Less is More: an Objective/Cost-Driven Approach to Reducing Uncertainty in the Appraisal of the Lam Reservoir, Central Yemen

Abstract

This paper presents a cost versus knowledge-gained appraisal strategy for reducing uncertainty in a tight sandstone reservoir within a short timeframe. More uncertainty should not necessarily demand more data acquisition, rather an approach that focuses on answering the big questions first.In the Shabwah Basin of central Yemen, sands from the Upper Jurassic Lam Member form fine-grained turbidite lobes. These tight sands, encountered while drilling to the Block S2 Basement reservoir, are currently undergoing appraisal as a potential unconventional oil resource. The presence of oil on surface correlates well with direct and indirect indications of natural fractures during drilling and wireline logging. In contrast, fluid sampling of thin hydrocarbon-bearing sands identified by magnetic resonance tools have so far been unsuccessful. Two possible production mechanisms have been identified in this sandstone reservoir: production from natural fractures and/or production from tight sands by hydraulic fracturing.The carefully risked appraisal strategy will target the key reservoir uncertainties, productivity and production mechanism, using three kinds of well re-completions: testing production from a well with natural fractures (with or without frac'ing), frac'ing and flowing a well with hydrocarbon-bearing tight sands and, drilling a slanted sidetrack well to increase exposure to natural fractures, with or without frac'ing.The appraisal approach is objective driven, not a wide-ranging data acquisition programme. There are many unknowns in the reservoir but this project focuses on reducing uncertainty efficiently by prioritising answers to the key objectives which are productivity and the production mechanism.A decision-tree based appraisal strategy has been developed with clearly defined exit points to control the cost of appraisal versus the value of information gained from the re-completion and drilling activities. This innovative, business-driven approach to reservoir appraisal maximises early uncertainty reduction by the most cost effective methods and can be used as a model for appraisal and early development projects in the industry.