of samples under coupled hydro-mechanical conditions.Velocitystepping(VS)and slide-hold-slide(SHS)experiments are performed on both bare and gouge-bearing surfaces of Xiashan basalt subjected to cyclic shear velocities at 1e30 mm/s,effective normal stresses of 1e5 MPa,and pore pressures of 70e300 kPa.The measured basalt friction coefficients are in the range of 0.67e0.74,which is sensitive to gouge thickness,normal stress,and water.Specifically,a reduction in friction coefficient is observed with an increment in gouge thickness,normal stress,and pore pressure.Based on the microscopic observation of the pre-and post-shearing sliding surfaces,this weakening effect in friction coefficient can be attributed to powder lubrication.Furthermore,the VS test results reveal predominantly velocity-strengthening behavior at investigated slip velocities,and this velocity strengthening behavior does not appear to be influenced by variations in normal stress,gouge thickness,and water.However,changes in sliding velocity and normal stress can lead to a shift between stable and unstable sliding.Specifically,stable sliding is favored by high sliding velocities and low normal stress applied in this study.Finally,we analyze the experimental data by calculating the rate-and-state parameters using the rate-and state-dependent friction(RSF)theory.Importantly,the calculated friction rate parameter(a-b)supports the velocity-strengthening behavior.Both frictional relaxation(Dmc)during hold periods and frictional healing(Dm)upon re-shearing are linearly proportional to the logarithmic hold time,which may be attributed to the growth in true contact area with hold time.This study sheds light on the roles of sliding velocity,and gouge thickness in controlling frictional strength and stability of basalt fractures.

Numerical study of hydraulic fracturing in the sectorial well-factory considering well interference and stress shadowing

In the Changqing Oilfield in northwest China, when traditional petroleum exploitation encounters forestry reserves or water source protection areas, sectorial well-factory design is proposed. The most distinct feature of a sectorial well-factory is the deviation of the well from the minimum horizontal principal stress, resulting in hydraulic fracture deflection after the initiation, along with possible well interference (i.e., fracture hit) and fracture coalescence in the oblique wells. Four indexes describing well deflection are then proposed according to fracture morphology. Several fracturing designs, including stage arrangement, fracturing sequences, and fracturing techniques are applied to study the feasibility of the sectorial well-factory design. The results show that the “gradual” or “sparse” stage arrangement, large injection rate, and simultaneous multifracture treatment can help to optimize the fracture morphology and stimulation design. However, the subsequent stress shadowing effect usually adversely affects the fracturing of adjacent wells. With a small initial horizontal stress difference, large injection rate and staggered stage arrangement can achieve ideal stimulation performance. Our results can provide a guidance for optimizing stimulation design in unconventional well-factory while taking into account environmental protection.

Numerical investigation on the effect of depletion-induced stress reorientation on infill well hydraulic fracture propagation

Depletion-induced stress change causes the redistribution of stress field in reservoirs,which can lead to the reorientation of principal stresses.Stress reorientation has a direct impact on fracture propagation of infill wells.To understand the effect of stress reorientation on the propagation of infill well’s fractures,an integrated simulation workflow that combines the reservoir flow calculation and the infill well hydraulic fracturing modeling is adopted.The reservoir simulation is computed to examine the relationship between the extent of stress reversal region and reservoir properties.Then,the hydraulic fracturing model considering the altered stress field for production is built to characterize the stress evolution of secondary fracturing.Numerical simulations show that stress reorientation may occur due to the decreasing of the horizontal stresses in an elliptical region around the parent well.Also,the initial stress difference is the driving factor for stress reorientation.However,the bottom hole pressure,permeability and other properties connected with fluid flow control timing of the stress reorientation.The decrease of the horizontal stresses around the parent well lead to asymmetrical propagation of a hydraulic fracture of the infill well.The study provides insights on understanding the influence of stress reorientation to the infill well fracturing treatment and interference between parent and infill wells.

Numerical investigation of refracturing with/without temporarily plugging diverters in tight reservoirs

Refracturing is an importa nt technique to tap the potential of reservoirs and boost production in depleted oil and gas fields.However,fracture propagation during refracturing,including both conventional refracturing and temporary-plugging refracturing remains poorly understood,especially for cases with non-uniform distribution of formation pressure due to long-term oil production and water injection.Therefore,taking pilot tests of refracturing with sidetracking horizontal wells in tight reservoirs in the Changqing Oilfield,China as an example,we establish a three-dimensional numerical model of conventional refracturing and a numerical model of temporary-plugging refracturing based on the discrete lattice method.Non-uniform distributions of formation pressure are imported in these models.We discuss the effects of key operating parameters such as injection rate,cluster spacing,and number of clusters on the propagation of multi-cluster fractures for conventional refracturing.For temporaryplugging refracturing,we examine the impacts of controlling factors such as the timing and number of temporary plugging on fracture propagation.In addition,we analyze a field case of temporaryplugging refracturing using well P3 in the Changqing Oilfield.The results show that fractures during re fracturing tend to propagate preferentially and dominantly in the depleted areas.Improved stimulation effect can be obtained with an optimal injection rate and a critical cluster spacing.The proposed model of temporary-plugging refracturing can well describe the temporary plugging of dominant existingfractures and the creation of new-fractures after fracturing fluid is forced to divert into other clusters from previous dominant clusters.Multiple temporary plugging can improve the balanced propagation of multi-cluster fractures and obtain the maximum fracture area.The established numerical model and research results provide theoretical guidance for the design and optimization of key operating parameters for refracturing,especially for temporary-plugging refracturing.

Effect of mineralogy on friction-dilation relationships for simulated faults:Implications for permeability evolution in caprock faults

This paper experimentally explores the frictional sliding behavior of two simulated gouges:one,a series of quartz–smectite mixtures,and the other,powdered natural rocks,aiming to evaluate and codify the effect of mineralogy on gouge dilation and frictional strength,stability,and healing.Specifically,velocity-stepping and slide-hold-slide experiments were performed in a double direct shear configuration to analyze frictional constitutive parameters at room temperature,under normal stresses of 10,20,and 40 MPa.Gouge dilation was measured based on the applied step-wise changes in shear velocity.The frictional response of the quartz–smectite mixtures and powdered natural rocks are affected by their phyllosilicate content.Frictional strength and healing rates decrease with increasing phyllosilicate content,and at 20 wt.%a transition from velocity-weakening to velocity-strengthening behavior was noted.For both suites of gouges,dilation is positively correlated with frictional strength and healing rates,and negatively correlated with frictional stability.Changes in the permeability of gouge-filled faults were estimated from changes in mean porosity,indexed through measured magnitudes of gouge dilation.This combined analysis implies that the reactivation of caprock faults filled with phyllosilicaterich gouges may have a strong influence on permeability evolution in caprock faults.

Evaluation of fault stability and seismic potential for Hutubi underground gas storage due to seasonal injection and extraction

The Hutubi gas field was put into production in 1998 and then converted into an underground gas storage(UGS)facility in 2013,and since then a cluster of earthquakes associated with seasonal injection and extraction activities have been recorded nearby.To evaluate the fault stability and seismic potential,we established a pseudo-3D geomechanical model to simulate the process of seasonal injection and extraction.Reservoir pore pressures from 1998 to 2019 were obtained through multiphase reservoir simulation and validated by history matching the field injection and production data.We then imported pore pressures into the geomechanical model to simulate the poroelastic perturbation on faults for over 20 years.The fidelity of this model was validated by comparing the simulated surface deformation with global positioning system(GPS)measured data.We used Coulomb failure stress(CFS)as the indicator for the likelihood of fault slippage.The simulation results show that the location of the induced earthquake cluster was within the positive Coulomb stress perturbation(DCFS)area,in which fault slippage was promoted.In addition,DCFS at the earthquake location kept increasing after the injection began.These findings could explain the induced earthquakes with the Coulomb failure stress theory.Furthermore,we conducted a parameter sensitivity study on the dominant factors such as the maximum operating pressure(MOP),frictional coefficient,and dip angle of the pre-existing fault.The results indicate that the magnitude of DCFS caused by seasonal injection and extraction decreases with distance;MOPs are constrained to 32.9,36.2,and 39.5 MPa according to different DCFS thresholds;the critical dip angle ranges are 0-20°and 80°-100°;and strengthening the fault friction can either increase or decrease the seismic potential.This study can help determine the MOP for Hutubi underground gas storage(HTB UGS)and provide a framework for simulating the potential causes of induced seismicity for other sites.

Experimental and numerical study of seepage-induced suffusion under K_(0) stress state

Suffusion in gap-graded soil involves selective erosion of fine particles through the pores formed by coarse particles under seepage forces.As the fines content(FC)decreases,the hydraulic and mechanical behavior of the soil will change,posing a huge threat to engineering safety.In this study,we first conduct a series of experimental tests of suffusion by using gap-graded soils and then analyze the evolution process of suffusion and the effect of the hydraulic gradient.Subsequently,according to the physical model,a discrete element method(DEM)numerical model with dynamic fluid mesh(DFM)is developed to extend the experimental study to the pore scale.Our results reveal the migration process of fines and the formation of erosion zones.A parametric study is then conducted to investigate the effect of the hydraulic gradient,FC,and K_(0) pressure(which limits the lateral displacement of the sample and applies vertical pressure)on eroded weight.The results show that the eroded weight increases with the increase of the hydraulic gradient and FC but decreases with the increase of K_(0) pressure.