Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Diagenetic And Compositional Controls Of Wetability In Siliceous Sedimentary Rocks, Monterey Formation, California


In spite of a century of production in California, the Monterey Formation still holds tremendous quantities of recoverable oil, and much of it may be in truly unconventional reservoirs. Maximizing recovery and developing accurate reserve estimates requires understanding all factors influencing reservoir quality. The importance of wetability variations in the fluid distribution of Monterey Formation porcelanite reservoirs is little known. Observed differences in charge and producibility of opal-CT- and quartz-phase rocks have previously been attributed to pore throat size and geometry that relate to the silica phase and clay content. Yet with significant variation in bound water content of common clay and silica phases, it is reasonable to expect differences in fluid-solid surface interactions exist and may influence oil movement through the rocks. This study will be the first comprehensive investigation of variation in wetability in siliceous sedimentary rocks as a result of silica phase, crystallographic ordering and clay content. Contact angle measurements directly demonstrate the affinity of a fluid to a substrate, thus eliminating variables inherent in other wetability measurement methods. Wetability measurements in this study will accordingly be determined by the contact angles of distilled water and mineral oil on a suite of Monterey Formation siliceous rocks collected from subsurface cores. Multiple samples from each silica phase — opal-A, opal-CT and quartz — will be measured over a range of clay content from 0 to 30% in both natural and hydrocarbon—cleaned states. The composition of each sample will be determined by XRD. For uniformity in measurement, samples are crushed and sieved to <500 micrometer, then cleaned of hydrocarbons through methylene chloride extraction process, and then compressed into discs in a hydraulic press. Water and oil are placed onto the sample discs and the drop process is photographed with a high speed camera tensiometer. Contact angles and surface free energy are then calculated. Multivariate statistical analysis will quantify the correlation between the variables of silica diagenesis (phase and ordering), clay content and wetability.