Lessons Learned and Performance Improvement: Drilling Case Study from Sarulla Geothermal Operation, North Sumatra

Abstract

A lessons-learned and continuous improvement approach used to improve drilling performance significantly during a five-well drilling campaign on one drilling pad at the Sarulla geothermal development field in North Sumatra, Indonesia. Drilling times were reduced by 73%, and costs decreased by 64% from the first to the fifth well. Drilling challenges on this pad included severe lost circulation, high vibration when drilling the surface hole sections, soft and swelling clays in shallow sections, along with sloughing Paleosol formations in the deeper section, and a corrosive drilling fluid environment, which resulted in drill pipe (DP) washouts and twist offs.
Optimized intermediate casing setting depths were developed to manage the sloughing formation issues. The use of nested liners was employed to stabilize the difficult hole sections already drilled but allowing drilling to commence safely with smaller hole sizes deeper. To help prevent intercepting previous wells and kickoff issues in the soft clay formation, the 26-in. surface hole section was “nudged” using directional drilling techniques. Formation clay samples were tested using the linear swelling method (LSM) and Xray diffraction (XRD) to optimize drilling fluids formulations specific for each section, and a set of best practices for drilling these formations was developed.
Other performance improvements that were applied include the following:
• Using a shock sub in the surface section
• Replacing drilling stabilizers with roller reamers to reduce the torque while drilling in the reservoir
• Using casing running tools (CRTs) to increase efficiency and safety of casing operations
• Using a custom-designed and built polycrystalline diamond compact (PDC) drill bit to help enhance the rate of
penetration (ROP)
• Modifying tubular inspection procedures to detect internal DP flaws