Multiscale Evaluation of Hydraulic Fracturing Effects via Borehole Acoustics

Hydraulic fracturing has become a common practice worldwide in enhancing hydrocarbon recovery, especially for unconventional and tight reservoirs. This paper presents several imaging technologies that can evaluate fracturing effects from the wellbore wall deep into formation, by using borehole array acoustics. The proposed solution makes use of varying investigation depths of different borehole waves, providing multi-scale perspectives about fracturing- induced fractures and heterogeneities. As a near-field approach, a new similarity method is presented to transform energy transitions of Stoneley wave into sensitive indicators of wellbore fractures. This method is a way to characterize the fracture locations on the wellbore wall, that are caused by hydraulic fracturing activity. In the middle-field, instead of using fast iterative solvers, the radial profiling method (also known as traveltime tomography) is revisited from a regularized inversion perspective to get more accurate slowness profiles. By inspecting ‘before-and-after’ changes, this approach reveals how formation has been stimulated or altered at the near-wellbore region that is around 1.0 meter away from the wellbore wall. Moreover, separation and migration of far-field reflections is performed to detect overall geometry and intensity of the fracturing-induced fracture network that can extend tens of meters into the formation. For the multi-scale evaluation system, details about the related theory, processing workflow, quality control and new interpretation methods are discussed and illustrated with an unconventional shale-oil case, which has multiple acoustic logging runs for both open hole and cased hole. In this example, fracturing operations have been implemented at two depth intervals. As a result, distributions of fracturing-induced fracture swarms at the wellbore wall are clearly characterized in terms of Stoneley wave properties. With P-wave tomography, the induced formation changes in the near wellbore region can be identified via enlarged alterations, which are 30+ inches away from borehole wall (as indicated by penetration depth of acoustic rays). Furthermore, the reflected shear-wave imaging technology detects more scattering objects and reveals how effects of the two- stage-fracturing are stacked in far-field. Findings via the multi-scale approaches well explain practical findings associated with the two hydraulic fracturing operations.