Fluid Identification Challenges in the Near Critical Fluids: Case Studies in Malaysia

Abstract

Abstract Reservoir fluid identification plays a crucial role in reservoir characterization and hydrocarbon volume estimation. Gas condensate reservoir is well known for its complex behaviour due to the nature of a near critical fluid. The reservoir pressure and temperature in such reservoirs are very close to the critical point, and therefore, small changes in reservoir condition will result in a change of fluid properties considerably. As a result, there exists a broad spectrum of reservoir fluids in this reservoir condition. Identifying reservoir fluid in the zones of interest is extremely challenging, especially when it is associated with overpressured low porosity shaly sandstone reservoir. It becomes difficult and at times impossible to definitively identify different types of formation fluids from the well logs alone. This paper presents challenges of fluid identification process during the exploration/appraisal campaign in such reservoirs, offshore Malaysia, where the operator needs to gather as much information and as quickly as possible to make immediate operation decisions and Field Development Plans (FDP). First part of this paper demonstrates an integration of available data including mud logs, gas chromatography, gas wetness ratio, well logs, formation pressure and DST in order to determine fluid types in a well where an expected reservoir fluid is oil. The result from a systematic integrated reservoir characterization performed later, however, has found that the reservoir fluid is gas condensate. The second part shows an extensive application of Downhole Fluid Analyzer (DFA) in the Wireline Formation Tester (WFT) tool to conclusively identify reservoir fluid types and their properties in-situ and in real time in the second well drilled in a different fault block. In this case, the use of WFT together with DFA has allowed identification and PVT property determination of a full range of downhole fluids including gas, retrograde gas, volatile oil and black oil. This suggests a number of compartments in such complex reservoirs. Introduction Reservoir fluid identification plays a crucial role in reservoir characterization and hydrocarbon volume estimation. In thick, porous and clean reservoirs, the process of fluid identification is straight forward. Initially, the bulk density and neutron porosity logs are used in combination with resistivity logs to identify reservoir fluid type. In clean reservoir, density porosity log will overlay neutron porosity log in water zone. In hydrocarbon bearing zone, density and neutron porosity logs will start crossing over each other. A very large density and neutron porosity log crossover together with high value of resistivity suggests that the formation is gas bearing. Normally, formation pressure gradients obtained from wireline formation tester (WFT) tools greatly help in identifying fluid types.