Remote real-time pore pressure monitoring using a combination of Logging-While-Drilling (LWD) services coupled with a predrill pore-pressure model provides significant insight into wellbore stability and allows for optimizing casing points. This paper presents the results of a job in the Gulf of Mexico (GoM) that allowed an operator to drill confidently in a very tight hydraulic envelope and eliminate a string of casing.
The real-time use of data from the LWD formation pressure and sonic tools provides confidence in geopressure predictions. These LWD measurements allow the predrill velocity-to-pore-pressure transforms established during predrill modeling to be updated while drilling using the velocities from the sonic tool and pressures from the LWD formation pressure tool. This calibrated transform is then applied to revise the predrill pore-pressure model while drilling, thus reducing the uncertainty in the pore-pressure prediction ahead of the bit. In this case, the predrill model used velocities extracted from a 3D mechanical earth model of the northern GoM based on velocities derived from checkshots and sonic logs. These velocity data are kriged to give a 3D velocity model with uncertainty estimates.
Using state of the art Logging-While-Drilling (LWD) technologies, a new methodology was initiated to optimize drilling performance on a Vermillion 338 well. Continuously updated LWD annular pressure measurements effectively gauge wellbore pressures and help the driller rapidly intervene in pressure and/or geomechanical wellbore stability issues. A complete understanding of the hydraulic forces on a borehole can increase the rate of penetration, provide greater safety, minimize casing strings, reduce or eliminate kicks and formation fracturing, and allow faster and less expensive completions.
The technique described in this paper allows for incorporating real-time measurements into a pre-drill model, thus reducing the uncertainty ahead of the bit and allowing the operator to extend both the 9-5/8-in. intermediate casing and 7-in. liner to TD. As a result, a critical casing string was pushed 1,287 ft deeper than planned and a pre-planned 5-in. liner eliminated. The reduction in casing expense, as well as slim-hole drilling and completion costs resulted in a savings to the operator of approximately $1.7 million.