Low-Temperature Geothermal Energy Extraction from Depleted Oil and Gas Fields in the Taranaki Region
Authors:   , Duggal Rohit 
Published: July 2024

<p><strong>Mature oil and gas fields have the potential to produce geothermal energy and contribute to a low-carbon sustainable energy future. These fields produce more hot water than hydrocarbons at the end of their economic life from reservoirs which are usually at lower temperatures (<190℃) as compared to conventional volcanic geothermal reservoirs (>220℃). Their prospective use in energy generation, potential to decarbonize various process heat applications, reduce reliance on fossil-produced energy, and an opportunity to reduce capital cost and risk of developing on already available infrastructure have emphasized the need for an effective energy extraction process. This effective process must assess the feasibility of these resources to produce energy over the lifetime of a project. In hydrocarbon-depleted fields, this feasibility must include additional energy required to maintain production, predict thermal exchange with the surrounding formation, and determine losses to transport energy to the end-user.</strong></p><p>The energy required to maintain production depends on several factors such as the remaining pressure in the reservoir, dynamic evolution of the reservoir during fluid production in terms of thermal and hydraulic characteristics, impact of the presence or absence of an aquifer responsible for providing pressure support, and thermal properties of the fluid and the formation. Hence, this research explores a mature oil field and a gas field and presents an analysis of energy recovery using production and a reinjection wellbore configuration. The influence of reservoir depth, wellbore location, production rate, permeability-thickness value, and geothermal gradient is also investigated. In addition, a sensitivity analysis of the key parameters and their impact on the effectiveness of energy recovery is also investigated. The next step in assessing energy recovery is to determine heat loss to the surrounding formation.</p><p>The current thermal losses depend on the thermal conductivity of the wellbore, reservoir, backfilling material, and steel tubing. However, the traditional practice of determining thermal losses is based on analytical and empirical correlations requiring administrative efforts of the modeller to refine the model set up. The conventional method assumes the value of heat transfer coefficient based on fully developed flow conditions which are less practical, especially near each feedzone. In addition, this conventional method has a time factor to dictate the heat exchange between the fluid and the formation; and is arbitrary. The value of this time factor depends on the knowledge and experience of the modeller. An inaccurate value leads the analysis towards either underestimating heat exchange or overestimating the thermal loss. To enhance the heat loss modelling in a rigorous manner, this research proposes a method to remove the dependence of determining heat losses on the modeller’s input; and rather evaluate by using the practical parameters from the simulator. The thermal exchange calculated using the proposed method is compared to heat loss values computed using the conventional method. This comparison shows that the proposed method is an improvement on the current convention. In particular, the proposed method removes the need to define a time factor and obtains the key parameter values from the simulation time steps. Subsequently, it does not produce inaccurate estimations of the heat exchange. The proposed method predicts heat loss values within less than a 1% difference. The benefits offered by the proposed method mean that it can be employed to calculate accurate heat exchange in low-temperature and low-flow rate settings. Such a benefit may serve as a starting point for engineers wishing to design an effective energy recovery system in depleted oil and gas fields.</p><p>Another challenge noted is the practice of assessing the effectiveness of heat transport to the end-user. The conventional method often ignores length-dependent thermal or pressure losses. Hence the identification of critical insulation thickness of the transmission pipelines to improve the amount of energy transported to the end-user is necessary. The study includes energy extraction from wellbores flowing large quantities of water and the wellbores that are unused due to dry or no hydrocarbon shows. The former is utilised in production and an injection wellbore configuration, while the latter utilises a downhole heat exchanger to extract energy. It is identified that the thermal losses are less than 2% in a 10 km pipeline. The optimum pipeline diameter is around 500 mm which leads to minimum pressure losses and the least pumping power requirement. From the analysis of energy recovery, this thesis also presents a procedure for assessing the levelized cost of energy for the user. Various parameters were identified to improve economics and energy transported based on the objective of the design of a direct-use heat system.</p><p>This thesis provides an in-depth analysis of energy recovery from depleted oil and gas fields. The contribution of this research will help energy engineers to accurately determine the feasibility of a geothermal system in depleted fields leading to an effective design of the energy recovery system. Finally, the contributions will help to increase the adoption of depleted wellbores resulting in more energy generation. The outcomes of this research can be realized to reduce inaccuracies in determining heat losses and perform optimisation. Also, a generalised procedure for calculating heat loss can be developed in the future.</p>