The north-central Gulf of Mexico area received rapid deposition of a basin-floor fan system consisting of interbedded muds, silts, and sandy turbidite deposits during the Pleistocene. Overpressure occurs at shallow depths when burial rates exceed the dewatering rates of sediment pore fluids. Two stratigraphic sequences in the region contain significant overpressure with elevated shallow-water flow risk within these units. We have used publicly available seismic and well data to identify the geomorphology and overpressure variation of these units. The previously described “Blue Unit” and its lateral extent, thickness, depth below sea level (BSL), and overpressure gradient have been revised. The Blue Unit extends from the northern portion of the Mississippi Canyon (MC) protraction area to as far south as the Atwater Valley (AT) protraction area. For the first time, the Green Unit’s lateral extent, thickness, depth BSL, and pore pressure are defined. The “Green Unit” was found to extend further south than the Blue Unit into the AT protraction area and further east in the Desoto Canyon protraction area. The tops of both units are highly incised by postdepositional erosional systems, whereas the base of each unit is well preserved. The top of the Blue Unit below the mud line (BML) varies from <70 m (<230 ft) in the north to as deep as 701 m (2300 ft) in the south, whereas the top of the Green Unit is as shallow as 300 m (985 ft) in the north to 901 m (2956 ft) in the south. Overpressure in the MC area has been reported just BML. The pore pressure gradient ranges from 0.47 to 0.52 psi/ft at the base of the Blue Unit and increases to 0.60 psi/ft within the Green Unit.

Deepwater basins around the world contain shallow sequences of overpressured, sand-prone sediments that can result in Shallow Water Flow (SWF) events. These events have frequently resulted in wellbore instability, increased man-hour exposure to potential HSSE risks as well as non-productive time (NPT) and have sometimes been the cause of the loss of the well while drilling the shallow (riserless) section for oil and gas exploration or development projects. Methods previously established to classify the magnitude of a SWF event have been used with partial success to identify the onset of a SWF event. The need existed to develop a system enabling early prediction, detection and mitigation of SWF events while drilling. Real-time monitoring of the riserless section of a marine well for SWF requires a system using a plurality of data feeds defined here as the SYSTEM. The data feeds include seismic data, remotely operated vehicle (ROV) video, and surface and downhole logging measurements. A SWF surveillance methodology, herein defined as a discharge category model (DCM), has been developed for early detection of a SWF event, prior to the onset of wellbore instability. The DCM focuses on baseline discharge categories (ranging from no flow to minor flow) prior to wellbore instability and taking into account the u-tube effects. Real-time monitoring of data feeds coupled with the DCM in the context of the SYSTEM has helped to mitigate SWF events. There have been no wells lost due to SWF events that have utilized the DCM in the context of the SYSTEM in various basins throughout the world. A total of 154 wells have been monitored globally using the DCM with 46 SWF events detected and mitigated before reaching a severity level that might compromise the well integrity from 2012 to 2019.

Extending the geohazards assessment throughout the overburden section above the main reservoir is a relatively recent practice. Traditionally, a geohazards assessment for the riserless section of wells has been a key element of pre-drill studies in offshore well planning. Potential geohazards such as seafloor/buried faults, gumbos, gas hydrates, shallow gas and shallow water flow are routinely investigated using seismic reflectors, seismic amplitude, extent of well-known problematic stratigraphic units, and offset wells drilling data. As a result, the understanding of the regional distribution of geohazards has increased significantly. Although the common geohazards as mentioned above are well known, the assessment has been challenging with deeper depths. The loss of seismic resolution with depth affects the interpretation of the stratigraphy, structure, pore pressure and fracture gradient. The uncertainty in the depth and inclination of key marker events including stratigraphic horizons, chronostratigraphic ties, etc. might lead to a wide margin of possible pore pressure and fracture gradient (PPFG) estimates. The velocity-effective stress transformations used for pre-drill PPFG from low resolution seismic data is also less reliable, compounding the error resulting from predicted stratigraphic markers (horizons). Seismic interpretations with low resolution are inadequate to identify thin over-pressured zones. The paper presents an integrated workflow that maximizes the predictability of geohazards for the entire reservoir overburden section. A variety of seismic volumes including amplitude reflection, amplitude versus offset (AVO), seismic inversion, seismic velocity, coherence data, etc. allows for the optimization of interpretations such as stratigraphy, structure, and rock properties. A detailed geologic model with advanced seismic processing techniques provides a high-resolution understanding of structure and stratigraphy, seismic attribute distributions, and spatial velocity variations. The model is useful to identify key faults, leak points, sealing intervals, and trapping mechanisms. Understanding the stratigraphic facies assists in mapping the intervals of pressure generation and retention zones. Considering these limitations, offset well data is integrated when available and utilized to characterize seismic facies and rock properties in sparse data environments. These data are then correlated with seismic reflection and velocity data to develop a well-constrained geologic model. Multiple types of seismic volumes with various frequencies, coverages, and penetration provide better control and understanding of the riserless section. This may include AVO and inversion volumes, which are not commonly used in a shallow geohazards assessment. Finally, a fully integrated geohazards assessment, from the seafloor to the main reservoir, results in an optimized drilling program and is developed to minimize the impact of geohazards and drilling risks along the wellbore trajectory.