Stimulated Reservoir Volume Size Research Articles

Fracture sealing and its impact on the percolation of subsurface fracture networks

Current uderstanding of the impact of fracture sealing on macro-scale fluid flow is insufficient. Here, we generate well-connected orthogonal fracture networks in two- and three-dimensions using a discrete fracture network method. The fracture sealing is simulated by dividing fractures into segments (2D) and blocks (3D). Impacts of fracture sealing on the percolation status of orthogonal fracture networks are systematically studied. Different aperture shapes, segment lengths, and spacing distributions are broadly considered. We find that a small amount of sealing can prevent the formation of spanning clusters, suggesting that global connectivity is rarely realized. Local clusters can trigger hydraulic responses if their sizes are large enough. The connectivity reduction caused by fracture sealing can be reversed by significant stress perturbations, such as hydraulic fracturing. The well-connected and critically oriented fractures become critically stressed and slide because of the increased pore pressure. Partially sealed and mechanically stable fractures can also contribute to fluid flow by enlarging the stimulated reservoir volume (SRV). A new procedure to estimate the SRV size is proposed, and the estimated results are qualitatively consistent with microseismicity maps and field observations.

Impacts of fracture properties on the formation and development of stimulated reservoir volume: A global sensitivity analysis

Stimulated reservoir volume (SRV), the highly permeable fracture network created by hydraulic fracturing, is essential for fluid production from low-permeable reservoirs. However, the configuration of SRV and its impacting factors are largely unknown. In this work, we adopt the stochastic discrete fracture network method to mimic natural fractures in subsurface formations and conduct a global sensitivity analysis with the Sobol method. The sensitivity of different fracture properties, including geometrical properties (fracture lengths, orientations, and center positions), mechanical properties (fracture roughness and fracture strength), fracture sealing properties (probabilities of open fractures and segment lengths), and the fracture intensity, are investigated in two and three-dimensional fracture networks. JRC-JCS model is adopted to identify critically stressed fractures. We find that critically stressed fractures compose the backbone of SRV, while partially open fractures can significantly enlarge the size of SRV by connecting more critically orientated fractures. The fracture roughness is the most influential factor for the total length (area) of critically stressed fractures. For the relative increase of SRV (RI) in 2D/3D fracture networks, the probability of open fractures is the most significant factor. The fracture lengths and center positions are essential factors for RI in 2D fracture networks but insignificant in 3D fracture networks. This work provides a realistic scenario of the subsurface structure and systematically investigates the influential factors of SRV, which is useful for estimating the size of SRV and predicting shale gas reservoirs’ production in an accurate and physically meaningful way.

Productivity Analysis of Volume Fractured Wells under Different Working Systems

The volume fracturing technique has been widely used to improve the productivity of ultralow-permeability reservoirs. This paper presents a new semianalytical model to simulate the pressure transient and production behaviour of finite conductivity vertical fractured wells with stimulated reservoir volume (SRV) in heterogeneous reservoirs. The model is based on the five-linear flow model, the Warren-Root model, and fracture conductivity influence function. The model is validated by comparing its results with a numerical model. One novelty of this model is its consideration of three different kinds of production prediction models. Constant rate, constant pressure, and compound working systems are taken into account. This paper illustrates the effects of the SRV size and shape, mobility ratio, initial flow rate, limiting wellbore pressure, and hydraulic fracture parameters under different working systems. Results show that the SRV and parameters of fractures have a significant influence on long-term well performance. Moreover, the initial rate can extend the constant rate period by 418%, and limiting wellbore pressure can effectively improve the cumulative recovery rate by 23%. Therefore, this model can predict long-term wells’ behaviour and provide practical guiding significance for hydraulic fracturing design.

ANALYTIC EVALUATION METHOD OF FRACTAL EFFECTIVE STIMULATED RESERVOIR VOLUME FOR FRACTURED WELLS IN UNCONVENTIONAL GAS RESERVOIRS

It has been proved that effective stimulated reservoir volume (ESRV) is a significant area dominant to the production of the fractured well in unconventional gas reservoirs. Although ESRV properties can be estimated based on the microseismic technology and the analysis of actual fracturing data, the operations are complicated and results are inaccurate. Due to the complex structure of stimulated reservoir volume (SRV) with fractal and chaotic characteristics, a fractal evaluation model for ESRV (ESRV-FEM) of fractured wells in unconventional gas (both tight and shale gas) reservoirs is developed. Multiple gas transport mechanisms, SRV and unstimulated reservoir volume (USRV) are included. According to the pressure transient analysis (PTA), influences of multiple transport mechanisms on gas transport behaviors in ESRV are conducted. Moreover, the fractal index representing the heterogeneity degree is applied to estimate the ESRV under different inter-porosity flow coefficients and storage ratio conditions based on the ESRV-FEM. In addition, the presented ESRV-FEM is validated by an actual field case. The results show that gas adsorption has a significant effect on the radial flow duration time in SRV, and the heterogeneity makes the radial flow on PTA curves no longer show a horizontal line with the value of 0.5. Calculated ESRV sizes are compared with the assumed ones under different fractal indexes. The stronger the heterogeneity, the smaller ESRV is. The ESRV size of a fractured well in shale gas reservoirs is only 52.11% of SRV size when the fractal index equals to 0.6. The ESRV-FEM presented in this paper is expected to provide an effective method for the evaluation of the ESRV of fractured wells in unconventional gas reservoirs.

Prediction and analysis of the stimulated reservoir volume for shale gas reservoirs based on rock failure mechanism

The ultra-low-permeability shale gas reservoir has a lot of well-developed natural fractures. It has been proven that hydraulic fracture growth pattern is usually a complex network fracture rather than conventional single planar fractures by micro-seismic monitoring, which can be explained as the shear and tensile failure of natural fractures or creation of new cracks due to the increase in reservoir pore pressure caused by fluid injection during the process of hydraulic fracturing. In order to simulate the network fracture growth, a mathematical model was established based on full tensor permeability, continuum method and fluid mass conservation equation. Firstly, the governing equation of fluid diffusivity based on permeability tensor was solved to obtain the reservoir pressure distribution. Then Mohr–Coulomb shear failure criterion and tensile failure criterion were used to decide whether the rock failed or not in any block on the basis of the calculated reservoir pressure. The grid-block permeability was modified according to the change of fracture aperture once any type of rock failure criterion was met within a grid block. Finally, the stimulated reservoir volume (SRV) zone was represented by an enhancement permeability zone. After calibrating the numerical solution of the model with the field micro-seismic information, a sensitivity study was performed to analyze the effects of some factors including initial reservoir pressure, injection fluid volume, natural fracture azimuth angle and horizontal stress difference on the SRV (shape, size, bandwidth and length). The results show that the SRV size increases with the increasing initial pore reservoir and injection fluid volume, but decreases with the increase in the horizontal principal stress difference and natural fracture azimuth angle. The SRV shape is always similar for different initial pore reservoir and injection fluid volume. The SRV is observed to become shorter in length and wider in bandwidth with the decrease in natural fracture azimuth angle and horizontal principal stress difference.

Performance analysis of fractured wells with elliptical SRV in shale reservoirs

A stimulated reservoir volume (SRV) exists around fractured wells in unconventional shale reservoirs, which leads to the flow characteristics of the fluid that are different from those in unstimulated formations. The existence of elliptical flow near vertical fractures is especially common and can provide a more comprehensive analysis than classical radial flow. This paper presented a new prediction composite model for fractured wells with elliptical SRV to analyze well performance in shale reservoirs. We developed the dual-porosity model to describe both the stimulated region and the unstimulated region in elliptical shale reservoirs. The gas flow follows Fick's Law between the matrix and the fractures due to the pressure difference and the concentration difference. In addition to the Laplace transform and Stehfest's algorithm, we also incorporate the modified Mathieu equations, including the angular Mathieu equation and the radial Mathieu equation, to obtain solutions of the elliptical flow model. The type curves are verified with the actual production data of the unconventional gas reservoir, and the sensitivities of the elliptical flow model are analyzed, including the inter-porosity coefficient λ, storativity ratio ω, adsorption index σ, and SRV size ξ1. The elliptical boundary characteristic parameter of SRV is a key parameter that is useful for describing and predicting the performances of fractured wells in composite shale reservoirs.

Production performance laws of vertical wells by volume fracturing in CBM reservoirs

Volume fracturing technology has been widely applied in the development of coalbed methane (CBM) reservoirs. As for the stimulated reservoir volume (SRV) created by volume fracturing, the seepage laws of fluids are described more accurately and rationally in the rectangular composite model than in the traditional radial composite model. However, the rectangular composite model considering SRV cannot be solved using the analytical or semi-analytical function method, and its solution from the linear flow model has larger errors. In view of this, SRV areas of CBM reservoirs were described by means of dual-medium model in this paper. The complex CBM migration mechanisms were investigated comprehensively, including adsorption, desorption, diffusion and seepage. A well testing model for rectangular composite fracturing wells in CBM reservoirs based on unsteady-state diffusion was built and solved using the boundary element method combined with Laplace transformation, Stehfest numerical inversion and computer programming technology. Thus, production performance laws of CBM reservoirs were clarified. The flow regimes of typical well testing curves were divided and the effects on change laws of production performance from the boundary size of gas reservoirs, permeability of volume fractured areas, adsorption gas content, reservoir permeability and SRV size were analyzed. Eventually, CBM reservoirs after the volume fracturing stimulation were described more accurately and rationally. This study provides a theoretical basis for a better understanding of the CBM migration laws and an approach to evaluating and developing CBM reservoirs efficiently and rationally.

A Semianalytical Model for Multiple-Fractured Horizontal Wells with SRV in Tight Oil Reservoirs

The paper developed a new semianalytical model for multiple-fractured horizontal wells (MFHWs) with stimulated reservoir volume (SRV) in tight oil reservoirs by combining source function theory with boundary element idea. The model is first validated by both analytical and numerical model. Then new type curves are established. Finally, the effects of SRV shape, SRV size, SRV permeability, and parameters of hydraulic fractures are discussed. Results show that SRV has great influence on the pressure response of MFHWs; the parameters of fractures, such as fracture distribution, length, and conductivity, also can affect the transient pressure of MFHWs. One novelty of this model is to consider the nonlinear flow around hydraulic fracture tips. The other novelty is the ability to model the shape of the SRV, production behavior of different fractures, and interfaces. Compared to numerical and analytic methods, this model can not only reduce extensive computing processing but also show high accuracy.

Transient pressure and production dynamics of multi-stage fractured horizontal wells in shale gas reservoirs with stimulated reservoir volume

Multi-stage fracturing in unconventional shale gas reservoirs always creates complex fracture network around horizontal wellbore. The existence of the stimulated reservoir volume (SRV) has significant impact on performance of fractured horizontal well, and various composite models are introduced to simulate gas flow caused by fractured horizontal wells with SRV. However, most of the existing models do not simultaneously taken into account fracture conductivity, non-uniform properties of hydraulic fractures, SRV region, multiple flowing mechanisms of shale gas.This paper extends previous work by presenting a more comprehensive model to simulate the performance of multi-stage fractured horizontal well in shale gas reservoirs. The model takes into account not only the existence of SRV, but also the influence of fracture conductivity, multiple flowing mechanisms (desorption, transient state and pseudo-steady state viscous flow and diffusion) in shale matrix as well as the effect of non-uniform fracture distribution. By employing the source function theory, Laplace transformation and discretization method, a relatively simple semi-analytical solution is obtained. After the verification of the presented model with other published results, the transient pressure and production dynamics are compared and analyzed. The influences of relevant parameters on performance of fractured horizontal wells are also discussed. It is found that the performance of fractured horizontal wells in shale gas reservoirs is mainly affected by the parameters related to SRV (the size of SRV, the density of fractures in SRV, the configuration of primary fractures in SRV), conductivity and distribution of hydraulic fractures, and desorption and diffusion in shale matrix. The model presented in this paper can be easily simplified to simulate simpler cases, such as performance of fractured horizontal wells in tight reservoirs. The model and corresponding conclusions presented in this study will be helpful to understand transient performance of multi-stage fractured horizontal wells in shale gas reservoirs and thus to effectively develop unconventional shale gas reservoirs.

Study of the CO2 ECBM and sequestration in coalbed methane reservoirs with SRV

Stimulated reservoir volume (SRV) and CO2-enhanced coalbed methane recovery (ECBM) are two popular technologies used to develop coalbed methane (CBM) reservoirs. Anthracite coal samples were selected from the CBM reservoir in the southern part of Qinshui Basin. The CO2 and CH4 displacement experiments were conducted using two different methods, “gas injection/desorption” and “desorption/gas injection/desorption”. Based on the indoor experiment, and using the reservoir geological properties, fluid properties and stimulated measure data of the target CBM reservoir, three numerical simulation models of 5-point well groups with SRV were built. These models were used to conduct influence factor analysis of CO2 storage and ECBM in the volume stimulated CBM reservoir. Using the above indoor experiments and numerical simulation experiments, the following conclusions were obtained: the method of “desorption/gas injection/desorption” is more favorable for CO2 replacing CH4 and realizing CO2 storage and ECBM. The SRV measures can increase the flow conductivity and improve the single well productivity of CBM. The SRV size, SRV flow conductivity and gas saturation of the cleat system are the key influence factors of CO2 storage and ECBM in volume stimulated CBM reservoir. The study described in this paper can offer technical support for implementing the synthesis technologies of SRV and CO2 ECBM and storage in CBM reservoirs.

Research on transient flow theory of a multiple fractured horizontal well in a composite shale gas reservoir based on the finite-element method

A shale gas reservoir, as a special type of gas reservoir, is significantly different from a conventional gas reservoir. Gas flow in shale involves a complex process of multiple flow mechanisms (including adsorption, diffusion, slippage, and viscous flow in multi-scaled systems of nano-to macro-porosity) that substantially deviates from Darcy flow. The multiple hydraulic fracturing procedure not only creates the stimulated reservoir volume (SRV) to improve production but also causes the flow in the shale to become more complex. In this work, we first propose a dual porosity continuum and discrete fracture model to describe the flow of MFH well in a rectangular composite shale gas reservoir. The finite element method based on the unstructured 3D tetrahedral meshes is used to obtain a numerical solution. Next, we assume a transient well index to correct the early calculation error caused by the mesh precision and obtain the complete pressure transient and production decline curves. Sensitivity analyses show that the Langmuir volume, equivalent apparent permeability, hydraulic fracture distribution and size of SRV have great impacts on the transition flow regime and production performance. The research results obtained in this paper can provide theoretical guidance for efficient and large-scale development of shale gas reservoirs.

A MIXED-FRACTAL FLOW MODEL FOR STIMULATED FRACTURED VERTICAL WELLS IN TIGHT OIL RESERVOIRS

Stimulated reservoir volume (SRV) with large fracture networks can be generated near hydraulic fractured vertical wells (HFVWs) in tight oil reservoirs. Statistics show that natural microfractures and fracture networks stimulated by SRV were self-similar in statistical sense. Currently, various analytical models have been presented to study pressure behaviors of HFVWs in tight oil reservoirs. However, most of the existing models did not take the distribution and self-similarity of fractures into consideration. To account for stimulated characteristic and self-similarity of fractures in tight oil reservoirs, a mixed-fractal flow model was presented. In this model, there are two distinct regions, stimulated region and unstimulated region. Dual-porosity model and single porosity model were used to model stimulated and unstimulated regions, respectively. Fractal geometry is employed to describe fractal permeability and porosity relationship (FPPR) in tight oil reservoirs. Solutions for the mixed-fractal flow model were derived in the Laplace domain and were validated among range of the reservoir parameters. The pressure transient behavior and production rate derivative were used to analyze flow regimes. The type curves show that the fluid flow in HFVWs can be divided into six main flow periods. Finally, effect of fractal parameters and SRV size on flow periods were also discussed. The results show that the SRV size and fractal parameters of fracture network have great effect on the former periods and fractal parameters of matrix mainly influence the later flow periods.

Production performance analysis of tight oil/gas reservoirs considering stimulated reservoir volume using elliptical flow

Most tight oil/gas reservoirs are naturally fractured with dual-porosity characteristics. After fracturing or re-fracturing, the stimulated reservoir volume (SRV) always exists around the wellbore. This paper demonstrates a composite elliptical mathematical model to analyze the pressure transient and rate transient behaviors in tight oil/gas reservoirs with SRV. This model focuses on two main dual-porosity media regions which are the inner region and outer region divided based on micro-seismic observation. The inner region which has higher flow capacity characterizes the SRV. The inter-porosity flow between matrix and fracture was assumed as unsteady flow for both the inner region and outer region. We solve the model with the elliptical flow model and using Mathieu modified functions, Laplace transform, and Stehfest algorithm comprehensively. The model solution was verified with previous work thoroughly. Also, we presented the pressure transient and rate transient type curves based on which seven flow regimes were recognized including the early wellbore storage period, skin factor period, linear flow period, transfer flow period, first radial flow period, transition flow period, and later radial flow period. Then, effect of four related parameters including SRV size, outer region fracture permeability, fracture half-length, and inner region matrix radius on tight oil/gas transient behavior were investigated. At last, we tested the model with field cases to analyze fractured wells performance in tight reservoirs. History matching results were shown according to real production data and micro-seismic results. The presented model and obtained results can enrich the production performance analysis methods for tight oil and gas reservoirs.

A composite model of hydraulic fractured horizontal well with stimulated reservoir volume in tight oil & gas reservoir

Due to the low porosity and permeability, multiple hydraulic fracturing is applied in tight oil & gas reservoir. A composite model of multiple fractured horizontal well is built with considering the stimulated reservoir volume (SRV), where the reservoir is divided into three main areas including fractures, stimulated reservoir volume and unstimulated reservoir. The dimension of fracture is reduced based on the discrete fracture model (DFM) to improve the computational efficiency, the stimulated reservoir volume follows the dual permeability model, and the unstimulated reservoir obeys to the single porosity model. The composite model is solved using the finite element method and verified by the classic analytical solution in dual porosity reservoir. The result shows that there are five main flow regimes for the multi-fractured horizontal well (MFHW) in the composite model including linear flow around the hydraulic fractures, pseudo-steady flow of transition area, inter-porosity flow period, radial flow of unstimulated reservoir and the outer boundary pseudo-steady flow. The bigger the size of SRV is, the later the pseudo-steady flow of transition area arrived, so according to the arrival time of pressure wave to the SRV outer boundary, namely the pseudo-steady flow of transition area appeared, the size of SRV can be determined; and the smaller main hydraulic fracture permeability is, the later the time of pseudo-steady flow of transition area arrived.