Calibrated Predrill Pore Pressure Prediction from 3D Seismic for the Cocuite Field, Veracruz Basin

Abstract

Abstract In order to optimize drilling operations in the Cocuite field in the Veracruz basin, Mexico, seismic velocities from a three-dimensional (3D) seismic survey were used to predict pore pressure. The accuracy of the seismic velocities was assessed using a comparison with interval velocities calculated by upscaling sonic logs and by inverting time/depth pairs from checkshots measured in wells in this area. To calibrate the velocity to pore pressure transform, available pressure tests and drilling mud weights were used to estimate formation pore pressure. A method to estimate the uncertainty in the predicted pore pressure is discussed. The technique can be used to help optimize drilling operations - avoid unnecessary kicks, develop casing points, and assist in reservoir development by evaluating pressures compartments for example. This technique resulted in 3D models that are less uncertain and of higher density than those obtained using conventional techniques and also improve drilling safety and economics in overpressured areas. A study of nonproductive time in the Veracruz basin concluded that in certain wells the mud weight was sufficiently high to fracture the formation, resulting in loss of drilling fluid. Use of the new pore pressure prediction to guide the choice of mud weight should help to alleviate the lost circulation problems. It will also improve the well design and lower the downtime, which will lead to improved economics and enhanced production from future wells in this area. Introduction Knowledge of formation pore pressure is a key requirement for optimal development decisions in overpressured formations. The pore pressure prediction described in this paper was made for the Cocuite field in the Veracruz basin, Mexico, to help optimize drilling operations in this area. The location of this field is shown in Fig.1. Prior to this project, 3D seismic data had been acquired over the Cocuite field. Seismic interval velocities picked using the Cocuite 3D seismic data were available as a function of depth on a grid with 500m by 450m spacing. The locations at which seismic interval velocities were available are shown in Fig. 2 together with the locations of wells in this area. The work presented in this paper used the seismic velocities from the Cocuite 3D survey to predict pore pressure based on a velocity to pore pressure transform established using data from the wells shown in Fig. 2. The accuracy of the seismic velocities was assessed using a comparison with interval velocities calculated by upscaling sonic logs and by inverting time/depth pairs from checkshots from the field; the velocities were in reasonable agreement. Fig. 1 - Location of the Cocuite field in the Veracruz basin.(AVAILABLE IN FULL PAPER) Fig. 2 - Velocity analysis location s and locations of wells used in this study.(AVAILABLE IN FULL PAPER) To perform the pore pressure prediction, it is assumed that velocity is a function of the vertical effective stress, defined as the difference between the total vertical stress and the pore pressure. The vertical effective stress was calculated from an integral of the density log using available well data. To calibrate the velocity to pore pressure transform, available pressure tests and drilling mud weights were used to estimate formation pore pressure.