Revealing Complex Oil Water Contacts Driven by Faulting

Abstract Oil-water contacts in mature fields where injection water results in complex sweep patterns display rugose morphologies. Understanding the driving factors behind the distribution of fluids is critical to advance reservoir understanding and therefore better exploit the reservoir. Mapping fluid boundaries with electromagnetic (EM) tools and integrating the resultant models with independent logging while drilling (LWD) tools is the key to increasing interpretation confidence. Deep EM LWD inversion allows mapping of resistivity boundaries in the region of 25 to 30 feet from the wellbore. In a perceived massive carbonate reservoir these boundaries are dominated by low resistivity water and high resistivity hydrocarbons. Mapping these fluids can reveal complexities in the reservoir architecture and lithologic variation which facilitates optimal well placement and improved reservoir understanding. Increasing the confidence in EM inversions through data integration increases the capability of higher resolution fluid distribution interpretations. In this example, a series of small sub-seismic stepped faults are revealed in the EM inversion results. The stepped appearance of the reservoir with sharp contrast between high and low resistivity zones reveals the underlying small scale reservoir structure. This supports two controls on fluid distribution: faulting and lithology. In some cases, hydrocarbon saturated porosities with high resistivities are at identical true vertical depths (TVD) to water flooded low resistivity zones. Faults are exhibited by thin, high apparent angle, low resistivity streaks, suggesting water movement parallel to the interpreted fault plane. However, zones immediately adjacent to interpreted faults retain high resistivity, suggesting water movement perpendicular to the fault plane is impeded. The position of the structural elements can be supported by analysis of density image logs displaying the same high angle features and the fluid distribution from analysis of EM images which are independent of the inversion results. Integration of multiple independent measures from multiple tools and methods reveals the sub-seismic reservoir architecture in detail. Understanding how complex structural and stratigraphic elements contribute to the distribution of fluids in a reservoir can help to define the controls on production-time fluid movement in complex systems. To adequately capture the resolution required for detailed fluid boundary interpretation, data acquisition programs must consider tradeoffs of depth-of-investigation and resolution. Assessment of production and water flooding controls at different scales demonstrates the strength of a holistic approach to data analysis and interpretation.