Cementing processes in geothermal well drilling: application and techniques

Abstract

During drilling of deep high temperature geothermal wells, zones of weak or fractured formations are encountered. These zones pose a challenge to the quality and effectiveness of the cementing process. Losses are encountered during cementing which require top jobs to complete and anchor the various casing strings back to surface with cement. The cement sheath should be able to withstand the cyclic thermal-induced loading through the operational life of a geothermal well. Cement is also used to plug zones of large circulation losses and to stabilize collapsing weak zones which slow down the drilling process. Well cementing is one of the operations which impacts the overall cost of well drilling process. In deciding the type of cement slurry to be placed for each of the casing strings in the well’s profile, adjustments are made on cement properties as per the strength of the open borehole walls (lithology). The open borehole wall’s strength is dependent on geological conditions of the section to be cemented. Cement properties such as viscosity, thickening time and strength are of prime consideration when engineering for the highest probability of successful primary cementing operations (Lecourtier and Cartalos, 1993). The viscosity and thickening time must be optimized so that the slurry remains pumpable long enough to place the cement across the desired zones and achieve the top of cement (TOC).

Additionally, cement should set as quickly as possible after pumping is stopped to minimize the wait on cement (WOC) time while achieving necessary structural integrity for retaining zonal isolation throughout the life of the well. The physical properties of cement need to be customized for the specific attributes of each well. Each well is unique in regard to pore pressure and the fracture gradient, bottom-hole temperature, bottom borehole pressure, physical properties of the formation, properties of the fluids already in the wellbore, wellbore deviation and annular space clearance. It is critical to perform cementing operations with cost and integrity of the operation in mind. With proper cementing procedures, drilling time and cost can be optimized (Halliburton, 2005). Cement manufactured to API specification class A is usually utilized in Kenya, and additives are added to improve its properties and customize it to a particular well environment. This report gives an outline of different cement placement methods, both for plugging large circulation losses, and for casing cementing under certain conditions. The methods will be compared to the method applied in Olkaria, Kenya for primary casing cementing and remedial top cementing of casing at eight hour intervals. Quality assessment of the cementing process, through laboratory tests and post cementing assessment, through the use of cement bonding logs and temperature logs, is also discussed.