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Liu, Lijun (China University of Petroleum) | Huang, Zhaoqin (China University of Petroleum) | Yao, Jun (China University of Petroleum) | Lei, Qinghua (ETH Zurich) | Di, Yuan (Peking University) | Wu, Yu-Shu (Colorado School of Mines) | Liu, Yongzan (Texas A&M University)
The object of this work is to develop an efficient coupled hydro-mechanical numerical model for two-phase flow in fractured vuggy porous media. The fluid flow in matrix and fractures is described by two-phase Darcy's equation, and the free flow in vugs is simplified with the assumption of multiphase instantaneous gravity differentiation. The modified Barton-Bandis's constitutive model is used to handle the nonlinear deformation of fractures. The fluid pressure is applied on the vug boundaries to model the vug deformation. Then the finite-volume (FVM) and finite-element (FEM) methods are used for space discretization of the flow and geomechanics equations, respectively. The coupled problem is iteratively solved using fixed-stress splitting method. Then a set of 2D and 3D simulation cases are conducted to investigate the effects of fractures and vugs on the flow and geomechanical behaviors. The results show that vugs can hinder the water breakthrough due to their storage effect, while water can quickly break through in the high-conductivity fractures. The significant effect of gravity on the saturation distribution can be observed in the 3D case. Besides, the stress concentration is much more obvious when vugs are present. 1. INTRODUCTION The coupled hydrology and mechanics of fractured vuggy porous media is an important issue in several fields, such as oil recovery in fractured vuggy carbonate reservoirs and mining engineering. The fractured vuggy porous media is usually characterized by its multiscale pore space, including porous matrix, natural fractures, and vugs (Okabe and Blunt, 2007). Due to the co-existence of porous flow, fracture flow and free flow, as well as their coupling with the deformation, the hydro-mechanical modeling in fractured vuggy porous media remains challenging, especially for two-phase flow. A series of models have been developed to study fluid flow in fractured vuggy porous media, including equivalent continuum model (Popov et al., 2009; Huang et al., 2011), triple continuum model (Wu et al., 2011), and discrete fracture-vug model (Girault & Rivière, 2009; Yao et al., 2010; Liu et al., 2020a). However, the former two models are too simplified to capture the dominating flow of large-scale fractures and vugs (Zhang et al., 2016), and most of current discrete fracture-vug models focus on single-phase flow.
Yao, Jun (School of Petroleum Engineering, China University of Petroleum, P.R. China) | Huang, Zhaoqin (School of Petroleum Engineering, China University of Petroleum, P.R. China) | Li, Yajun (School of Petroleum Engineering, China University of Petroleum, P.R. China) | Wang, Chenchen (School of Petroleum Engineering, China University of Petroleum, P.R. China) | Lv, Xinrui (School of Petroleum Engineering, China University of Petroleum, P.R. China)
Abstract Modeling and numerical simulations of fractured vuggy porous media is a challenging problem due to the presence of cavities called macro vugs which are connected via discrete fractures networks. The main difficulty is the co-existence of porous and free-flow region in such media on macro scale. In this study, a novel conceptual discontinuum model i.e. discrete fracture-vug network (DFVN) model has been proposed to this problem. In DFVN conceptual model, naturally fractured vuggy porous rock masses are considered as a composite porous material, consisting of (1) macro fractures system, (2) porous rock matrix system, and (3) macro vugs system. Macro fractures and vugs are embedded in porous rock, and the isolated vugs are connected via discrete fracture network. We model the fractured vuggy media on macroscopic scale using Navier-Stokes equations within the vugular region, Darcy's law within the porous flow region including porous rock matrix system and macro fractures system, and a Beavers-Joseph-Saffman boundary on the interface between two regions. A standard Galerkin finite element method is implemetated for the solution of DFVN model. A good match with analytical and numerical solutions for Poiseuille flow in a free/porous channel was achieved, which verified the accuracy of our finite element numerical scheme. Both 2D DFVN models with homogeneous isotropic rock matrix and heterogeneous anisotropic rock matrix are simulated and studied. The numerical results have shown that DFVN model provides a natural way of modeling realistic fluid flow in fractured vuggy porous media.
Han, Songcai (School of Petroleum Engineering / China University of Petroleum (East China)) | Cheng, Yuanfang (School of Petroleum Engineering / China University of Petroleum (East China)) | Gao, Qi (School of Petroleum Engineering / China University of Petroleum (East China)) | Yan, Chuanliang (School of Petroleum Engineering / China University of Petroleum (East China)) | Wei, Jia (School of Petroleum Engineering / China University of Petroleum (East China)) | Zhang, Jincheng (Research Institute of Petroleum Engineering)
ABSTRACT: A fully coupled thermal-hydraulic-mechanical (THM) model was developed to investigate the heat extraction process of an artificial enhanced geothermal system (EGS). The random fracture network in the stimulated geothermal reservoir was generated by the fractal theory. The local thermal non-equilibrium (LTNE) theory was adopted to simulate the heat exchange between the rock matrix and the injection cold fluid. Temperature-dependent fluid thermodynamic properties and pressure-dependent fracture/pore permeability were also incorporated into the thermo-poroelastic coupling model by some empirical formulas. The proposed multiphysics model was validated by several analytical solutions. The evolution of temperature, effective stress and reservoir permeability during heat extraction was analyzed in detail. The sensitivity of heat production performance to heat convection coefficient, injection rate, injection temperature, fracture network morphology was discussed. Results indicate that at the early stage, the interconnected large-scale fractures around wellbore dominate the mass and heat transport. Some scattered small-scale fractures contribute to uniformly propelling the cooling of the heat rock mass. The change in effective stress associated with the thermo-poroelastic effect may induce fracture shear dilation and pore expansion, resulting in the permeability enhancement of the overall reservoir. In some regions, where the temperature drop is insignificant and the pore pressure decreases, thereby inducing compressive stress to make fractures closure. It is more practical to use the LNTE theory to analyze the heat extraction process in fractured geothermal reservoirs. The heat extraction efficiency, thermal breakthrough time and service-life of an EGS are seriously affected by the hydraulic conductivity and connectivity of fracture networks under constant injection rates and injection temperatures. Generating a complex and scattered fracture network but without preferential channels is conducive to extracting more heat from geothermal reservoirs.
Geothermal resources due to their renewability, cleanness and universality have attracted broad attention around the world. The hot dry rock(HDR) type accounts for about 90% of the total geothermal resources and thus has greater development potential and prospects (Xu et al., 2018). Generally, the HDR reservoir possess extra-low porosity and ultra-low permeability, which results in the difficulty of obtaining economic heat efficiency without stimulation treatment (Breede et al., 2013). Consequently, permeability enhancement techniques play a critical role in these nearly impervious reservoirs (Gao et al., 2017a, 2019b; Kumari et al., 2018). Enhanced geothermal system (EGS) is widely regarded as an efficient method to extract heat from the HDR (Tester et al., 2006; Kumari et al., 2018). The key is to induce a complex fracture network for fluid circulation between the injection well and the production well by a series of stimulation techniques, such as hydraulic fracturing, waterless fracturing, chemical treatment. Recently, cryogenic fracturing such liquid nitrogen and liquid carbon dioxide has aroused many researchers’ interest due the strong thermal shock effect (King. 1983; Cha et al., 2014; Han et al., 2018). So, it may be promising technology to further improve the stimulated reservoir volume (SRV) of the geothermal reservoir. Actually, mine practices have confirmed that the combination of thermal-chemical-hydraulic fracturing technology can induce a more pronounced stimulation volume in geothermal reservoirs than using a single stimulation method (Vidal et al., 2016; Lengliné et al., 2017).
This paper presents a stress-induced variable aperture model to characterise the effect of polyaxial stress conditions on the permeability of a three-dimensional (3D) fractured sedimentary layer. The 3D fracture network is created by extruding a 2D outcrop pattern of a limestone bed that exhibits a ladder structure consisting of a “through-going” joint set abutted by later short fractures. Geomechanical modelling of the fractured rock is achieved by the 3D finite-discrete element method (FEMDEM), which can capture the deformation of matrix blocks, the variation of stress fields, the reactivation of pre-existing fractures and the propagation of new cracks. A joint constitutive model (JCM) is implemented to simulate the rough wall interaction behaviour of individual fractures associated with roughness characteristics. The combined JCM-FEMDEM model gives realistic fracture behaviour with respect to opening, closing, shearing and dilatancy, and includes the recognition of the important size effect. A series of 3D geomechanical simulations is conducted for the fractured rock under various polyaxial in-situ stresses. Fluid flow is further modelled for the stressed but static solid skeletons based on the hybrid finite element-finite volume method (FEFVM). The magnitude of the equivalent permeability varies significantly with respect to the change of stress ratio.
Natural fractures are ubiquitous in crustal rocks in the form of faults, bedding planes, joints and veins over different length scales (Lei and Wang, 2016). These naturally occurring discontinuities often comprise complex networks and dominate the geomechanical and hydromechanical behaviour of subsurface media (Rutqvist and Stephansson, 2003). The understanding of the nontrivial effects of fractures on the overall behaviour of such highly disordered geological media has important implications for many engineering applications including geothermal energy, nuclear repository safety and petroleum recovery.
Discrete fracture networks (DFNs) are often used to mimic naturally faulted or jointed geological structures (Dershowitz and Einstein, 1988). Compared to the conventional dual porosity model (Warren and Root, 1963) and analytical solution for mathematically idealised discontinuity networks (Oda, 1985), the discrete fracture approach possesses the advantage of explicit representation of fracture geometries together with specific description of hydraulic transmissivity (Herbert, 1996). Flow properties, such as the block or equivalent permeability tensor, of a finite-sized fracture system can be studied from steady state fluid flow modelling (Lang et al., 2014; Renard and de Marsily, 1997).
Hu, Yu (Department of Petroleum Geology & Geology, School of Geosciences, University of Aberdeen) | Gan, Quan (Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology) | Hurst, Andrew (Department of Petroleum Geology & Geology, School of Geosciences, University of Aberdeen) | Elsworth, Derek (Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology)
Abstract Sand injectite complexes comprise kilometer-scale clastic intrusion networks that act as effective conduits for the migration, accumulation and then recovery of hydrocarbons and other fluids. An equivalent continuum model is constructed to represent a sand injectite reservoir, coupling stress and fluid flow in fractured rock using the continuum simulator TOUGHREACT coupled with FLAC3D to follow deformation and fluid flow. A permeability model, which uses staged percolation models, is proposed to improve permeability estimation of fracture networks by accommodating four different levels of fracture connectivity. This permeability model is confirmed against field and laboratory data, corresponding to the different connectivities of fracture networks. The new constitutive permeability model is incorporated into the coupled hydro-mechanical simulator framework and applied to sand injectites with the analysis of permeability evolution mechanisms and mechanical sensitivity. The results indicate that when the magnitudes of principal stresses increase in a constant ratio, normal closure is the dominant mechanism in reducing fracture aperture and thereby permeability. Conversely, the evolution of stress difference can accentuate aperture and permeability due to an increase in shear dilation for critically or near-critically oriented fractures. Also, the evolution of aperture and related permeability of fractured rock are more sensitive at lower stress states than at higher stress states due to the hyperbolic relationship between normal stress and normal closure of the fractures.