First Successful LWD NMR T1 and T2 Measurements in Unconventional Source Rock with Harsh Drilling Environment: A Case Study from Kuwait

Abstract

The thickness and thermal maturity of the Najmah formation presents a favorable prospect for commercial exploitation of unconventional shale gas. Estimating reservoir storage volume is one of many formation-evaluation objectives when assessing an unconventional reservoir. The presence of kerogen has significant effects on several downhole logging sensor measurements. An operator used nuclear magnetic resonance (NMR) measurements to evaluate total fluid-filled porosity for a reliable estimate of the reservoir storage volume in source rock. While wireline NMR has historically provided a large segment of unconventional reservoir logging, logging-while-drilling (LWD) NMR provides an alternative method for assessing the total fluid-filled porosity within unconventional reservoirs. This paper provides a case study in the Najmah Shale Formation, where an operator used an LWD penta-combo system (gamma ray, electromagnetic wave resistivity, azimuthal bulk density, thermal neutron porosity, high-frequency multipole azimuthal sonic, ultrasonic caliper, and NMR sensors) in real time. The system provided petrophysical and geomechanical evaluation with well inclinations greater than 50° in 16-ppg oil-based mud. T1 and T2 measurements were acquired with an LWD NMR sensor while drilling and during wiping passes. These measurements provided an additional dimension to differentiate clay-bound water from organic hydrocarbons, with good correlation to existing core data in real time. Reservoir characterization of the Najmah Formation using LWD measurements is currently underway to provide a better understanding of reservoir quality, hydrocarbon potential, and reservoir storage capacity of this unconventional source rock. Acoustic velocities play an important role as input to geomechanical information, including the stress regime, principal stress orientation identification, and rock moduli estimation. An azimuthal acoustic tool measuring shear velocities at 360° around the borehole provides answers for the first two attributes. Estimation of formation rock moduli requires accurate acoustic properties and density log inputs, as provided by high-frequency sonic and azimuthal density measurements. In wells above 60° inclination, it becomes almost impossible to run wireline logging, thus LWD quad-combo with a high-frequency azimuthal acoustic tool is a viable solution for this environment. Additionally, it is an advantage to run LWD while in drilling mode to save rig time and optimize well construction costs.