Geothermal Pre-Drilling Decision Optimization: Methodologies and Case Histories

Abstract Geothermal formations are hot, often hard, highly fractured and under-pressured. They often contain corrosive fluids and some formation fluids that have very high solids content. These harsh environments mean that drilling is usually difficult. Challenges include degradation of drilling fluids with associated variances in fluids properties, difficulty in managing mud systems, slow rate of penetration, short bit life and lost circulation. The potential for fines migration and induced formation damage in geothermal wells is significantly high due to weakening of attaching electrostatic forces under high temperatures and as a result of thermal contraction. Through case histories, this paper presents drilling challenges and the mechanisms of reservoir damages. The paper will also show the workflow and methodology of using the integrated geosciences analysis in pre-planning to mitigate the challenges related to geothermal activities. Understanding geothermal reservoirs challenges requires a systematic workflow including but not limited to the following: structural geology, mineralogy, geochemistry, drilling fluid chemistry, high-temperature rock-water-fluids interactions, drill bit selection, and geomechanics modeling. ThermoChemo-Poroelasticity stability analysis is also an important consideration. Lab work to properly select the drilling fluids chemicals is required to optimize the drilling fluids parameters and simulating bottom hole temperature. The outcomes from geology, mineralogy, geochemistry and geomechanics will be considered for optimum drilling fluids selection and fluids formulation optimization. The ultimate outcomes include but are not limited to MWT limits (Window), Breakout width, Pmud to trigger slip, drilling fluids formulation effects, drilling bits selection and surface parameters optimization. For reliable performance in high-temperature environments, we need to consider the following: Know your geothermal reservoir; rock type, mineralogy, geochemistry, structural controls, geomechanics and Thermo/Chemo-Poro-elasticity conditions.Matching your injected water chemistry to formation water chemistry is very important, especially in high TDS geothermal brines. Incompatible total dissolved solids (TDS) concentrations will alter the ion carrying capacity, disturb the natural reservoir equilibrium and can lead to formation damage.Optimizing drilling fluid selection.Hydraulics, gel breaking, swab and surge including thermal effects.High-performance drill bits to keep you in the hole longer, reducing trips and saving you moneyAdvanced drilling technologies to deliver fast, efficient wellbore construction, including specially engineered motors for extreme operating environments, automated drilling systems, and high-temperature MWD technologies.