Drilling for hydrocarbon deposits beneath the salt canopy in the Gulf of Mexico (GOM) has become common in deepwater; however, such drilling is made more difficult because of geological events that can cause difficulties in the drilling process, leading to expensive sidetracks or unwanted extra casing operations. To make operations economically feasible and to accomplish demanding rig utilization schedules, wells are planned to be drilled using one bottomhole assembly (BHA) from casing to casing at optimized rates of penetration (ROP), accurately placing the well while minimizing drilling and geological risks. Sediment inclusions, salt creep, rubble zones, narrow mud-weight windows along with well control events and mud losses, wellbore stability events and thin tar zones are some of those geological events found when drilling through salt and subsalt. Rotary steerable systems (RSS), reamers and synthetic oil-base muds have provided solutions to most of these challenges.
This paper presents case histories of two deepwater wells drilled in the GOM. The first case, describes how, after having drilled through a long salt interval, tar was unexpectedly penetrated, requiring sidetracking in two different bypasses at high inclinations. Previous experiences of the operator and the industry indicated that tar cannot be detected in seismic data or accurately correlated from well to well because its short extension, and its presence could prevent reaching programmed total depths. Because of the inability to drill through the few sequences of tar intervals, it was required to sidetrack the well on two different occasions at high inclinations to reach the planned borehole total depth with the planned casing program.
The second case presents the history of the deepest sidetrack performed in the GOM. This well was sidetracked for geological resons, subsalt at 28,822 ft measured Depth (MD), off of a cement plug.
A push-the-bit RSS (and a reamer in the first case history) was used to sidetrack off cement plugs in both cases. Both cases were planned to sidetrack on the low side; however, this practice commonly used for downhole motors, proved ineffective in both wells. Both cases indicated that good cement was placed in the lower side of the hole, and so a setting to the right of the high side of the toolface was used successfully to deflect the wellbore. The technique, difficulties, and best practices resulting from drilling these high inclination sidetracks are addressed in this paper.