Minimizing uncertainties in reservoir monitoring and evaluation with combination Oof Cased hole formation resistivity and Pulsed Neutron Logging in Jasmine field, Gulf of Thailand

Abstract

Abstract For a producing field, oil-water contact movement, gas cap expansion/shrinkage and residual oil saturation (thus the sweep efficiency) are key reservoir dynamic properties for resource estimation and proper reservoir management. Jasmine field in the Gulf of Thailand first began oil production in June 2005. Light oil & gas has been produced from multiple high permeability fluvial sandstones of Miocene-Oligocene age. After more than seven years of development and production, reservoir monitoring becomes even more critical in decision making for well recompletion, drilling of new infill wells and overall fluids management. To understand fluid movement, pulsed neutron logging, including inelastic spectrum (carbon-oxygen ratio, C/O), thermal decay time (Sigma) and cased formation resistivity have been widely employed to evaluate formations through casing. However, each technology has its advantage and limitations. There are three major challenges for formation evaluation through casing in Jasmine field: Formation water is relatively fresh so it is very difficult to distinguish oil from fresh water with pulsed neutron capture (PNC) Sigma.In undertaking hydraulic workover operations, sea water has to be used to kill producing wells before logging, and therefore deep invasion is experienced in perforated zones with high permeability (multi Darcy) sandstone reservoirs. This can result in high uncertainty in oil saturations derived from C/O and partially to cased hole resistivity.In the deeper Oligocene reservoir section, massive shaly sands with low resistivity low contrast (LRLC) pay with high water saturation are present. Fluid typing in such strata is problematic even with open hole resistivity data. To address these various challenges, a combination of cased hole resistivity and pulsed neutron inelastic spectrum, thermal decay porosity & Sigma data have been employed used to reduce water saturation uncertainty and achieve multiple reservoir monitoring and formation evaluation objectives. Results from several logging campaigns since 2009 demonstrate the advantages of using these technologies in-parallel. Utilizing case studies, this paper summarizes the technical challenges and reservoir monitoring solutions as applied in the Jasmine field, including: Gas cap dynamics & OWC movementUnproduced perforated zones in commingled completionsUnswept portions of perforated zonesLow resistivity, low contrast shaly sand evaluation through casingReducing uncertainty in saturation estimation on sands killed by sea waterInputs to resource estimation, reservoir simulation and production optimization