Planning for the deep vertical exploration well, in shallow waters off the West Coast of India commenced more than a year ahead of drilling operations. The well was targeted to 4500m to explore deep objectives in a four-way closure. Bottom hole pressure was expected to be in excess of 12000 psi and formation temperatures were predicted to be higher than 400°F (205°C). A major challenge was mobilizing special high pressure equipment to an area with limited oilfield infrastructure. The successful drilling and formation evaluation of the well has validated the efficacy of the design philosophy, highlighted the critical importance of mud hydraulics, HP/HT rheology, temperature modeling, real time detection of pressure transition zones, and will significantly impact future exploratory drilling operations in the area.

Pore-pressure prediction in a mature hydrocarbon province with producing fields bears issues different from an exploration or immature basin setting. Appraisal and production activities can introduce anadditional set of complications that needs to be considered to make well-constrained pore-pressurepredictions. Problems for infill drilling can arise from severe changes in rock strength (fracture gradientreduction), caused by depletion of reservoirs through production, or development drilling in neighboringfields, if there is pressure communication via a common aquifer. Increases above initial pressure can be caused by crossflow from overpressured reservoir layers through poor cement bonds or abandonments. Reservoirs can also receive additional pressure through water or gas injection. The Luconia gas province is a mature basin and several stages of successful exploration, appraisal, development and infill-drilling campaigns. Shell, supported by its partners, has successfully overcome the above mentioned issues by thorough and innovative pore-pressure prediction approaches. In addition to the standard bracketing of uncertainties, 3 key process elements are employed:proper framing of the pore-pressure prediction and continued interfaces with all stakeholders anda 'scenario-based'prediction approach, that captures all possible effects caused by appraisal and production of the target reservoir or neighboring fields, and that predicts their impact on the pore pressure of the reservoir to be drilled.Risks identified are captured and mitigated via a 5 point pore-pressure prediction that spans a facilities design range as well as a wider drilling range. The drilling-range end members have a low probability of occurring, but have to be captured to enable hardware selection that ensures safe drilling execution. State of the art technologies are employed to achieve this, including 4D seismic, regional databases, innovative modeling methodologies and full integration in a well-delivery process.

This paper was prepared for presentation at the 1999 SPE/IADC Middle East Drilling Technology Conference held in Abu Dhabi, UAE, 8–10 November 1999.