Fracture Spacing Research Articles

Thermal destressing: Implications for short-circuiting in enhanced geothermal systems

Viable Enhanced Geothermal Systems (EGS) require (1) access to large reservoir volumes at high temperatures and (2) sufficient permeability for large production flow rates. However, working fluid injection might quickly decrease the recoverable heat energy. Thermal destressing increases fracture transmissivity and reduces the production temperature. Large fracture openings cause the injection fluid to localize in a single fracture, or a few dominant fractures, and leads to thermal short-circuiting within the EGS hydraulic network. This work provides novel numerical simulations of a multi-fracture EGS that models the fracture aperture from initially closed to mechanically open. Fractures are modeled as discontinuities within a three dimensional thermo-poroelastic rock mass. We derived weak-form equations of mass balance, energy balance, and mechanical contact for fractures and wells and coupled those equations to the Comsol Multiphysics base package. The simulations investigate the full reservoir behavior through coupling of fluid flow through wellbores and fractures with the surrounding rock. Results show that large in-situ stresses contribute to a uniform fluid flow distribution in the fracture network because high compressive contact stresses decrease fracture compressibility and prevent large fracture openings. Initial or late large fracture openings cause thermal short-circuiting and are more likely to arise in locations with low initial stress and compliant fractures. Geometrical and operational properties of the EGS hydraulic network can minimize the severity of thermal short-circuiting including wellbore diameter, fracture spacing, injector/producer lateral spacing, EGS doublet orientation, and number of transmissive fractures. Thermo-mechanical interference between fracture stages causes (1) larger fracture opening displacements and (2) thermal short-circuiting earlier in time. Distinct initial fracture geometries and compliance causes the EGS hydraulic network to flow a non-uniform distribution of the injection fluid from the beginning and tends to decrease heat recovery.

The geometrical properties of fracture corridors

Fracture corridors are densely fractured zones that may form preferential drains for fluids in the surface and subsurface. Their accurate characterization and modeling are of utmost importance to predictive hydrodynamic models. This paper details their key geometrical attributes and shows that they can be predicted, based on a comprehensive study of surface and subsurface data.The spacing, length and width of fracture corridors all exhibit multifractal characteristics, with a very good consistency between the different data sources. Their width appears to be limited to a maximum of ∼250–300 m, while their fracture density is correlated to their width by a negative power law relationship, but with a significant variability at all scales. The constitutive fractures in the corridors also show multifractal properties. The fracture spacings typically fall in the cm-m range regardless of the corridor size, and moderately increase with increasing corridor size. The fracture heights increase more significantly with increasing corridor size, but with a constant distribution when normalized to the corridor height. All these elements are indicative of typically scale-independent fracture corridor development mechanisms, consistent with a recent model.The data acquired form a key resource from which a new method and tools are proposed for the modeling of fracture corridors in surface and subsurface contexts, with applications to aquifers, hydrocarbon reservoirs or underground CO2 storage sites.

Effects of Forward and Reverse Shear Displacements on Geometric and Hydraulic Characteristics of Single Rough Fracture by the Finite Volume Method

Abstract Shear displacement will lead to the change of rock fracture space and then affect seepage characteristics of the fracture, but for the same rock fracture, whether the spatial geometry and seepage characteristics of the fracture can be consistent under the forward and reverse shear displacements is a new question. In this paper, the 2D rough fracture profile was used to establish models of different shear displacements in the forward and reverse directions without contact zone, and the geometric distribution characteristics of the fracture space with shear displacements were analyzed. The FVM (finite volume method) was adopted to calculate and simulate the hydraulic characteristics of the relative seepage direction (forward and reverse flow) under different pressure gradients at various shear displacement models. The results showed that under the same shear displacement, the spatial geometry characteristics of forward and reverse shear displacements are consistent after the initial angle of the fracture profile is eliminated. The slope of equivalent hydraulic aperture decreases with the shear displacement, and the amplitude of the non-Darcy coefficient difference increases with the shear displacement, which are inconsistent in the forward and reverse directions, which are negatively correlated with the directional roughness of the initial fracture profile. It shows that the directional roughness inconsistency between the forward and reverse directions of fracture profile is the primary factor leading to the difference of seepage characteristic parameters under the forward and reverse shear displacements.

Numerical and experimental investigation of production performance of in-situ conversion of shale oil by air injection

In-situ conversion of oil shale process has been appealing from economic and energy security perspectives. Conventional in-situ upgrading techniques use electric heaters to heat oil shale, not shale oil. However, the efficiency of electrical heating method is very slow which requires preheating more than a year. Most conventional heating technologies focused on using different heat injection methods converting the shale oil reservoirs. As shale is a poor heat conductor, many of attempts are still in the laboratory stage. The shale oil matrix is very tight and the pore scale ranges from micro to nano-meter. In this paper, we propose to inject air into hydraulically fractured horizontal wells to create in-situ combustion of shale oil in ultra-low permeability formations. Heat is introduced into the formation through multistage fractured horizontal wells, which enhances the contact area of exposed kerogen. This process uses combustion reactants as fuel feed stock to generate perpetual heating. Firstly, we used the laboratory tube combustion of shale oil results to validate the simulation model. Then, we upscale the simulation model to investigate the well productivity performance by in-situ upgrading of shale oil by air injection.The main objective of this work is to determine the technical feasibility of recovering shale oil resources by air injection. It involves the application of hydraulic fracturing technology to enhance the kerogen exposure to oxygen. Heat flows from the fracture into shale oil formation, gradually converting the solid kerogen into mobile oil and gas which can be produced via fractures to the production wells. Our simulation results in this paper have shown the potential of in-situ combustion process to improved oil recovery in shale oil reservoirs. Simulation result indicates that the key parameters controlling the well productivity include air injection rate, injected gas composition and fracture spacing. The available literature provides limited studies on the applicability of in-situ combustion of shale oil in very low permeability formations. The paper provides a comprehensive model that allows kerogen to undergo thermal cracking and oxidation process. The results of this study indicate that the developed kinetic model is feasible to model the kerogen in-situ conversion process induced by air injection.

Bed thickness-dependent fracturing and inter-bed coupling define the nonlinear fracture spacing-bed thickness relationship in layered rocks: Numerical modeling

Bed-normal fractures or joints are common in sedimentary basins (reservoirs). Fracture spacing S is an important parameter affecting both the mechanical stability and permeability of rock masses. Many studies have focused on predicting this parameter from the bed thickness T. Field measurements do show that S and T are related but frequently in a very nonlinear way so that at T > T0, S stabilizes at a certain value S0. Both T0 and S0 strongly vary in different settings. To understand this S(T) relationship, we conducted 2-D finite-difference modeling of a three-stratigraphic-layer system, where interfaces between competent and incompetent layers have different shear strength values τ. The models reproduce not only a general trend of the S(T) dependence in nature but also the observed variability of T0 and S0. These parameters are controlled in the models mostly by τ. T0 and S0 rapidly decrease with the τ increase, but the ratio T0/S0 ≈ 1.5 remains constant. At small T, the S(T) curves are linear, with slopes κ, whose reciprocal (1/κ) is linearly related to τ and increases with τ increase. The strong nonlinearity of S(T) relationship is explained by the T-dependence of the fracturing process obtained in the models.

A Comprehensive Model to Evaluate Hydraulic Fracture Spacing Coupling with Fluid Transport and Stress Shadow in Tight Oil Reservoirs

A Comprehensive Model to Evaluate Hydraulic Fracture Spacing Coupling with Fluid Transport and Stress Shadow in Tight Oil Reservoirs

Global sensitivity analysis of CO2 storage in fractured aquifers via computer experiments

• Accelerated CO 2 storage sensitivity analysis via Computer Experiments as surrogate modes. • Rigorous dynamic models use to model non-reactive CO 2 storage mechanisms, inspired by the Teapot Dome in Wyoming dataset. • Demonstration that a network of connected fractures in an eolian sandstone shifts the importance of CO 2 storage controlling parameters. In this work, we use the “Design and Analysis of Computer Experiments” (DACE) technique to analyze the sensitivity of CO 2 storage in fractured aquifers. The aquifer static or geogical model was inspired by the Tensleep formation at Teapot Dome, a potential CO 2 storage target in Wyoming. The CO 2 storage performance in aquifers is comprehensively simulated in our dynamic model by considering all the trapping mechanisms, except for mineralization, i.e. structural, dissolution and residual, as well as local capillary trapping. Operational strategies are investigated to achieve an effective trade-off between higher CO 2 trapping efficiency and delayed CO 2 breakthrough at producers. The effect of natural fractures is explored by running single-porosity, dual-permeability and dual-porosity models. Results show that different models produce significantly different trapping efficiencies, which indicates that natural fractures and their type impact the CO 2 storage practice appreciably. To carry out the global sensitivity analysis, two 100-point computer experiments were designed using the modified space-filling method considering a bottom aquifer, matrix permeability and porosity, fracture permeability and spacing, and water Total Dissolved Solids (TDS). Results show that an active bottom aquifer reduces trapping efficiency noticeably. In addition, dissolution trapping becomes the dominant trapping mechanism. The variability of CO 2 trapping efficiency is explained primarily by fracture permeability, then depth, and finally matrix permeability. In contrast, in the absence of aquifer drive, the variability is almost completely explained by matrix permeability. Furthermore, our results provide guidance to the collection of information as to reduce the most dominant uncertainties.

Estimating the Geological Structure and Transport Properties of Denied and Limited Access Sites

AbstractOnsite measurements are the standard for establishing geological conditions and related transport properties. However, making direct measurements is not always possible, either because a location is too remote, too vast, or access is outright denied. The latter is typically the case at nuclear test sites, or military installations. Here we describe a systematic method to characterize geology and estimate fracture width, spacing, tortuosity, permeability, and porosity at locations where direct measurements cannot be made. Fractures are treated as fractals with their respective fractal dimensions determined using surface images. Tortuosity in the geological matrix (i.e., bulk media) is determined using rock class with the latter also being used to determine distributions for permeability and porosity. The approach is tested using areal images and rock class for the Nevada Test Site and validated by comparison to field and core measurements. Past work has shown that testing a nuclear weapon underground can affect not only the migration time for gases to the surface but also their isotopic ratios. Accurately modeling gas transport and isotopic fractionation requires knowledge of the geology. However, suspected tests are typically conducted at sites to which the international community has no access.

Research on the propagation mechanism of hydraulic fractures in infill horizontal wells

In recent years, infill horizontal well technology has been used to develop oil and gas in the remaining oil areas of unconventional low-permeability reservoirs. However, the initial fractures in parent wells will affect the hydraulic fractures formed by fracturing infilling horizontal wells. The interaction mechanisms between initial fractures and artificial fractures in infill horizontal wells are still unclear. Combined with the boundary element method and the maximum circumferential tensile stress criterion, a numerical model of hydraulic fracturing that can simulate the evolution of fracture trajectory and stress field was established. The analytical solution of the hydraulic fracture-induced stress field was used to verify the accuracy of the model. Using this model, propagation of hydraulic fractures in infill horizontal wells under different conditions was analyzed. Simulation results show that both the fracture spacing and well spacing have a significant impact on the propagation trajectory of hydraulic fractures in infill horizontal wells. The shorter the fracture spacing and well spacing is, the stronger the inter-fracture stress interference between the initial fractures and hydraulic fractures is. Reasonable fracture spacing and well spacing can enhance the induced stress field and form a complex fracture network in the reservoir. Too small well spacing may cause artificial fractures to communicate with initial fractures, thereby reducing hydraulic fracturing efficiency and limiting the stimulation volume of the reservoir.

Rate-transient analysis for estimating the linear flow parameters of communicating wells using the dynamic drainage area (DDA) concept

Development of unconventional reservoirs using multi-fractured horizontal wells (MFHWs) has led to closer well and hydraulic fracture spacing. Tighter placement of MFHWs has consequently led to increased instances of well interference, particularly through hydraulic fractures. In addition to possible problems posed for development, fracture communication that persists during production complicates rate-transient analysis (RTA) because models/methods for RTA have mostly been developed for single-well analysis. Availability of RTA methods specifically developed for communicating wells can reduce errors associated with hydraulic fracture and reservoir characterization and can lead to improved decision making.A semi-analytical method for history matching MFHWs communicating through hydraulic fractures has recently been developed using the dynamic drainage area (DDA) concept. In the current work, the DDA concept is employed to develop a new straight-line analysis (SLA) method for the transient linear flow regime that can be applied to two communicating MFHWs. The change in the drainage area of a single well due to communication challenges the use of the DDA concept. However, when the second well comes on production, reinitialization of the calculations for the first well helps to facilitate the implementation of the DDA concept.The new DDA-corrected RTA (SLA) method for communicating wells is verified against the results of numerical simulation. Several synthetic cases are generated using numerical simulation to verify the accuracy of the developed method using various reservoir/fracture properties. The new method applied to the simulated cases allows the fracture half-length (xf) to be estimated within 10% of simulation model input. The DDA-corrected RTA method has also been applied to a field dataset, previously analyzed by the authors using the DDA approach to history match well production data. This dataset consists of six wells, drilled into two separate reservoir layers (three wells per layer) and from two adjacent pads, which exhibit strong well-pair communication. For this dataset, the calculated linear flow parameters for these wells are compared. The results demonstrate that there is a difference between the linear flow parameters of wells completed in the upper layer and lower layers, which could be attributed to either differences in effective fracture area or permeability.This new approach can help practitioners quickly and accurately estimate reservoir and fracture properties in these scenarios. The method can therefore be used to improve hydraulic fracture design, well forecasting and development planning where persistent well communication through hydraulic fractures is occurring.

Numerical Simulation of Well Type Optimization in Tridimensional Development of Multi-Layer Shale Gas Reservoir

Aimed at the development of shale gas reservoirs with large reservoir thickness and multiple layers, this paper carried out a numerical simulation study on the optimization of three different well types: horizontal well, deviated well, and vertical well. To make the model more in line with the characteristics of shale gas reservoirs, a two-phase gas–water seepage mathematical model of shale gas reservoirs was established, considering the adsorption and desorption of shale gas, Knudsen diffusion effect, and stress sensitivity effect. The embedded discrete fracture model was used to describe hydraulic fracture and natural fracture. Based on Fortran language, a numerical simulator for multi-layer development of shale gas reservoirs was compiled, and the calculation results were compared with the actual production data of Barnett shale gas reservoirs to verify the reliability of the numerical simulator. The spread range of hydraulic fractures in the reservoir with different natural fracture densities is calculated by the simulation to determine well spacing and fracture spacing. The orthogonal experimental design method is then used to optimize the best combination of well spacing and fracture spacing for different well types. The results show that the well productivity of the high-density (0.012 m/m2) natural fractures reservoir > the well productivity of the medium-density (0.006 m/m2) natural fractures reservoir > the well productivity of the low-density (0.001 m/m2) natural fractures reservoir. According to the design of the orthogonal test, it can be seen that the most significant factor affecting the productivity of horizontal wells is the fracture spacing in the Y direction. For deviated wells and vertical wells, the X-direction well spacing has the greatest impact on its productivity.

A rapid, simplified, hybrid modeling approach for simulating solute transport in discrete fracture networks

In this paper, a hybrid model is developed to simulate solute transport in discrete fracture networks (DFNs) considering mass exchange between the fracture and the surrounding matrix. The developed model considers three mechanisms: advection and dispersion along the fracture, molecular diffusion within the fracture and into the matrix, and adsorption within the matrix. Furthermore, the developed model can predict solute transport at fracture intersections and at the discrete fracture network outlet for both constant and pulse injections at inlet boundaries. The developed model predictions are compared to those of an existing analytical model; the results indicate the former is approximately 250 times faster than the later, and this efficiency increases with network complexity. In addition, the developed model is compared to a computational fluid dynamics (CFD) model employing Navier Stokes equations, and the comparison indicates the former has a lower dispersion compared to the later. Nonetheless, the developed model can approximately predict solute transport simulated by the CFD model when assuming a higher value of dispersivity. Finally, the results in a high density DFN indicate that large diffusion coefficients, small apertures, and large fracture spacings reduce solute migration in fracture networks.

Long‐Wavelength Propagation in Fractured Rock Masses (3D Stress Field)

AbstractFractured rocks affect a wide range of natural processes and engineering systems. In most cases, the seismic characterization of fractured rock masses in the field involves wavelengths much longer than the fracture spacing; reproducing this condition in the laboratory is experimentally challenging. This experimental investigation explores the effect of fracture rock fabric and the 3D stress field on P wave propagation in the long‐wavelength regime using a large‐scale true triaxial device. P wave velocities increase with stress in the propagation direction and follow a power law of the form Vp = α(σ’/kPa)β; analyses and experimental results show that stress‐sensitive fracture stiffness and fracture density define the α‐factor and β‐exponent; conversely, long‐wavelength velocity versus stress data can be analyzed to identify the stress‐dependent fracture stiffness. P wave velocities exhibit hysteretic behavior caused by inelastic fracture deformation and fabric changes. During deviatoric loading, the P wave velocity decreases in the two constant‐stress directions due to the development of internal force chains and the ensuing three‐dimensional deformation. Following a load increment, time‐dependent contact deformations result in P wave velocity changes during the first several hours for the tested carbonate rocks; the asymptotic change in velocity is more pronounced for higher stress changes and stress levels. The fracture network geometry that defines the rock fabric acts as a low‐pass filter to wave propagation, so that wavelengths must be longer than two times the fracture spacing to propagate (Brillouin dispersion); the long‐wavelength velocity and the fracture spacing determine the cutoff frequency. Fabric anisotropy contributes to anisotropic low‐pass filtering effects in the rock mass.

Publisher’s Note

In-situ conversion of oil shale process has been appealing from economic and energy security perspectives. Conventional in-situ upgrading techniques use electric heaters to heat oil shale, not shale oil. However, the efficiency of electrical heating method is very slow which requires preheating more than a year. Most conventional heating technologies focused on using different heat injection methods converting the shale oil reservoirs. As shale is a poor heat conductor, many of attempts are still in the laboratory stage. The shale oil matrix is very tight and the pore scale ranges from micro to nano-meter. In this paper, we propose to inject air into hydraulically fractured horizontal wells to create in-situ combustion of shale oil in ultra-low permeability formations. Heat is introduced into the formation through multistage fractured horizontal wells, which enhances the contact area of exposed kerogen. This process uses combustion reactants as fuel feed stock to generate perpetual heating. Firstly, we used the laboratory tube combustion of shale oil results to validate the simulation model. Then, we upscale the simulation model to investigate the well productivity performance by in-situ upgrading of shale oil by air injection. The main objective of this work is to determine the technical feasibility of recovering shale oil resources by air injection. It involves the application of hydraulic fracturing technology to enhance the kerogen exposure to oxygen. Heat flows from the fracture into shale oil formation, gradually converting the solid kerogen into mobile oil and gas which can be produced via fractures to the production wells. Our simulation results in this paper have shown the potential of in-situ combustion process to improved oil recovery in shale oil reservoirs. Simulation result indicates that the key parameters controlling the well productivity include air injection rate, injected gas composition and fracture spacing. The available literature provides limited studies on the applicability of in-situ combustion of shale oil in very low permeability formations. The paper provides a comprehensive model that allows kerogen to undergo thermal cracking and oxidation process. The results of this study indicate that the developed kinetic model is feasible to model the kerogen in-situ conversion process induced by air injection.

Optimization of Fracturing Parameters with Machine-Learning and Evolutionary Algorithm Methods

Oil production from tight oil reservoirs has become economically feasible because of the combination of horizontal drilling and multistage hydraulic fracturing. Optimal fracture design plays a critical role in successful economical production from a tight oil reservoir. However, many complex parameters such as fracture spacing and fracture half-length make fracturing treatments costly and uncertain. To improve fracture design, it is essential to determine reasonable ranges for these parameters and to evaluate their effects on well performance and economic feasibility. In traditional analytical and numerical simulation methods, many simplifications and assumptions are introduced for artificial fracture characterization and gas percolation mechanisms, and their implementation process remains complicated and computationally inefficient. Most previous studies on big data-driven fracturing parameter optimization have been based on only a single output, such as expected ultimate recovery, and few studies have integrated machine learning with evolutionary algorithms to optimize fracturing parameters based on time-series production prediction and economic objectives. This study proposed a novel approach, combining a data-driven model with evolutionary optimization algorithms to optimize fracturing parameters. We established a significant number of static and dynamic data sets representing the geological and developmental characteristics of tight oil reservoirs from numerical simulation. Four production-prediction models based on machine-learning methods—support vector machine, gradient-boosted decision tree, random forest, and multilayer perception—were constructed as mapping functions between static properties and dynamic production. Then, to optimize the fracturing parameters, the best machine-learning-based production predictive model was coupled with four evolutionary algorithms—genetic algorithm, differential evolution algorithm, simulated annealing algorithm, and particle swarm optimization—to investigate the highest net present value (NPV). The results show that among the four production-prediction models established, multilayer perception (MLP) has the best prediction performance. Among the evolutionary algorithms, particle swarm optimization (PSO) not only has the fastest convergence speed but also the highest net present value. The optimal fracturing parameters for the study area were identified. The hybrid MLP-PSO model represents a robust and convenient method to forecast the time-series production and to optimize fracturing parameters by reducing manual tuning.

Simulation of the Fracturing Process of Inclusions Embedded in Rock Matrix under Compression

Typical parallel fractures are often observed in the outcrops of inclusions in the field. To reveal the failure mechanism of inclusions embedded in rock matrix, a series of heterogeneous models are established and tested based on the damage mechanics, statistical strength theory, and continuum mechanics. The results show that, with the spacing between two adjacent fractures decreasing, the stress is firstly transferred from negative to positive, then from positive to negative. Stress transition is profound for the fracture spacing. Meanwhile, three types of fractures, i.e., consecutive fracture, non-consecutive fracture, and debonding fracture, are found, which are consistent with the observed modes in the field. Multiple inclusions are often fractured easier than an isolated inclusion due to the stress disturbance between inclusions and newly generated fractures. Either in single or multiple inclusions, tensile stresses inside the inclusions are the main driving force for fracture initiation and propagation. Besides, although the material heterogeneity has a small effect on the stress variation, it has an evident impact on the fracturing mode of inclusions. The stiffness ratio is critical for the stress transition and failure pattern; the interface debonding occurs earlier than the fracture initiation inside the inclusion when the stiffness ratio is relatively high. Additionally, the inclusions content only affects the sequence of fracture initiation rather than the final fracture spacing pattern.

Experimental study on the multiple fracture simultaneous propagation during extremely limited-entry fracturing

Horizontal well with multi-stage fracturing is one of the most effective stimulation methods for unconventional reservoirs, e.g. tight oil/gas or shale. To maximize reservoir stimulation volume (SRV), tighter fracture spacing and fewer perforations are distributed in one stage during extreme limited-entry fracturing (XLEF) in recent years. However, the fracture geometries and injection pressure curve are not clear when multiple fractures with close spacing were created simultaneously in the perforated wellbore during XLEF. This study investigated the multiple fracture simultaneous propagation in the XLEF perforated wellbore based on the true tri-axial fracturing experiments. Critical factors of horizontal stress difference (HSD), the number of perforation clusters, helical/in-plane perforated method, number of perforations per cluster and fracturing fluid flowrate were investigated in detail. The results showed that, firstly, compared to one fracture produced by the helical perforated method, XLEF with the in-plane perforated method has a higher breakdown pressure and could simultaneously create multiple transverse fractures. Secondly, longitudinal fractures and a small number of curved transverse fractures occurred simultaneously under lower HSD conditions, while multiple parallel transverse fractures could be created under high HSD conditions. Thirdly, increasing the number of perforations per cluster will reduce perforation cluster effectiveness, and increasing the number of clusters will lead to the merging of multiple fractures. Finally, three relationships between pressure response and fracture geometries during XLEF, e.g. single transverse fracture, multiple transverse fractures, co-existence of longitudinal and transverse fractures, have been revealed. This study provides a meaningful perspective for the multiple fracture propagation in the perforated wellbore, which could help the field fracturing design during XLEF.

A novel image-based approach for interactive characterization of rock fracture spacing in a tunnel face

This paper presents a novel integrated method for interactive characterization of fracture spacing in rock tunnel sections. The main procedure includes four steps: (1) Automatic extraction of fracture traces, (2) digitization of trace maps, (3) disconnection and grouping of traces, and (4) interactive measurement of fracture set spacing, total spacing, and surface rock quality designation (S-RQD) value. To evaluate the performance of the proposed method, sample images were obtained by employing a photogrammetry-based scheme in tunnel faces. Experiments were then conducted to determine the optimal parameter values (i.e. distance threshold, angle threshold, and number of fracture trace grouping) for characterizing rock fracture spacing. By applying the identified optimal parameters involved in the model, the proposed method could lead to excellent qualitative results to a new tunnel face. To perform a quantitative analysis, three methods (i.e. field, straightening, and the proposed method) were employed in the same study and comparisons were made. The proposed method agrees well with the field measurement in terms of the maximum and average values of measured spacing distribution. Overall, the proposed method has reasonably good accuracy and interactive advantage for estimating the ultimate fracture spacing and S-RQD. It can be a possible extension of existing methods for fracture spacing characterization for two-dimensional (2D) rock tunnel faces.

Parametric effects on fracture geometries from multi-fracture propagation emanating from neighbouring wellbores in quasi-brittle rocks

An integrated approach is presented using field data input from measured geological information into numerical simulation for understanding effects of induced stress on geometries of multiple fracture propagation. We establish a benchmark study based on comparison of field result with numerical computations. The comparison then acts as reference measures for studying effects of changing in-situ stress, fracturing fluid viscosity and fracture spacing on propagation and geometries of multiple fractures between neighbouring wellbores in an undisclosed gas field. This leverages understanding of more complexities associated with inter-well multiple fracture growth that are idealized as straight from certain perspectives. Although some studies focus on stress interference from pre-existing fractures, actual fracture propagation geometries may be far from the idealized scenarios. Therefore, the stress shadow effects between growing multiple hydraulic fractures, if not taken into account, can lead to unrealistic estimation of hydraulic fracture trajectories. Consequently, more attention should be paid to the actual propagation of hydraulic fractures. Actual field geologic information is provided through well-logging and field mapping data. Very short fractures were propagated in these wells before the operation was terminated due to technical problems. The reservoir depth in the area is about 2170 m. At such depth, quasi-brittleness of shale should be accounted for by using relevant methods that capture rock ductility such as traction separation law. Abaqus commercial software is used to conduct the numerical computation using extended finite element method based on cohesive zone modeling. Application of this technique is further validated using Kristianovich-Geertsma-de Klerk analytical solution. This study is important in field implementation of infill drilling with well-informed mechanism of fracture interference in the inter-well region.

Quantitative investigation of multi-fracture morphology during TPDF through true tri-axial fracturing experiments and CT scanning

Due to the reservoir heterogeneity and the stress shadow effect, multiple hydraulic fractures within one fracturing segment cannot be initiated simultaneously and propagate evenly, which will cause a low effectiveness of reservoir stimulation. Temporary plugging and diverting fracturing (TPDF) is considered to be a potential uniform-stimulation method for creating multiple fractures simultaneously in the oilfield. However, the multi-fracture propagation morphology during TPDF is not clear now. The purpose of this study is to quantitatively investigate the multi-fracture propagation morphology during TPDF through true tri-axial fracturing experiments and CT scanning. Critical parameters such as fracture spacing, number of perforation clusters, the viscosity of fracturing fluid, and the in-situ stress have been investigated. The fracture geometry before and after diversion have been quantitively analyzed based on the two-dimensional CT slices and three-dimensional reconstruction method. The main conclusions are as follows: (1) When injecting the high viscosity fluid or perforating at the location with low in-situ stress, multiple hydraulic fractures would simultaneously propagate. Otherwise, only one hydraulic fracture was created during the initial fracturing stage (IFS) for most tests. (2) The perforation cluster effectiveness (PCE) has increased from 26.62% during the IFS to 88.86% after using diverters. (3) The diverted fracture volume has no apparent correlation with the pressure peak and peak frequency during the diversion fracturing stage (DFS) but is positively correlated with water-work. (4) Four types of plugging behavior in shale could be controlled by adjusting the diverter recipe and diverter injection time, and the plugging behavior includes plugging the natural fracture in the wellbore, plugging the previous hydraulic fractures, plugging the fracture tip and plugging the bedding.