Sourceless Neutron-Density Porosity Determination: Fit-for-Purpose Formation Evaluation with Significant HS&E Benefits

Abstract

Chemical radioactive sources are commonly deployed in logging-while-drilling (LWD) assemblies to measure formation bulk density and neutron porosity. These measurements are widely used in porosity evaluation, lithology determination, clay volume estimates and fluid identification. Deploying logging tools with chemical sources poses Health, Safety and Environment (HS&E) risks requiring specific procedures throughout the entire process of planning, logistics and execution of well construction. Obtaining similar, identically equal or better formation bulk density and neutron porosity measurements without chemical sources has many advantages, particularly from the HS&E perspective in both standard and high-risk developments. Advances in LWD technology have enabled the neutron porosity measurement to be obtained by employing an electronic neutron generator instead of a chemical source. These electronic neutron generators have been used for many years in different wireline tools and are now also available in LWD tools. The bulk density measurement has, until recently, only been measured utilizing a chemical source. Recent developments use the gamma rays induced by the interaction of high-energy neutrons (from the electronic neutron generator) with the formation as an "in-situ" source for subsequent derivation of bulk density. Studies of the sourceless neutron-gamma-density (SNGD) data acquired in 15 to 20 horizontal and vertical wells have shown the accuracy obtained is within the specified limits, as compared to the traditional gamma-gamma-density (GGD) data. The typical density differentials observed in these environments are in the 0.01 to 0.02 g/cm3 range, which results in a porosity and saturation "error" in the order of 0.002 to 0.004 V/V. This level of accuracy is probably acceptable in a number of applications (e.g., infill drilling in mature fields), but may not be acceptable in other scenarios (e.g., more complex lithologies, extremely tight reservoirs), unless the HS&E considerations are overriding. The HS&E benefits are important to keep in focus as they have a direct human benefit through the entire process of equipment preparation, transportation, wellsite operation and potential bottomhole assembly abandonment. Associated with this there is also the economic benefits related to less rig time required for source handling, and the potentially large savings in a lost-in-hole (LIH) abandonment scenario, in the locations where this is applicable. All these different aspects, HS&E, financial and technical, associated with the SNGD system need to be evaluated from a holistic viewpoint to ensure the most appropriate, fit-for-purpose acquisition program is selected in each case.