Well Testing and Interpretation Challenges of Discharging “Big Bore” and “Super Big Bore” Wells

Abstract

Big bore production wells have been drilled and are in full operation at the Ngatamariki and Kawerau geothermal fields. The recent Ngatamariki drilling campaign saw the drilling of a ‘super’ big bore production well with 18-5/8” cemented production casing, narrowing down to 13-5/8” cemented production casing and completed with 10-3/4” perforated liner in a 12-¼” hole and a 7” liner in an 8-½” hole. Big bore and super big bore production wells are drilled up to 3,000 meters and produce geothermal fluid up to 1,100 t/hr at 1250 kJ/kg discharge enthalpy. The successful drilling and operation of these wells have provided significant production contributions to the geothermal operations of Mighty River Power (MRP). The large mass flow and large casing sizes of the wells have presented unique challenges in full discharge downhole well test operations and well test results interpretation. MRP and Western Energy Services (WES) have successfully logged and analyzed big bore and super big bore wells. The operational challenges required additional planning to ensure quality data and risk mitigation. The study presents the operational set-up that was required to safely run a wire line log at full discharge conditions. In addition, the study presents wellbore modeling results that highlight the challenges in understanding and modeling two phase flow behavior in high flow and large diameter conditions.