Integrated Formation Evaluation Using Novel Neutron Correction in the Presence of Trace Elements

Abstract

The estimation of formation porosity in particular and formation volumetrics in general are key to any reservoir evaluation as they establish the hydrocarbon storage capacity. Compensated neutron porosity is one of the most common measurements incorporated in any log-based volumetric analysis. However, this type of measurement is strongly affected by the presence of trace elements, especially in clay rich formations. Trace elements, such as boron and gadolinium, have very high neutron capture cross-sections and absorb large amounts of thermal neutrons in the formation. Whenever these elements are present in concentrations exceeding just a few parts per million they have a significant effect on overall thermal neutron capture, yielding high apparent neutron porosity. When the measurement is included in an integrated log analysis system, this results in inaccurate formation porosity and matrix volumes. A reliable neutron porosity correction method is therefore needed to improve petrophysical evaluation in in such logging environments. In this work, an integrated model is proposed to correct the compensated neutron response using pulsed neutron spectroscopy measurements and other conventional log data. A joined inversion method is implemented which obtains an improved neutron response by removing the undesirable contributions by the trace elements which are not a part of the formation petrophysical model. The proposed model is validated, by applying it to several data sets from different shale plays in North America and Latin America, and comparing the results against core analysis data. Due to presence of rich trace elements in the formation, without the correction model, the original processing results indicate up to 50% higher formation porosity when compared with core measurements. After applying the proposed correction, the porosity comparison is dramatically improved, matching the reference core porosity. The proposed model offers an effective, integrated analysis method, in challenging unconventional clay-rich shale environments, for correct estimation of formation porosity and volumes of mineral constituents.