A New Perspective to Shallow Water Flow (SWF) and Sinking Well-Head Preventions in Deep Water
A new study integrating the seismic velocity profile with a proposed subsurface geopressure partition sheds light on one of the possible main causes of shallow water flow (SWF) and sinking well head in deep water. The Bureau of Ocean Energy Management (BOEM), previously known as the Minerals Management Service (MMS), reported 157 cases of SWF in the Gulf of Mexico. Most of these cases occurred in the Mississippi and Green Canyons areas where the late Pleistocene depositional fan was active. Occasionally, conductor casings and well heads sink and get lost in these areas as well.
Study of the pressure gradients of sand vs. shale in the proposed subsurface zones (A, B, C, and D) points to a possible source of these two events. The fragile nature of the unconsolidated shallow hydrostatic zone A is mostly responsible for the loss of well head. This shallow zone gradually transforms to a compacted hydrodynamic system (zone B), associated with a dewatering process that can lead to SWF.
Calculating the linear pressure gradient in the sand beds vs. the feasible formation pressure in the shale layers in zone B is the backbone of this shallow water flow study. The sand rapidly flows upward at a linear gradient (0.536 * z – 228) ranging from 0.53 to 0.59 psi/ft. On the other hand, slow compaction of shale and dewatering follows an exponential pressure gradient rate of 1.49 * ln(z – MLdepth) – α. During drilling, penetrating the interface between the shale and the underlying sand causes sand overflow that overcomes the mud pressure and SWF takes place due to the pressure differential between the sand and the shale.
Mitigating these events should be attempted before drilling any wells in the deepwater. Seismic velocity, sequence stratigraphy and geopressure modeling can identify potentially problematic zones so that precautions can be taken to combat and avoid these challenges during drilling operation. Choosing the right depth for casings and adjusting the value of the mud up during drilling to avoid SWF are recommended in this article to avoid potential well abandonments.
AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015