| Abstract |
A deep well drilling program was undertaken in the Bulalo Geothermal Field from 2002 to 2004, with a total of 10 makeup wells drilled to between 9,300 ft and 11,300 ft (MD). Well design was challenging due to a combination of a shallow reservoir top (2,500 ft) with the high temperatures (>640ºF) and corresponding saturation pressures (>2,040 psig) encountered at depths of >8,000 ft and the potential impact these conditions could have on the casing and wellhead. This was demonstrated during initial testing of Bul-103, where the wellhead pressure increased to over 1,850 psi while the well was on bleed. This remains the highest wellhead pressure ever recorded in the field. The well design used for the first three new wells, Bul-104 to 106, was overly conservative due to the incident at Bul-103 and also proved difficult to implement, primarily due to the need to run and cement the control casing string after the well had been drilled into the reservoir and circulation had been lost. After extensive multi-disciplinary discussions, modifications were made to better match the reservoir conditions, based on data obtained from deep wells. The modified design included running and cementing the control casing prior to losing circulation and allowing cross-flow to occur in the well into shallow permeable zones, with the objective of reducing wellbore pressures. The required shallow permeability was verified by injection tests prior to completing the well into the deep reservoir. The results from the wells showed that the modified design was successful, with measured pressures within the casing and at the wellhead being well within the design criteria. |