ABSTRACT

The identification, quantification, and implications of bitumen are concerns for a deep (>5000-m) tight gas reservoir in the Amin and Miqrat Formations in the Sultanate of Oman. Hydraulic fracturing is required to yield economical production rates in these reservoirs. The bitumen, which is believed to result from oil to gas cracking process, poses an additional challenge because it directly degrades storage capacity, destroys the permeability, and results in erroneous computed saturations. It is, thus, imperative that these factors are taken into consideration in reserves and productivity predictions that could ultimately impact overall development planning.

An evaluation workflow was devised to directly quantify bitumen and provide accurate volumetric analysis. The methodology is based on the integration of the triplecombo, nuclear magnetic resonance (NMR), and pulsed neutron spectroscopy logs in an openhole environment. The NMR effectively identifies bitumen through its reduced porosity against density log. This difference arises because NMR is incapable of measuring fast relaxation of the hydrogen contained in bitumen. However, the NMR porosity is also reduced due to the reduced hydrogen index of gas. Spectroscopy analysis quantifies the concentrations of various elements contained in the reservoir rocks. This includes carbon concentration contained in the rock, gas, and bitumen. The total organic carbon (TOC) is derived by subtracting inorganic carbon contained in the rock. The measured TOC is essentially due to the bitumen as it is negligible in gas. The integration of these different measurements is an effective means to differentiate and quantify different pore constituents contained in our subject reservoirs.

In cased and completed wells, we have developed an evaluation workflow that uses the slim pulsed neutron tool outputs. This includes sigma that is sensitive to saline water, hydrogen index (HI) and fast neutron cross section that are sensitive to gas, and TOC that is predominately due to bitumen.

The integrated volumetric analysis is done using an iterative linear solver. The analysis provides the total porosity, bitumen volume, and true gas saturation. The log predictions were verified against pyrolysis and thin-section analyses on core samples from similar wells. The production logs directly demonstrated the influence of the bitumen presence on the well productivity. Repeating the analysis in different areas of the reservoir further established the uneven bitumen distribution and its consequences across the reservoir. Our analysis provided the basis for more representative static and dynamic modeling.

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