Background
The study area is located in the Gulf of Mexico in 2000+ feet of water.
Pre-Drill Offset Well Analysis
For each offset well, the vertical stress (SV) or overburden, was calculated and a normal compaction trend line analysis was performed to estimate Pp. Disequilibrium of sand and shale pressure observed from pressure tests was taken into account using centroid and buoyancy effects. A slightly conservative approach was taken when estimating the FG. FG was assumed to be equal to the minimum horizontal stress (Shmin) and was calibrated to FIT/LOT/XLOT data from each offset well.
Application to Planned Well
A synthetic GR log that incorporated the potential sand bodies was created. SV and shale Pp for the planned well were calculated in a manner consistent with the offset wells using a stretch density log and the interval velocity along the planned well path. Centroid and buoyancy effects were considered to evaluate the potential for pressure inflation in sands prognosed from 2D seismic. FG was calculated in the same manner as in the offset well analyses. The resulting pre-drill Pp profile highlighted the risk of encountering pressure inflation in 2 shallower sands (Sand 1 and Sand 2) as well as in the deeper target sands.
Real-Time Monitoring & Model Updates
The competitor modeling the well in real-time and operator disregarded the risk posed by the shallow sands, basing the mud program on shale pressure alone and failing to increase the MW before drilling into the first potential sand. No drilling problems were encountered while drilling the depth interval of the prognosed Sand 1. The lack of drilling problems in this interval may have been taken as evidence that the sand pressure was not elevated. However, LWD Gamma showed that Sand 1 was not present. Since there was no sand, there was no pressure inflation and no need to increase the MW. The MW was also not raised before drilling into the second potential sand. Sand 2 was present and the well took a kick, consistent with the pre-drill model. After approximately 1 week of trying to regain control of the well, the decision was made to sidetrack the well.
Potential Cost Savings to the Operator
Incorporating the pre-drill model with the real-time monitoring, perhaps by watching for sands on the LWD Gamma and increasing MW immediately when Sand 2 was encountered, could have prevented a costly and dangerous well control incident and saved the operator approximately $10 MM.
|
|
|
|
|
|
