Converting an Oil Field into a Geothermal One: the Use of a Decision Matrix Procedure

Abstract

In current energy outlook and oil price trends any operation in extending the life of oil&gas fields is welcome. Mature oil fields are characterised by a large amount of co-produced water. The produced water must be treated continuously and could not be delivered to the environment. Oil dewatering and water injection operations tend to increase operational expenditure (OPEX). Therefore, oil companies are actively seeking more innovative ways to reduce operating costs and to extend the life of their ageing fields. From these assumptions, the waste heat recovery from the produced stream could be a quite interesting option. The main advantages are: the improvement of the entire economic cycle of oil production processes; reduction of the reservoir abandonment costs; increase of the life time and the recovery factor of the reservoir; improvement of the economic feasibility of the geothermal plant having avoided drilling costs (generally corresponding to the 50% of total costs of the project). This study presents an unconventional lifecycle management in two steps implemented on one of the largest European oil fields, Villafortuna Trecate oil field. The first step considers the co-production of hydrocarbons and geothermal energy. In order to evaluate the production of electrical energy from the produced hot fluids, has been decided to use a binary cycle plant based on an Organic Rankine Cycle (ORC), which have a fixed power generation. The analysis was conducted over the entire range of water cut. The second step of lifecycle management starts when hydrocarbon reservoir is depleted and the wells produce only hot water. To convert the field into a geothermal energy production one, two different solutions have been compared: the use of produced geothermal fluid and the implementation of a wellbore heat exchanger. The first solution assumes that the plant keep on producing geothermal energy as in the first step of lifecycle management. No changes to the production plant are forecast. In the second scenario the bottom well will be closed and a wellbore heat exchanger, made by two coaxial pipes, is inserted into the well; the heat carrier fluid is pumped into the annulus, gradually heated by rocks, it is recovered from the internal tube. This type of well completion avoid to produce geothermal fluids, its reinjection and reduce the environmental impact. Also corrosion and scaling problems are avoided. To select the best refitting solution for Trecate plant, a decision matrix procedure was developed. This matrix can be exploited for other fields, even in case of new geothermal plant, to choose the best solution between the traditional plant (production – injection wells) and a wellbore heat exchanger plant.