UNLOCKING THE POTENTIAL OF UNCONVENTIONAL RESERVOIRS THROUGH NEW GENERATION NMR T1/T2 LOGGING MEASUREMENTS INTEGRATED WITH ADVANCED WIRELINE LOGS

摘要

The fundamental challenges in formation evaluation of unconventional shale reservoirs are the estimation of producible hydrocarbons and identification of target zones for horizontal wells. Modern gamma-ray spectroscopy tools can estimate the in-situ total organic carbon (TOC) which is an important parameter for determining the hydrocarbon potential of shale reservoirs. However, differentiating the components of TOC into kerogen and bound and free hydrocarbon remains an outstanding problem. Resistivity and dielectric measurements are sensitive only to water-filled porosities, and therefore these measurements cannot be used for partitioning the TOC components. Similarly, nuclear magnetic resonance (NMR) T2 cutoff-based methods for the estimation of clay-bound, capillary-bound, and free fluids are generally not valid because the responses of the different fluids overlap in the T2 domain. In this paper, we describe a novel methodology for quantitative estimation of fluid volumes in unconventional shale reservoirs from NMR T1-T2 measurements. The methodology is based on the mathematical concepts of data analytics for identifying underlying features and systematic relationships in the data without any a-priori information. The NMR T1-T2 data in an entire logged interval are arranged in a database matrix and subsequently resolved as a product of two positive matrices using an automatic feature extraction technique. The first matrix contains the T1-T2 signatures of the different fluids and the second matrix contains the respective volumetric fractions of each fluid type. The methodology is robust and provides accurate extraction of fluid signatures even in low porosity and low signal-to-noise ratio conditions. The extracted fluid signatures could be easily visualized for gaining further petrophysical insights. The methodology is used to predict continuous logs of fluid volumes in unconventional shale reservoir. High resolution T1-T2 measurements were obtained in an Eagle Ford shale play using a new-generation wireline NMR logging tool. The higher operating frequency of the tool and a new acquisition scheme implemented in the tool result in enhanced sensitivity to the T1-T2 contrasts of fluids and to the fast-relaxing components typical of shale reservoirs. The signatures of different fluids such as bitumen, hydrocarbons in organic matter and inter-particle pores, clay-bound and inter-particle water could be resolved using the methodology described. The fluid volumes show good comparison with core measurements obtained using Dean Stark and tight rock analysis. Integration of fluid volumes from T1-T2 measurements with other advanced wireline logs such as spectroscopy and dielectric measurements provides a complete formation evaluation in shales. A second field example from a heavy oil reservoir shows that the methodology could accurately resolve fluid signatures including heavy oil, clay-bound and capillary-bound water.