Sourceless Formation Evaluation and Geomechanics While Drilling: A Case History

Abstract

A combination of nuclear magnetic-resonance (NMR) and azimuthal acoustic tools has recently become an optional sourceless porosity logging-while-drilling (LWD) tool. This tool combination was used in Kuwait as a part of the bottomhole assembly (BHA) only once before, but this time, we were able to draw a comparison with density-neutron logs taken from the same well as it was run in wipe mode for a specific interval. A deviated 8.5-in. pilot hole was planned through sand-shale sequences in a geological section which is deemed to be critical from a borehole-stability point of view. Borehole deterioration and significant variations in pore pressure can increase the risk for the drillstring to become stuck in hole. The use of conventional porosity tools in an LWD BHA with radioactive sources would elevate the risks to an environmental-hazard level. In addition, retrievable-based sources can also be problematic during extraction in high-angle wells. LWD NMR and azimuthal acoustic tools, free of radioactive sources, were run in an LWD tool combination, with the primary objective to measure porosity, pressure prediction, and possible anisotropy using a centralized four-axis acoustic caliper. Previous experiences in the same geological setting encountered borehole enlargements by approximately 5 in. after only 15 to 16 hours. LWD helped bypass this issue and acquire high-quality data before borehole enlargement occurred. Additionally, indications of the presence of possible rock anisotropy were observed. The results were then correlated and matched to offset well data as well as with existing field geomechanics knowledge. This paper presents the planning, design, and execution of using LWD NMR and azimuthal acoustic tools in this well. It also discusses the viability, integrity, and robustness of logged data as well as interpreted results. Optimization of real-time drilling operations and petrophysical data-acquisition requirements are also investigated to improve future field development and overall reservoir-management strategies.