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Evaluation of Naturally Fractured Reservoirs in Indonesia Using Formation Imaging and Sonic Logs
Electrical and ultrasonic borehole images along with sonic logs can be used to characterize fracture systems. For electrical imaging tools, algorithms exist to determine from Rm and Rxo the fracture aperture, fracture continuity, the frequency with which fractures intersect the borehole wall (fracture density) and, from these values, the fracture porosity of the formation. Electrical image methods, however, assume that all conductive fractures are open and can transmit fluids. In reality, the majority of fractured reservoirs flow hydrocarbons from only a few isolated fractures, or else from a specific fracture "set" or "system". The picture is further complicated by clay-filled fractures that are difficult to distinguish from open fractures on both electrical and acoustic images.
Sonic Stoneley wave analysis from low frequency sonic logging modes provides a key analytical tool that is complimentary to the image logs. Stoneley waves are essentially insensitive to non-productive fractures. Open fractures that extend significant distances from the borehole are easily identifiable. Relative intensity and size of fracturing are also easily observed on the Stoneley analysis, though the vertical resolution of the Stoneley measurement is lower than that of the borehole imaging tools. The methodology is firstly to identify zones of open fractures on the Stoneley analysis, then evaluate the orientation and density of fractures using a higher resolution image log. Sonic cross-dipole measurements can locate vertical fractures and indicate tectonic stress imbalance direction.
Examples from Indonesia are presented, in which sonic logs and borehole images combined have been used to characterize fractures in reservoir lithologies that include igneous and metamorphic basement, interbedded volcanics and volcaniclastics, and limestones. The conclusions are, in a number of these examples, supported by production tests.
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