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The lifetime of geothermal projects mainly depends on the thermal breakthrough (thermal breakthrough occurs when a cold water front reaches the producer). Currently, geothermal-energy production is marginally economical because of its uncertainties and risks associated with the subsurface such as lifetime, flow rate, temperature. Lifetime of a geothermal reservoir plays the most important role in the use of geothermal energy because it mainly determines whether or not geothermal-energy production is economically viable. Through optimization of the well positions from one or more geothermal doublets in a homogeneous or heterogeneous reservoir, the profitability of the project, which is largely dependent on the time of compositional breakthrough, temperature breakthrough and the rate of temperature decline, can be improved. This thesis studies optimization of the well positions such that the Net Present Value (NPV) of a project is maximized in a 2D geothermal reservoir for the selected heterogeneity structure. For this purpose an automated, gradient-based optimization method is used. The approach is based on the concept to surround the wells, whose locations have to be optimized, by so-called pseudo-wells. The reservoir simulations are performed using the Finite Element Method in the program COMSOL Multiphysics 4.2a. The major features of the simulation results are discussed in detail. The compositional front moves faster than the thermal front (the ratio of these two is the thermal retardation factor). Breakthrough of water with altered composition will therefore occur at an early stage in the doublet lifetime. Reservoir heterogeneities influence the time at which thermal and compositional breakthrough occur and also determine the rate at which temperature and composition decline after breakthrough. The temperature and compositional decline curves after breakthrough are generally steeper in a homogeneous reservoir than in a heterogeneous reservoir. Therefore, the thermal breakthrough does not necessarily mean the end of the lifetime of a doublet. It is also shown that the effect of heterogeneities on the thermal retardation factor is small. Three successful optimization sequences in two different reservoirs are described in this thesis. It is shown that, from an economical standpoint, is makes little sense to assume a doublet lifetime of more than 30 years. Furthermore, the effectiveness at which a geothermal doublet is able to deplete a reservoir (recovery factor) and profitability of a geothermal doublet are closely interlinked. However, a higher recovery factor does not necessarily mean that the doublet is more profitable and vice versa. There exists an optimum well spacing for doublets positioned in homogeneous reservoirs, such that additional gain of later breakthrough (when placing the production well further away from the injector) is negated by the loss in pressure support of the injection well. This optimum well spacing is found to be an important factor, influencing the profitability in homogeneous and heterogeneous reservoirs. In addition, it is found that the optimum well spacing of a doublet for greenhouse heating is the same as the optimum well spacing of a doublet for spatial heating. The heat production from an aquifer can be maximized through the usage of multiple doublet layouts. It is found that, even in a heterogeneous reservoir, it is best to use a checkers-board well arrangement, which is more effective than a tramrail well arrangement.

Recently, an amount of methane was produced with hot water in the Ammerlaan geothermal field. The presence of methane influences fluid properties and fluid flow in the well, leading to changes in the pressure and temperature drop in the producing well. This study presents a model for non-isothermal two-phase flow in geothermal wells. In this model, two-phase flow regimes and their corresponding temperature and pressure drop calculations are coupled. The pressure drop, temperature drop and the flow regime are calculated for different well parameters for the Ammerlaan field. The simulation results are used to give a general overview of optimal well diameter and well inflow rate. Furthermore, the impact of two-phase flow regimes is shown in these simulations. We found that for a larger well diameter, pressure drop decreases. Pressure drop increases drastically when slug or even churn flow is reached. Smaller well inflow leads to a decrease in pressure drop. Based on the simulation results, the optimal combination of a well diameter of 0,3 meter and well inflow of 40 kg/s are found for the Ammerlaan field. Besides Ammerlaan, other fields with different salinity and gas contents are investigated in terms of pressure and temperature drop for two-phase flow in a geothermal well. It is found that fields with a low salinity and low gas content are more favorable for geothermal production.

We investigate non-isothermal compositional flow in methane-rich geothermal aquifers by coupling a thermodynamic model and a dynamic flow model. For the thermodynamic model, we develop a MATLAB program, which calculates the thermodynamic equilibrium of the H2OCH4-NaCl mixtures at high pressure conditions and reservoir temperatures. In the same program we calculate the transport properties. The dynamic flow model is solved using finite element simulations with the NegSat solution approach. We add artificial diffusion and adaptive mesh refinement to obtain a stable solution. Our interest is in the extraction of geothermal energy and our field of study is a reservoir in the Delft Sandstone Member (2200 m depth) and a reservoir in the Main Buntsandstein Subgroup (4000 m depth), both in the West Netherlands Basin. We consider the production of hot methane-rich water and the injection of cold water without methane for both reservoirs. The objectives of the study are to: 1) determine under which circumstances free methane gas can be present in geothermal reservoirs; 2) quantify how heat recovery is influenced by gas evolution in the reservoir (i.e., the release of solution gas into a free gas phase), and 3) analyze the possibilities for optimal operational conditions for heat recovery and the gas-to-water ratio. Given the pressure, temperature, salt concentration, thermodynamic equilibrium and gas-water ratio, we can determine the initial phase state of the reservoir and the possibility of gas evolution during production. Solubility and phase density calculations of H2O-CH4-NaCl mixtures are included in the thermodynamic model. In our simulations, the methane that the reservoirs contain initially is dissolved in the salt water and we investigate cases for different methane concentrations. For high methane concentrations, gas evolution occurs upon the pressure drop at the production well. Using simulation results we show that, even if the initial amount of dissolved methane approaches the solubility limit, the influence of gas evolution on heat extraction is very limited during production and injection. Furthermore, there is no noticeable effect of gas evolution on the water production rate and the production gas-water ratio. Gas saturations throughout the reservoir remain lower than 0.5% and are too low to alter the heat transfer and to cause upward migration of the evolved solution gas. The low gas saturations have a very low mobility and methane is therefore trapped in the gas phase. As the compositional front of the injected water, with high methane concentration downstream and no methane concentration upstream, progresses into the reservoir, the trapped methane gas will dissolve again in the injected water. Furthermore, the effect of a two-phase region on the relative permeability of water is very low for the conditions studied by us. Near the production well, where an increased pressure drop causes higher gas saturations (up to 2.2%), water flow rate is only reduced by 0.5-1.8% at maximum and changing operational conditions to optimize the subsurface flow regarding this two-phase flow would not result in noticeable improvements.