Field N is a complex oil and gas field in the north of the Sultanate of Oman producing from several reservoirs. In this paper, the focus will be on Shuaiba gas reservoir, which is characterized by large variations in rock properties. An additional challenge is limited core data available and big gas effect observed from the logs. The described work was undertaken as part of an FDP update.
Gas correction effect and permeability modeling in such complex carbonate reservoir with the limited availability of representative core data is challenging. The density and neutron logs are affected by the presence of gas, which required advanced correction methods. The 1st applied technique of porosity correction comprised an iterative method of density and micro resistivity with simultaneous solution for residual gas saturation and corrected porosity. In addition, a new modification of gas correction was tried from Density-Neutron gas separation along with known PVT parameters was used for porosity calculation and gas corrections.
The log response of the Shuaiba reservoir in Field N looks like a classical illustration of gradual change of saturation from water leg to the transition zone and gas leg. However, flank well has been perforated close to the anticipated contact and produced gas for more than 200 days without water. As a result of comprehensive data analysis, a significant vertical heterogeneity of the reservoir was established. It was found that Gas saturation is a good indicator of permeability. A good relationship between fluid mobility from Wireline Formation Test data and water saturation from logs for gas leg was established. In the gas leg, all water is considered as irreducible, thus this is in fact relationship between mobility and irreducible water saturation. The relationship was then calibrated to permeability using the available core. As a result of this work, significant heterogeneity of the Shuaiba reservoir was captured in case of limited core data available. The remaining gas quantified for optimizing future development.
As part of this Study, an advanced gas correction method was applied resulting in more reliable Porosity evaluation. Besides, a new modification of the method for gas corrections from Density-Neutron separation was tested and showed good results.