The inherent uncertainty in establishing reservoir connectivity has always been an issue for reservoir management. Standard correlation methods using logs, cores and seismic data are sometimes inadequate, whereas an extended production test may be too expensive or non feasible. Increasingly, geochemical techniques are being deployed to determine reservoir connectivity based on the compositional differences in the reservoir fluid.
In a number of reservoirs around the world, carbon dioxide (CO2) is a critical gas composition. Examples from two such reservoirs, one from the Browse Basin in Australia and the other from the Malay Basin in Malaysia will be discussed in this paper. The CO2 content can vary from very low concentrations in one zone to significantly high in others in the same field. In addition, accurate quantification of CO2 from reservoir fluid samples can be difficult especially if some water is also present in the collected samples. This is due to the nature of CO2 which easily reacts with water, the source of which could be mud filtrate or formation water. As a result, in a well drilled with water based mud (WBM), contamination needs to be mitigated in samples captured for the purpose of quantification of CO2 in a given reservoir.
This paper presents field examples from the Asia Pacific region where a new generation Wireline Formation Tester (WFT) tool together with an advanced Downhole Fluid Analyzer (DFA) was used to quantify CO2 in real time as well as acquire high quality PVT samples. As the analyses of hydrocarbon samples from previous exploration wells within the same regions had shown significant variation in CO2 content, new and improved sampling procedures together with advanced DFA measurements were required to accurately measure and quantify CO2 concentrations in a number of reservoirs with varying fluid compositions. In this paper operational considerations and challenges of acquiring high quality PVT samples for different reservoir fluids under varying conditions are also discussed.