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The purpose of this paper is to discuss emissions from geothermal power plants in New Zealand. Geothermal energy has been considered as a clean and sustainable source of energy. However, the geothermal fluid also contains the greenhouse gases which are emitted to the atmosphere after the electricity generation process. Releasing the gases to the environment after the electricity generation process is neither environmentally suitable and may not be economically feasible if the price of carbon increases. These emissions have become an issue of major concern with the draft advice of the climate change commission recommending that high emitting power stations be closed by 2030. Reinjection of the fluid and the gases back to the reservoir or using CO2 for industrial and agriculture purpose seems to be a viable solution towards achieving a zero-carbon emission target. Reinjection of separated brine is routinely carried out and plays an important role in the geothermal system as it provides pressure support which acts as a barrier to cold water recharge to the reservoir and reduces environmental impacts by deposing water back to the reservoir. Though reinjection provides a feasible solution to the water disposal problem, it can also harm to the reservoir by creating problem such as thermal breakthrough. Therefore, we need well planned and modelled strategies for reinjection systems, that are site-specific as the geologic setting differs from site to site. This paper will provide a qualitative understanding of the various reinjection strategies in New Zealand and the response of the geothermal system to those strategies. If the CO2 gas from a geothermal power plant is reinjected into the reservoir there may be impacts on surface features which may impact economic and cultural factors. This work will outline a study that will be undertaken to optimize the strategy for reinjecting CO2 gas into a geothermal reservoir. The optimization will consider the impacts on power generation, surface features and other factors. Index Terms—Greenhouse gas emissions, geothermal, reservoir, gases, reinjection, emissions.

Heat exchangers play an important role in binary geothermal power plants. The presence of some non-condensable gases in the geothermal fluid impacts the efficiency of the heat exchangers. These Noncondensable gases are released into the environment after separating them from the geothermal fluid used for electricity generation. They have become an issue of significant concern for the geothermal industry. The reinjection of gases seems to be a solution for mitigating emissions. However, we cannot ignore the impact of those gases on the geothermal power plants. Different factors affect the efficiency of heat exchangers in a binary geothermal powerplant, such as the mass flow rate, temperature, and fluid pressure. The gas concentration also plays a significant role in heat transfer between the fluids in the heat exchanger, impacting the efficiency of the heat exchanger. This study analyzes the variation in the heat exchanger's efficiency for different gas concentrations in the fluid at varying temperature conditions. The calculation was based on all the primary inputs and outputs from the heat exchangers—the measures for heat loss to the working fluid estimate the energy loss in the system. Also, the liquid's gas concentration at the heat exchanger’s output helps understand the heat transfer behavior to the working fluid in the heat exchanger.

Greenhouse gas emissions from geothermal power plants are a barrier to the growth of geothermal energy at present and in the future. The discharge of the gases associated with the electricity generation process from geothermal fluids to the atmosphere contradicts New Zealand's target to achieve net-zero emissions by 2050. These gases are dominated by carbon dioxide. Reinjecting the gases back into the reservoir by dissolving them in the separated brine could be one way of reducing emissions from the geothermal electricity generation process. Besides reducing greenhouse gas emissions, reinjection may help in mitigating silica scaling problems by making the reservoir fluids more acidic. This helps in the dissolution of minerals, which further helps in improving the permeability of the reservoir and contributes to variation in output from the production well. This paper discusses an analysis of the impact of the concentration of gases present in the reservoir on the mass flow rate from the production well. This is a result of changes in the thermodynamic properties of the fluid present in the reservoir. The analysis of the main parameters that contribute to the predicted mass flows will be presented. Well flows are estimated from production wells over the range of conditions found in New Zealand's geothermal production fields. It demonstrates the impact of reinjection of gases on different types of geothermal reservoirs through changes in mass flow rate from the production well.

Extracting energy from petroleum fields is an attractive topic for research. In the Taranaki region, some of the depleted oil and gas reservoirs are prospective candidates for low-temperature geothermal development. This is because the hydrocarbon wells produce a large amount of coproduced water at the surface. Therefore, we analyse one of the hydrocarbon fields encountering fractured reservoirs in the Taranaki region. We aim to estimate the production rate and well separations to sustain generation capacity from the field for 10 years. We used the Volsung reservoir simulator to simulate the reservoir models using the MINC (the Multiple INteracting Continua) formulations and representative rock properties. Then we analysed the behaviour of the petroleum reservoir over time with the extraction of geothermal energy. It was found that a flow rate of 180 kg/s is sufficient to produce 6 MW of electricity and could be sustained for at least 10 years. The work suggests that the theoretical maximum production rate between production and reinjection well separated by 500 m would be 220 kg/s before the production enthalpy declined below 20% of the initial value. But the field has several wells and larger separation between the production and injection wells. The model of the field has seen production rates of 180 kg/s to sustain a generation capacity of more than 6 MW without the evidence of any thermal breakthrough from the injection well over a period of 10 year. The model demonstrates the effective movement of the warmer fluids from the lower formations to the upper reservoir for higher production rates before the arrival of the cold waterfront from the reinjection well. For future analysis, it is recommended to conduct optimisation of the operation strategy in the field. A detailed thermo-economic study could also be performed to analyse the benefits of ultimate oil recovery, aquifer effect, and the onsite power demand.

The primary purpose of this paper is to discuss the greenhouse gas emissions from geothermal power plants in New Zealand. Geothermal energy has been considered a clean, economical, environmentally friendly, and sustainable source of energy. However, emission from geothermal power plants is a barrier to the growth of geothermal in the present and the future. The emissions have become an issue of primary concern after the report presented by the climate change commission recommending the closing of high greenhouse gas emitting geothermal power plants by 2030. Also, to reach New Zealand targets of achieving net-zero emissions by 2050, the discharge of gases to the atmosphere is to be reduced. The reinjection of separated brine plays an important role in the geothermal system. It provides pressure support that acts as a barrier to cold water recharge and reduces environmental impacts by deposing water back to the reservoir. Reinjection of gases could show different behaviour depending upon the reservoir conditions. This paper gives an outline of the use of geothermal energy for electricity production. It also shows the impact of gases on the flow rate from the well for a single-phase liquid dominated reservoir which showed an increase in flow rate from the well when the gases are present in the reservoir.

Geothermal energy is the most sustainable energy source, and geothermal reservoirs are an essential component of the geothermal system. The geothermal pool of liquid contains the geothermal fluid, including some non-condensable gases. These gases majorly consist of carbon dioxide. There is an impact of these gases present in the geothermal fluid that comes to the surface through production wells on the well's productivity, heat transfer capacity of heat exchangers etc. The paper demonstrates a process model showing the fluid flow from the production well to the heat exchangers. It is a system process model from the feeds in the wells from the geothermal reservoir to the heat exchangers. In addition, the model provides us with an estimate of the megawatts obtained for a given concentration of carbon dioxide in the reservoir. Finally, the calculations estimate how much the concentration of carbon dioxide changes the productivity of the well and the heat transfer capacity of the heat exchangers.

Hydrocarbon fields host several unused wells with temperatures suitable for low to medium-temperature geothermal energy extraction. The number of these wells increases every year with the depletion of petroleum reserves. Plug and abandonment of a well is a cost-intensive process, and these wells will sit idle after abandonment. However, an opportunity exists to use these drilled holes for geothermal extraction and extend their operational lifetime. This way, the cost of drilling new wells for geothermal exploration and development may be avoided. Also, it is environmentally beneficial and avoids any potential hazards from accidental spillage. To realize this motivation, the energy output from unused wells found in the Taranaki region, New Zealand will be evaluated in a Ph.D. programme. This is a major oil and gas producing region in the country and has many unused wells in place. These wells reach up to 5064m depth and up to 172 °C bottomhole temperatures are found in them. Due to fundamental differences between typical geothermal fields and hydrocarbon fields, it is important to study the thermal and hydraulic performance of these wells. Results obtained in the preliminary work for Ph.D. proposal discussing the potential of geothermal energy extraction will be presented. We will also describe the influence of the reservoir and fluid parameters on the thermal and hydraulic performance. Finally, we address some of the unique challenges and recommendations during energy extraction from these fields.