SPE Reservoir Evaluation & Engineering
Volume 14, Number 4, August 2011, pp. 473-484

SPE-133086-PA

Recovery Mechanisms and Oil Recovery From a Tight, Fractured Basement Reservoir, Yemen

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DOI  More information 10.2118/133086-PA http://dx.doi.org/10.2118/133086-PA

Citation

  • Legrand, N., de Kok, J., Neff, P., and Clemens, T. 2011. Recovery Mechanisms and Oil Recovery From a Tight, Fractured Basement Reservoir, Yemen. SPE Res Eval & Eng  14 (4): 473-484. SPE-133086-PA. doi: 10.2118/133086-PA. 

Discipline Categories

  • 6.4 Primary and Enhanced Recovery Processes

Keywords

  • Fractured basement, Solution gas drive, Fractured reservoir, Gas injection

Summary

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 background fractures.

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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History

  • 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