Predicting the Geopressure Tolerance Window in Deep Water: Safe Drilling is Walking a Fine Line, Case Histories from GoM

 
Exploring for hydrocarbon in deep water has considerably changed the oil and gas industry premise. Great prospects have been abandoned due to their drilling complexity. The substantially increased water depth has impacted the subsurface geopressure profile and consequently all aspects of exploration, from seismic acquisition to drilling. 
The behavior of the subsurface geopressure profile is driven by subsea water depth and stresses created by sedimentation rate, lithology, and structural setting. Principal Stress reduction due to subsea water depth is the main reason of the narrowing safe drilling envelope between the pore and fracture pressures i.e. Drilling Tolerance Window (DTW). This leads to limited maneuverability to control and overcome the surge of formation pressure using mud weight. 
In the deep water there are four subsurface pressure zones. They are, from the top to the bottom: (1) the shallow free flow, (2) hydrodynamic, (3) transition (top of geopressure), and (4) geopressured. Each zone has it’s own pressure gradient dictated by stresses and compartmentalization. Several seismic velocity – pore pressure transformation models are used to identify these zones and predict their pressure gradients. The presence of nearby offset wells is very helpful in calibrating the prediction model. However, calibration of the forecast model should also continue during drilling.
The most common drilling challenges in the upper two zones are limited to Loss of Circulation (LOC) and Shallow Water Flow (SWF). On the other hand, in the transition and lower geopressured zones, higher pressure has recently emerged as a new challenge, especially during testing deep prospects. Hard kicks, high torques, mud cuts, Flow-Kill-LOC (loss of circulation) phenomenon and even uncontrollable underground and surface blow outs can take place in these deep geopressured zones.
In this study, several case histories from East Breaks and Mississippi Canyon areas (Neogene) exhibit the drilling challenges due to narrow DTW. The examined wells show that excess pressure created by compartmentalization between the shale and sand is usually amplified by the presence of hydrocarbon. This leads to even narrower DTW and hazardous events. Drilling the interface between seal and pay sand in the geopressured section is the most troublesome drilling zone e.g. Deep Horizon in MC 252 (Macondo prospect).
Moreover, drilling the new play trend at the Salt Toe (Wilcox / Neogene ) has suffered a number of drilling setbacks in some prospective areas. This is usually a result of narrow DTW, especially at the base of the salt.  Losses of circulation followed by formation breakdown are common at the salt-sediment lower interface. Side tracks and bypasses to overcome these problems can substantially increase the exploration cost.  Exploring Jack, St. Malo, and Atlantis fields in deep water frontier area exhibit the importance of defining this window ahead of drilling, especially in Sub Dirty Salt prospects.
To safely walk this fine line, several steps need to be taken. Before drilling, a calibrated geopressure model should be diligently assessed from seismic velocities and offset wells. The optimal processed velocity should be screened, QCed and tailored for pore pressure predictions. The predicted geopressure profile should be enhanced by adding the pressure transgression and regression risks due to the geological building blocks setting. 
During drilling, model parameters and mud weight ought to be tuned frequently, especially across the shale – sand interface and potential pay zones. The excess pressure due to the presence of hydrocarbon column can create a sudden pressure surge and mud weight has to be adjusted accordingly. Setting casing and cementing operations must be done in gas free mud.
Drilling for deep prospective objects in the deep water is sensitive to the pressure – fracture subsurface envelope. Understanding and applying the previous recommended steps to managed pressure drilling can circumvent any future disastrous hydrocarbon spill. The Blowout Preventer (BOP) should be the last line of defense to control hard kicks, pressure surges and unexpected underground – surface blow outs.