Geothermal Wells, What Can Be Learned From The Oil Industry, A Technology Perspective

Abstract

Waterflooding is the most widely used secondary recovery method in the oil industry. The key to the success of these projects is proper planning and execution. Obviously, this is not different from what is required for a geothermal project. From decades of experience the oil industry has encountered numerous problems in their waterfloods, ranging from loss of injectivity, out-of-zone injection, early water breakthrough, loss of reservoir containment, injector-producer shortcutting, scaling issues etc. A number of these problems are equally applicable to geothermal wells and need to be addressed at the project design phase: -Is injection really occurring under matrix conditions? If this is unclear and fractured injection takes place a different strategy needs to be applied. -Injectivity decline and water quality specification are closely linked. Quite often water quality is not properly monitored and results in decreased injection rates which can lead to inefficient geothermal project. - In case of fractured injection, how will the fracture grow over time? This can lead to injector-producer short- circuiting. Even to the extent that the fracture grows into the overlaying seal which can jeopardize the right of the licence to operate and/or environmental impact. - What is the risk of thermal fracking if the cold water is injected into the warm reservoir. Realising the magnitude of the above problems Shell has developed a suite of tools to address the above issues in waterfloods which can equally help in designing and optimising geothermal projects. - Quantification of fracture growth as a function of water quality and injection rate is done by a tool called PWRI- FRAC. - On-site coreflood testing at real-life conditions, measuring formation impairment as function of water quality. To that end a mobile core flow test rig has been designed. - Estimating the injectivity decline over time using a rock impairment simulation tool (FORDAM). With this the extent of formation damage under matrix injection due to contaminated solids in the injected water. - Surveillance and analysis of the data are key to manage the injection of water and avoid risk as mentioned before. Typical data that should be collected is, rates, pressures, temperatures, water quality, but also scale tendencies should be monitored in case the conditions change, this can be done with tools like OLI. In addition, regular well test should be done to monitor fracture growth, typically Injection Fall-Off (IFO) tests are conducted. Rates and volumes injected can be depicted in so called Hall plots which can indicate if fractured injection occurs. All of the above can be applied to geothermal wells and leads to more successful projects and, more importantly, to safe operation of these wells.

AAPG Datapages/Search and Discovery Article #90345 © 2018 AAPG European Region, Geothermal Cross Over Technology Workshop, Part II, Utrecht, The Netherlands, April 17-18, 2018