Low Resistivity Pay Study with 3D X-Ray Microscopy

Abstract Digital imaging of the rock samples is one of the keys to address the increasing complexity of drilling targets. With this in mind, Saudi Aramco embarked on the journey of digitization and analysis of its vast subsurface rock samples collection called digital core. Digital core is a process of generating digital twins of the rock samples by digitizing the samples and predicting rocks properties by simulating physical and chemical processes at the pore scale. This paper presents the fundamentals of the digital core and its business application with regards to the carbonate low resistivity pay (LRP) characterization. LRPs are reservoirs from which water free hydrocarbons are produced in the presence of erroneously interpreted water saturations. LRPs are often identified within formations with multi-modal pore-size characteristics identified on the basis of well logging, testing and core observations. The following study addresses the characterization of a microporous carbonate LRP formation using digital rock physics (DRP) analysis in various facies recognized in 62 core plugs through digital imaging. A comprehensive DRP workflow was utilized to produce 3D digital rock models of rock samples through multi-resolution X-ray tomographic imaging and application of a machine-learning algorithm to characterize the internal fabric of the rock samples based upon the abundance of microporosity. The physical samples were also analyzed using mercury injection capillary pressure (MICP) to allow calibration and to validate the 3D digital rock models. Numerical simulation of the electrical current flow through the samples showed the variation of Archie’s saturation exponent "n" with water saturation, i.e., the increasing influence of the microporous regions as water saturation is decreased. The meso- and macro-pores were found to produce the moveable hydrocarbons due to their lower capillary pressure while the micro-pores holding immobile formation water. The DRP models were then used to simulate the drainage relative permeability curves to identify the water saturation causing the first water in-flow, and separate immobile and free-fluids. The wireline logs saturation profile was updated with these results. As shown by digital experiments, conducted on LRP, the water-filled micro-pores provide a continuous path for electric current, masking the hydrocarbons and overestimating the water saturation. Saturation calculation could be significantly refined, if LRP wireline petrophysical models are calibrated using DRP data by incorporating the modelled Archie’s saturation exponent and first water in-flow from relative permeability curves.