Overview
Research addressed the simulation of high-enthalpy fractured geothermal reservoirs, focusing on the interplay of non-isothermal, multiphase, multicomponent flow, and mineral scaling, specifically halite precipitation. A new compositional flow model was developed to manage these complexities. The model was verified against an established simulator and applied to a 2D fractured reservoir to predict operational challenges.
Research Context
Simulating high-enthalpy fractured geothermal reservoirs presents challenges due to several coupled processes. These include non-isothermal, multiphase, and multicomponent flow, strongly nonlinear thermodynamics, and the significant influence of fractures. An additional complexity arises from mineral scaling, such as halite precipitation, which can degrade reservoir permeability and reduce well productivity. Current approaches often struggle with these combined factors.
Approach
The research developed a new compositional flow model grounded in a persistent set of primary variables: pressure, enthalpy, and overall salt mass fraction. This formulation was designed to inherently handle phase transitions without requiring manual switching, thereby enhancing numerical stability. The model integrates a discrete fracture-matrix approach to represent the fractured reservoir structure.
A key aspect of the model's design involves an efficient, robust correlation-based phase-behavior linearisation of saltwater thermodynamics. This approach replaces more computationally intensive on-the-fly phase separation calculations.
To capture the impact of mineral scaling, the model incorporates the Kozeny-Carman relation. This allows for the dynamic modeling of porosity and permeability reduction caused by halite precipitation within the reservoir.
The model was implemented within the open-source PorePy framework. For verification, a 1D salt dissolution benchmark case was used, comparing the new model's results against those from the established closed-source simulator, CSMP++. This verification involved conditions reflecting geothermal environments and included transitions between single- and multi-phase regions.
Following verification, the model was applied to a 2D halite-saturated fractured reservoir scenario. This application involved simulating injection and production processes to assess the model's ability to predict halite precipitation patterns and their consequences for permeability damage and energy recovery.
Findings
- The new compositional flow model, utilizing pressure, enthalpy, and overall salt mass fraction as primary variables, effectively handles phase transitions without manual switching, contributing to numerical stability.
- The model integrates a discrete fracture-matrix approach and employs a correlation-based phase-behaviour linearisation for saltwater thermodynamics, replacing expensive phase separation calculations.
- Dynamic modeling of porosity and permeability reduction due to halite precipitation is incorporated using the Kozeny-Carman relation.
- Verification through a 1D salt dissolution benchmark against the CSMP++ simulator demonstrated strong agreement across geothermal conditions involving transitions between single- and multi-phase regions.
- Application to a 2D halite-saturated fractured reservoir with injection and production showed the model's capability to predict halite precipitation patterns and quantify their impact on permeability damage and energy recovery.
- Numerical results further indicated the model's utility in predicting operational challenges such as wellbore blockage and in assessing the role of fracture connectivity.
Why This Matters
The developed open-source numerical tool provides a methodology for analyzing complex heat and mass transport processes that involve mineral scaling within high-enthalpy fractured geothermal systems. This capability can assist in predicting operational challenges like wellbore blockage and assessing the impact of fracture connectivity, which are critical for the sustainable management of geothermal resources.