A Multi-Disciplinary Approach to Identifying Geohazards for the Reservoir Overburden Section with Loss of Deep Seismic Resolution
Authors: Berger William Bill J. , Metz Zachary Ian , Gao Baiyuan , Przybylski Piotr Antoni , Bisrat Shishay , Jere Chaitanya B. , Deshpande Asim A. 
Published: April 2020

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.