Naturally fractured carbonate reservoirs hold well over 100 billion barrels of heavy oil worldwide. Thermally Assisted Gas Oil Gravity Drainage (TAGOGD) is a new and novel thermal EOR technique, which has applicability in selected reservoirs. In conventional isothermal GOGD, vertical fractures cause the gas-oil contact in the fracture system to advance ahead of the gas-oil contact within the matrix blocks, causing the oil in these blocks to become mobile. The addition of heat in the fractures generates additional hydrocarbon gas cap, lowers the viscosity of the oil, and accelerates conventional GOGD, as seen in the 220 cp heavy-oil Qarn Alam field in Oman. Pilot results in the Qarn Alam field support the commerciality of this process, and a first-of-it's-kind steam injection project is being implemented. The economic success of the Qarn Alam project depends on the ability to credibly predict steam requirements and oil production. Two key oil production mechanisms are heat transport through the fractures and into the matrix, and subsequent gas cap generation due to thermal volatilization of the oil. The process mechanisms involved in TAGOGD were validated through laboratory experiments, while the field forecast model results were validated by history matching pilot performance data. A fully integrated workflow of fracture characterization, integrated reservoir physics, and static and dynamic modeling has enabled uncertainties and risks involved in developing the Qarn Alam field to be managed in a scenario based design approach. Introduction The Qarn Alam field is a highly fractured carbonate field that lies atop a salt diapir in Northern Oman. The 6 km long and 3 km wide field forms a relatively high-relief anticline with a N-NE by S-SW orientation. The reservoir is relatively compact dome-shaped structure, with a maximum oil column of 165 m. The main oil bearing reservoirs, the Shuaiba and Kharaib formations, are separated by a very low permeability oil bearing zone called the Hawar. The crest of the Shuaiba is located at 212 mss, and the original oil water contact is ∼375 mss. Fracturing occurs throughout all zones, and is believed to be contiguous and in hydraulic communication with a very active aquifer. The initial oil saturation is about 95% and initial water saturation is connate water. The matrix porosity is high (∼30%), while the matrix permeability ranges between 5 md-20 md. Under primary production, the reservoir produces on average about 100 m3/day of 16o API "heavy" oil, at a GOR of 10 m3/m3.

No abstract

Significant volumes of heavy oil remain in fractured carbonate reservoirs worldwide. Some of these reservoirs are good candidates for the application of thermally assisted gas-oil-gravity-drainage (TA-GOGD), a novel EOR technique. Unlike a normal steam flood, the steam is used as a heating agent only to enhance the existing drive mechanisms. The elegance of TA-GOGD is that the fracture network is both used for the distribution of steam (heat) and the recovery of the oil. The number of wells can therefore be kept to a minimum compared to conventional steam floods. Following encouraging pilot results in a field in Oman, a steam injection project is heading for implementation, a first of its kind on this scale. Studies to date indicate that recovery factors of 25–50% with Oil-Steam-Ratios of 0.2 -0.4 m3/ton of steam are feasible. The success of the project is critically dependent on the field-wide presence of conductive fractures and the ability to characterize them. Both stochastic and deterministic studies were tried, but the latter method is now favoured as it allows the use of geological and dynamic understanding as input to the modelling and honours existing faults, deformation mechanism and the conceptual model. Fracture characterisation is to some extent still an art and outputs are'only static scenarios'. Therefore results should be validated with dynamic data as much as possible. The dynamic models are thermal and dual permeability, with compositional dependencies: a complexity that is rarely encountered. Explicit fracture block models are used to verify that the heating rate and GOGD are captured properly, in particular for irregularly shaped fracture patterns. A new fully integrated workflow of fracture characterisation with static and dynamic modelling has enabled uncertainties and risks to be managed in a scenario based approach. Introduction Primary production performance such as that of Qarn Alam Qarn Alam Field is located in central Oman south of the Shuaiba is only expected to recover some 3–5% of the oil in western Hajar Mountains. This large oil accumulation is place over any reasonable time frame due to low matrix trapped in the Cretaceous Shuaiba, Kharaib and Lekhwair permeability and high oil viscosity on gravity drainage rates. limestone units at a depth of around 200–400m sub sea. The Recoveries via matrix floods of water, polymer or steam were anti-clinal structure is a result of a deep salt diaper, with discounted as development options due to the pervasive significant crestal faulting and fracturing.fracturing observed in the field which would encourage the flooding agents to completely bypass the matrix. The field was discovered in 1972 and placed on primary production in 1975. The produced oil was found to be 16 ° API with a viscosity of 220cP. During the primary production period from1975 to 1995, the first year showed a large peak in oil mainly from emptying of the fracture network with a minor contribution from fluid expansion due to pressure reduction. At the end of the first year, production had declined to a very low sustainable rate interpreted to be from gravity drainage, from a combination of gas-oil (GOGD) from the secondary gas cap and oil-water (OWGD) below the fracture gas-oil contact (FGOC). The reservoir then consists of a matrix with very little drainage and a fracture network with a thin oil rim below the secondary gas cap and above the fracture oil-water contact (FOWC), figure1.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractSignificant volumes of heavy oil remain in fractured carbonate reservoirs worldwide. Some of these reservoirs are good candidates for the application of thermally assisted gas-oil-gravity-drainage (TA-GOGD), a novel EOR technique. Unlike a normal steam flood, the steam is used as a heating agent only to enhance the existing drive mechanisms. The elegance of TA-GOGD is that the fracture network is both used for the distribution of steam (heat) and the recovery of the oil. The number of wells can therefore be kept to a minimum compared to conventional steam floods. Following encouraging pilot results in a field in Oman, a steam injection project is heading for implementation, a first of its kind on this scale. Studies to date indicate that recovery factors of 25-50% with Oil-Steam-Ratios of 0.2 -0.4 m3/ton of steam are feasible. The success of the project is critically dependent on the field-wide presence of conductive fractures and the ability to characterize them. Both stochastic and deterministic studies were tried, but the latter method is now favoured as it allows the use of geological and dynamic understanding as input to the modelling and honours existing faults, deformation mechanism and the conceptual model. Fracture characterisation is to some extent still an art and outputs are 'only static scenarios'. Therefore results should be validated with dynamic data as much as possible. The dynamic models are thermal and dual permeability, with compositional dependencies: a complexity that is rarely encountered. Explicit fracture block models are used to verify that the heating rate and GOGD are captured properly, in particular for irregularly shaped fracture patterns. A new fully integrated workflow of fracture characterisation with static and dynamic modelling has enabled uncertainties and risks to be managed in a scenario based approach.

No abstract