Petrophyscial and Fluid Flow Properties of a Tight Carbonate Source Rock Using Digital Rock Physics

Abstract

The evaluation of unconventional formations is complicated by the structurally heterogeneous nature of fine-grained strata and their intricate pore networks, which are interdependent on many geologic factors including total organic carbon (TOC) content, mineralogy, maturity and grain-size. The low permeability of unconventional rocks requires massive hydraulic fracturing to enhance connectivity and increase permeability for the flow. To design an effective fracturing technique, it is necessary to have a good understanding of the reservoir characteristics and fluid flow properties at multiple scales. In this work, representative core plug samples from a tight carbonate source rock in the Middle East were characterized at the coreand pore-scale levels using a Digital Rock Physics (DRP) workflow. The tight nature of the carbonate rocks prevented the use of conventional methods in measuring special core analysis (SCAL) data. Two-dimensional Scanning Electron Microscopy (SEM) and three-dimensional Focused Ion Beam (FIB)-SEM analysis were studied to characterize the organic matter content in the samples together with (organic and inorganic) porosity and matrix permeability. The FIB-SEM images in 3D were also used to determine petrophysical and fluid flow (SCAL) properties in primary drainage and imbibition modes. A clear trend was observed between porosity and permeability related to identified rock fabrics and organic matter in the core. The organic matter was found to have an effect on the imbibition two-phase flow relative permeability and capillary pressure behavior and hysteresis trends among the analyzed samples. The data obtained from DRP provided information that can enhance the understanding of the pore systems and fluid flow properties in tight formations, which cannot be derived accurately using conventional methods. Introduction Unconventional reservoirs differ largely from conventional sandstone and carbonate reservoirs in their origin, geologic evolution and current occurrence. They comprise a variety of rocks that are composed of extremely finegrained particles with very low porosity and permeability values in the order of few porosity units and nano-darcy range, respectively. Unconventional formations are very complex at the core scale, and exhibit large vertical variations in lithology and Total Organic Carbon (TOC) at a small scale that renders core characterization and sweet spot detection very challenging. They are also very complex at the nano-scale where the pores have different porosity types that are detected within kerogen and rock matrix. These complexities led to further research and development of advanced application of high resolution imaging to characterize mineralogy, porosity and rock facies so that accurate evaluation of the sweet spot locations could be made for further detailed petrophysical and petrographic studies (Walls and Armbruster, 2012). In recent DRP work from the Middle East the depositional facies was found to have great effect on pore types, rock fabric and porositypermeability relations (Almarzooq et al., 2014). URTeC 2015 Page 2550