Abqaiq Hajifa Reservoir: Geologic Attributes Controlling Hydrocarbon Production and Water Injection

Abstract

ABSTRACT The Hanifa reservoir at Abqaiq Field consists entirely of mud-supported limestones that lack megascopic pore spaces. Porosity in the Hanifa reservoir is in the form of micropore spaces, spaces ≤10 microns in size. Microporosity is evidenced by high porosities (5-32%) within these fine-grained rocks based on porosity logs and coreplug analyses, absence of any visible porosity (even in thin sections) to account for the high pore volumes, and 2000X SEM images that show a crystal framework texture composed of microrhombic (day-size) caldte crystals with 2-5 micron size pore spaces between caldte crystals. The microporosity is considered to reflect retention of primary intercrystalline spaces within the precursor lime mud sediment, and with little introduction of allochthonous caldte to ocdude pore spaces. Flow meters indicate that the reservoir is capable of produdng/injecting large volumes of oil/water, but with no stratigraphic predictability to the flow, and that thin (2-10 ft) low porosity (<15%) intervals can contribute over 60% of the entire flow. These reservoir attributes, coupled with the low "matrix" permeabilities (0.1-10 md) of the reservoir indicate the presence of an apparent permeability that is controlling fluid flow. Core studies revealed that this apparent permeability is in the form of high angle fractures. The fractures are ≤ 1 mm wide, contain hydrocarbon residue and caldte cement, and many are in dose assodation with high amplitude stylolites, suggesting a genetic link between stylolitization and fracturing. Bore hole imaging log data is providing value insight into fradure location, abundance, orientation, and size in non-cored wells. The Hanifa is separated from the giant Arab-D reservoir by over 450 feet of fine-grained and impermeable carbonates of the Jubaila Formation. However, these two reservoirs are in pressure-fluid communication via a network of fractures through the Jubaila carbonates. This reservoir communication together with the reservoir heterogeneity of the Hanifa, in the form of micropores and assodated fractures, provides a challenge for reservoir geology and reservoir engineering to formulate a development plan, involving horizontal producers, to mitigate reservoir communication and to effidently and effectively extract the reserves within the Abqaiq Hanifa reservoir.