Enhanced Seismic Velocity Processing to Improve 3D Geopressure Evaluation

Abstract

Geopressure evaluations are key considerations for successful well planning and drilling. The necessity to satisfy the world needs of oil & gas presses Oil Companies to drill in conditions that are getting harder and harder in terms of geopressure environment. The pore pressure prediction accuracy has a large impact on the cost of a project and is especially critical for drilling hightemperature/high-pressure (HP/HT) wells where there is a small margin between pore and fracture pressures. This paper outlines the most accurate methodologies to estimate seismic velocities to be used for the pore pressure prediction. In exploration, pore pressure prediction is critical for the evaluation of vertical and lateral sealing, the estimation of maximum possible column of hydrocarbons in place, and consequently to rank the prospects and evaluate the economics. As far as concerns drilling, the geopressure information is important to define the best well design in order to reduce costs, NPT (Not Productive Time), wellbore stability, sand production and reservoir damages. Generally, in complex and deep areas, conventional velocity fields derived from seismic time processing are often not accurate enough to make a correct pore pressure prediction. For this reason, several methods have been developed trying to obtain more appropriate velocity fields. In this paper we compare some of them, mainly CVA (Continuous Velocity Analysis) calculated in the time domain versus Grid Tomography, applied in the depth domain. In particular, Eni proprietary software based on reflection tomography, able to integrate geological information in the Grid Tomography loop, has been successfully applied in North Africa, where the exploration goal is to reach deep HP/HT targets. Besides, in that region, strong pressure anomalies and pressure compartments are frequently present in the shallower layers, due to compaction disequilibrium and vertical pressure transfer along faults or through windows in salt layer. These phenomena have created serious problems during drilling operations in the past and a good geopressure prediction can considerably reduce drilling costs and risks. The application of the proposed methodology to build the seismic velocity field has been important both in defining more detailed pore/over pressure volumes and structural models by an iterative pre stack depth migration workflow.

Pore Pressure Prediction in HP/HT environment

“…The Grid Tomography is based on ray tracing techniques to compute the residual moveouts and takes information not only from geological consistent reflecting horizons, but also from local events, seismically coherent, interpretable just in restricted areas of the model. Also in this case, the possibility to depart from a definite horizons based model using further information from extra seismic events, gives the opportunity to highlight velocity anomalies in the updated velocity field often associated to pore pressure anomalies (Cibin P.2006, 2008, 2010and Colin Sayers, 2000.…”