Pore pressures for the shallow sub-seafloor sediments at the Atlantis field are estimated by analysis of P and S wave velocities observed seismic data recorded by a patch of ocean-bottom multicomponent nodes. A modified Eaton’s algorithm for pressure prediction is applied, and estimated overpressure locations are compared to known hazardous shallow water flow zones evaluated from a batch-set drilling project.
Overpressures, or abnormally low effective pressures, are hazardous in drilling operations and in the construction of deep water seafloor facilities. Knowing the locations of excess pore pressure in the shallow subsurface prior to drilling wells is important in improving safety, planning operations, and mitigating overpressure related constructions problems. Application of Eaton’s relation is a common method to transform seismic P-wave velocities in the subsurface sediments into pore pressure predictions. Typically, these velocities are obtained from conventional seismic data using streamer acquisition. With developing technology, multicomponent marine seismic node data present unique opportunities to study both compressional (PP) and converted shear (PS) wave reflections. Because shear wave velocities have increased sensitivity to the type of pore fluid content, lithology, and most importantly, effective pressures, a more detailed pore pressure analysis using seismic velocities is possible. Thus, a modified Eaton’s relation is used to incorporate converted shear wave velocities into estimates of pore pressure (effective pressure) in the shallow sub-sea sediments.
Eaton’s method infers the ratio of seismic wave velocities in normally and abnormally pressured zones relates to the ratio of effective stresses from the same zones through an empirically determined exponent. Estimated effective stress less than that expected in normally pressured situations is an indication of the presence of overpressure. We apply this methodology to estimate pore pressure in a sample dataset from Atlantis field, southern Green Canyon, GOM (Figure 1). One concern in Atlantis drilling operations is encountering high risk shallow water flow (SWF) zones that may contain excess pore pressure. In this study, the interest is to first obtain P and S wave velocity values from the dataset and then transform these values to pore pressure predictions in the shallow sediments immediately below the deep water seafloor. Proper conditioning of node-recorded seismic data to analyze PP and PS wave velocities is significant in the prediction of pore pressure. Optimal conditioning includes application of a deconvolution algorithm to enhance shallow subsurface reflections and implementing asymmetric ray tracing to extract seismic P and S wave velocities. Deviations in wave velocity values will yield different predicted pore pressure values. The framework to properly manage marine multicomponent seismic data, specifically accurate wave velocity estimates, is required for accurate pressure predictions.
Eaton’s (1969) equation for estimating pore pressure trends from seismic velocities is applied using observed PS-wave velocity values obtained from six nodes in Atlantis Field. Eaton’s method relates pore pressure to deviations in Pwave velocity from an established depth trend. As inputs to Eaton’s equation, Hamilton (1972, 1976) and Eberhart- Phillips et al. (1989) regressions are used to represent velocities in a normally pressured marine environment.
