Comprehensive PVT Data Review, Fluid Characterization and Grouping Study for Block X in Sarawak, Malaysia
Authors: Laing Lionel Kallang , Shirai Seiji , Rosli Kamal Amri , A. Talabi Oluwole , Mohammad Fawzi Nurul Suhaila 
Published: March 2022

Block X of offshore Sarawak consists of stacked and multi-layer reservoirs that produce gas, condensate and oil where the production is in-commingle from many wells. 47 surface PVT samples were taken and analyzed from every reservoir during the exploration and initial production periods. Despite the number of samples and a series of PVT studies had been conducted, fluid PVT and its reservoir modelling application remain an uncertainty for Block X. Therefore, a comprehensive 3-phase PVT study was conducted, and its improved results will be implemented in the upcoming simulation model to represent fluid interactions not only within each reservoir but also between different reservoirs that are produced in commingle.

Firstly, a detailed quality check and validation were performed on every sample using a systematic process proposed in Paredes et al. (2014) to identify high-quality samples. These samples were ranked based on their Fluid Sample Quality index (FSQI), and the best samples were carried forward for further analysis. Initial PVT grouping analysis was performed by plotting observed saturation pressure, composition, CGR and other key variables versus depth for the selected samples. The existing PVT models and compositional characterization, which were reviewed and found to be satisfactory except for the matching quality of liquid saturation, were used to generate predicted profile trends using Compositional gradient experiments and compared to the data to define the PVT data that could be grouped together.

Next, modelling and calibration of Equation of State (EOS) parameters to match the observed properties from lab experiments were performed for each PVT group using the best sample from each group as identified by its FQSI value. The results of the new PVT calibrations showed improvements over the existing models with the variance between the group PVT model and lab observations ranging from 0.1 -2.2% in saturation pressure and 0.5 – 17.8 % in CGR. This indicated that reasonable group PVT models had been obtained. Despite, uncertainty for one of the PVT groups remained high as its fluid needed to be adjusted due to a large inconsistency between the observed gas-oil-contact (GOC) and the observed saturation pressure even when its sample's FQSI was good.

Finally, the new PVT models were validated with the existing dynamic simulation models by initializing them close to the original sampling conditions and applying a compositional gradient. Comparisons with previous models show improvements between 3 − 58% when compared to the sample and early production data. Significant uncertainty remains for the reservoir or PVT group where the fluid adjustment was performed due to its limited production for further calibration. In addition, improvements were not immediately reflected in the dynamic history match of the existing models because of the variation in separator conditions during field life and the uncertainty of wells’ zonal contributions from commingled production; these are aspects for future work.