Wellbore stability : principles and analysis in geothermal well drilling

Abstract

Drilling a stable geothermal well that experiences least drilling challenges is key to delivering a successful well that meets the set objective of either being a production or reinjection well. Wellbore instabilities encountered during drilling can add to the overall cost of the well by consumption of more materials and extension of well completion time. Olkaria geothermal field in Kenya is a high temperature field and wells are designed with 20" Surface Casing, 13⅜" Anchor Casing, 9⅝" Production Casing and the production section is lined with 7" perforated Liner. Drilling progress is affected by various downhole challenges such as loss of drilling fluid circulation and borehole wall collapse that lead to stuck drilling string, problems in landing casings and liners and in extreme cases loss of irretrievable part of drill string and abandonment of the well. Well sections with less drilling problems affecting drilling progress have high percentage of time spent on drilling activity but wells that encountered downhole challenges have less drilling time compared to other activities that do not add to the well depth. Geothermal wells in Olkaria at well pad OW-731 and well RN-33 in Reykjanes Iceland have been used in this report. Reassessment of minimum casing setting depths for 3000 m deep Olkaria wells was made according to the The African Union Code of Practice for Geothermal Drilling (2016). The criteria applied for this report was for the formation temperature and pressure to follow the boiling pressure for depth (BPD) curve based on a water level at 700 m and the effective containment pressure resulting to a vertical Production Casing depth of 1450 m.

The pressure pivot point is lacking in the directional well indicating need for a deeper production casing setting depth. Minimum stress〖 S〗_h calculated using Eaton´s formula and overburden stress S_v form the maximum and minimum field stresses used to calculate effective hoop, radial and vertical stresses on the wellbore wall. 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 measured clockwise from North (0° azimuth). In directional wells, the hoop stresses are dependent on the well inclination and azimuth. Directional wells at OW-731 pad are inclined to approximately 20° from the vertical at different azimuths but indicate difference effective stresses. Well RN-33 with an inclination angle of 30° at azimuth of 171° has the highest hoop stresses at 96°/276° followed by OW-731D (200°), OW-731B (225°), OW-731A (135°) and OW-731C (270°) with the least measured clockwise from North (0° azimuth). Mohr´s circle diagrams using effective stresses at different depths and drilling fluid densities 0, 500, 800, 1000 1200 and 1800 Kg/m3, indicate compressive failure that induces wellbore collapse during loss of circulation at all depths. Tensile failure that can result in fracturing occurs in all depths at 1.8 SG because of high radial stresses. Wellbore stability is maintained with drilling fluid density between 0.8-1.2 SG. The average of estimated formation pressure and calculated minimum stress gives a ratio of 0.60 to 0.73 for minimum stress that corresponds to an ECD of 0.60 to 0.93 SG from 750 m to 3000 m giving a range of drilling fluid variation.