Pore Pressure Prediction From 3-D Seismics

Abstract

Abstract Pore pressure prediction from seismic interval velocity is not a new topic, Pressures predicted using this technique have been typically used to assist the drilling engineer in the design of casing and drilling mud programs for three decades1. Implemented this way the technology is one dimensional. That is, the prediction involves no spatial component. Not to minimize the applicability of pore pressure prediction to drilling, the extension of pore pressureprediction to three dimensions for exploration applications is a new and significant topic. Introduction Rock property and reflectivity models are used in exploration to predict the changes in seismic response as a function of reservoir fluid. This modeling can be also viewed as onedimensional. 3-D pore pressure information is an important addition to the precision of rock property and reflectivitymodels. That is, spatial variations in elastic properties are integrated into seismic amplitude and Amplitude Versus Offset (AVO) models. The elastic properties of shale trap seals can vary as greatly as a function of pore pressure as reservoir-sand elastic properties vary as a function of pore fluid. 3-D pore pressure information adds the spatial component to the precision ofrock property and reflectivity models. As these models are used to understand seismic amplitude and AVO anomalies, the importance of 3-D pore pressure prediction to exploration is significant. Pore Pressure Prediction Pore pressure gradient is a measure of the change in the pressure exerted on fluids in the pore spaces of buried rocks as a function of depth. These pressure gradients vary as a function of depositional history, compaction, mineralogy, depth of burial and other environmental conditions. Thenormally pressure section has a pore pressure gradient equal to that of a water column unimpeded by permeability. It issaid that the reservoir is in hydraulic communication with the surface. Sections where the flow of pore fluids is restricted, for what-ever mechanism, are called abnormally pressured or geo-pressured. Abnormally pressured sections can be underpressured but are more typically over-pressured (Fig. 1). Correlation to velocity. The empirical relationship between seismic interval velocity and pore pressure gradient is exploited for the prediction of pore pressure gradients in areas where direct measurements are impractical2. These velocities are a product of seismic trace data processing for normal move-out (NMO) correction, dip move-out (DMO) correction and the event migration for correct structural imaging. Seismic migration velocities are a precise measure of aspecific average velocity type called RMS velocities (for Root Mean Squared velocities). From RMS velocity. interval velocity is calculated. Various workers have recognized that pore pressure gradient is related to interval velocityl,2,3 (Fig. 2). Although valid petrophysical models exists for this correlation, typicalpublished work exploit empirical relationships, Most empirical techniques exploited for pore pressure prediction recognize that the logarithm of the interval travel time(reciprocal of interval velocity) is linearly related to the logarithm of depth for the normally (hydrostatic) pressuredregions of the subsurface.