The Role of Fractures in the Khuff Reservoir Performance, Onshore Fields, Saudi Arabia

Mohammed S. Ameen¹, Ismail M. Buhidma², Zillur Rahim², and Carl W. Anderson^3
1Exploration Technical Services Department, Structural Geology & Rock Mechanics Group, Saudi Aramco, Dhahran, Saudi Arabia
²Reservoir Management Department, Saudi Aramco, Dhahran, Saudi Arabia
³Reservoir Characterization Department, Saudi Aramco, Dhahran, Saudi Arabia

The performance of the Permo-Triassic Khuff reservoirs in the onshore fields, Eastern Saudi Arabia, is heterogeneous, and the pre-production geochemical and pressure data indicate that the different reservoirs within the Khuff have similar pressure profiles and hydrocarbon composition. These phenomena were frequently attributed to natural fractures. This paper presents observations that show that wells which exhibit clear evidence of enhanced performance from natural fractures are few. The study is based on core, borehole image, 3D seismic, pressure transient analysis, and well performance data.

We identified the following challenges to the perceived major role of fractures in the Khuff reservoir performance:
1. Individual fractures are small (mesoscopic and microscopic), and do not extend between the Khuff reservoirs.

2. Most fractures are fully mineralized because the depth of burial of the Khuff reservoir (> 10000 ft) and its carbonate and anhydritic composition render the fractures susceptible to healing. Pressure solution seams, along bedding and pre-existing fractures (tectonic stylolites) and veins attributed to the crack-seal mechanism are common.

3. Most observed faults are small and below seismic resolution.

4. In all producing fields, the initial vertical pressure gradient changes during production indicate no communication between the different Khuff reservoir units.

5. Wells drilled perpendicular to the dominant ENE fracture trend which is parallel to the current maximum horizontal in-situ stress, show little difference in productivity compared to those drilled parallel to the fracture trend and maximum horizontal in-situ stresses.

6. Sustained losses of circulation during the drilling of Khuff wells are rare.

Therefore fractures impact on the Khuff reservoir performance of the study area is considerably less than perceived.

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