Exploring for hydrocarbon in deep water since the early 1970s has considerably changed the seismic acquisition, drilling and production practices. It requires floating drilling rigs, rather than the fixed jack ups which were used on the shelf since the late 1940s and are still in use today.
The most common drilling challenges in the deep water were limited to Loss of Circulation (LOC) and Shallow Water Flow (SWF). The LOC and SWF caused drilling hurdles resulting in high costs to test a prospect and sometimes led to the abandonment of the well. High pressure in the deeper subsurface formations that are being drilled today has emerged as a new challenge.
The decade from 1969 to 1979 witnessed three massive spills from offshore oil wells around the world: the IXTOC 1 spill off Mexico in 1979, the Ekofisk spill in the North Sea in 1977, and the 1969 spill off the coast of Santa Barbara. On April 20, 2010, a loss of well control occurred and resulted in an explosion and fire on the Offshore Drilling Unit Deepwater Horizon. Eleven lives were lost in this incident, the rig subsequently sank, and the resulting release of oil has been declared a spill of national significance with oil threatening sensitive coastlines and resources in the Gulf of Mexico. Current information indicates that a high pressure hydrocarbon reservoir is the main cause of this tragedy. The BOP, as a last resort in a sequence of preventative measures, did not function in response to an unexpected pressure surge.
The behavior of the subsurface geopressure profile is driven by subsea water depth, sedimentation rate, lithology, stratigraphy, structural setting, and stresses. In deep water, stress reduction due to subsea water depth is the main cause of the narrow Drilling Tolerance Window (DTW). This leads to a limited maneuverability to control the formation pressure using mud weight pressure. Mud pressure is the first line of defense to combat bore- hole’s flow, well kicks, gas infusion, and blowout. The Blowout Preventer (BOP) is built and designed, as the last line of defense, to control the hard kick of an unexpected pressure.
There are many schools of thoughts explaining the causes of abnormal pressures and the techniques to detect pressures changes in subsurface. We know pore pressure progresses with depth. In the deep water there are four zones. They are from the top to the bottom: (1) the shallow free flow, (2)hydrodynamic, (3) transition, and (4) geopressured zones. Each zone is characterized by a pressure gradient dictated by lithology and stress. Some of the drilling problems in the upper two zones are SWF, LOC and the presence of hydrates. Riseless drilling was developed to tolerate the mud pressure that is needed to stay within the DTW and reduce the number of casing seats to reach exploration objectives.
In the lower two zones, pressure exponentially accelerates in the shale (Seals) and becomes hydrostatic in the sand beds (Reservoirs). The excess pressure created by compartmentalization between the shale and sand is usually amplified by the presence of hydrocarbon. Drilling the interface between shale seal and pay sand is the most troublesome drilling zone. Hard kicks, mud cuts, well flow, and blowouts are common at the seal-reservoir boundary and well prognoses should account for that.
Before drilling, seismic velocity, offset wells and modeling are used to predict pore-pressure. Calibration of the model should be performed while drilling. Failure to incorporate the geological setting, structural pattern, and the expected hydrocarbon column height can lead to faltering well prognoses and unforeseen events.
There are several lines of defense that should be employed before relying on the BOP.
Before Drilling:
- Perform sub-surface geopressure modeling using seismic velocities.
- Calibrate the model using the offset wells. In the Deepwater Horizon case, located in Mississippi Canyon block 252, there are several wells close by. Two of them are in Mississippi Canyon block 252 (Texaco) and two miles due south in Mississippi Canyon block 296 (ENI).
- Design well prognoses to account for geopressure compartmentalization, in addition to the expected hydrocarbon column.
During Drilling:
- Observe the balance between the formation pressure and the mud pressure.
- Increase mud weight to combat the flow, kicks and bore-hole instability as needed and calibrate the model accordingly.
Casing seats and cementing:
- Cementing the casing should be done in a gas free mud. Presence of gas in mud causes lower integrity cement bond. The cement shoe should be able to stand firm against the formation’s high pressure, especially with the presence of hydrocarbon. Repeated circulation should take place to clear the dissolved gases in the mud.
- Run Cement Bond Logs to QC the annulus cement infill. The presence of gaps in the cement requires squeezing additional cement behind the casing to increase cement integrity.
- Test the casing shoe to a pressure value exceeding the hydrocarbon charged formation.
In the recent event, it seems that the bottom cementing plug was not able to hold the formation pressure, especially with the presence of hydrocarbon. At this point the hydrocarbon gushed out through the casing to the drilling ship.
Reliance on the BOP without careful and diligent use of additional lines of defense, such as those listed above, may result in other catastrophic events in deep water exploration.