First Field Application of Downhole CO2 Measurement in Asia Pacific

Abstract

Abstract 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. Introduction The first generation Downhole Fluid Analyzer (DFA) tool was introduced together with the launch of the new and improved Wireline Formation Testers (WFT) in the early 1990's. Its main objective was to identify in-situ reservoir fluids and to obtain fluid fractions, i.e. water/oil fractions, and to monitor contamination cleaning up process prior to capturing fluid samples 1. This was done by the application of optical principles for continuous analysis of fluids in the WFT flowline. The first generation of DFA tools had a visible and near-infrared absorption spectrometer for fluid discrimination and a refractometer for free gas detection. Since then, other generations of the DFA tools have been introduced to obtain additional reservoir fluid information. DFA tools today are used for such applications as:to evaluate downhole fluid sample contamination 2to measure Gas-Oil-Ratio (GOR) and Condensate-Gas-Ratio (CGR)to provide fluid composition, i.e. C1, C2-C5, C6+to identify when the flowing pressure falls below saturation pressureto identify compositional grading 3to identify reservoir compartmentalization 4,5to measure in-situ pH6to monitor the cleaning-up process using downhole pH when sampling formation water in a well drilled with WBM7