Fluid and Reservoir Characterization from Wireline Formation Tester: Field Examples from Santos Basin Reservoir

Abstract

The objective of this study is to dynamically characterize the reservoir and fluid distribution in a Santos Basin field by using well data including in-situ fluid properties and dynamic reservoir properties from wireline formation tester (WFT), openhole logs, and a simplified structural model of the field. Reservoir pressure measurement, pressure gradients, and fluid identification and sampling are part of the routine wireline suite service to evaluate offshore wells. The ability to extract high-quality reservoir properties from interval pressure transient tests (IPTT) and to measure fluid properties in real time are critical to characterizing laminated reservoirs in early stages, before production tests. The challenge of characterizing the reservoir and downhole fluid in different intervals of the reservoir as well as assessing reservoir connectivity in early stages of the field evaluation is ably addressed by deploying the latest WFT technologies, including radial probes entry for higher-quality interval IPTT and downhole fluid properties measurement. These measurements and methodology using a dissolved-asphaltene equation of state (EOS) enabled the investigation of compartmentalization or lack of thermodynamic equilibrium in some of the reservoir intervals, providing information for future well data acquisition and field planning decisions. Among downhole fluid measurements, optical density of the formation fluid linearly correlates with reservoir fluid asphaltene content, which in combination with a gas-liquid-asphaltene equilibria modeling enables operators to reduce uncertainty in reservoir connectivity assessment. Different fluid units were identified, on top of each other and most probably disconnected from each other. The technique used is based on comparing the fluid coloration of different collected samples that could be in communication to establish if they could be in equilibrium (same fluid unit) or not. From an understanding of the fluid distribution and permeabilities of the field, reservoir scenarios are developed to optimize and design future well data acquisition and to improve field development plans.