Quantifying Velocity-Geology-Pressure Relationships in Core: The Foundations of a 4-D Seismic Feasibility Study in the Ichthys Gas-Condensate Field, NW Australia

Abstract

The effectiveness of 4D (time-lapse) seismic reservoir monitoring depends on the ability to reliably detect changes in seismic response due to production-driven changes in fluid saturation and effective pressure. JOGMEC and INPEX have collaborated to test the feasibility of 4D seismic monitoring in the Ichthys gas-condensate field in the Browse Basin, offshore NW Australia. The foundation of that study, presented here, is an examination of the effects of geological factors and effective pressure (P_eff) changes on elastic wave velocity in core plug samples.

P- and S-wave velocities were measured on 55 Brewster Member sandstone core plug samples, at angles of 0°, 45°, and 90° relative to bedding, and under confining pressure of 1000 to 11000 psi. Only hydrostatic pressure conditions will be discussed here. Porosity, permeability, and density were measured on the same plugs, along with X-Ray CT scanning to observe internal fabrics. Plug ends were used to prepare thin sections, with which mineralogy was quantified using QEMSCAN. Quartz cement abundance and grain size statistics were estimated on 40 prioritized samples using a combination of QEMSCAN and cathodoluminescence (CL).

Porosity is the dominant control on velocity, while scatter in the porosity-velocity data is clearly explained by clay content. Velocity measured parallel to bedding is strongly controlled by porosity, while velocity measurements oriented perpendicular to bedding are more sensitive to clays and bedding fabrics. Similarly, pressure sensitivity is consistently higher for velocity measured perpendicular to bedding. Thomsen’s anisotropy parameters, ε and γ, decrease with increasing effective pressure; ε and γ averaged over all analyzed samples at each pressure step approach zero. Anisotropy parameter δ, however, shows no relationship to pressure. Functions for geology-velocity, geology-velocity-pressure, and anisotropy relationships for all wave types and orientations were defined using multiple linear regression.

The results of this core analysis have subsequently been used for rock physics analysis and forward seismic modeling. The recognition and quantification of anisotropy in the core analysis has significant implications for subsequent modeling, as higher pressure sensitivity values determined from vertical velocity measurements will yield larger modeled seismic responses than values determined from horizontal velocity measurements.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018