Sedimentary rocks are formed through the accumulation of small particles in aquatic environments and may subsequently be cemented by minerals formed within the rocks.. Such rocks have been of significant interest for oil and gas exploration and production for decades and become increasingly important for permanent CO2 storage as part of the integrated Carbon Capture and Storage (CCS) technology. Consequently, the economic and environmental importance of sedimentary rocks in the subsurface has instigated the development of various approaches and technologies associated with their characterization and their representation in geological models with the aim being to improve the results from dynamic simulations. Sedimentary reservoirs are inherently heterogeneous in their lithology. This lithological heterogeneity in the form of sedimentary structures such as cross and planar bedding and as interfaces between massive beddings is known to impact fluid flow and fluid-rock reactions in the subsurface. Such heterogeneity exists at mm- to cm-scale and is typically not accounted for in conventional geological models due to technical restrictions posed by the wireline log resolution and the inefficiency of reservoir modelling softwares to handle such fine-scale information.
Overall, it is evident that multiphase flow and reactive transport simulations based on conventional models fail to appropriately account for the impact of sub-meter scale sedimentary heterogeneity. To address this challenge, researchers from The University of Melbourne in Australia: Achyut Mishra (PhD candidate), Dr. Kuncho D. Kurtev and led by Professor Ralf R. Haese developed a novel workflow which could allow building reservoir models where such fine-scale lithological heterogeneity can be integrated, thereby overcoming the limitations associated with log resolution and modelling softwares. Their approach entailed a workflow where information from the wireline logs was coupled to the high-resolution data from core samples. Their work is currently published in the research journal, Marine and Petroleum Geology.
To begin with, a lithotype log of the composite rock types was derived and was used to downscale the depositional facies model to the scale of rock types. Rock types are nested within the coarser-resolution depositional facies intervals to complement the conventional reservoir modelling approach. The workflow was applied to the data available from the CO2CRC’s Otway Research Facility site where sedimentary heterogeneity is known to exist at scale of 5 mm.
The authors reported that the developed approach had two main advantages: first, mm- to cm-scale lithological heterogeneity could be accounted for in the definition of composite rock types and second, technical pitfalls associated with the characterization of grid cell properties at different reservoir resolutions were overcome. In fact, the workflow could determine the number of composite rock types and their composition with statistical significance at different discretization scales. This step was important as it bypassed any bias associated with the choice of upscaling domain size and lead to a correspondence of properties during the downscaling of the coarsely discretized depositional facies model.
In summary, the study by Professor Haese and his research team demonstrated a novel approach for systematically modifying the conventional lithotype logs in terms of composite rock types. The team reported that the modified lithotype logs were useful in obtaining reduced yet statistically significant variograms while accounting for the spatial correlation in the properties. In a statement to Advances in Engineering, Professor Haese who is also the Director of Peter Cook Centre for CCS Research highlighted that their composite rock type method would allow to represent fine scale heterogeneity to be directly accounted for in static reservoir models. He further pointed to the fact that the outcomes from the workflow were backed by core data which gives confidence in the utilizing and generalizing the presented approach.
Achyut Mishra, Kuncho D. Kurtev, Ralf R. Haese. Composite rock types as part of a workflow for the integration of mm-to cm-scale lithological heterogeneity in static reservoir models. Marine and Petroleum Geology 114 (2020) 104240.