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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.

Vertical seismic profile (VSP) data is a powerful calibration tool that helps to link the time (surface seismic) & depth domains (well logs). Conventional rig source VSP data is not recommended to be used in a highly deviated well for seismic calibration. Excessive ray bending from the source-receiver offset will cause errors in the vertical time calculations, and this data will not be reliable to calibrate the surface seismic. This paper will discuss a case-study for a highly deviated offshore well using a vertical incident VSP (VIVSP) source geometry. A VIVSP is a unique type of geometry that places the source above the receiver array. This is usually done using a supply vessel for an offshore environment. Placing the air gun source above the receiver ensures vertical raypaths, and this gives the most accurate TWT result. Planning is crucial for this type of VSP to calculate the number of receiver arrays & source positions. 2D ray trace modelling using the surface seismic interval velocity & VIVSP geometry was used to verify the data would be within tolerance for vertical raypaths, and to optimize the acquisition time. VSP data was acquired using the optimized vertical incidence geometry. Data quality was excellent which gave a reliable time-depth profile for most of the well interval. VSP data was processed to corridor stack. and CDP transform was used to create an image below the well trajectory. This data was used to calibrate seismic velocity during seismic processing, as well to tie surface seismic to well.

Vertical seismic profile (VSP) data is a powerful calibration tool that helps to link the time (surface seismic) & depth domains (well logs). Conventional rig source VSP data is not recommended to be used in a highly deviated well for seismic calibration. Excessive ray bending from the source-receiver offset will cause errors in the vertical time calculations, and this data will not be reliable to calibrate the surface seismic. This paper will discuss a case-study for a highly deviated offshore well using a vertical incident VSP (VIVSP) source geometry. A VIVSP is a unique type of geometry that places the source above the receiver array. This is usually done using a supply vessel for an offshore environment. Placing the air gun source above the receiver ensures vertical raypaths, and this gives the most accurate TWT result. Planning is crucial for this type of VSP to calculate the number of receiver arrays & source positions. 2D ray trace modelling using the surface seismic interval velocity & VIVSP geometry was used to verify the data would be within tolerance for vertical raypaths, and to optimize the acquisition time. VSP data was acquired using the optimized vertical incidence geometry. Data quality was excellent which gave a reliable time-depth profile for most of the well interval. VSP data was processed to corridor stack. and CDP transform was used to create an image below the well trajectory. This data was used to calibrate seismic velocity during seismic processing, as well to tie surface seismic to well.