SS: Technologies To Identify Salt-Related Deep-Water Drilling Hazards
Authors: Cullen Paul , Taylor Jack , Thomas William , Whitehead Paul , Brudy Martin , Van Der Zee Wouter 
Published: July 2010

Drilling through a salt canopy can potentially provide a faster route to reach a sub-salt objective rather than drilling through a thick, overpressured sedimentary section in a supra-salt mini-basin. However, numerous geological factors can complicate the drilling, leading to expensive sidetracking or casing operations. These factors include isolated sediment inclusions within the salt, continuous or discontinuous sediment inclusions between sutured salt bodies, rapidly creeping salt, and anomalous pressure conditions such as low fracture pressures and/or narrow pore pressure-fracture pressure differentials at the base salt exit point. Sediment inclusions may be normally pressured or overpressured, possess low fracture pressures and contain variable fluid types including tar which can result in stuck drill pipe, well control events or fluid losses. Significant drilling costs could be eliminated if these hazards could be identified and avoided in the well planning process. In this paper we discuss the potential for recent developments in seismic technology including wide azimuth acquisition, 3-D multiple attenuation, sub-salt velocity determination, pre-stack depth migration, LWD borehole seismic tools and advances in geomechanical modeling using 3-D finite element analysis to impact well planning and drilling operations. Continued investment focus on these technologies could result in mitigation or avoidance of these geological hazards and greatly increase the efficiency of drilling deep-water wells to reduce finding and development costs.

Drilling Hazards

Some of the most active and high profile hydrocarbon plays currently being explored and developed around the world lie below a salt canopy of variable depth, geometry and thickness. Creeping salt is a problem in certain areas of the Gulf of Mexico (GOM), particularly the northern GOM salt trend 1 and also in the Campos Basin of Brazil due to high temperatures and high differential stresses 2. It can generally be managed through the use of mud weights close to the overburden (90-100%) and the use of reaming to open up the well bore prior to casing. Casing collapse is a rare but potential consequence of salt creep 3.

Overpressured shales are commonly encountered as inclusions or in sutures within the salt and may require increased mud weights to control the flow of the plastic shales into the well bore. The problem may be further complicated by adjacent geomechanically weak lithologies that fracture due to elevated mud weights, leading to a complex well control situation that may ultimately require plugging and sidetracking.

Tar has been encountered in various situations in wells in the deep-water GOM such as inclusions and sutures within salt, in deep rooted faults and fractures beneath salt and in pore spaces within the target reservoir intervals. These tar occurrences present significant drilling challenges as the flow of tar into the wellbore will " ball?? around the bit, requiring a trip to change out the bit, or pack off the drill string necessitating the use of jars and large overpulls to retrieve the drilling assembly, or may even result in sidetracking.

Any of these events can add significantly to the final well cost and negatively impact the economics of the project. In order to engineer a well plan that mitigates these risks, identification of such hazards must extend from the mudline to TD.