Hydraulic fracturing of low permeability formations has long been used to improve productivity. A lot of effort has gone into optimizing the effectiveness of this expensive and complex reservoir stimulation technique from using extensive geomechanics studies to sophisticated 3D modeling techniques. What is lacking is a reliable and consistent method for post evaluation of the created hydraulic fracture to feed back into the planning of future jobs, particularly when the stimulation did not produce the expected results.
A method for evaluating the success of a hydraulic fracturing operation has been tested in several wells in the Changqing oilfield with encouraging results. Located in the Ordoz basin in China, Changqing oilfield is comprised of low permeability sand-shale sequences with permeability in the range from 0.5–2 md. Confining the hydraulic fracture to the sand layer, without breaking through into nearby depleted or water bearing zones is a major challenge in this area. Thin shale barriers between the sands, and low strength contrast between the shales and sands means that very accurate planning is required before fracturing the sands.
The evaluation technique utilizes a sophisticated cased-hole sonic measurement to evaluate the height and orientation of the hydraulic fracture. An experimental technique to evaluate fracture effectiveness was also tested, and the initial results are presented in this paper.
This hydraulic fracture evaluation technique has provided much better understanding of the performance of the fracturing operations, and has resulted in better quality fracturing jobs in the area.