Cost model for geothermal wells

Abstract

The cost of drilling geothermal wells is estimated to be about 40% of the total investment cost for a new high temperature geothermal plant. This makes geothermal plants more expensive to build than conventional fuel fired power plants, and as a result the cost of the wells becomes a key consideration when determining the economic viability of a geothermal field. Obtaining accurate costs for geothermal wells is, therefore, very important as it quantifies a substantial percentage of the cost of the geothermal project. This will help in future planning and budgeting of geothermal projects. Accurate well cost records also make it possible to carry out an analysis of drilling-cost-with-depth and evaluate the benefits of selecting different drilling technologies and materials for various geothermal fields and regions and, further, couple them with the energy-recovery-with-depth for the field.

The purpose of this paper is to develop a cost model for high temperature geothermal wells that allows for the estimation of well costs from a few key input variables such as well depth, number and size of casing intervals, and well trajectory. The model uses two input parameters, the criteria where the well design is established and a price book where all the unit costs are listed. The cost model then calculates the drilling materials required to drill each section of the specified well to completion. The cost of these materials is then automatically calculated using the unit cost that is automatically picked from the price book. The summary sheet then gives the total cost of the well. The paper also describes the well cost structure, the factors that affect the cost of the well and items considered when pricing a geothermal well. There is surprisingly little published data available on the breakdown of geothermal drilling costs due to the competitive nature of the geothermal drilling industry and confidentiality clauses. As a result, the data used for the price book for this model are estimated based on best guess values. The cost model divides the well cost into three major parts: pre-spud, drilling, and completion costs. The pre-spud costs include all the costs prior to spud-in, while the drilling costs are all the costs incurred while making the hole. This includes the rig rental cost, materials and the supervision cost. This is where the bulk of the cost lies. Finally the completion costs are the costs incurred after achieving total depth, prior to rig release. The model does not include the cost of monitoring the well after drilling nor that for flow testing.