Harnessing Geothermal Energy from Mature Onshore Oil Fields- the Wytch Farm Case Study

All mature oil fields co-produce significant volumes of water with oil, which is a consequence of waterflooding and/or aquifer influx. Water cut increases with the life of the field, sometimes reaching as high as 99 %, leading to decrease in oil revenues. Such volumes of co-produced water require costly treatment to remove the oil before it can be safely disposed of or re-injected back into the reservoir. Typically, these waste streams of water have temperatures in the range of 65 to 150 °C. This combination of moderate temperatures and large volumes is favourable for generating electricity or for use in district heating. Being able to harness geothermal energy from existing oil and gas fields could extend their lifespan by delaying their economic cut-off point. This would be achieved by generating new revenue streams, using in-field generated power to offset fuel costs and reduce operational expenditure, delaying decommissioning liabilities, and increasing ultimate oil recovery. The emissions of greenhouse gases could also be reduced by providing renewable, low carbon energy. The objective of this study is to assess the potential for geothermal exploitation of the co-produced water from mature onshore hydrocarbon fields in the UK. The Wytch Farm oil field, located in Dorset, Southern England, was chosen as the case study, with the necessary data provided by the field’s operator. The available geothermal energy could be harnessed in the form of electricity, via an Organic Rankine Cycle (ORC) power plant, or as thermal energy for direct use, via heat exchangers. An analytical model is developed to compute the power generated from co-produced water by ORC technology over a range of operating conditions and with different working fluids. Flexibility is built into the model to account for time-varying properties, such as increasing water cut and decreasing reservoir temperature over the life of the field. At 65°C, the co-produced water at Wytch Farm represents a low- to moderate-temperature geothermal resource, with associated low cycle thermal efficiencies. However, in comparison with gas turbines or diesel engines, the ORC’s thermal efficiency is not critical in power generation, as the fuel source is essentially free and abundant. The results of this study indicate that, for the power generation option, a maximum of 1.4 MW of electricity could be produced at site M at the Wytch Farm oil field, and 850 kW at site F. With the combined heat and power (CHP) scenario, a total of 2600 households could be served with district heating from Site M, with a total thermal energy of 37 GWh delivered annually. However, these results must be interpreted vis-à-vis overall economics and users’ availability