Unconventional reservoirs have low porosity and complex mineral composition containing quartz, feldspar, calcite, dolomite, pyrite and kerogen, which may seriously reduce the accuracy of the porosity measurement. The multi-detector pulsed neutron logging technique was already used for determining porosity through the combination of inelastic and capture gamma ray information in different spacing. In this paper, the new parameter, which is characterized by thermal neutron count ratio and lithology factor based on element content, is proposed to determine porosity from the three-detector pulsed neutron element logging in unconventional reservoir.
To evaluate mineral composition, lithology, and gas/oil/water saturation in unconventional reservoir, a new multi-detector pulsed neutron logging tool was put out. The instrument consists of two He-3 thermal neutron detectors and a LaBr3 gamma detector. Therefore, the combination of thermal neutron count ratio between near detector and long detector with lithology factor of element content can measure neutron porosity and eliminate the influence of complex lithology.
Based on some calibration pit data measured in laboratory, as well as the numerical simulation method, the influences of different lithological characters and mineral types on the neutron count ratio were studied. Meanwhile, large numbers of stratigraphic models with different lithological characters and different mineral compositions were established using Monte Carlo simulation method, and the content of silicon, calcium, hydrogen, oxygen, magnesium, aluminum and iron under different stratigraphic conditions was determined by the spectral element solution. A regression analysis was conducted to establish the relationship between the content of elements and the lithologic factor. The count ratio difference stemming from different lithological and mineral compositions was eliminated through a combination of lithological correction factor and thermal neutron count ratio. Different mineral compositions of stratigraphic simulation models were set up for verification. The absolute error of porosity measurement was less than 1.0p.u. in the formations with porosity less than 15p.u., which verified the accuracy of this method for neutron porosity evaluation in complex lithological characters formations. Two field examples were processed by this new parameter which in combination of thermal neutron count ratio and formation elements content information from the three-detector pulsed neutron instrument, which indicated a good accuracy for unconventional oil and gas reservoir evaluation.