This paper presents a case study of Field Sigma, a large gas condensate field in Pakistan. Sigma comprises two primary sandstone reservoir formations, X and Y, where an oil rim was discovered during the late field life. A re-development plan utilizing an integrated reservoir simulation model was formulated to enhance oil production from the oil rim beneath a gas cap. The key challenges faced included reservoir pressure depletion, reservoir heterogeneity, complex fault geometry, and fluid contact uncertainty. The study aims to introduce a novel concept for optimizing oil production from the oil rim reservoir through an integrated reservoir study.
In this study, an independent seismic interpretation was conducted on the most recent 3D seismic data, focusing on mapping the major faults that impact the static/dynamic reservoir model and evaluating the transmissibility of the mapped faults iteratively during the history-matching phase. A comprehensive petrophysical study was conducted to calculate various petrophysical parameters on a field scale, providing an updated and consistent analysis that minimized uncertainties from the previous interpretations. Field Sigma has a complex anticline structure with intricate fault geometries and thrust sections. Thus, the static model's structural grid was created using the Volume-Based Modelling (VBM) method, chosen over Corner Point Gridding to represent the complex nature of the field's structure accurately.
The reservoir engineering data was thoroughly analyzed and incorporated into a dynamic simulation model. The history-matched, compositional model was used to generate production forecasts. From the various evaluated well locations, Eight economically favorable infill and appraisal sites were identified, with 5 in the Southeastern compartment and 3 in the sub-thrust area of the Northeastern compartment. To appraise the Free Water Level (FWL), Sigma-5 was proposed as an appraisal well in the Southeasternern compartment and after achieving the appraisal objective, in case of water production, the well had the option to sidetrack in an up-dip location. The infill wells Sigma-6,7 and 9 were proposed between Sigma-3 & 4 to drain the remaining hydrocarbon volumes and were considered comparatively less risky as compared to the other wells. However, due to the limited dynamic data, there exists uncertainty in the reservoir connectivity and there is a possibility of encountering further depleted pressures as compared to the simulated pressures. The appraisal well Sigma-8 is proposed to evaluate the eastern extent of the Southeastern compartment, however, this area has high-depth uncertainty and limited well control. All three wells identified in the Northeastern compartment target the Sub-thrust region and were considered high-risk wells.
The paper emphasizes the crucial role of data integration from diverse sources in the re-development of a complex mature oil-rim brownfield through the combination of geological knowledge, reservoir-level petrophysical evaluation, incorporation of core data, production history, reservoir understanding, and critical data acquisition during the infill drilling, the project team gained the confidence to devise and execute successful re-development strategy. The iterative creation of robust static and dynamic models provides a valuable planning resource for future endeavors. The methodology outlined in the paper holds broad applicability to typical field developments, establishing it as a valuable industry practice.
