Ronglei Zhang

Coupled Geomechanical and Reactive Geochemical Model for Fluid and Heat Flow: Application for Enhanced Geothermal Reservoir

A major concern in development of fractured reservoirs in Enhanced Geothermal Systems (EGS) is to achieve and maintain adequate injectivity, while avoiding short-circuiting flow paths. The injection performance and flow paths are dominated by the permeability distribution of fracture network for EGS reservoirs. The evolution of reservoir permeability can be affected by rock deformation, induced by the change in temperature or pressure around the injector, and chemical reactions between injection fluid and reservoir rock minerals. Thus the change in permeability due to geomechanical deformation and mineral precipitation/dissolution could have a major impact on reservoir long-term performance. A coupled thermal-hydrologicalmechanical-chemical (THMC) model is in general necessary to examine the reservoir behavior in EGS. This paper presents a numerical model, TOUGH2-EGS, for simulating coupled THMC processes in enhanced geothermal reservoirs. This simulator is built by coupling mean stress calculation and reactive geochemistry into the existing framework of TOUGH2 (Pruess et al., 1999) and TOUGHREACT (Xu et al., 2004a), the well-established numerical simulators for geothermal reservoir simulation. The geomechanical model is fully-coupled as mean stress equations are solved simultaneously with fluid and heat flow equations. After solution of the fluid, heat, and stress equations, the flow velocity and phase saturations are used for reactive geochemical transport simulation in order to sequentially couple reactive geochemistry at each time step. We perform coupled THMC simulations to examine a prototypical EGS reservoir for permeability evolution at the vicinity of the injection well. The simulation results demonstrate the strong influence of rock deformation effects in the short and intermediate term, and long-term influence of chemical effects. It is observed that the permeability enhancement by thermalmechanical effect can be counteracted by the chemical precipitation of minerals, initially dissolved into the low temperature injected water. We analyze the sensitivity of temperature of injected water on the coupled geomechanical and geochemical effects, and conclude that the temperature of injected water could be modified to maintain or even enhance the reservoir permeability and the injection performance. Introduction The successful development of enhanced geothermal systems (EGS) highly depends on the reservoir fracture network of hot dry rock (HDR) and its hydraulic properties, especially the reservoir permeability. The geomechanical processes under subsurface reservoir condition are prevalent in the EGS applications. For example, Tsang (1999) investigated and claimed that hydraulic properties of fracture rocks are subjected to change under reservoir mechanical effects. Rutqvist et al. (2002) presented the correlations between reservoir in-situ stress and the porosity, permeability and capillary pressure. It is also well known that the EGS production processes, such as the cold water injection and steam or hot fluid extraction, have strong thermo-poro-elastic effects on EGS reservoirs. On the other hand, the strong impacts of geochemical reaction on the EGS reservoirs have been observed in the commercial EGS fields and studied for carbon dioxide (CO2) based geothermal system in the past few years. Kiryukhin et al. (2004) modeled the reactive chemical process based on the field data from tens of geothermal fields in Kamchatka (Russia) and Japan. In addition, Xu et al. (2004b) presented the reactive transport model of injection well scaling and acidizing at Tiwi field in Philippines. Montalvo et al. (2005) studied the calcite and silica scaling problems with exploratory model for Ahuachapan

Non-Darcy displacement in linear composite and radial aquifer during CO2 sequestration

This paper presents Buckley-Leverett type analytical solutions for non-Darcy displacement of two immiscible fluids in linear and radial composite porous media. High velocity or non-Darcy flow commonly occurs in the vicinity of the wellbore because of smaller flowing cross-sectional areas. However, the effect of such non-Darcy flow has been traditionally ignored. To examine the physical behaviour of multiphase immiscible fluids in non-Darcy displacement, an extended Buckley-Leverett type of solution is discussed. There exists a Buckley-Leverett type solution for describing non-Darcy displacement in a linear homogeneous reservoir. This work extends the solution to flow in linear and radial composite flow systems. We present several new Buckley-Leverett type analytical solutions for non-Darcy flow in more complicated flow geometries of linear and radial composite reservoirs, based on non-Darcy flow models of Forchheimer and Barree-Conway. As application examples, we use the analytical solutions to verify num...

A Fully Coupled Model of Nonisothermal Multiphase Flow, Solute Transport and Reactive Chemistry in Porous Media

Over the past decades, geochemical reaction has been identified through experiments in different processes, e.g. the CO2 EOR process, the CO2 sequestration, the enhanced geothermal system. Research has gradually led to the recognition that chemical reactions between injected fluid and mineral rock have significant impacts on fluid dynamics and rock properties in these processes. However, for the majority of the reactive transport simulators, the sequential calculation processes of fluid flow, solute transport, and reactive geochemistry result in numerical instability and computation efficiency problems. In this paper, we present a fully coupled computational framework to simulate reactive solute transport in porous media for mixtures having an arbitrary number of phases. The framework is designed to keep a unified computational structure for different physical processes. This fully coupled simulator focuses on: (1) the fluid flow, solute transport, and chemical reactions within a threephase mixture, (2) physically and chemically heterogeneous porous and fractured rocks, (3) the non-isothermal effect on fluid properties and reaction processes, and (4) the kinetics of fluid-rock and gas-rock interactions. In addition, a system of partial differential equations is formed to represent the physical and chemical processes of reactive solute transport. A flexible approach of integral finite difference is employed to to obtain the residuals of the equation system. Jacobin matrix for NewtonRaphson iteration is generated by numerical calculation, which helps the future parallelization of the fully coupled simulator. Finally, the fully coupled model is validated using the TOUGHREACT simulator. Examples with practical interests will be discussed, including CO2 flooding in a reservoir, supercritical CO2 injection into a saline aquifer, and cold water injection into a natural geothermal reservoir. This type of simulation is very important for modeling of physical processes, especially for CO2 EOR and storage, and geothermal resources development. Inroduction Reactive fluid flow and geochemical species transport that occur in subsurface reservoirs have been of increasing interest to researchers in the subjects of CO2 geological sequestration, CO2 EOR process, enhanced geothermal system, or even waterflooding and other EOR processes. The chemical reaction path has been observed in these processes when subjected to fluid injection in the subsurface reservoir. The nonisothermal reactive solute transport phenomena involed in these processes are thermal-hydrological-chemical (THC) processes. However, the reaction paths may be slightly different due to the different fluid flow mechanisms related to these processes. CO2 geological sequestration and CO2 EOR are two effective solutions to store CO2 from burning fossil fuels in geological formations and petroleum reservoirs. Saline aquifers and petroleum reservoirs have the largest capacity among the many options for long-term geological sequestration. They are large underground formations saturated with brine water or hydrcarbons, and are often rich in dissolved minerals. CO2 is injected into these formations as a supercritical fluid with a liquid-like density and a gas-like viscosity. It is believed that geochemical reaction between CO2 and rock minerals in the aqueous–based system dominates the long-term fate of CO2 sequestrated in geological formations. Two types of geochemical reactions between CO2 and rock minerals have been identified by experiments, i.e. reactions between dissolved CO2 and rock minerals, and reactions between supercritical CO2 and rock minerals. The chemical mechanism between dissolved CO2 and rock mineral has been well understood. The acid H2CO3 is formed by the dissolution of CO2 in an aqueous solution, and it dissociates in the brine to release H + . The carbonate minerals are dissolved into the aqueous phase under this weak acid

Non-Darcy Displacement in Linear Composite and Radial Flow Porous Media

This paper presents Buckley-Leverett type analytical solutions for non-Darcy displacement of two immiscible fluids in linear and radial composite porous media. High velocity or non-Darcy flow commonly occurs in the vicinity of wellbore because of smaller flowing cross-sectional area, however, the effect of such non-Darcy has been traditionally ignored. To examine physical behavior of multiphase immiscible fluid nonDarcy displacement, an extended Buckley-Leverett type of solution is discussed. There exists a Buckley-Leverett type solution for describing non-Darcy displacement in a linear homogeneous reservoir. This work extends the solution to flow in linear and radial composite flow systems. We present several new Buckley-Leverett type analytical solutions for non-Darcy flow in more complicated flow geometry of linear and radial composite reservoirs, based on non-Darcy flow models of Forchheimer and Barree-Conway. As application examples, we use the analytical solutions to verify numerical simulation results as well as to discuss non-Darcy displacement behavior. The results show how non-Darcy displacement in linear and radial composite systems are controlled not only by relative permeability, but also non-Darcy coefficients, characteristic length, injection rates, and as well as discontinuities in saturation profile across the interfaces between adjacent flow domains. Introduction Multiphase flow and displacement occurs in a large variety of subsurface systems ranging from gas, oil, and geothermal reservoirs, vadose zone hydrology, and soil sciences. In oil and gas industry, fluid displacement has long been used as an effective EOR process. Buckley and Leverett [1942] established the fundamental principle for flow and displacement of immiscible fluids through porous media in their classic study of fractional flow theory. Their solution involves the displacement process of two incompressible, immiscible fluids in a one-dimensional, homogeneous system without considering capillary effect. The solution, then, has been extended in many aspects e.g. including capillary effects [Yortsos and Fokas, 1983; Chen, 1988; Mc-Whorter and Sunada, 1990], heterogeneous reservoir, linear composite,Wu [1993]. The effects of non-Darcy or high-velocity flow regimes in porous media have long been noticed and investigated for porous media flow (e.g., Tek et al., 1962; Scheidegger, 1972; Katzand Lee, 1990;Wu, 2002).

Efficiency of a Community-Scale Borehole Thermal Energy Storage Technique for Solar Thermal Energy

Solar thermal has been quite efficient in harvesting solar energy, but has not been used widely at the community-scale as thermal energy is difficult to store. Borehole thermal energy storage (BTES) has been recently researched by several countries for its suitability in storing excess heat generated from solar thermal panels during the summer times. The first community-scale BTES system in North America was installed in the town of Okotoke, Alberta, Canada in 2006 in order to supply partial winter heating energy for 52 residual houses. To better understand the working principles of BTES and to improve BTES performance for future applications at larger scales, a three-dimensional heat transfer model is established, using the 5year in-situ monitoring data. The model realistically imposes the time-dependent heat injection and withdrawals rates measured at the site. A total of 10 continuous years of annual cycle are simulated. The modeling results are compared with the measured temperature data over the simulation times and space. The time-dependent temperature distributions within the borehole region agree well with the measured temperature profiles. The predicted energy recovery efficiency approaches to 27% after 10 years, which also compares well with the current year efficiency of 25% at the site.

Fully Coupled Thermal-Hydraulic-Mechanical Reservoir Simulation with Non-Isothermal Multiphase Compositional Modeling

We present the development and application of a multi-physical simulator for evaluating the combined thermal-hydraulic-mechanical behaviors of petroleum reservoirs. The simulator combines non-isothermal multiphase compositional modeling with coupled geomechanical simulation module. The simulator consists of two major modules, namely, the fluid and heat flow module and the geomechanical module. An isenthalpic flash calculation approach is implemented in the fluid and heat flow module. In the flash calculation module, a nested approach is adopted, in which PT flash calculations are conducted in the inner loop and temperature is updated in the outer loop. The iteration is continued until both the fugacity and energy stopping criteria are satisfied. An improved version of the Beltrami-Michell equation, called extended Beltrami-Michell equation, has been derived and implemented in the geomechanical simulation module to simulate heterogeneous and plastic behavior of formation rocks. The three normal stress components inside the stress tensor are solved simultaneously with the pressure and enthalpy in the fluid/ heat module, ensuring the mass/energy conservation. The newly-derived extension of the Beltrami-Michell equation is capable of handling materials with changing mechanical properties. This way, the simulator is able to capture the phase change as well as the poro-mechanical effects on rock deformation induced by fluid injection/extraction. The multi-physics simulator is built on an object-oriented parallel simulation framework, with a speedup factor up to hundreds. Introduction The recovery of oil/gas and thermal energy from petroleum/geothermal reservoirs typically involves complex thermal-hydraulic-mechanical (THM) processes. For instance, problems with rock failure in the vicinity of an injection well induced by cold water injection and permeability-porosity change during production require the simulator to be able to accurately predict the transient pressure, temperature, and stress fields of the reservoir. Recently, coupled simulation of thermal-hydraulic-mechanical processes in reservoirs has become an appealing subject in reservoir simulation (Fung et al., 1994; Wan et al., 2003). Rutqvist et al. (2002)

Coupled geomechanical and reactive geochemical model for fluid, heat flow and convective mixing: application for CO2 geological sequestration into saline aquifer with heterogeneity

The significance of thermal-hydrological-mechanical-chemical (THMC) processes is well-identified in the operation of CO2 geo-sequestration. Geomechanical and geochemical effects may significantly affect aqueous phase composition, porosity and permeability of the formation, which in turn influence fluid advection, convection and transport. A sequentially coupled mathematical algorithm is employed to simulate reactive transport of water, CO2 gas and species in subsurface formation with geomechanics, which is bale to model the THMC processes of the fluid advection and convection, heat and solute transport in aqueous and gaseous phase, mean stress, and geochemical reactions under both equilibrium and kinetic conditions. A 2D model with complex chemical compositions is presented to analyse the THMC processes quantitatively, including geomechamical effect due to CO2 injection, dispersion and convective mixing due to CO2 dissolution, mineral alteration due to chemical reaction of water, CO2 and rock minerals, coupled effects of geochemical reaction and geomechanics.