NEUTRON POROSITY AND FORMATION DENSITY ACQUISITION WITHOUT CHEMICAL SOURCES IN LARGE CARBONATE RESERVOIRS IN THE MIDDLE EAST–A CASE STUDY

摘要

A new logging-while-drilling (LWD) tool uses a pulsedneutron generator (PNG) and a suite of detectors todetermine neutron porosity and formation density fromthe interaction of high-energy neutrons. Unliketraditional neutron porosity and gamma-gamma-density(GGD) measurements acquired with Am~(241)Be and Cs~(137)chemical sources, the new measurement does notrequire chemical sources for either neutron porosity orformation density. The absence of chemical sourcesdramatically reduces acquisition and transportationrisks previously associated with neutron porosity andformation density logging programs.Similar to traditional neutron porosity measurementsthe sourceless neutron porosity is based on themeasurement of thermal neutrons. The new sourcelessneutron-gamma-density (SNGD) formation densitymeasurement is based on the detection of neutroninducedgamma rays at a gamma ray detector placed farfrom the neutron source. High-energy neutrons from thePNG excite formation nuclei during inelastic collisions.The excited nuclei produce a gamma ray cloud, whichserves as the source for the SNGD measurement. Thegamma rays arriving at the detector exhibit significantdensity sensitivity. The tool is characterized to provideaccurate neutron porosity and formation densitymeasurements.Traditional sourced density measurements werecompared with the new sourceless density measurementin multiple field wells in gas- and oil-saturated intervalsfrom a large carbonate reservoir in the Middle East.Due to the high-energy neutron source and longersource/detector spacing, the SNGD has a deeper depthof investigation (DOI) than GGD, but also less axialresolution. The collocation of the source and detectorswith neutron porosity and similar DOI simplifiesneutron-density log evaluation as any drilling-fluidrelatedinvasion effects are comparable; this isparticularly relevant to gas- and light-hydrocarbonsaturatedreservoirs. Related to this is the sourcelessneutron porosity measurement. It offers a truerhydrogen index (HI) measurement than traditionalthermal neutron porosity tools and is advantageous inboth shaly reservoirs and in clean gas-saturatedreservoirs.