ABSTRACT: Hydraulic fracturing is one of the most effective conventional reservoir-stimulation techniques currently utilized in the petroleum industry. Hydraulic fracture (HF) – Natural fracture (NF) interactions during the hydrofracturing process are complex in space and time. Although in the past five decades, several numerical, analytical and experimental studies have explored and characterized the interactions between natural fractures and hydraulic fractures, there remains differing opinions on the fundamental behaviors of hydraulic fractures when and where they interact with natural fractures. Here, we present a comprehensive review of the propositions and limitations of these studies in order to emphasize the need for further studies on NF-HF interactions and highlight the important fundamental questions that are yet to be answered. Our review focuses on previous experimental, analytical and numerical studies that investigated the impact of geomechanical and geometrical properties, and operational parameters on the result of NF-HF intersections during fracture initiation and propagation. We envisage that advancement in our understanding of NF-HF interactions will help in optimizing the fracture-stimulation processes during hydrofracturing operations.

ABSTRACT: Hydraulic fracturing is the best method to improve production in deep shale, and the complex natural fracture affects the fracture propagation. In this study, numerous true tri-axial experiments were conducted on hydrostone to study the effect of the natural fracture complexity. The position and the relative angle of natural fracture were mainly studied. The acoustic emission was used to detect the fracture propagation and the pumping curve was analyzed. The results showed that the three-dimensional fracture morphologies can be classified into five ways: arresting, bypassing, crossing, vertical extension and slipping. Symmetrical natural fracture around the wellbore helped to increase the fracture height extension. When natural fractures distributed asymmetrically, the hydraulic fracture showed obvious single wing crack. The hydraulic fracture tended to activate the natural fracture and changed direction along the natural fracture with a small approaching angle. The pumping curve rising in the propagation stage showed the fracture arresting the natural fracture. More complex fracture network corresponded to more fluctuated curve. The results give a deep understanding of the effect of the natural fracture complexity in 3D and help to predict the fracture intersection morphology in the field.

ABSTRACT: Characterization of rock permeability change during the fracture network evolution is critical to the design and assessment of geological storage facilities. Under geo-storage conditions, rock fractures propagate during damage processes and re-bond during healing. However, natural heterogeneities in rocks significantly reduce experimental repeatability. Moreover, healing is a time-consuming process and requires high pressure or temperature to achieve. In this study, we resort to the 3D printing technology for specimen preparation so that complex rock microstructure changes are precisely controlled. We validate the specimen preparation by comparing experimental permeability test results with analytical solutions. Channel networks with different tortuosity, porosity, fracture branch, and structural anisotropy are designed to mimic various microstructure changes in rocks. To fabricate fractured rocks, we also embed heterogeneous fracture networks into 3D printed specimen and modify fracture geometry to account for anisotropic microstructural changes. Experimental findings revealed the evolution trend of permeability with these structural characterization parameters. They also match well with those obtained from computational fractured rock models under identical hydro-mechanically coupled conditions. This study is expected to provide new insights into the dependence of rock permeability on its microstructures under complex geo-storage conditions.

ABSTRACT: The purpose of this paper is to present a numerical code for rock mechanics modelling of discontinuities such as fractures, joints and faults. We implement this in an open-source Matlab Reservoir Simulation Toolbox designated for rapid prototyping of geomechanics, reservoir modelling and simulation problems. Appropriate and explicit modelling of such discontinuities become a central element of the scientific discussions. In particular, we contemplate processes when fractures open, close and slip due to stress changes in the subsurface. Existing numerical codes use either long-syntax programming languages where details of implementation are not explicitly shown or commercial simulators that often lack the flexibility for implementing new ideas. We use the recently developed Virtual Element Method (VEM) and solve contact problems that allow fractures to dynamically interact with the surrounding rock mass and each other, transferring contacting stress across faces due to closure and friction. This method has advantages over standard Finite Elements or Finite Volumes in solving the contact problem because it handles complex, not necessarily convex grid geometries and hanging nodes. For the contact problems, we implement contact detection, normal and tangential contact interaction algorithms. We provide numerical examples and match to analytical solutions in order to validate employed algorithms.

ABSTRACT: Ordovician carbonate rocks make up the main reservoir stratum in the Tuofutai region of the Tahe Oilfield. The rocks exhibit extensive formations of the faults, fractures and caves which results in a rather complex stress distribution. To determine the variation in the stress field and the rotation of the principal stress, fracture distribution was obtained by logging data and FRACMAN numerical analysis was conducted. The results showed that: 1) two conjugate faults and fractures were recognized by FMI images and logging data, and the trends of the fractures were about NNW343° and NNE28°, which means the trend of maximum horizontal principle stress is about NNE10. 2) The vertical stress is the maximum principle stress at about154-170MPa, the maximum horizontal principle stress is about 122-135MPa and the minimum horizontal principle stress is about 89-95MPa when the depth is 6200-6700 m. 3) The value of each principle stress decreased about 5-12MPa in the fault zone and the degradation is about 8-10%. The trend of maximum horizontal principle stress varied from NNE5° to NNE30° at the ends of the fault. 4) The value of each principle stress decreased about 3-5MPa with fracture development and the degradation is about 3-5%. The trends usually held steady; 5) The value of maximum horizontal principle stress decreased about 20-30MPa around the caves and the minimum principle stress decreased about 10-20MPa; and the maximum horizontal principle stress varied from NNW282° to NNE70°. 6) The stress field in the surrounding rock was mainly concentrated within the distance of 0.5 times of the cave radius. The rotation of the principal stress mainly occurred at the end of faults and around the caves.

ABSTRACT: It is important to determine well spacing for maximizing oil recovery in shale reservoirs with complex hydraulic and natural fracture geometries. The optimal number of well placement should be achieved to minimize well interference. Although there are many simulation studies to optimize well spacing, very few studies have been performed to combine the fracture propagation model and reservoir simulation considering complex fracture configurations. The goal of this study is to fill this gap by integrating a complex fracture model and reservoir model through a non-intrusive embedded discrete fracture model (EDFM) method. The fracture model was applied to predict complex fracture geometries. After that, such fractures were swiftly transferred to traditional reservoir simulators based on the non-intrusive EDFM technology. We applied the workflow to examine the effect of well number with varying well spacing on well performance in the Permian basin with and without considering natural fractures. Finally, the optimal well spacing was discussed. This study can provide key insights into optimization strategies for well spacing in shale reservoirs.

ABSTRACT: Previous studies on the strength and deformability of pillars in hard-rock mines have demonstrated that the influence of natural fractures diminishes with increasing pillar width. Jointing conditions and fracture intensity/intersections play a critical role in the behaviour of slender pillars (width to height ratio less than 0.5). An important challenge is the characterization of the intact rock damage as a function of the in-situ fracture network and loading conditions. In this paper, we present advanced pre-processing tools to measure fracture intensity and fracture density prior to loading and relate these measures to the evolution of damage (intensity and density) with increasing pillar load. A hybrid finite-discrete element method with fracture mechanics capabilities is used to simulate intact rock failure and allows to incorporate discrete fracture systems. This method compared to continuum modelling has the advantage of fully considering both brittle failure and kinematic mechanisms. This research considers four synthetic pillar models with different embedded discrete fracture traces of similar total length. Newly developed post-processing tools are able capture the location of damage events track the intensity/density of rock damage as the simulation progresses and relate these changes to the modelled pillar strength. The results of this synthetic rock mass pillar study provide useful insights on damage evolution and rock bridge strength characterisation in pillars. Our results show that pillars with different initial jointing conditions may yield similar strength but undergo different stress paths. Detailed interpretation of the results has allowed the contribution of damage to block creation with increasing load to be highlighted. The roles of structurally-controlled failure and brittle mechanisms are discussed and implications on pillar design and potential future trends in pillar damage characterization presented.

ABSTRACT: Hydraulic fracturing is the principal stimulation technique used to improve the oil and gas recovery and enhance production from unconventional resources. In open hole completion, the direction of the initiated hydraulic fracture around the wellbore is mostly influenced by existence of natural fractures or rock inhomogeneity. After passing the stress perturbed zone near wellbore, the fracture will propagate perpendicular to the minimum principal stress direction. The fracture turning and tortuosity around the wellbore significantly increases the pressure drop and difficulty in placement of proppant. The use of a notch, as a predefined crack in the preferred fracture propagation direction is commonly done in lab experimental studies to assist the fracture to initiate more easily and direct the fracture propagation near wellbore. The use of notch in field hydraulic fracturing applications has also been reported. In this study we numerically simulate the impact of the notch geometry on fracture initiation and propagation pressure and geometry. The XSite, a lattice based model was used to simulate some lab experimental data and the results showed the importance of notch length and its orientation on fracture propagation.

ABSTRACT: The Rock Engineering System (RES) approach provides a framework for geotechnical engineering design, and has been used successfully to support a wide range of rock engineering. RES solutions require the identification of the principal parameters that play a role in the process under examination. The relationship between these parameters and other related variables are then assessed to form an interaction matrix that represents the system. Previous studies have considered blast optimization using an RES approach. However, these approaches consider few details regarding the existing fracture network. How a rock mass reacts to a blast is dependent on the rock itself and on the discontinuities within the rock mass. A set of discrete fracture network (DFN) realizations generated and assessed using MoFrac are used to derive inputs for a RES fragmentation interaction matrix. Parameters representing block size and rock mass integrity derived from fracture network models are identified that can be incorporated into a RES solution. A new parameter for inclusion in existing solutions measuring fracture intensity variability is also presented. From analysis of a fracture network, estimations of in situ block size distribution, fracture set intensity, and variability are presented that can be integrated in a RES approach to blast optimization.

ABSTRACT: Pre-existing natural fracture networks in unconventional reservoirs present at least two major challenges for refracturing and infill well design. First, spatial characteristics of pre-existing natural fracture networks impact how a hydraulic fracture will propagate. Second, the mechanical/chemical characteristics of a natural fracture impact how the reservoir depletes, which might lead to the reservoir's anisotropic effective permeability and depleted zone profile not being as predicted when only considering isotropic reservoir permeability. This paper uses an integrated approach to investigate the challenges imposed by pre-existing natural fractures and their depletion effect on refracturing, as well as completion plans for placing and fracturing infill wells in naturally fractured unconventional reservoirs. Single-well multiple-stage refracturing, parent-child well interference, and infill wells under different scenarios are simulated with consideration to natural fracture networks. Then, engineering options are evaluated to optimize fracture placement to help minimize negative depletion effects and maximize hydrocarbon recovery in parent-child naturally fractured unconventional reservoirs.

ABSTRACT: Stimulation of pre-existing natural fractures is considered to contribute significantly to hydrocarbon production from unconventional shale reservoirs. An effective natural-fracture simulation may occur in mixed normal and shear modes due to the misalignment of the natural fractures with respect to the principal stresses. This mixed-mode failure is triggered especially upon intersection of these natural fractures with hydraulic fractures and the fracturing fluid infiltration into the natural fractures. Among various influential parameters on the fracture intersection behavior, hydraulic-natural fracture intersection angle and initial reservoir pore pressure carry the highest uncertainty in shale resources characterization. To address this uncertainty, we adopted poroelastic fracture-intersection and propagation models on the basis of a commonly used pore-pressure cohesive zone model. For the natural fracture reactivation, we obtained a threshold horizontal stress contrast beyond which the natural fracture does not undergo slip. We propose an iterative algorithm using numerical experiments to retrieve the threshold horizontal stress contrast for 16 permutations of initial reservoir pore pressure and intersection angle. Our simulations reveal that with increasing intersection angle, the natural fracture reactivation may occur at a higher or lower horizontal stress contrast depending on the initial reservoir pore pressure.

ABSTRACT: The Valley of the Kings (also known as King's Valley, KV) in Egypt is recognized as one of the richest archeological world heritage sites. Thousands of tourists visit the valley each day and its rock-cut tombs, of which many were excavated close to or into the steep cliffs of the valley. The rock mass properties and environmental conditions of the area were gathered from literature to conduct a preliminary stability assessment of the fractured cliff above the 18 th Dynasty tomb KV42 constructed for Hatshepsut-Meryet-Ra, the wife of Thutmose III, reused by a mayor of Thebes, Sennefer, and his family members. A 2D numerical model was developed in RS2® using the Shear Strength Reduction (SSR) modelling method, which indicates the importance of vertical fractures on the cliff stability. Preliminary results suggest that failure could be more influenced by the orientation of vertical joint sets rather than bedding planes. We installed a monitoring station to record the local environmental parameters such as temperature and relative humidity along with crack meter readings and discuss the impact of daily and seasonal climatic variations on vertical discontinuity opening and closing cycles. A coupled thermal and geomechanical model is recommended to be developed in further stages of this research to validate the effects of climatic variables on the extension of fractures and cliff stability at KV42.

ABSTRACT: Pre-existing natural fracture networks (NFNs) are ubiquitous in many unconventional oil and gas formations. Hydraulic fracturing (HF) is used to stimulate these formations; many traditional HF analyses and modeling efforts have largely ignored (or highly simplified) the NFN effect on HF stimulation results. NFNs are characterized by geometric properties (including spatial distribution, length, and height) and mechanical properties (including tensile strength, cohesive strength, and shear stiffness). This paper explicitly represents NFNs in a validated, state-of-the-art geo-mechanical simulation model, which couples rock mechanics, fluid mechanics, and proppant transport to enable HF stimulation process modeling. Simulations are used to systematically study NFN properties and their effect on HF stimulation outcomes, including fracture length, propped area, and fracture complexity. All NFN properties (and other formation and stimulation treatment variables) together affect the final stimulated network; examples from the US Eagle Ford and Spraberry formations demonstrate some of these effects. In addition to NFN properties, fracture toughness and horizontal stress anisotropy properties were varied to provide additional insight into the results. Study results demonstrate that NFNs may crucially affect HF stimulation outcomes and, therefore, indicate the importance of mapping NFNs and their mechanical properties, and considering them in designing optimal HF stimulation jobs in particular formations.

ABSTRACT: The geometry of hydraulic fractures is a major concern, especially in highly fractured reservoirs, once it can severely affect oil productivity. Furthermore, interaction of hydraulic fractures with natural fractures can affect locally the in-situ stress state that may cause slippage of the natural fracture. In some cases, stress shadowing effects can even close the hydraulic fracture. Stress rotation around the natural fracture can make hydraulic fracture geometry complex, demanding more computational capacity and time to run numerical simulations. The neural network is a tool that can help real time daily operations when a quick decision is necessary and there is no time to run numerous simulations. This paper focuses on the development of an artificial neural network to predict the interaction between natural and hydraulic fractures. The artificial neural network predicts if hydraulic fractures cross or open natural fractures. In-situ stresses, fracture energy, friction angle of the natural fracture, and angle of approach between fractures are the required input for the artificial neural network, while mode of interaction such as opening, or crossing is the output. The database to train the neural network includes experimental data available in literature and results from numerical simulations. A considerable number of numerical simulations using interface elements to represent fractures provide a suitable database to build an accurate neural network. The neural network was trained for predicting fracture interaction with more than 90% accuracy for opening or crossing behavior. Results were compared with experimental data available in literature and numerical simulations. The results show that for higher angles of approach and higher stress differential, there is a strong tendency to fracture crossing. This is in agreement with the experimental conclusions presented by previous publications in the literature. Additionally, intermediary stress differentials, low values of cohesion and friction angle, and approach angle below 60° define conditions with a strong tendency to open the natural fracture. The scenarios predicted by the neural networks is in agreement with the rock mechanics concepts and with the expected tendencies, which gives more reliability to the neural network.

ABSTRACT The aim of this paper is the proposition of a new approach and a specific workflow to characterize and model the natural fractures using as case study the Cambro-Ordovician reservoir analog in the Mouydir basin, Algeria. The fracture analysis is based on two approaches. The first approach is the analysis of global fractures that affect the area of study. The fracture map is generated from the combination of curvature attribute, illumination attribute, geological maps, satellite images, and digital elevation models. The outcomes are the determination of fracture sets, fractures length's distributions, correlation coefficients, power law coefficients, and fractal dimensions. The second approach is the analysis of the fractures affecting the basement and the different Cambro-Ordovician units, by using the same input data and determining the same parameters and outputs. The 3D deterministic fracture models for each unit are built to show the fractures distribution in space and their superposition helps to determine their origin, their relationship, their kinematics, which illustrate the impact of the basement's fractures on the sedimentary cover.

ABSTRACT: This paper shows how to integrate several disciplines to characterize and model one of the most important tight oil fractured reservoirs in Algeria to explain its productivity. Seismic data, seismic attributes, well correlation, core, and borehole data are integrated to understand the type and origin of fractures, their spatial distribution, and their impact on the reservoir parameters. The Discrete Fracture Network (DFN) models are generated to display the fault and the fracture sets and show the different mechanical units and compare their relationship with the reservoir units. DFN model at field scale show a lateral variability due to the impact of the Hercynian Unconformity (HU) on the different Hamra Quartzite’ units. The overall DFN shows that the upper unit of Hamra Quartzite appears intensely fractured and fracture sets appear connected and can contribute significantly to the productivity of this reservoir. The existence of the upper unit if the Hercynian Unconformity does not erode it would be the best indicator of the sweet spot in the Hassi Guettar field.

ABSTRACT: The fractal dimension is applied to verify whether the 2-D fracture networks that affect the basement and the Cambro-Ordovician reservoirs analog in north Hoggar shield have fractal dimension. The Ajjers, In-Tahouite, and Tamadjert units compose the Cambro-Ordovician reservoirs analog. They are characterized by stiff tectonic style, showing dense fracture network that affects all the Paleozoic series. The statistical analysis of the length distribution of fracture in the Cambro-Ordovician reservoir analog indicates several fault sets orientation where the fracture length follows a power-law distribution. The fractal dimension Dm is estimated using the center distribution and box-counting algorithms. According to geometry and structure, the fractures can be divided into Major and Minor fracture where the fractal analysis is based on several approaches; analyzing the fractures by sets at different scales. All these approaches show that the fractures affecting the basement and the Cambro-Ordovician units in the area of study are fractal where their fractal dimension D m oscillates between 1 and 2.

ABSTRACT: Discrete Fracture Network (DFN) modeling is an effective tool to estimate the distribution of rock block sizes. However, for stochastically generated DFN, the clustering of rock blocks is often difficult to emulate, and the locations of high fracture intensity areas are often randomized or arbitrary leading to not-so-realistic DFN models and potentially inaccurate estimates of block sizes. This paper follows a geostatistical approach to model the spatial variability of fracture intensities. This approach is demonstrated through a basement excavation project in Sydney and provides a comparison case study of two DFN modelling methodologies using (1) Geostatistical interpolation of lineal (1-D) fracture intensity (P 10 ) using Ordinary Kriging (OK) and, (2) Random Distribution (RD) of P 10 . Four validation criteria were established to calibrate and provide quality assurance for the DFN models. The results of DFN validation indicate the OK model was able to return better DFN validation measures in P 10 , P 21 and visual sections. For P 32 validation measure, both the OK and RD methodologies are comparable. The Block Size Distribution (BSD) curves generated using the OK model have significant differences when compared to the RD model. This is especially true when assessing the BSD curves for each individual excavation face.

ABSTRACT: In view of the complicated flow mechanism of shale gas flow, multi-scale media and multiple pressure systems of organic matter/inorganic matrix pores/natural fractures and discrete hydraulic fractures are established to accurately describe the gas flow in shale reservoir. The mathematic model considers mechanisms of non-equilibrium desorption of the adsorbed gas, the viscous flow and Knudsen diffusion of the free gas, and the influence of inorganic matrix pores, natural fractures and hydraulic fractures pressure system on shale deformation. A fully coupled solid-fluid coupling model is established, and it is solved with finite element method. It is found from the results that the pressure drop in natural fractures and matrix pores is the main influence factor that causes stress sensitivity during production, and natural fractures play a more important role. With the increase of stimulated reservoir volume, the stress sensitivity is more significant, especially in the early production stage. The proposed fully coupled solid-fluid model of shale gas flow have the ability to study the influence of solid deformation on different pore pressure systems, which consider the non-linear flow mechanisms. The model is also of great value for predicting the shale gas production more accurate.

ABSTRACT: In computational simulations of drilling mud flow in fractured formations, consideration of interactions between the fracture networks and the porous rock requires an appropriate coupling procedure from the local fractures to the scale of typical finite elements in poromechanics problems. A hydro-mechanically coupled model is developed in this paper to characterize drilling mud flow in naturally fractured formations. Especially, the mud leak-off from fractures into the matrix is described by a new method. The finite element equations are derived for the model and integrated over time. By exploiting the partition-of-unity property of the finite element shape functions, the position and direction of the fractures are independent from the underlying discretization. The resulting discrete equations are non-linear due to the non-linearity of the leak-off effect, and the Newton–Raphson method is used to solve the nonlinear equations. Simulation results show that the fracture deformation and leak-off effect both have significant impacts on mud loss volume, especially the expansion of the fracture network due to mud invasion can further increase the cumulative mud loss volume. The presented model improves the accuracy of the mud loss prediction, and thereby results in a more realistic characterization of naturally fractured reservoirs.