Integrating Permeability Data from DST Interpretation to Characterize Deepwater Gas Turbidite Reservoir in Niger Delta: Lessons Learnt

Abstract

Data acquisition in deep water environments is critical in de-risking subsurface uncertainties and data must be of high quality before development plans can be put in place and sanctioned, given the expensive associated field development costs. In exploration and appraisal drilling campaigns for deepwater environments, one of such data acquisition is via the Drill Stem Test (DST), essential to understand the turbidite reservoirs; characterize and reconstruct the depositional environment. One key challenge in the deepwater environment is the connectivity and thinly bedded nature of the reservoirs with obvious vertical heterogeneity and varying layer flow properties. DST is commonly used to acquire formation pressures, reservoir fluid samples and determine the productivity of the formation. In this study, the DST was run in a deep offshore gas reservoir with a net thickness of 49ft over a perforated interval; 13,215 ft MD to 13,294 ft MD. The objective was to quantify average formation dynamic flow capacity (permeability-thickness product), well productivity, prove sufficient gas resources for further development of the structure and obtain information about the lateral extent and architecture of the sand bodies. A series of multi-rate drawdowns and build-ups were extracted from the DST and used for the analysis. The Kappa Ecrin well test interpretation software was used to interpret the DST and from the test, formation properties such as skin and permeability as well as insights about the Infinite-acting radial flow (IARF) and boundaries effects were evaluated. This paper demonstrates the use of DST interpreted results with core data to estimate the range of permeability uncertainty for varying stratigraphy or facies in a deepwater gas reservoir in the Niger delta. Although, the reservoir boundaries could not be fully detected due to limited pressure Build-up (PBU) periods, various model scenarios were evaluated to estimate the range of permeability values using the limited transient phase. Thus, outcome of the interpretation was used to quantify the range of permeability from both the DST and the core plug permeability measurements within the reservoir ranging from 250mD to 1900mD. Finally, this paper also used permeability data from wireline log and DSTs to complement geological and seismic data interpretations and establish the range of uncertainties in permeability.