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Geological Modeling of Fluvial Reservoir for Horizontal Well Development in a Southern Central Oman Heavy Oil Field

Abstract

The oldest oil field in Southern Central Oman produces more than 120,000 of heavy oil per day from reservoir sands deposited in a fluvial depositional system of the Permian Gharif reservoir. The complex fluvial reservoir architecture is defined by a rich data set of more than 2000 vertical and horizontal wells drilled on closely spaced patterns, since 2005. As expected, the addition of well data also added complexity in the geology. The challenge is to build a full field 3D geological model that captures local variability and geologic detail within a consistent framework of fieldwide stratigraphy across 10 km, for input to simulation for design and operational optimization of this thermal recovery EOR of the reservoir. Geological modeling efforts began in 2006 to build a single “evergreen” model that supported business strategic activities such as reserves assessment, flow simulation for forecasting type curve model, surveillance and be sufficiently detailed to plan and steer horizontal wells. Stratigraphy framework that was generated using lithostratigraphy well correlation during period 2006–2012 model that used for simulation was questioned due to unable to capture reservoir heterogeneity, unanticipated field performance and the inability to history match the well performance of specific patterns and/or injector/producer pairs. In 2013, a sequence-stratigraphic interpretation was proposed as a method to better capture the field's geologic heterogeneity. The methods attempt to correlate time-equivalent and genetically related strata, a sequence-stratigraphic interpretation is a chronostratigraphic interpretation. The geological model covers a field area of about 200 km2. The vertical injector well data describe the vertical geologic profile of the reservoir. The horizontal producer wells describe the lateral continuity of the reservoir sands. The geological modeling integrates multi-disciplinary data from sedimentology, petrophysics, sequence stratigraphy, geophysics, and geostatistical analysis of the well data. The chronostratigraphic framework teases out the regional cycles of deposition that are important to correlating well pairs. The geological model covers a field area of about 200 km2. The vertical injector well data describe the vertical geologic profile of the reservoir. The horizontal producer wells describe the lateral continuity of the reservoir sands. The geological modeling integrates multi-disciplinary data from sedimentology, petrophysics, sequence stratigraphy, geophysics, and geostatistical analysis of the well data. The chronostratigraphic framework teases out the regional cycles of deposition that are important to correlating well pairs. The reservoir corresponds to the proximal part in the south east of the field to medial-distal part toward north west of an Early-Middle Permian Gharif Formation of a non-marine fluvial deposit. The reservoirs are heterogeneous and layered, mostly consists of stacked fluvial channel deposits interbedded with flood-plain shales with paleosols. The channel amalgamation rate varied with cyclic of base-level variations, which directly controlled the reservoir geometry. During periods of low accommodation, channels were amalgamated, forming highly heterogeneous sand sheets. As the accommodation increased, channels became progressively isolated within flood-plain shales. Estimation of channel dimensions in the fluvial reservoir is based on established sedimentology relationships between vertical and horizontal channel and bar dimensions in modern fluvial systems. These estimates are calculated from core, wells and outcrops. These results are used to guide the dimensions and spatial distribution in the geologic model. Five cycles that constituted the reservoir layering framework were identified. Geostatistical simulations of lithofacies distribution within these units were computed using the Truncated Gaussian method. The use of modeling result is to drill a horizontal well for each of the major chronostratigraphic units. Local regions of the reservoir that are anomalous to the regional stratigraphy have adjusted design in order to optimize the cost of development and production.