Research Projects and Publications

Wellbore Stability - Principles and Analysis in Geothermal Well Drilling

Geothermal Engineering and Exploration ÍSOR

Author:  Samuel Ikinya Nganga
Year: 2018
Supervisors:  Juliet Newson and Björn Már Sveinbjörnsson


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. Well bore instabilities encountered during drilling 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 located in the Kenyan Rift System and is mainly dominated by normal faulting running in North-South direction. Geothermal wells in Olkaria are designed with 20" Surface Casing, 13⅜" Anchor Casing, 9⅝" Production Casing and the production section is lined with 7" perforated casing. Drilling progress is affected by various downhole challenges encountered that affect the stability of the well bore during drilling. 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 and abandonment of the well.

Well bore instabilities, in five geothermal wells in Olkaria at well pad OW-731, one vertical and four directional and a directional well RN-33 in Reykjanes Iceland haves been used in this report. Hard formation in the Surface Casing section, loss of drilling fluid circulation, collapsing formations and loss of cement during casing cementing were experienced during drilling of these wells. Revising minimum casing setting depths for Olkaria wells using pressure profiles of the vertical well sets Production Casing depth at 800m using boiling pressure for depth (BPD) based on water level at 700m and at 1100m using the pressure pivot point. The pressure pivot point is lacking in the directional well indicating need for a deeper production casing. . Minimum stress σ3 (fracture pressure) calculated using Eaton´s formula and overburden stress σ1  form the maximum and minimum field stresses used to calculate effective wellbore effective hoop, radial and vertical stresses. 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.  Directional wells at OW-731 pad are inclined to approximately 20° from the vertical at different azimuths but indicate difference in stress levels. 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.

Plotting Mohr´s circle diagrams using maximum effective stresses at different depths and drilling fluid densities 500, 800, 1000 1200 and 1800 kg/m3, indicate well bore stability is improved with increase of drilling weight as compressive hoop stresses that induce well bore collapse are reduced. Mohr's Criteria at drilling fluid weight less than SG of 1.0 show the effective stresses plot above the failure line at all depths but below the line for 1.8 SG at all depths The mid-point of the mud window corresponds to hydrostatic at 1.0 SG at 3000m. But at high drilling fluid density probability of tensile fracture increases that causing loss of drilling fluid circulation.