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AAPG Bulletin


AAPG Bulletin, V. 92, No. 1 (January 2008), P. 31-52.

Copyright copy2008. The American Association of Petroleum Geologists. All rights reserved.


A new technology for the characterization of microfractured reservoirs (test case: Unayzah reservoir, Wudayhi field, Saudi Arabia)

Mohammed S. Ameen,1 Ernest A. Hailwood2

1Geological Technical Services Division, Saudi Aramco, P.O. Box 2817, Dhahran 31311, Saudi Arabia; [email protected]
2Core Magnetics, The Green, Sedbergh LA10 5JS, United Kingdom


This article presents a test case of a new technology using artificially enhanced anisotropy of magnetic susceptibility (referred to here as EAMS) for the characterization of microfractured reservoirs. These are reservoirs in which microfractures are essential to porosity and/or permeability. A conventional geological characterization is costly, time consuming, and difficult to quantify in terms of assessing fracture impact on porosity and permeability. Therefore, an efficient and effective method is required to characterize these microfractures and to determine their contribution to porosity and permeability. The EAMS technology, which we developed and tested, allows rapid analysis that bridges reservoir geology and engineering. Using petrography, the margin of error to detect microfractures that impact porosity and/or permeability is 43%; however, it requires three times the sampling rate of the new EAMS technology.

The lower part of the Unayzah reservoir (Unayzah-B/C) in the Wudayhi field, Saudi Arabia, where fractures were studied and microfractures are known to impact reservoir performance, is used to develop and verify the EAMS technology. The results show that EAMS-derived microfracture fabric strikes east-northeast–west-southwest, consistent with that obtained by geological means. The effective-porosity profile obtained from EAMS tests is similar to that of the conventionally acquired porosity. Open microfractures in tested samples increase mean values of reservoir effective porosity by 36–50% in Unayzah-B/C. The occurrence of connected microfractures is estimated to cause an increase in average permeability of 75% in Unayzah-B/C. This is in agreement with the fact that wells in microfractured Unayzah-B/C have 4.5–14 times the productivity of wells as nonfractured sections of this reservoir. A maximum permeability trend of northeast-southwest permeability anisotropy is detected. The implementation of the EAMS technology in other fields with microfractured reservoirs will directly impact operational and simulation effort.

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