An Improved Method for Oil-Water Contact Determination in Connected Fractured-Caved Reservoir Modeling Restricted by Data of Oil Displacement by Cyclic Water Injection

Abstract

Abstract With complicated accumulation mechanism of fractured-caved carbonate formations, characterization of reservoir and fluid distribution has great uncertainty due to varied oil-water contact and discontinuous reservoir, which brings difficulties to refined reservoir characterization and development index prediction. However, wells are always completed in upper part of the reservoir, instead of drilled through the pay zone, leaving the oil-water contact unknown below the well. It's urgent for a reservoir unit to confirm the oil-water contact. In the study, a method is proposed to constrain the pore volume and aquifer volume in a fractured-caved model based on the projection of water injection data. A material balance equation set of cyclic water injection in the fractured-caved reservoir is used to calculate the total volume of the connected reservoir drained by wells and distinguish the volume proportion of water in the fluid, according to the formation pressure recovery caused by the certain water injection volume in each cycle. A geological model is built via multi-attribute modeling technology and then modified by the outcome of former dynamic reserve evaluation. The seismic attribute threshold of reservoir extraction is controlled by the total reservoir volume, thus the volume of geological model determined. The depth of oil-water contact is adjusted to fit the oil and water volumes related to the water volume ratio, finally a reasonable oil-water contact obtained. Fractured-caved reservoir model and oil-water contact depth are further verified and corrected by drilling and completion data, oil testing and water production data. The feasibility and reliability of the method are demonstrated by a case study in Tarim Oilfield, where the depth of oil-water contact is deduced as −6342m from the first producer in the reservoir unit, and the oil testing of second producer supports it. The method can be applied to single wells or well groups in fractured-caved reservoir that have not yet seen water breakthrough after water injection. It can improve the geological understanding of the reservoir, evaluate the development potential of water injection, and predict the water breakthrough, helpful to the adjustment of development measures.