Extending Pulsed Neutron Logging Application in Reducing uncertainty in Low-Porosity, Complex Lithology Exploratory Field – A Detailed Case Study

Abstract

Abstract The assessment of oil and gas saturation behind casing by using Pulsed Neutron Logging technology (PNL) is not new to the industry. However, the assessment of low-porosity reservoirs has always presented challenges to the application of PNL. This is further complicated by the presence complex unknown lithology with rugose borehole architecture. In this paper, a detailed case study is discussed highlighting the job planning, logging execution, and integrated post-processing interpretation workflow with the obtained results. In the subject well, limited open hole data was available and the rugosity of the wellbore posed some challenge to the conventional porosity measurements. After the review of the available data, a multidetector pulsed neutron tool (MDPNT) was deployed. Capture data, inelastic data as well as spectral gamma ray data were acquired. The statistical uncertainty of the acquired data was assessed real time to determine the number of inelastic passes required with the associated logging speed. Special hole size correction models were applied to correct for the impact of rugosity in the tool measurement. Detailed mineralogical evaluation was done by comparing the capture elemental yields against the inelastic yields to ascertain the appropriate mineral model. Spectral gamma ray data was used for clay typing and also to confirm any possible presence of organic carbon/kerogen. Oil saturation was computed from carbon-oxygen measurement with detailed borehole corrections applied to correct for the effect of hole rugosity. Porosity data was computed from the pulsed neutron porosity indicators – the inelastic and capture ratios. Porosity was predominantly below 5% with mixed carbonate observed across the oil zone. Clay typing showed varied mix of various clay types with prevalent presence of chlorite and montmorillonite. Despite the very low porosity and low salinity, accurate hydrocarbon saturation was computed which was used perforation planning. The value of pulsed neutron logging in newly drilled exploratory well was demonstrated in this case study. The importance of proper job planning, detailed environmental corrections and real-time assessment of statistical and saturation uncertainty was showcased, in order to achieve results with high confidence despite the low-porosity challenge.