Abstract
Reservoirs in the D-18 fields, offshore Sarawak, Malaysia, are a combination of structurally and stratigraphically trapped sand bodies. Potential hydrocarbons are accumulated in the stacked sand bodies separated by weaker sealing shale bodies and regional coal seams. The fields have significantly depleted pressure following several years of production from different reservoir levels.
Reservoir pressure depletion results in a lower fracture gradient and a narrower drilling mud window. While a lower mud weight is necessary to drill through heavily depleted sands, the lower mud weight may result in excessive wellbore failure in normally pressured, weak formations. A geomechanical assessment is performed to aid coiled tubing horizontal drilling in the depleted reservoirs, which are interbedded with normal-pressure, weak shaly formations.
A geomechanical model was developed using available data from offset wells. A stress path factor is estimated to model the fracture gradient decrease from production-induced depletion. A safe mud window is optimized using the reduced fracture gradient in depleted sands and the hole collapse pressure in the normal-pressure weak shales. The operational mud window is optimized for hole size and annulus. The wells were drilled successfully using the recommended mud windows, without losses into the depleted sands or hole collapse in weak, normal-pressure formations.
The paper presents the workflow for geomechanical model building incorporating the depletion effect, the safe coiled tubing drilling (CTD) program design assessing various risks and highlights various CTD operational issues and the lessons learned.
