Summary

Three-Dimensional Full Tensor Gradiometry (3D FTG) acquires ultra-sensitive measurements of the Earth’s (vector) gravity gradient field. Departures from simple weakening of the field in the vertical direction are due to subsurface variations in density. We have undertaken a numerical examination of the feasibility of using this system for detecting lateral density contrasts in subsurface layers during reservoir monitoring. Our gravity modeling focuses on the additional value added by taking account of the horizontal components of gravity gradient in imaging local targets. A regularized inversion algorithm that can describe and predict the dynamic behavior of a hydrocarbon reservoir has been developed and tested on synthetic FTG data based on realistic petrophysical models. Our approach also yields estimates of uncertainty in hydrocarbon production data. Results show that the technique is particularly useful for direct monitoring of gas-oil contact or temperature front expansion during CO2 injection in heavy-oil reservoirs at shallow or moderate depths.

Introduction

3D FTG technology has improved dramatically over the past five years with advances in both acquisition and processing methods. Although the pioneers of gravity in exploration routinely exploited gradiometric methods using instruments such as the Eötvös torsion balance, Bell et al. (1997) were the first to write at any length about the potential applications of FTG. In the United States, this technology has already been applied to sub-salt exploration in the Gulf of Mexico. Coburn (2002) describes some case studies that used FTG and 3D seismic imaging to delineate the base of salt. 3D FTG monitors the first derivative of the gravity vector field (Figure 1). The instrument consists of a multiple accelerometer system comprising three gravity gradient instruments (GGIs). The system measures the full spectrum of the multi-component gravity gradient field as well as the magnitude of the gravity field itself. In principle, nine tensor components mathematically arranged in a 3 matrix are measured. However, four tensor components are redundant, so only five independent tensor measurements (say, GXX= GXX- GYY, GZZ, GXZ , GYZ, and GXY) are recorded. The unit ofgravity in common use is the mGal, equal in SI units to 10µms-2. For gravity gradients, the unit in common use is theEötvös (Eö) which is equivalent to 0.1 mGal/km.

Method

The goal of 4D FTG is to exploit high-resolution measurements of the tensor at different times to characterize changes in density (alternatively, movements of mass) within the rockmass during the intervening intervals. Interpretation of high-resolution FTG data requires reliable and efficient inversion methods that focus on the additional value provided by horizontal component information in imaging local targets. Following our recent feasibility study (Vasilevsky et al., 2003), we have developed a regularized inversion scheme designed to enhance production-related density anomalies within the reservoir. The components of the gradient tensor are used to constrain the reservoir model at a prospect level by defining the edges and shape of the time-lapse anomaly as well as estimating lateral density changes that might otherwise be masked by acquisition and processing artifacts.

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