Summary

The Lower Cretaceous Shu'aiba formation (Shaybah field) in southeastern Saudi Arabia is a heterogeneous carbonate formation with various facies because of diagenetic alteration of the original rock fabric. Preliminary laboratory rock-mechanics studies indicated that the Shu'aiba carbonates are mechanically weak, with the majority of the rocks tested yielding very low strength values (less than 3,000 psi) when compared to samples from other carbonate reservoirs (Salamy and Finkbeiner 2002). On the basis of laboratory-derived rock-strength data from triaxial tests, the formation appears to behave in a plastic manner that strengthens the wellbore. To understand the stress behavior of the Shaybah rocks better, a set of pore volume compressibility tests was conducted in which the changes taking place within the samples were observed and quantified using an X-ray computerized-tomography (CT) scanner. A new technique involving dual-energy CT scanning was used to obtain the pore volume compressibility values, which were compared against the conventional triaxial-cell-generated data and published results by Harari et al. (1995) on cores taken from the same reservoir. Although the magnitude of the pore volume compressibility was the same between different setups, visual data showed some interesting behavior of the Shaybah cores including movement of grains in an irreversible manner even for a relatively low imposed effective stress of 2,000 psi. Some of the advantages of the new CT-based analysis technique include the generation of multiple pore volume compressibility curves from the same plug (a band of data, each at a different slice location), the "visualization" of the changes, the possibility of using either hydrostatic or triaxial cells to make the test more case-specific, and the possibility of measuring permeability under different stress conditions before, during, and after the test.

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