The Establishment of a Novel Hydraulic Flow Unit Model for a Mature Carbonate Reservoir Based on Water Saturation

ABSTRACT / EXECUTIVE SUMMARY Establishing a new geological model with updated reservoir properties was deemed essential to support the new reservoir simulation model for this Middle Cretaceous carbonate reservoir. Historically, the reservoir simulation team had to apply permeability multipliers on the order of ten to accurately reflect and match historical production data from the reservoir. Previous geological models failed to account for the high-permeability features within the reservoir. The presence of these features was suggested by production data from many wells. Evidence for these existed in the well log resistivity profile and the presence of these high-permeability layers had been documented in technical reports from the 1980’s. It is important to note that these high-permeability layers had never been identified in the recovered cores, despite the collection of more than forty cores from the reservoir since the 1930’s. A significant variation in well log resistivity values is observed in most wells within what is classified as a clean carbonate reservoir interval. These layers exhibit high water saturation and appear to be well developed in the same interval across most wells, demonstrating consistency in thickness and severity throughout much of the field. The decrease in resistivity values is often accompanied by a reduction in porosity, and the layering has been described in previous publications as post-depositional rock leaching at the phreatic-vadose interface (water table). Subsequent production and noise logs have indicated that these high-water saturation layers are the primary producing zones in many wells, underscoring the importance of capturing these features in the geological reservoir model. Consequently, a "Rock-typing" scheme was adopted to map these enhanced permeability features using resistivity logs that capture these layers in most wells. This was achieved by developing a Hydraulic Flow Unit (HFU) methodology based on Rock Quality Index (RQI), with corresponding Saturation Height functions that assisted in establishing an improved static model to aid in reservoir simulation.