Advanced Formation Testing and PVT Sampling in Deep Gas Condensate Reservoir: Case Study from Malaysia

Abstract

Abstract Downhole sampling in gas condensate reservoir is well known to be challenging due to the nature of the near critical fluids. Reservoir fluid properties can change dramatically with slightly changes in reservoir pressure and temperature. As a result, accurate and representative PVT data are essential for reservoir fluid modeling and field development purpose. This paper presents the first downhole gas condensate sampling in a deep gas condensate field, offshore East Malaysia. Samples collected from the previous surface tests showed large variation in Condensate Gas Ratio (CGR), from 50 to 200 stb/mmscf. This resulted in large uncertainty in the dew point pressure, condensate yield, reservoir fluid productivity, and also reservoir fluid typing, i.e. rich or lean gas condensate reservoirs. As a result, there are strong needs to acquire high quality downhole samples to reduce these uncertainties, which may affect the entire field development plan. This paper illustrates several key challenges in this particular case study; for example, the risk of stuck tools required us to minimize stationary time during clean up process. Typically in the same area, the usual time allowed per station is less than 2 hours. However, since this gas condensate sampling job is much more complex in its fluid behaviour than other types of reservoir fluids, the wireline formation tester tool were allowed to be slightly longer at each station. Furthermore, best possible downhole fluid samples were requested for PVT modeling. As a result, minimizing OBM contamination and controlling drawdown pressure drop in low mobility reservoir were added into operation challenges. The use of new technology Wireline Formation Tools (WFT) such as focused sampling probe and in-situ fluid analyzer were introduced to ensure that job objectives can be met with limited time per station. The focused sampling approach was used to collect representative PVT sampling in several stations. The direct comparison between conventional and focused sample probes in the same zone will also be presented in this paper. During the pump-out period, the guard and sampling flowline pump rates were controlled and pressure drop between each flowline was monitored closely in the real time to ensure that the clean single phase reservoir gas is flowing though the sample flowline. When the clean gas condensate with minimum OBM contamination is detected from different measurements such as stabilized GOR, live density and composition using the in-situ fluid analyzer tool, PVT samples were collected at each station. It seems like the focused sampling technique procedure is straight forward. However, when different types of mud, i.e. WBM and OBM, was used in different hole sections, the two-phase flowing behaviour, i.e. gas condensate in OBM and in WBM behave differently, and therefore, this two-phase flowing effect requires different operation procedures for this particular reservoir in such a limit time frame.