Technology Focus: Formation Evaluation (August 2009)

Abstract

Technology Focus Generally, the entire interval between the surface casing and total depth is logged, but far too often we neglect to take core. While the expense of coring and core analysis is not small, it usually is only a fraction of total well cost; yet it remains an uphill struggle to convince management that the project will benefit from the knowledge gained. With the quest for reserves leading to exploration for and development of reservoirs with ever-more-complex porosity systems at greater depths and higher temperatures where log responses become suspect, I would argue that taking core has never been more important. There is no denying that logs provide greater statistical coverage of a larger volume; have a better depth reference; can identify fluid contacts, missed pay, and swept zones; measure reservoir pressure; and can be calibrated to seismic data for reservoir-property mapping. However, visual inspection and laboratory analysis of core provide many key data that logs simply cannot provide, with applications for every discipline. Core confirms the lithology and mineralogy of reservoirs; calibrates estimates of fundamental rock properties such as porosity, saturation, and net thickness; and remains the only true measure of permeability. Core shows how fluids occupy and flow within the reservoir pore space; enables formation-damage studies; and supplies mechanical properties to allow faster and safer drilling and better-designed completions. Despite issues with cleaning, storage, restoration to native state, and scaling up, core still provides the best estimates of many of the crucial inputs for accurate reservoir modeling, even though logging-contractor marketing literature would have us believe otherwise. Logs cannot characterize a reservoir if knowledge of the rock is absent, so subsequent modeling must rely on uncalibrated and unverified log-derived correlations and analogs. The inevitable consequence is greater uncertainty. Formation Evaluation additional reading available at OnePetro: www.onepetro.org IPTC 12837 • "Accurate NMR Fluid Typing Using Functional T1/T2 Ratio and Fluid-Component Decomposition" by Boqin Sun, Chevron Energy Technology, et al. SPE 117728 • "Reservoir Rock Typing From Crest to Flank: Is There a Link?" by R.E. Mahmoud Basioni, Abu Dhabi Company, et al. IPTC 12328 • "Data-Acquisition and Formation-Evaluation Strategies in Anisotropic, Tight Gas Reservoirs of the Sultanate of Oman" by H.J. de Koningh, SPE, Petroleum Development Oman, et al.