Step Change in Reservoir Monitoring in Complex Borehole Environments

Abstract

Reservoir monitoring carried out using previous-generation pulsed neutron logging tools worked well in ideal borehole conditions. However, evaluations were complicated in non-ideal borehole environments, such as gas in the borehole, which affects capture cross section, sigma, and thermal neutron porosity measurements, changing borehole fluid holdup, which confuses carbon-oxygen interpretation, and identifying hydrocarbon type using only neutron porosity when oil density and hydrogen index are very low or open hole (OH) data are unavailable. A new-generation pulsed neutron logging tool has been introduced that benefits from a high output neutron generator, two LaBr3 detectors, one yttrium aluminum perovskite (YAP) detector, one neutron source monitor, and an improved acquisition sequence. It provides self-compensated measurements of sigma and thermal neutron porosity, along with full capture and inelastic spectroscopy, including total organic carbon (TOC) and carbon-oxygen ratios. This tool also measures a new formation property called the fast neutron cross section (FNXS), which provides a gas saturation estimate independent of conventional methods. All measurements are recorded in the same logging pass, thus reducing overall logging operation time. Pulsed neutron measurements were acquired in lateral wells using the new generation tool in the A field, onshore Abu Dhabi. Through lateral sections with changing oil, water, and gas holdups in the borehole, and in changing completion environments, robust sigma and neutron porosity measurements were acquired with the help of the automatic self-compensation algorithm. Neutron porosity helped quantify gas saturations where the OH data are available and of good quality. However, in zones where it is not possible to use the neutron porosity by itself (for example, in zones with missing or uncertain OH results), the FNXS measurement provided an independent estimate of gas presence and saturation. FNXS of brine (7.5 1/m), calcite (7.5), and oil (6.0 to 7.0), are similar and strongly contrast with the FNXS of gas (1.5 to 2.5). Thus, the measurement is insensitive to porosity by itself but highly sensitive to gas presence. A crossplot of thermal neutron porosity (TPHI) and FNXS provides a robust estimate of gas saturation in wells where OH results are uncertain or not available. This paper presents, through multiple examples, a first comprehensive look at the various challenges faced while logging lateral wells in a light oil environment and showcases how a combination of self-compensated measurements coupled with the new measurement of FNXS can make data interpretation more robust in complex borehole and completion environments.