Propagation Of Multiple Fractures Research Articles

Stress-Dependent Unstable Dynamic Propagation of Three-Dimensional Multiple Hydraulic Fractures with Improved Fracturing Sequences in Heterogeneous Reservoirs: Numerical Cases Study via Poroelastic Effective Medium Model

Multistage hydrofracturing of horizontal wells is one of the key technologies for deep tight oil and gas resource exploitation. The stable and parallel propagation of multiple fractures formed by m...

Numerical simulation study on fracture penetration behavior in a reservoir with multiple coal seams

Coal seams are widely deposited in marine-continent transitional facies shale reservoirs. For this type of reservoir, horizontal wells and hydraulic fracturing technology are the main ways to exploit the oil and gas resources. However, multiple layers with uneven thickness and significant differences in physical and mechanical properties among them lead to insufficient vertical fracture propagation, limiting the application of horizontal well and hydraulic fracturing technology. Therefore, to reveal the influence of in situ stress conditions and injection parameters on the vertical propagation of fractures, numerical models with multiple coal seams based on logging lithology were established and the behavior of fractures crossing coal seams under different conditions was studied based on the extended finite element method. In addition, the fracture width and pressure at the injection point were also studied. The simulation results showed that the uneven thickness of the coal seam limited the vertical penetration of the fractures compared with that of a homogeneous formation as the fractures tend to pass through the thinner coal seam. Due to the physical properties difference and stress shadow effect, multiple clusters of fractures were prone to deflection in the coal seam, which made it difficult for hydraulic fractures to grow in height. However, the stress shadow effect in the process of multiple fractures propagation also has a positive side. The stress shadow becomes stronger as the difference in fluid distribution increases, leading to a difference in the growth rate of the fracture up and down, and ultimately improving the overall penetration effect of the fractures. Based on the work presented in this paper, the perforation spacing can be optimized to either avoid the stress shadow effect or effectively utilize its positive influence. Meanwhile, the results can guide the placement of horizontal well and the selection of injection rate.

Characterisation of fracture evolution of a single cemented brittle grain using in-situ X-ray computed tomography

In this paper, the diametrical fragmentation of a single brittle glass bead cemented by ductile epoxy resin is studied using in-situ X-ray computed tomography, focusing on the fractures within the grain that underpin its crushability. Key quantities of interest include the evolution of fracture regions, paths and orientations, as well as the evolution of fracture statistics within the grain. The complex nature of the fracture process is revealed by qualitative and quantitative analysis based on both continuous X-ray radiography and discrete tomography. A series of image processing techniques are used and developed to obtain the statistical details of fragment volume, surface area, morphological geometry, fracture spatial locality and fracture preferential orientations. The single cemented grain failure consists of an initial splitting fracture process creating large and irregular glass fragments, followed by a further rapid propagation of multiple fractures through the cleavage fragments in the form of severe comminution that tends to generate smaller and more regular fragments. Despite the whole fragmentation process shows a shifting trend of the overall median anisotropy of fragments and a decreasing trend of conversion rate between fracture energy and input energy, it still maintains a constant fragment surface area power law distribution and appears to remain scale-invariant over time. The central region of the glass bead is markedly more susceptible to fracture with a constant preferred fracture orientation along the loading direction, indicating that the tensile-activated fracture is the dominant fracture mechanism during diametrical fragmentation.

Virtual multi‐dimensional internal bonds model with fracture energy conservation for three‐dimensional numerical simulation of laboratory scale fluid pressurized fracturing

AbstractStrain‐softening models have proved effective in treating fracturing under mechanical load. However, these models tend to suffer from mesh‐size dependency due to failure in preserving the fracture energy. The Virtual Multi‐Dimensional Internal Bond Model (VMIB) is derived from a particle‐based constitutive law at the micro scale that is implemented in a 3D Finite Element Method (FEM), in which material softening and energy dissipation occur in the “representative elementary volume”. However, according to the classic fracture mechanics theory, energy dissipation is due to the creation of new fracture surfaces instead of homogenized softening within the volume element. Therefore, the dissipated strain energy has to be consistent with the fracture energy during failure process. We present an improved VMIB model to bridge the energy storage in a volume and dissipation over a fracture surface. This is accomplished by developing a 3D virtual bond potential that incorporates the material fracture energy and eliminates the mesh‐size sensitivity. The virtual bond potential considers both the critical fracture energy and element size. The 3D model is calibrated and verified simulations of a group of three‐point‐bend tests. Then, by incorporating the coupled three‐dimensional element partition method, the model is applied to a series of laboratory scale fluid pressurized fracturing experiments. Multiple fracture propagation from closely‐staged cluster from a laboratory test is simulated. The comparisons between numerical and experimental results indicate that the model captures the fractures growth influenced by the stress boundary conditions and the stress shadow. The predicted breakdown pressure reasonably agrees with the experiment data.

Numerical modeling of multiple fractures competition propagation in the heterogeneous layered formation

Separate-layer fracturing technology is one of the key stimulation technologies in the multi-layer reservoirs However, multiple fractures cannot evenly propagate due to the strong heterogeneity of the layers. In this paper, a three-dimensional model considering heterogeneity and fluid partitioning is developed to simulate the competitive propagation of multiple fractures in heterogeneous layered formation. The fracture pressure, fracture morphology, fluid volume and fracture length were systematically analyzed under different heterogeneous physical parameters. The following conclusions were obtained: (1) Formations with high rock strength, low permeability or high horizontal minimum principal stress will increase the fracture pressure and reduce the fluid volume within the fracture and fracture length. (2) Hydraulic fractures are easy to propagate into the interlayers if the pay zones have high rock strength and low permeability. (3) In heterogeneous layered formations, hydraulic fractures are more likely to gain competitive advantage in the high Young’s modulus and high permeability formations, while have the disadvantage in the high rock strength and high minimum principal stress formations. Poisson’s ratio has a little effect on the fracture propagation. This paper provides some helpful design guidance for the separate-layer fracturing in the heterogeneous layered formation.

Optimization method of fracture parameters in multistage fracturing of horizontal wells

ABSTRACT Analyzing the simultaneous propagation of multiple fractures in horizontal wells is a challenging task. Although many fracture propagation models exist, the primary controlling factors are unclear. A fast and relatively accurate optimization method is urgently needed. In this study, a semi-analytical model of fracture propagation is established that couples stress interaction with fracture propagation using the energy balance principle, which can run a complete simulation process within 10 seconds. The simulation results show that the pumping rate, perforation cluster number, and fracturing fluid viscosity are the primary factors controlling fracture propagation. For two areas with different elastic modulus, the process of parameter optimization is analyzed. The reservoir with an elastic modulus of 10 MPa has an optimal perforation cluster number of 5, while the reservoir with an elastic modulus of 35 MPa has an optimal perforation cluster number of 8. The results show that the area with larger elastic modulus need to set a larger number of clusters, while the area with smaller elastic modulus have a smaller number of perforation clusters. This work aims to provide a practical and simple workflow for optimizing the fracture design of fractured horizontal wells.

Multiple Fracture Growth in Modified Zipper Fracturing

Operational parameters, such as multiple-cluster and well spacing, flow rates, and injection time, play a vital role in the deviation and growth of multiple-cluster hydraulic fractures during the implementation of modified zipper fracturing technology. Fundamental analysis of the dynamic zipper fracture growth because of changing flow rates, well spacing, and injection time does not appear to have been studied in the literature, although it is of critical importance for fracturing optimization. In this study, a fully coupled two-dimensional (2D) extended finite-element method (XFEM)–cohesive zone model (CZM) in combination with a phantom-node method (CPNM) was used in ABAQUS to investigate the initiation and propagation of multiple hydraulic fractures with 10 injection clusters that are distributed in two horizontal boreholes. This study aims to assist fracturing design to achieve minimal deviation in multiple fracture propagation and create deeper and much wider fractures between two horizontal boreholes of a modified zipper fracture (MZF) completion pattern. The growth of a hydraulic fracture in an XFEM region were verified by Khristianovic–Geertsma–de Klerk (KGD) zero toughness solution and good agreement with a negligible error of <2.5% was obtained. A series of simulation scenarios was carried out with various flow rates and three well spacings of 40, 60, and 80 m, over a long injection period to identify the optimum injection time in which a fracture grows straight with minimal deviation. The results show that to create smooth and deeper multiple hydraulic fractures in zipper fracturing, an increase in the flow rate is preferred, but the duration of the injection period should be optimized. The results of this study could be used to better address operational difficulties, such as multiple-fracture crossing and closure that is associated with unfavorable injection times, well spacing, and flow rates in MZF operations.

A New Method to Improve the Fracturing Effect of Coal Seams by Using Preset Slots and Induced Stress Shadows

Efficient extraction of coal bed methane before coal mining is essential to eliminate the risk of coal-gas outbursts. However, stimulation technologies should be implemented to enhance the conductivity of the coal seam. In this study, we propose a novel method to create a complex fracture network in underground coal mines with the integration of multiple hydraulic slotting and hydraulic fracturing. In this method, hydraulic slots are used to direct hydraulic fractures and initialize branch fractures, while hydraulic fracturing is used to extend the fractures. Given the mutually exclusive and attractive propagation of multiple fractures, a relatively evenly distributed fracture network can be generated. The results show that (1) the dynamically induced stress shadows of hydraulic fractures can cause exclusive and attractive propagation of multiple hydraulic fractures; (2) a preset slot that deviates from the principal stress can direct hydraulic fractures to a certain extent and generate branch fractures; and (3) with a staggered distribution of preset slots, a relatively large volume of the coal seam in both the minimum and maximum horizontal stress directions can be stimulated, creating a complex fracture network including many vertical branch fractures and a large area of horizontally layered directional fractures.

An integrated numerical model for coupled poro-hydro-mechanics and fracture propagation using embedded meshes

Integrated models for fluid-driven fracture propagation and general multiphase flow in porous media are valuable to the study and engineering of several systems, including hydraulic fracturing, underground disposal of waste, and geohazard mitigation across such applications. This work extends the coupled model multiphase flow and poromechanical model of Ren et al. (2018) to admit fracture propagation (FP). The coupled XFEM-EDFM scheme utilizes a separate fracture mesh that is embedded on a static background mesh. The onset and dynamics of fracture propagation are governed by the equivalent stress intensity factor (SIF) criterion. A domain-integral method (J integral) is applied to compute this information. An adaptive time-marching scheme is proposed to rapidly restrict and grow temporal resolution to match the underlying time-scales. The proposed model is verified with analytical solutions, and shows the capability to accurately and adaptively co-simulate fluid transport and deformation as well as the propagation of multiple fractures.

Quantitative analysis of the sequential initiation and simultaneous propagation of multiple fractures in shale gas/oil formations

The excessive reach of new fractures during hydraulic fracturing can interact with the pre-existing fracture system during reservoir stimulation. This phenomenon is known as Frac Driven Interaction (FDI), which can greatly compromise the production performance of the wells. The key to minimizing the impact of FDI is to effectively control the length of fractures through a proper fracturing design. This study provides a computer program that outputs the length and initiation time of each fracture to quantitively analyze the sequential initiation and propagation of multiple fractures. Parametric analysis results show that increasing the injection rate, apparent viscosity, and the dilatancy of the fluid can help shorten fracture length and increase fracture count, which ultimately mitigate fracture interaction. Higher dilatancy mainly contributes to the resultant populated length of fractures, while a higher injection rate mainly contributes to the resultant number of fractures. The results also show that a better estimation of critical fracture width is critical to improving the accuracy of the simulation. The effect of various parameters on the FDI index is analyzed i.e. decreasing the injection time from 2h to 1h has led to a 46% decrease of fracture length while the number of fractures is only reduced from 10 to 8. It is found that factors that lead to a higher energy dissipation rate can result in shorter fractures and higher fracture counts. The methodology and results of this paper can deepen our understanding of the fracture interaction mechanism, which may also be used as a guideline and tool to control fracture length and count to mitigate FDI issues.

Fracture-Hit Detection Using LF-DAS Signals Measured during Multifracture Propagation in Unconventional Reservoirs

SummaryLow-frequency distributed acoustic sensing (LF-DAS) data are a powerful attribute to detect fracture hits and characterize fracture geometry during multistage hydraulic fracturing treatments in unconventional reservoirs. The DAS data in low-frequency bands linearly correlate with strain and strain rate induced by dynamic fracture propagation. Due to the complexity of multiple-fracture propagation in unconventional reservoirs, the measured signals from different wells exhibit various characteristics. Mechanisms causing the differences are not well understood, which makes the interpretation of real LF-DAS data and detection of fracture hits very challenging. Hence, it is necessary to relate the observed strain/strain-rate signatures to specific fracture patterns based on the physical model of rock deformation during fracture propagation and to quantitatively characterize signatures surrounding fracture hits. In this study, we have applied our in-house fracture propagation model to simulate simultaneous multiple-fracture propagation as well as fracture-induced strain and strain-rate responses along an offset monitor well. Then a general guideline for fracture-hit detection is proposed based on quantitative analysis of strain/strain-rate responses during multiple-fracture propagation. Finally, a set of field examples are presented to demonstrate the potential of LF-DAS data on hydraulic fracturing monitoring.During multiple-fracture propagation, a “heart-shaped” zone with positive strain rates may be identified for each fracture before the fracture hit. Immediately after the fracture encounters the monitor well, part of the fiber within the fracture path keeps being extended, while the fiber sections off the path become compressed. Three 1D features along the channel (location) axis are designed to detect fracture hits. The features are maximum strain rate, the summation of strain rates, and summation of strain-rate amplitudes. Channels with fracture hits usually exhibit significant peak values of the three features. However, the characteristic signatures can be less detectable when the gauge length is close to the cluster spacing. Connections between fracture-hit locations and cluster perforations clearly reflect the fracture propagation direction. The field examples illustrate the complexity of real LF-DAS signals and demonstrate the adequacy of the proposed guideline for fracture-hit detection with multicluster completion. The fractures propagate nearly perpendicular to the horizontal wellbore in this unconventional shale formation. In addition, four to five fractures out of eight perforation clusters can propagate 396.24 m (1,300 ft) and hit the monitor well, and the “heel-biased” fracture pattern is observed (fractures that do not hit the monitor well are usually close to the toe side). Fracturing fluid leaking off into the previous stage can also be diagnosed, which could negatively affect the completion efficiency.

Hydraulic Fracture Propagation in a Poro-Elastic Medium with Time-Dependent Injection Schedule Using the Time-Stepped Linear Superposition Method (TLSM)

The Time-Stepped Linear Superposition Method (TLSM) has been used previously to model and analyze the propagation of multiple competitive hydraulic fractures with constant internal pressure loads. This paper extends the TLSM methodology, by including a time-dependent injection schedule using pressure data from a typical diagnostic fracture injection test (DFIT). In addition, the effect of poro-elasticity in reservoir rocks is accounted for in the TLSM models presented here. The propagation of multiple hydraulic fractures using TLSM-based codes preserves infinite resolution by side-stepping grid refinement. First, the TLSM methodology is briefly outlined, together with the modifications required to account for variable time-dependent pressure and poro-elasticity in reservoir rock. Next, real world DFIT data are used in TLSM to model the propagation of multiple dynamic fractures and study the effect of time-dependent pressure and poro-elasticity on the development of hydraulic fracture networks. TLSM-based codes can quantify and visualize the effects of time-dependent pressure, and poro-elasticity can be effectively analyzed, using DFIT data, supported by dynamic visualizations of the changes in spatial stress concentrations during the fracture propagation process. The results from this study may help develop fracture treatment solutions with improved control of the fracture network created while avoiding the occurrence of fracture hits.

Use of Mathematical Model to Predict the Maximum Permissible Stage Injection Time for Mitigating Frac-Driven Interactions in Hydraulic-Fracturing Shale Gas/Oil Wells

Abstract Frac-driven interactions (FDIs) often lead to sharp decline in gas and oil production rates of wells in shale gas/oil reservoirs. How to minimize the FDI is an open problem in the oil and gas industry. Xiao et al.’s (2019, “An Analytical Model for Describing Sequential Initiation and Simultaneous Propagation of Multiple Fractures in Hydraulic Fracturing Shale Oil/Gas Formations,” Energy Sci Eng., 7(5), pp. 1514–1526.) analytical model for two-fracture systems was extended in this study to obtain a general model for handling multiple fractures. The general model was used to identify engineering factors affecting the maximum permissible stage fluid injection time for minimizing FDI. On the basis of model results obtained, we found that increasing fluid injection rate can create more short fractures and thus increase the maximum permissible stage injection time before FDI occurs. Use of dilatant type of fracturing fluid (n &amp;gt; 1) can reduce the growth of long fractures, promote the creation of more short fractures, and thus increase the maximum permissible stage injection time before FDI occurs. It is also expected that injecting dilatant type of fracturing fluid at high rate will allow for longer injection time and thus larger injection volume, resulting in larger stimulated reservoir volume (SRV) with higher fracture intensity and thus higher well productivity and hydrocarbon recovery factor.

An integrated model for non-isothermal multi-phase flow, geomechanics and fracture propagation

This paper presents a fully integrated poro-thermo-elastic hydraulic fracturing and three-phase black-oil reservoir simulator. Both implicit-pressure-explicit-saturation and fully implicit solution algorithms are presented. The energy balance equation is solved in a coupled manner to calculate the temperature change caused by fluid injection for multi-phase flow including the effects of both conduction and convection. Associated poro-thermo-elastic stresses are rigorously computed and their impact on subsurface stress changes and fracture propagation are discussed.This allows us, for the first time, to implicitly couple 3-D reservoir simulations with the transport and settling of multiple proppants as the fracture propagates, dilates and closes. Fracturing fluid leak-off is modeled explicitly. Thermal and poroelastic stresses are fully integrated based on the coupled solution of the energy balance and the flow and mechanics equations. The new simulator is fully parallelized through domain decomposition with message-passing between domains.The model is first validated with well-known problems with analytical solutions and verified with commercial software. This new simulator is then used to study two field-scale geomechanics and fracturing cases. The first case is the simulation of water-injection-induced fracture propagation by the combined poro-thermo-elasticity effect. The second case presents an integrated simulation of multi-stage, multiple fracture propagation with different types of proppant injected and shut-in with proppant settling. After the creation of the multiple fractures, multi-phase production from the well with fracture closure during long-term production with proppant embedment is simulated.

An Equivalent Representation of Multiple Hydraulic Fractures with a Fewer Number of Fractures

Summary Multiple hydraulic fractures are often generated simultaneously from a wellbore to increase efficiency of reservoir stimulation. Numerical modeling of such a system of fractures is computationally costly, especially if the goal is to simulate numerous stages, each containing multiple fractures, on different wells, which is the current trend in the petroleum industry. To address the challenge, this study defines a method and a workflow to represent the simultaneous propagation of multiple fractures with a reduced number of equivalent fractures that accurately describes the overall fracture geometry, the created surface area, the propped surface area, the fluid leakoff, and the resulting induced stresses, as resulting from the original configuration. A hybrid approach is used, in which a combination of physical modeling and data science is involved. We first develop a database of numerical solutions using a fully coupled hydraulic fracturing simulator. The equivalent fracture representation is quantified for each set of problem parameters presented in the database. Then, the results of the database solutions are used to tackle more general cases with field pumping schedules and rock properties. Several numerical examples are presented to validate and illustrate the developed concept.

Preloading Depleted Parent Wells To Avoid Fracture Hits: Some Important Design Considerations

Summary Mitigating the negative impact of fracture hits on production from parent and child wells is challenging. This work shows the impact of parent-well depletion and repressurization on child-well fracture propagation and parent-well productivity. The goal of this study is to develop a method to better manage production/injection in the parent well so that the performance of the child well can be improved by minimizing fracture interference and fracture hits. A fully integrated equation-of-state compositional hydraulic fracturing and reservoir simulator has been developed to seamlessly model fluid production/injection (water or gas) in the parent well and model propagation of multiple fractures from the child well. The effects of drawdown rate and production time is presented for a typical shale play for three different fluid types: black oil, volatile oil, and dry gas. The results show that different reservoir fluids and drawdown strategies for the parent wells result in different stress distributions in the depleted zone, and this affects fracture propagation in the child well. Different strategies were studied to repressurize the parent well by varying the injected fluids (gas vs. water), the volumes of the preload fluid, and so on. It was found that fracture hits can be avoided if the fluid injection strategy is designed appropriately. In some poorly designed preloading strategies, fracture hits are still observed. Last, the impact of preloading on the parent-well productivity was analyzed. When water was used for preloading, water blocking was observed in the reservoir, and it caused damage to the parent well. However, when gas was injected for preloading, the oil recovery from the parent well was observed to increase. Such simulations of parent–child well interactions provide much-needed quantification to predict and mitigate the damage caused by depletion, fracture interference, and fracture hits.

A novel approach of tight oil reservoirs stimulation based on fracture controlling optimization and design

Abstract To deal with the stress interference caused by simultaneous propagation of multiple fractures and the wettability reversal and physical property changes of the reservoir caused by fracturing fluid getting in during large-volume fracturing of tight oil reservoirs through a horizontal well, a non-planar 3D fracture growth model was built, wettability reversal characterizing parameters and change of relative permeability curve were introduced to correct the production prediction model of fractured horizontal well, a fracturing design optimization software (FrSmart) by integrating geological and engineering data was developed, and a fracturing design optimization approach for tight oil reservoirs based on fracture control was worked out. The adaptability of the method was analyzed and the fracture parameters of horizontal wells in tight oil reservoirs were optimized. The simulation results show that fracturing technology based on fracture control is suitable for tight oil reservoirs, and by optimizing fracture parameters, this technology makes it possible to produce the maximum amount of reserves in the well-controlled unit of unconventional reservoirs. The key points of fracturing design optimization based on fracture control include increasing lateral length of and reducing the row spacing between horizontal wells, increasing perforation clusters in one stage to decrease the spacing of neighboring fractures, and also avoiding interference of old and new fracturing wells. Field tests show that this technology can increase single well production and ultimate recovery. Using this technology in developing unconventional resources such as tight oil reservoirs in China will enhance the economics significantly.

Integrating Reservoir Geomechanics with Multiple Fracture Propagation and Proppant Placement

Summary This paper presents the formulation and results from a coupled finite-volume (FV)/finite-area (FA) model for simulating the propagation of multiple hydraulically driven fractures in two and three dimensions at the wellbore and pad scale. The proposed method captures realistic representations of local heterogeneities, layering, fracture turning, poroelasticity, interactions with other fractures, and proppant transport. We account for competitive fluid and proppant distribution between multiple fractures from the wellbore. Details of the model formulation and its efficient numerical implementation are provided, along with numerical studies comparing the model with both analytical solutions and field results. The results demonstrate the effectiveness of the proposed method for the comprehensive modeling of hydraulically driven fractures in three dimensions at a pad scale.

New insights of natural fractures growth and stimulation optimization based on a three-dimensional cohesive zone model

A fully coupled fluid-solid three-dimensional model based on cohesive zone method is presented to simulate the interactions between hydraulic fracture (HF) and natural fracture (NF). Unknown interaction mechanisms of the crossing, arresting and bypassing are revealed. The interactions between HF and NF in three dimensions are shown to be more complex than in two dimensions. Numerical results demonstrate that vertical stress contrast plays a significant effect on the fracture geometry for single and multiple stages fracturing treatments. Multiple fractures propagation of different fracturing sequences under the influence of NFs distribution and in-situ stresses is investigated. For Texas-two step fracturing method, the propagation of interior HF in horizontal direction will be terminated by stress perturbation in closely spaced HFs. A method to weaken severe mechanical interactions and enlarge the separations of NFs is proposed. The findings provide some new insights for stimulation design to create fracture network.

Numerical investigation of single and multiple fractures propagation in naturally fractured reservoirs

The properties of natural fractures (NFs), including fracture size, aperture width, and mechanical properties, etc., cannot be neglected when developing a model of hydraulic fracturing. Without considering the geological characterisation of NF properties, hydraulic fracture simulations will give much less accurate prediction of complex fracture propagation pattern. In this research, a novel two-dimensional finite-discrete element method (FDEM) model has been developed to describe complex fracture propagation in unconventional formations. We developed a natural fracture network builder by considering natural fractures geological observations. Simulations have been conducted to investigate single fracture and complex fracture network propagation in naturally fractured reservoirs. In hydraulic fracturing treatments, opening of natural fractures is determined by geological properties of NFs. For multiple fractures propagation in naturally fractured reservoirs, stress shadowing effect plays a key role in fracture network evolution. This work provides a framework for more realistic prediction of complex fracture geometry in naturally fractured formations. [Received: December 11, 2017; Accepted: July 6, 2018]