Cementing Challenges in Geothermal Wells
Geothermal energy is becoming an important source of energy and its importance will be increasing in the future. When we drill geothermal wells, we encounter high temperature zones and may also encounter high pressure areas. Cementing in high temperature environments such as geothermal wells is very challenging. The survey that was sent to High-Pressure- High-Temperature (HPHT) professionals at the HPHT Summit meeting in 2012, showed Cement Design is one of the biggest concerns for HPHT operations and it is one of their technology gaps. Temperatures as high as 200 °C-400 °C could destabilize the setting of the cement. If the well has both high temperature and high pressure, the cementing process becomes much more complex. This article discusses various aspects of cementing procedures and considerations for geothermal wells including crucial problems encountered in cementing of geothermal wells as well as cements design considerations, and some technology solutions. Some of the crucial problems encountered in geothermal wells that will be discussed in this paper include; a) High Temperatures which can cause a reduction in the compressive strength and an increase in the permeability of cement b) Lost Circulation which is one of the most common problem encountered in geothermal wells either during drilling or cementing c) Thick Filter Cake which can lead to differential sticking problems. d) CO2 Attack which is another problem found in high temperature geothermal wells that can result in cement carbonation. The CO2 in the fluids also can corrode cement and casing. Cementing in geothermal wells is more complex than ordinary oil and gas wells. Bottomhole temperatures could reach 400 °C sometimes creating some failure in the cementing process. Cement systems for geothermic environments are normally designed to have a compressive strength of at least1000 psi and possess no more than 0.1 mD water permeability (API Task Group on Cements for Geothermal Wells, 1985). The formation waters are often highly saline, corrosive and contain toxic heavy metals; as a result, the set cement must be designed to be resistant to degradation of saline brines and other destructive chemicals. Some of the Cement Design Consideration Technology for Geothermal wells that will be discussed in this paper include; Portland Cement Systems, Alternate Cement compositions with calcium phosphate or calcium aluminosilicate, Light Weight Cementing, Foam Cementing, use of Glass Bubbles, Latex slurry additives to avoid gas migration, Cementing lost circulation fibers, Reverse Cementing, use of External casing packers, Plugback cementing, and Two-Stage cementing.
AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019