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Results
3D Poroelastic Analysis of Rock Failure Around a Hydraulic Fracture
Ghassemi, A. (Harold Vance Department of Petroleum Engineering, Texas A&M University) | Zhou, X.X. (Harold Vance Department of Petroleum Engineering, Texas A&M University) | Rawal, C. (Harold Vance Department of Petroleum Engineering, Texas A&M University)
ABSTRACT: Three-dimensional stress and pore pressure distributions around a hydraulic fracture are numerically calculated to analyze the potential for formation failure resulting from pressurization of the hydraulic fracture. The three-dimensional numerical model used combines the finite element method and the poroelastic displacement discontinuity method. Elements of the model formulation and solution procedures are first presented. Then, the problem of constant water injection into a rectangular fracture in Barnett shale is presented. Using the Mohr-Coulomb failure criterion with a tension cut-off, results show that rock failure can occur in the vicinity of the fracture, especially near the fracture tips. The dominant failure mode is tension in the close vicinity of the fracture where the pore pressure attains its highest values. Shear failure potential exists away from the fracture walls where shear stresses are sufficiently high for the relatively weak rock. The extent of the potential failure zone increases with increasing injection rate. 1. INTRODUCTION Hydraulic fracturing by water injection is extensively used to stimulate unconventional gas and geothermal reservoirs. The water is pumped at a high pressure into a selected section of the wellbore to create and extend a fracture(s) into the reservoir rock. The applied pressure in the fracture(s) re-distributes the pore pressure and stresses around the main fracture causing rock failure by fracture initiation and/or activation of discontinuities such as joints and bedding planes. The net result is often enhancement of the formation permeability. The rock failure process is often accompanied by micro-seismicity that can provide useful information regarding the stimulated volume. The literature pertaining to the subject of rock failure around a hydraulic fracture is not extensive. Warpinski et al. [1] presented a semi-analytical method to calculate the stress and pore pressure variations induced by a hydraulic fracture, and evaluated the likelihood and potential causes of micro-seismic activity in the vicinity of a major fracture. The semi-analytical method was based on simple crack geometry and approximation of the pore pressure in the reservoir without flow considerations in the fracture. Palmer et al. [2] adopted a 2D model to study the impact of stimulation of Barnett shale permeability enhancement. Ge and Ghassemi [3] also used a 2D approach and studied the impact of the in-situ the stress, pore pressure, as well as poroelastic and thermoelastic phenomena on the rock failure around a hydraulic fracture. The resulting stresses were also used to calculate the stimulated volume and the permeability enhancement. In this paper, we present a 3D poroelastic numerical model for analysis of stress distribution around an irregularly-shaped fracture. The model is applied to study the potential for rock failure in the vicinity of the fracture. The model couples fluid flow in the fracture with poroelastic deformation of the reservoir matrix to calculate the pore pressure and stresses in the rock. Numerical examples are presented to highlight the characteristics of stress distributions as well as the effect of fluid injection rate on the extent of the potential rock failure zone for a large rectangular fracture.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.46)
- Geology > Petroleum Play Type > Unconventional Play > Shale Play > Shale Gas Play (0.46)
ABSTRACT: Fracture aperture and fluid flow are affected by poro-thermo-mechanical processes and mineral precipitation/dissolution. In this paper, we study these phenomena by development and application of a three-dimensional porothermo- mechanical model with silica dissolution/precipitation effects. The solid mechanics aspect of the problem is treated using a poro- and thermoelastic displacement discontinuity method, while the solute transport and heat transport in the fracture are solved using the finite element method. The single component solute reactivity in the fracture is considered using temperature dependant reaction kinetics. The model is applied to simulate the impact of water circulation on the dynamics of fracture permeability in enhanced geothermal systems. Injecting under-saturated cold geothermal fluid causes large silica mass dissolution in the fracture in a zone that extended towards the extraction well over time, increasing the fracture aperture in this zone. Fluid pressure near the injection well initially increases with injection and aperture reduction in response to leak-off, however, pressure decreases as cooling proceeds. Thermo- and poroealstic stresses are induced in the reservoir matrix that cause secondary fracturing and possibly induce seismicity. 1. INTRODUCTION During extraction of the thermal energy from subsurface, fluid flows through natural fracture/fracture networks and interacts to the adjacent rock-matrix in the reservoir. This circulation of low-temperature fluid in fractures leads to the variation in the geometry of fractures which can be described as its response to mechanical, thermal and chemical processes. Different aspects of thermal and mechanical processes have been studied by the researchers [1, 2, 3, 4, 5,6]. For example thermo-elastic effects are reported dominant near the injection when compared to those of poro-elasticity and under some conditions; silica reactivity may govern permeability [3]. Furthermore, experimental studies [7, 8, 9] also show that chemical precipitation and dissolution of minerals significantly affect fracture aperture. In general, simulating poro-thermo-elastic-chemical mechanisms involves solving a set of equations each for fluid flow, heat transport, solute transport/reactions and elastic response of the reservoir and more importantly, these processes are coupled. In present work, we use a partially coupled poro-thermo-elastic approach [6] and the displacement discontinuity method [5] to compute the poro-thermo-mechanical processes. On the other hand, we use finite element method for reactive flow and heat transport to find the solution for their distributions in the fracture. The solute reactivity and solubility in fracture is considered using a temperature dependent formulation (e.g., [10, 11]) and presented in detail in following subsections. 2. GOVERNING EQUATIONS The physical and chemical processes associated to the geothermal injection/extraction are represented and described by a number of equations, which obtained by considering constitutive models, transport, and balance laws (.e.g. fluid momentum, fluid continuity). 3. NUMERICAL SOLUTION In this study, the system of equations((3)-(9); (10)-(13); (14)-(17)) is solved using combined finite element and boundary element method. For example, we use Galerkin's finite element method to model fluid flow, heat and solute transport in the fracture, whereas boundary element is used to compute diffusive fluid flow, conductive heat flow and mineral diffusive transport in the reservoir matrix (three-dimensional space).
- Energy > Oil & Gas > Upstream (1.00)
- Energy > Renewable > Geothermal > Geothermal Resource (0.48)