The fractured basement field in Yemen described in this paper is
characterized by two types of fracturing: background fractures with a very low
effective permeability of less than 0.001 md and fracture corridors with an
effective permeability of up to several millidarcies. Except for some
dissolution porosity related to fracture corridors, no significant matrix
porosity is encountered (total porosity is only 1.15%). Approximately one-half
of the oil in place is contained in the fracture corridors and one-half in the
Production from this field commenced in 2007. It is currently produced by
depletion. Compositional grading has been observed in the 3,120-ft oil column.
Despite the fact that the oil is close to bubblepoint pressure at the top of
the reservoir, a moderate increase in gas/oil ratio (GOR) has been seen.
Detailed studies using material balance and discrete-fracture-network (DFN)
models revealed that the reason for the slow increase in GOR is the low
permeability of the background fractures. The low permeability leads to viscous
forces being dominant over gravity forces and, hence, limited gravity
segregation of gas and oil.
Because of the relatively small viscosity difference between the gas and the
oil in this field (μo/μg = 6.5), the gas
mobility is not much higher than the oil mobility at low gas saturations.
Hence, oil and gas are produced effectively from the background fractures into
the fracture corridors and the reservoir pressure is not depleting as fast as
in reservoirs with higher viscosity difference between gas and oil. This
results in a more effective solution-gas-drive recovery mechanism than that
expected for a conventional reservoir.
A number of reservoir-management strategies have been investigated. The
results indicate that the low permeability of the fracture corridors and very
low permeability of the background fractures lead to low recovery factors of
14% for gas injection. However, the efficiency of solution-gas drive is higher
than in conventional reservoirs.
© 2011. Society of Petroleum Engineers
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- Original manuscript received:
29 June 2010
- Meeting paper published:
20 September 2010
- Revised manuscript received:
24 February 2011
- Manuscript approved:
14 April 2011
- Published online:
3 August 2011
- Version of record:
15 August 2011