Quantifying Bitumen Plugging in Tight Gas Reservoir using NMR and Pulsed-Neutron Spectroscopy Logging

摘要

The identification, quantification, and implications ofbitumen are concerns for a deep (>5000-m) tight gasreservoir in the Amin and Miqrat Formations in theSultanate of Oman. Hydraulic fracturing is required toyield economical production rates in these reservoirs.The bitumen, which is believed to result from oil to gascracking process, poses an additional challenge becauseit directly degrades storage capacity, destroys thepermeability, and results in erroneous computedsaturations. It is, thus, imperative that these factors aretaken into consideration in reserves and productivitypredictions that could ultimately impact overalldevelopment planning.An evaluation workflow was devised to directly quantifybitumen and provide accurate volumetric analysis. Themethodology is based on the integration of the triplecombo,nuclear magnetic resonance (NMR), and pulsedneutron spectroscopy logs in an openhole environment.The NMR effectively identifies bitumen through itsreduced porosity against density log. This differencearises because NMR is incapable of measuring fastrelaxation of the hydrogen contained in bitumen.However, the NMR porosity is also reduced due to thereduced hydrogen index of gas. Spectroscopy analysisquantifies the concentrations of various elementscontained in the reservoir rocks. This includes carbonconcentration contained in the rock, gas, and bitumen.The total organic carbon (TOC) is derived by subtractinginorganic carbon contained in the rock. The measuredTOC is essentially due to the bitumen as it is negligiblein gas. The integration of these different measurementsis an effective means to differentiate and quantifydifferent pore constituents contained in our subjectreservoirs.In cased and completed wells, we have developed anevaluation workflow that uses the slim pulsed neutrontool outputs. This includes sigma that is sensitive tosaline water, hydrogen index (HI) and fast neutron crosssection that are sensitive to gas, and TOC that ispredominately due to bitumen.The integrated volumetric analysis is done using aniterative linear solver. The analysis provides the totalporosity, bitumen volume, and true gas saturation. Thelog predictions were verified against pyrolysis and thinsectionanalyses on core samples from similar wells. Theproduction logs directly demonstrated the influence ofthe bitumen presence on the well productivity. Repeatingthe analysis in different areas of the reservoir furtherestablished the uneven bitumen distribution and itsconsequences across the reservoir. Our analysis providedthe basis for more representative static and dynamicmodeling.