Characterization of a New Litho-Density Logging-While-Drilling Tool to Ensure High-Confidence Reservoir Data

Abstract

Nuclear logging-while-drilling (LWD) tools delivering gamma-gamma density, photoelectric factor (Pe) and neutron-porosity measurements are essential services for the oil and gas industry to calculate accurate asset NPV (Net Present Value). As a part of the triple-combo bottom-hole assembly, neutron and formation density measurements enable accurate quantification of porosity, hydrocarbons volume and type, and through 360° coverage density imaging, bed boundary and structural measurements, and real-time reservoir navigation (Ellis & Singer, 2007), (Wahl, Tittman, & Johnstone, 1964). One of the crucial steps in tool development is the precise characterization and correction of any wellbore induced effects, in order to ensure accurate and reliable true formation properties to be evaluated, leading to a high confidence in formation properties calculated from them. In this paper we describe how stringent radiation-transport simulations of the instrument response are utilized during the characterization process for the case of a high-temperature 4.75-in combined neutron-porosity and litho-density tool, designed to be utilized for demanding rotary-steerable drilling applications. The tool covers application ranges of temperature up to 175°C, pressures of up to 30,000 psi and dogleg severities of 15°/100 ft during rotation and corrosion resistance in demanding oil and gas drilling applications (Reckmann, et al., 2022). The nuclear measurements are improved by integral stabilizers and fluid displacers. Moreover, the tool determines borehole size shape orientation using multi-axis acoustic azimuthal caliper coupled with an in-situ measurement of mud sound speed and mud density. All nuclear and acoustic measurements are implemented with digital data acquisition chains and correction algorithms utilizing model-based design. While neutron-porosity tools are characterized by nuclear simulation techniques since several decades, gamma-gamma density logging-while drilling tools, typically employing a 137Cs logging source and two scintillation gamma-ray detectors, are often characterized experimentally in a formation laboratory. The caveat of this procedure is that only a limited variety of formations, tool standoffs and drilling fluids can be covered. Another approach is the application of nuclear Monte-Carlo simulations, either to aid an experimental characterization with additional datapoints, or to derive a complete characterization that is scaled to reality in a later experimental test.