Integrated Petrophysical Evaluation of Unconventional Formations, in the Delaware Basin, With a Customized NMR Acquisition

Abstract

Unconventional shale reservoirs have complex lithology, porosity and fluid systems where hydrocarbons are stored in both inter- and intra-particle pores. Hence, conventional log measurements and petrophysical techniques often fail to provide a consistent analysis for key petrophysical properties such as porosity and hydrocarbon saturation. Nuclear magnetic resonance (NMR) logging has gained an importance in unconventional resource evaluation, for its promise to allow in-situ differentiation of various fluid phases and their potential to flow. However, in unconventional reservoirs, pore sizes may range from only a few nanometers to a few hundred micrometers: in these small pores the NMR signal relaxes rapidly, making it challenging to measure using standard NMR acquisition modes and methodologies. This paper introduces a custom NMR log acquisition mode that is designed to measure these fast-relaxing components, to improve porosity identification and fluid typing in unconventional shale reservoirs. This is implemented through both tool improvements and novel processing techniques in the T2 and T1-T2 logging modes. Furthermore, the methodology can optionally incorporate other conventional and advanced measurements (such as, elemental spectroscopy, and multi frequency dielectric) as part of an integrated system to further constrain and compute key petrophysical properties. The methodology was applied to the Bone Spring and Wolfcamp formations of the Delaware Basin, integrating the NMR data with open hole log data, including density, porosity, and total organic content from elemental spectroscopy logs to identify organic rich zones. Integration of multifrequency dielectric measurements further increases confidence in estimating water filled porosity and hydrocarbon saturation within the reservoir. This methodology results in improved quantification of key reservoir properties such as porosity and hydrocarbon saturation. Compared to existing methodologies, benefits include increased logging speed, therefore saving rig time, improved hydrocarbon typing and quantification, and the ability to integrate with laboratory-based measurements, advanced logging suites and cuttings analysis to better identify and quantify productive zones. We present a case study applying the new NMR acquisition mode and demonstrating the improvements in quantification of the reservoir properties.