Real Time Checkshots While Drilling - Reducing the Risk In Exploration Drilling

Abstract

Drilling exploration wells always carry increased risk given the lack of offset wells. One of the main uncertainties usually is the depth prognosis derived from surface seismic depth converted data.Checkshot surveys at the wellsite traditionally provide the time to depth conversion for drilling targets by providing timedepth pairs as the measurement. Given a vertically incident survey, this furnishes the most accurate seismic time-depth relationship for processing and drilling decisions. This paper describes the successful implementation and application of logging-while-drilling (LWD) seismic while drilling technology. Acquiring checkshots while drilling in an exploration field offshore East Malaysia, enabled real-time drilling decisions to set casing accurately and safely above the over-pressured formation top. This technique was employed on two successive wells; Well A and Well B in the same field.For Well A, a total of sixteen real-time while drilling checkshots were acquired. The refined depth model from the real-time checkshots indicated that the target depth was 87 m deeper than that suggested by the pre-drill velocity model. As a result casing was pushed and set deeper successfully.On Well B, a total of thirteen real-time while drilling checkshots were acquired which indicated the target to be 150 m shallower than original prognosis and casing was set earlier accordingly. Drilling then proceeded ahead safely with the correct mud weight and the well was completed successfully.