Gas Oil Gravity Drainage (GOGD) of the Qarn Alam fractured low permeability carbonate reservoir is being enhanced by steam injection in the world's first full field development carbonate thermal development. Unlike a normal steam flood, the steam is used as a heating agent to enhance the existing gravity drainage mechanisms, and project has proved to be viable based on encouraging pilot results. The elegance of the thermally assisted - GOGD is that the fracture network is both used for distribution of steam and recovery of the oil. The number of wells can therefore be kept to a minimum compared to conventional matrix steam floods. Whereas the primary production performance of the Qarn Alam under cold GOGD is only expected to recover 3–5 % of the oil in place, studies to date indicate that the recovery factor under steam injection at 18,000 tonne per day will be in the range 20–35 % with Oil Steam Ratio of 0.16 -0.3 m3 oil /tonne of steam. The learning from the Pilot has not only helped understand the subsurface uncertainties but also provided significant insight into the engineering design and operational issues which are being managed right upfront during design. Energy consumption in the full field development of this project is reduced by banking on the benefit from co-generation of power and steam. Utilisation of co-generation will minimise CO2 emission and reduce gas import. The project will be under construction from 2007, with full rate steam injection reached by late 2009. The paper not only discusses how the project addresses the reservoir management challenges of this complex recovery mechanism but will address some of the engineering and operational challenges that are being managed. Introduction Qarn Alam Field is located in central Oman south of the western Hajar Mountains. This large oil accumulation is trapped in shallow Cretaceouslimestone units at a depth of around 200–400m sub sea. The anti-clinal structure is a result of a deep salt diaper, with significant crestal faulting and fracturing. The field was discovered in 1972 and placed on primary production in 1975. The 16° API oil with a viscosity of 220cP has been produced from the 29% porosity, low permeability (5–14mD) limestone. . During the primary production period from 1975 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), figure 1. Primary production performance such as that of Qarn Alam is only expected to recover some 3–5% of the oil in place over any reasonable time frame due to low matrix permeability and high oil viscosity on gravity drainage rates. Recoveries via matrix floods of water, polymer or steam were discounted as development options due to the pervasive fracturing observed in the field which would encourage the flooding agents to completely bypass the matrix.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractGas Oil Gravity Drainage (GOGD) of the Qarn Alam fractured low permeability carbonate reservoir is being enhanced by steam injection in the world's first full field development carbonate thermal development. Unlike a normal steam flood, the steam is used as a heating agent to enhance the existing gravity drainage mechanisms, and project has proved to be viable based on encouraging pilot results. The elegance of the thermally assisted -GOGD is that the fracture network is both used for distribution of steam and recovery of the oil. The number of wells can therefore be kept to a minimum compared to conventional matrix steam floods. Whereas the primary production performance of the Qarn Alam under cold GOGD is only expected to recover 3-5 % of the oil in place, studies to date indicate that the recovery factor under steam injection at 18,000 tonne per day will be in the range 20-35 % with Oil Steam Ratio of 0.16 -0.3 m3 oil /tonne of steam. The learning from the Pilot has not only helped understand the subsurface uncertainties but also provided significant insight into the engineering design and operational issues which are being managed right upfront during design. Energy consumption in the full field development of this project is reduced by banking on the benefit from cogeneration of power and steam. Utilisation of co-generation will minimise CO2 emission and reduce gas import. The project will be under construction from 2007, with full rate steam injection reached by late 2009. The paper not only discusses how the project addresses the reservoir management challenges of this complex recovery mechanism but will address some of the engineering and operational challenges that are being managed.

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Thermally Assisted Gas Oil Gravity Drainage of a fractured carbonate heavy oil field in Oman is starting the full field phase. Unlike a normal steam flood, steam is used as a heating agent to enhance the existing gravity drainage mechanisms. The project has been piloted successfully. The project start-up sequence consists of increasing off take from deviated producers followed by steam injection and aquifer pump-off. Steam will progressively fill the fractures whilst heated oil drains down in the matrix blocks and accumulates in an oil rim below the steam in the fractures. The fracture oil rim will be lowered by approximately 100m. Horizontal producers will be completed in the final fracture oil rim position. As no analogues exist a large degree of flexibility has been incorporated in the field development plan to cover uncertainties including caprock integrity, erratic oil rim movement and heterogeneous steam distribution. To facilitate decision making an enhanced reservoir surveillance, modelling and management system has been built. Reservoir pressures, oil rim positions, temperatures and rock strain data are obtained from a range of observation wells. Further data are obtained from surface uplift, microseismic monitoring and fluid sampling. Static, fracture, dynamic and geomechanical reservoir models have guided the design of the reservoir surveillance program and underlie the operating guidelines for the reservoir. These provide operator reactions for a wide range of "what-if" events. A corporate real time data portal has been optimised for the unique requirements of this project allowing for analysis of all data streams. A system allows 3D display, through time, of reservoir data and model forecasts to ensure optimum performance analysis. Initial application has resulted in identification of optimised well configurations and start-up sequence giving higher oil forecasts. The learning is being applied further in Oman and has wider application.

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe large permeability contrast between fractures and matrix in highly fractured carbonate reservoirs has been a hindrance to the efficient recovery of the oil from the matrix. Gas oil gravity drainage (GOGD) has been the most appealing option to date. However it is a slow process. Cyclic Pressure Pumping (CPP) is proposed which has the potential of increasing the rate of recovery over GOGD under certain conditions. CPP utilises the fact that due to the much lower permeability in the matrix, the matrix pressure lags behind the fracture pressure during a rapid pressure change. A sudden drop in pressure in the fractures results in expansion of fluids in the adjoining matrix which are discharged into the fracture system. The fluids in the fracture system can then be produced. The reservoir can be re-pressurised by gas injection and depressurised by producing the fluids in an alternate manner.This process originally called batch gas cycling was suggested in the 50s for improving recovery in nonhomogeneous reservoirs over conventional gas flooding. It was tried successfully in a small field in the late 50s. Laboratory experiments in the 70s on low-permeability matrix in contact with high-permeability fractures indicated the effectiveness of the process.The process was re-discovered during reservoir production optimisation simulation studies in a heavy oil steam injection project in Oman and will be applied to minimise oil loss during steam plant shut downs. Early field observations support the process. The paper presents a study of the process using both dual permeability and single porosity simulation modeling. It also highlights the conditions which are favorable to the process as well as certain practical problems that could be encountered in implementing it.

Globally large volumes of heavy oil are currently locked in shallow low permeability reservoirs. If they contain sufficient natural fractures then one of the most viable recovery processes is thermally assisted gas oil gravity drainage. In this process steam is injected into the fracture system from where it heats the oil in the matrix which reduces the oil viscosity and accelerates gravity drainage. The economic viability of this process is largely determined by the spacing of the fractures. Large blocks take longer to heat than small blocks, and the correct description and thermal simulation of these across the reservoir is critical to decision making. Arriving at the appropriate reservoir description of the fractures for full field simulation modelling requires input of the geological matrix and fracture model scenarios, a spacing averaging method per grid block and the geometric shape factor term per grid block. Each of these contains its own uncertainties. The objective of this paper is to access each parameter and their impact on recovery. For a given heterogeneous geological description, determination of the appropriate average spacing per simulation grid block is a non standard operation. A number of techniques have been assessed including arithmetic and square weighted averages. In addition a number of thermal shape factors used in dual permeability/porosity simulators are tested against single porosity results. Impact on recovery and produced fluid temperatures of the above parameters was investigated through multiple reservoir simulation models. A history matching approach was proposed and used in matching a pilot steam injection scheme. This included matching the measured temperatures, oil rim position, and production data gathered during a pilot. Conclusions are made regarding the importance and relative impact of the fracture characterisation, fracture spacing averaging and shape factors on recovery. 1. Introduction In naturally fractured carbonate reservoirs, the matrix which contains most of the oil is surrounded by a system of fractures of very little volume but with permeabilities that are several orders of magnitude higher than that of the matrix. In such a reservoir it is difficult to apply any pressure differential to the oil in the matrix to cause the oil to flow out by a conventional displacement process between injectors and producers. The injected fluid simply flows through the fracture system bypassing the oil in the matrix. If however gas is introduced into the fracture system such that the gas-oil contact (GOC) in the fracture system is deeper than the GOC in the matrix, then a hydrostatic imbalance is created. The oil in the matrix above the fracture GOC is surrounded by gas and is forced to drain down wards by virtue of its higher density ultimately into the fracture oil rim much the same way as in a U tube. As the oil drains from the matrix it is replaced by gas and the oil collecting in the fracture system can then be produced. This process is called gas-oil gravity drainage (GOGD). From Darcy's law the drainage rate can be derived as 1,2,3 Equation….. (1) kv is the matrix vertical permeability, kro is the oil relative permeability in presence of gas in the matrix, A is the reservoir area, µo is the oil viscosity, is the gas-oil density difference, g is the acceleration due to gravity, pc is the gas-oil capillary pressure in the matrix and z is distance upwards.

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