Wellbore stability analysis in geothermal well drilling

Abstract

Drilling progress is affected by various downhole challenges encountered during drilling that impact on wellbore stability. The main challenges are loss of circulation and borehole wall collapse that lead to stuck drilling string, problems in landing casings and liners and in extreme cases loss of drill string components and abandonment of the well. Wellbore instabilities encountered during drilling can add to the overall cost of a geothermal well by consumption of more materials and extension of well completion time. Minimum stress σ3 (fracture pressure) from Eaton´s formula and overburden stress σ1 form the maximum and minimum principal field stresses in normal faulting systems such as Olkaria in the Kenyan rift. These stresses are used to calculate effective hoop, radial and vertical stresses at the wellbore wall that control borehole failure if the formation strength is exceeded. Maximum compressive hoop stress occurs at 90° and 270° and minimum hoop stress at 0° and 180° in vertical well indicating the direction of minimum and maximum horizontal stresses, respectively. In directional wells the hoop stresses are dependent on the well inclination and azimuth. Radial stress emanates from drilling fluid pressure and varies with variation in fluid density. Variation of stresses around wellbore wall are demonstrated using vertical and directional wells at OW-731 in Olkaria geothermal field Kenya by developing Mohr’s circles at different drilling fluid densities.