Predicting Abnormal Pressure from 2-D Seismic Velocity Modeling

Abstract

Seismic velocities are the only data available, before drilling, on which to base a quantitative, present-day estimate of abnormal pressure. Recent advances in seismic velocity processing have enabled us to obtain, using an in-house approach, an optimized 2-D interval velocity field and consequently to better define the lateral extension of pressure regimes. The methodology, interpretation and quantification of overpressure-related anomalies are supported by case studies, selected in sand-shale dominated Tertiary basins, offshore West Africa. Another advantage of this approach is that it can also account for the presence of reservoir-potential intervals at great depth and thus provide significant insight, horn a prospective standpoint, into very poorly explored areas. Although at the outset the 2-D seismic tool legitimately merits being favored, optimization of the final predictive pressure model, prior to drilling, will depend upon the uccess of its combined use with other concepts and approaches, pertaining to structural geology, sedimentology, rock mechanics and fluid dynamics. INTRODUCTION In a poorly explored area, abnormal pressure regimes cannot be reasonably estimated, quantitatively, without preliminary consideration of seismic-derived data which are often the only data available before drilling. The use of seismic velocities, in a 1-D predictive approach, was introduced by Pennebaker (1968) and Reynolds (1970) for use in the Gulf Coast area. It aimed at detecting the top of under compacted shalesd evaluating the intensity of the under compaction related overpressures. With increased computer capacities and the recent inhouse advances in pre-processing and stack velocity processing, we were able to significantly optimize the 2-D interval velocity field. The results obtained contribute not only to more extensive knowledge of the lateral distribution and intensity of abnormal pressure fields, but also to the prediction of reservoir potentiality within and/or beneath overpressured intervals. Backed up by selected case studies, the present paper successively discusses:the methodology used, from original stack velocities through to the 2-D abnormal pressure regimethe characterization of seismic anomalies and their significance in terms of shale compaction desequilibrium, natural open fracturing, dilatant fault zones or reservoir-potential intervalsthe pressure regimes associated with the different types of anomaly This will lead us to conclude on the major contributions and limitations of this 2-D approach and to stress the necessity of integrating 2-D seismic modeling within a multi-disciplinary approach. METHODOLOGY The methodology used in this 2-D method is supported by a first case study. This case study concerns a 6.5 km long seismic section covering CDPS 640 to 1180. The objectives of the well to be drilled at CDP 840 were to test the deep Miocene structures below 3 seconds TWT, in a downthrown block limited to the North by a major roll-over fault system (see Figure 1). From stack velocities to the final pressure model, 6 major steps are involved. Step 1: Stack velocity field The prime objective of this preliminary step is to optimize stack velocities for each seismic trace, from CMP 640 to 1180, and at each marker down to 4.2 sTWT. This was accomplished using in-house processing, presenting a significant improvement over former techniques.

Overpressures: Causal Mechanisms, Conventional and Hydromechanical Approaches

“…2). Other empirical relationships are still used for predicting abnormal pressures before drilling, from low frequency, seismic-derived interval velocities (Grauls et al, 1995). The success ratio obtained from seismic predictions, was close to 65% considering a relative uncertainty on overpressure estimate (OVP) less than or equal to 10% (delta OVP/OVP < or = 0.1).…”

Use of Compressional and Shear Wave Velocity for Overpressure Detection

“…The decrease in the P-wave velocity with increasing pore pressure has been used for overpressure detection (e.g., Grauls et al, 1995;Moos and Zwart, 1998). However, velocity does not uniquely indicate pore pressure because it also depends, among other factors, on porosity, mineralogy, and texture of rock.…”