Geothermal Field Development Strategies Based on Economic and Fault Stability Analysis

Abstract

The main objective of this thesis is to assess the combined influences of specified reservoir conditions and operational parameters on the profitability of a geothermal project and on the potential for fault reactivation. The aim is to propose potential development strategies to maximize the profitability and minimize the potential for slip and reactivation of pre-existing critically stressed faults. The reservoir conditions of interest in this study include the fault permeability, the fault throw and the friction coefficient of the sandstone. The operational parameters of interest are the flowrate, the injection temperature of the re-injected water and the distance between the wells and the fault. As a case study a simplified homogeneous 3D box-shaped reservoir model is simulated based on the Delft Sandstone Member in the West Netherlands Basin, using the Delft Advanced Research Terra Simulator (DARTS). Reservoir production data and local pore pressure data are generated with DARTS and serve as the input data for the fault stability model and the economic model, which are both built in Python. The fault stability model is built based on the method of Mohr circles, regional stress values in the Delft area and the failure criterion of sandstone and allows to assess the fault slip tendency of a pre-existing fault. The economic model is based on the Dutch fiscal system and policies and includes the costs of the phases of a geothermal project and required energy calculations. The outputs of the model allow to assess the profitability of a geothermal project based on the Net Present Value (NPV). Outcomes of this study have shown that the profitability and the fault stability depend highly on the joint influences of specific reservoir conditions and operational options. Sealing faults generally have a negative influence on both the NPV outcomes and the fault stability as the presence leads to decreasing heat production, higher pumping costs and higher pressure build up near the fault. This influence is strengthened when the wells are placed close to the fault, while it is reduced by placing the wells far from the fault. The flowrate and the used injection temperature are found to be the most important operational options, regardless of the reservoir conditions. Combining the highest possible flowrates and the lowest possible injection temperature maximizes the NPV outcomes. As it is found that NPV outcomes increase by a factor 6 when increasing the flowrate 2.5 times and decreasing the injection temperature by 5 K may increase the NPV values up to 19\% to 43\% depending on the flowrate. With respect to the reservoir conditions the study has shown that the fault permeability and the sandstone friction coefficient are the most important influencing reservoir conditions, compared to the fault throw. The risk for fault instability increases with decreasing value of the friction coefficient and of the fault permeability. With respect to the operational options the potential for fault slip is minimized when the lowest possible flowrate is combined with the highest possible injection temperature. However, the use of a 5 K higher injection temperature allows the use of a 600 m3/day higher flowrate. Placing the wells minimally 200 m from a fault in the homogeneous reservoir and using a minimum flowrate of 7200 m3/day maximizes the NPV outcomes and fault reactivation is reduced as much as possible. The assessment of the fault stability and the profitability is however very sensitive to the reservoir conditions, which is explicitly found from the results comparing a homogeneous and a heterogeneous reservoir. This makes the potential development strategies extremely prone to heterogeneity effects and subsurface conditions which makes them highly dependent on locations specific properties. Though, the general influences of the reservoir conditions and operational options on a heterogeneous reservoir are similar to those found for the homogeneous reservoir.