The Geothermal Potential Held within Carboniferous Sediments of the East Midlands: A New Estimation Based on Oilfield Data

Carboniferous sediments have, to date, been largely ignored when UK geothermal resource assessments have been made. Resources located within deep sedimentary Mesozoic basins, and those associated with radiothermal granites have formed the main focus of resource quantification in recent years. There has been no attempt to formally quantify the resource located within Carboniferous sediments due to their complex structural and diagenetic history. The East Midlands Petroleum Province is the onshore extension of the Southern North Sea Basin. Oil reserves are typically found in Upper Carboniferous sandstone units, and rarely in Lower Carboniferous (Dinantian) Limestones. Exploration within the East Midlands has led to the discovery of over 30 separate fields. In 2011, IGas Energy PLC (IGas) purchased and now operates 16 of these fields. The well records and production data that were obtained as a result of this procurement has been used to produce a first quantification of the geothermal resource held within Carboniferous strata. Using known production data, Horner-corrected formation temperatures and oil/water specific gravity from 23 fields, a value of stored heat has been obtained for each field. In total, the geothermal resource has been approximated as being between 1.74 MWt and 4.36 MWt. Given these fields cover only 0.78% of the East Midlands total area, the potential for a larger geothermal resource base is likely to exist. Removal and sale of heat from the co-produced water will improve the economics of tail end production by lowering the effective total operating expenditure. Reinjection of the cooled water could also help increase the recovery factor of the reservoir; the cooled water having a higher viscosity and hence lower mobility ratio contrast with the oil than would hot water.