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TX 75083-3836, U.S.A., fax +1-972-952-9435. AbstractThe paper describes the development of an equation of state (EoS) for two reservoirs of the Dukhan field, the Arab C and the Arab D reservoirs. The paper also describes the development of compositional maps for both the reservoirs. In absence of pre-production samples, an iterative approach was used to estimate composition at a known depth for the Arab C reservoir. Equilibrium contact mixing (ECM) method along with an iterative approach was used to estimate oil composition at the gasoil contact depth for the Arab D reservoir. By examining the heptanes-plus (C 7+ ) properties of the samples collected from the depleted Arab C reservoir, existence of compositional gradient in the reservoir at discovery conditions was confirmed. It was shown that the existence of near constant C 7+ properties in the oil-column of the depleted Arab D reservoir does not preclude the existence of compositional gradient at discovery conditions. By incorporating the gravity segregation and the thermal diffusion effects, oil composition maps for the Arab C and the Arab D (excluding Diyab sector) were developed. It was shown that the inclusion of thermal diffusion effect resulted in very small changes in component concentration for the Arab C reservoir. The large saturation pressure difference between three sectors of the Arab D and the fourth sector, Diyab was then modeled using molecular diffusion. For the Arab D gas-cap, the gas composition at the gas-oil contact depth was calculated by utilizing the tuned EoS and the oil composition at the gas-oil contact. The gravity segregation and the thermal diffusion significantly changed pentanes-plus (C 5+ ) and C 7+ distribution in the gas-cap gas with 23% and 30% reduction in mole% from the gas-oil contact to the crest of the structure.

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After 70 years of production, more than 30% of the Arab C STOOIP has been recovered through various mechanisms including natural depletion, water flooding, gas-lift implementation and horizontal well development. Extending production into future years requires a strategic approach, focusing on innovative development optimization in order to target the remaining oil saturation. An aggressive drilling and intervention programme is ongoing to tap into the remaining oil. In addition to coupling evergreen reservoir models and flood-front surveillance, enhanced waterflood and CO2 WAG hold the greatest strategic potential to maximize recovery. Integration of a recently acquired, high-resolution 3D seismic survey complements the data available for subsurface description and characterization, positively impacting reservoir model history matching metrics. Utilizing the models to set appropriate production and injection targets and vice versa adjusting the models to new data acquired in the field maintains a tight coupling between our models and surveillance monitoring. The evergreen models facilitate optimization of infill drilling locations targeting bypassed/remaining oil for sweep improvements. Surveillance monitoring of flood-front encroachment and pressure behavior indicates that the permeable and connected beach and shoal lime grainstones of the Lower Arab C are sweeping according to simulation predictions. Conversely, the Upper Arab C exhibits a less favorable waterflooding potential due to the thin-bedded (1-2ft) grain- and mud-prone peritidal deposits that maintain significant lateral and vertical permeability contrasts. Sensitivity tests in our high-resolution full-field models support the addition of a line drive pattern to the existing peripheral waterflood for the Upper Arab C. CO2 WAG has also been identified and tested in the model as a potential EOR mechanism to improve recovery. Engineering studies are underway to develop the infrastructural requirements for a CO2 pilot. It is envisaged that the combined development strategy of both enhanced waterflood and CO2 EOR will greatly assist in producing the difficult oil and maximizing recovery in the process.

A reservoir study was conducted to plan a near-miscible CO2 gas injection pilot in the Dukhan Field with the following study guidelines:Determine overall objectives and timeframe for the pilotDesign the pilot to meet the objectives and timeframePlan the pilot in a portion of the field with representative reservoir descriptionPropose a surveillance and monitoring plan to ensure high quality information is obtained for interpreting the pilot performanceDesign the pilot so the recovery processes are scalable to larger well spacing for reservoir-wide application Using these guidelines, the following pilot objectives were developed:Provide data on well injectivity and productivityReduce uncertainty in the estimation of incremental oil recovery from CO2 gas injection relative to waterfloodProvide data to allow commercial scale well and facility CO2 gas injection designMinimize the impact on current operations outside the pilot area The results of the reservoir modeling study provided the detail for well orientation and placement to satisfy the pilot objectives at a reasonable cost and within appropriate timeframe for information gathering, analysis, and commercialization decisions. The reservoir model pilot area selected is one kilometer square segment in a crestal structural location that does not contain active wells and is near existing surface facilities. The reservoir description data used for the segment models were extracted from history matched full-field reservoir models. The proposed pilot wells are located within close proximity of each other to achieve gas breakthrough and oil bank arrival within the proposed pilot evaluation period of two years. The pilot design ensures that the pilot would allow for data gathering of key project design information; such as injectivity, gas breakthrough timing, oil displacement, and reservoir sweep. Introduction The large, mature Dukhan Field is located onshore Qatar, approximately 80km west of Doha. It is a north-south plunging anticline approximately 70km long by 8km wide. The field was discovered in 1939 and first production occurred in 1949. The field has more than 750 well penetrations. The major oil reservoirs are the Upper Jurassic Arab C and Arab D. There are lesser amounts of oil and gas in the Middle Jurassic and non-associated gas in the Permo-Triassic Khuff formation. The Arab C reservoir was initially filled with over 1400ft of oil between the structural crest and the oil-water contact. The Arab D reservoir was initially filled with over 700ft of oil rim in equilibrium with 700ft of gas-cap.