Estimation of fracture compliance from tubewaves generated at a fracture intersecting a borehole

Abstract

SUMMARYUnderstanding fracture compliance is important for characterizing fracture networks and for inferring fluid flow in the subsurface. In an attempt to estimate fracture compliance in the field, we developed a new model to understand tubewave generation at a fracture intersecting a borehole. Solving the dispersion relation in the fracture, amplitude ratios of generated tubewave to incident P-wave were studied over all frequency ranges. Based on the observations from the model, we propose that measuring amplitude ratios near a transition frequency can help constrain fracture compliance and aperture. The transition frequency corresponds to the regime where the viscous skin depth in the fracture is comparable to its aperture. However, measurements in the high frequency limit can place a lower bound on fracture compliance. Comparing the model to a previously published VSP dataset, we argue that compliance values of the order 10 −10 − 10 −9 m/Pa may be possible in the field.

Numerical investigation of seismic source location using distributed acoustic sensing for rockfall monitoring

“…tangential direction of the fibre cable (Bakku 2015;Baird et al 2020). The strain rates of the channels in each gauge length are averaged as the measurements obtained at the centre point of the gauge length (Dean et al 2017).…”

Quality Control of DAS VSP Data in Desert Environment Using Simulations and Matching Filters

“…Overstuffing a cable with optical fiber, cable relaxation between clamps, excessive cable length at the well head, fiber refractive index measurement imperfection, or inaccurate depth of the end of the fiber are leading factors contributing to depth calibration errors in DAS data [19][20][21]. Bakku [22], Ellmauthaler et al [23], and Wu et al [24] developed techniques to overcome the depth calibration challenge, mainly, by referencing the depth of DAS channel depth in the wellbore to certain features with known positioning such as plug depth, perforation shots, tubing clamps, or the end of the cable. However, these features can still be uncertain due to, for example, uneven fiber distribution between these reference features, subsurface compression, or fiber breakage.…”

Fracture characterization from attenuation of Stoneley waves across a fracture

“…This sets some limitations on the application of this theory to well log data. However, we could collect low frequency data when tube waves are excited in the borehole in a VSP setting (Bakku et al, 2011). Micro-seismic events accompanying hydraulic fracturing are reported to have a frequency band ranging from as low as 30 Hz (Fehler and Phillips, 1991) to kHz (Song and Toksoz, 2011).…”

“…In the absence of data spanning over a range of frequencies from 10s of Hz to 1 kHz, the transmission coefficients observed in the high frequency regime can be explained by a range of possible combinations of aperture and compliance. However, if we assume a large aperture, we can constrain the minimum compliance required to have the observed attenuation as described in Bakku et al (2011).…”