ABSTRACT:

We predict pore pressure and stresses from seismic velocity over a volume around a salt dome in Green Canyon 955, Gulf of Mexico. The salt dome significantly changes the magnitude and orientation of principal stresses in surrounding rocks. To account for these changes, we use the Full-Effective-Stress (FES) method. This new method uses a geomechanical model to account for stress changes caused by the salt dome and the state boundary surface of the rocks to account for the contributions of all three principal stresses to the rock velocity. In this study, we develop a 3D geomechanical model based on seismic interpretation of the salt-dome geometry and show that pore pressures, least principal stresses, and mud-weight windows that this model predicts significantly differ from those that a simplified, axisymmetric (2D) model would predict. Our study shows that the FES method with a 3D geomechanical model is necessary for accurate prediction of pore pressure and stresses from velocity near salt.

1. Introduction

Accurate estimation of in situ pore pressure in geological formations is crucial for hydrocarbon exploration and production. Pore pressure affects the fracture gradient in rocks sealing hydrocarbon reservoirs, which is a control on the hydrocarbon column in the reservoir (Flemings et al., 2002). It also affects the effective stresses in the reservoirs and thus their tightness and permeability. Pore pressure is a critical input for safe and economic design of wellbores (Zoback, 2010).

Pore pressure in areas under deposition is typically greater than hydrostatic pressure. As sediments are deposited, their weight causes vertical compression of underlying sediments. This compression requires expulsion of pore water from the sediment pores. Because sediments are composed mostly of low-permeability mudrocks, pore water expulsion cannot keep pace with sediment deposition; as a result, pore water bears a fraction of the weight of overlying sediments as overpressure, i.e., pore pressure in excess of hydrostatic pressure (Gibson, 1958; Terzaghi and Peck, 1948), and sediments cannot compress to the degree they would under hydrostatic pressure. The more sediments are overpressured, the more they are undercompressed.

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