Flow Unit Characterization of a Shallow Shelf Carbonate Reservoir: North Robertson Unit, West Texas
Authors: Davies David K. , Vessell Richard K. 
Published: December 2007

Abstract
A model is developed of a heterogeneous carbonate reservoir based fundamentally on measurement of pore geometrical parameters. Pore level reservoir modeling results in improved accuracy in the prediction of rock types, permeability and identification of flow units. Pore geometrical attributes are integrated with wireline log data to allow for i) log-based identification of intervals of rock with different capillary characteristics. and ii) field-wide, log-based prediction of permeability. Twelve hydraulic flow units are identified through integration of data concerning rock type distribution. and depositional environments. Maps of permeability thickness (kH) for each flow unit reveal significant stratigraphic compartmentalization. Future development drilling using uniform well spacing patterns is not appropriate. The location and design of infill drilling patterns should be geologically targeted for prudent, cost-effective field development.

Introduction
This study focuses on the use of pore geometrical attributes to predict permeability and define hydraulic flow units in a mature, heterogeneous, shallow shelf carbonate reservoir. The purpose of this study is to identify and map individual hydraulic flow units to aid determination of the potential for continued development drilling. The study is funded by the US Department of Energy as part of its Class II Oil Program for shallow shelf carbonate (SSC) reservoirs. One objective of the program is to demonstrate advanced reservoir characterization tools that will result in a significant increase of reserves.
SSC reservoirs in the USA originally contained &gt;68 BBO (about one-seventh of all the oil discovered in the Lower 48 States). Recovery efficiency is low; some 20 BBO have been produced and current technology may only yield an additional 4 BBO. The problem of low recovery efficiency in SSC reservoirs is not restricted to the USA — it is a worldwide phenomenon. SSC reservoirs share a number of common characteristics, including:A high degree of areal and vertical heterogeneity, relatively low porosity and relatively low permeabilityReservoir compartmentalization, resulting in poor vertical and lateral continuity of the reservoir flow units and poor sweep efficiencyPoor balancing of rates of injection and production, and early water breakthrough in certain areas of the reservoir. This indicates poor pressure and fluid communication and limited repressuringPorosity and saturation as determined from analysis of wireline logs do not accurately reflect reservoir quality and performanceMany injection and production wells are not optimally completed with regard to placement of perforations, and the stimulation treatment can be inadequate for optimal production and injection practices
The North Robertson Unit exhibits all of these characteristics.

North Robertson Unit
The North Robertson Unit (NRU) was the single largest waterflood installed in the onshore, lower 48 states of the USA during the 1980's. The unit covers 5,633 acres, has 259 wells and uses a 40 acre 5-spot waterflood pattern with 20 acre nominal well spacing. The field was on primary production from 1954 to 1987: the secondary waterflood has been in place since 1987. Currently, the field has 144 active producing wells, 109 active injection wells and 6 water supply wells. An objective of this current study is to identify the areas of the unit with the best potential for additional infill drilling (planned 10 acre spacing).
The original oil in place is estimated at 260 MMSTB with an estimated ultimate recovery factor of 13.5% (primary recovery = 7.5%, secondary recovery = 6%) based on the current production and workover schedule.
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