Mechanical Compliance of Individual Fractures in a Heterogeneous Rock Mass From Production‐Type Full‐Waveform Sonic Data. Issue 8 (9th August 2022)

• Record Type: Journal Article
• Authors: Zhou, Zhenya; Caspari, Eva; Barbosa, Nicolás D.; Greenwood, Andrew; Holliger, Klaus
• Is Part Of: Journal of geophysical research. Volume 127:Issue 8(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-09
• Subjects: fracture compliance -- full‐waveform sonic -- geophysical borehole method -- linear slip model -- hydraulic stimulation; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2022JB024302
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: The mechanical fracture compliance is of interest in a number of geoscientific applications. Seismic borehole methods, especially full‐waveform sonic (FWS) data, have indicated their potential to infer the compliance of macroscopic fractures under in situ conditions. These approaches rely on the assumption of a homogeneous background embedding the fractures and, as of yet, compliance estimates for individual fractures are limited to static FWS measurements. In this work, we assess the potential of inferring the compliance of individual fractures from standard, production‐type FWS data in the presence of background heterogeneity. We first perform a comparative test on synthetic data to evaluate three approaches known as the transmission, phase, and group time delay methods. The results indicate that the former two produce adequate compliance estimates for scenarios with a strongly heterogeneous background or a damage zone around the fracture. These two methods are then applied to two FWS data sets acquired before and after a hydraulic stimulation campaign in a crystalline rock, which allows to test them on natural and man‐made fractures. The transmission method turned out to be unsuitable for the considered data due to its reliance on amplitudes. Conversely, the travel time behavior remained stable and the phase time delay method produced robust and consistent estimates. The results for a newly created hydro‐fracture imply the capability of resolving remarkably small compliance values of the order of 10 −14 m/Pa. This estimate is one order‐of‐magnitude smaller than that for the natural fracture, which may help to distinguish between these two fracture types. Plain Language Summary: Fracture characterization is of interest in a number of geoscientific applications. Seismic borehole methods in general and sonic log measurements in particular have the potential to infer an important mechanical fracture property, known as compliance, under in situ conditions. These methods exploit that fractures reduce amplitudes and delay the travel times of seismic waves and rely on the assumption of a homogeneous embedding rock mass. Here, we assess the potential of inferring compliance from standard, production‐type sonic data in the presence of background heterogeneity. We first perform a modeling study to evaluate two methods based only on travel‐time delays and one accounting additionally for the amplitude decay. The latter and one of the time delay methods produce adequate results for heterogeneous scenarios. These two methods are then applied to data sets acquired before and after small man‐made fractures were created. The amplitude method turned out to be unsuitable due to inconsistencies in the amplitudes, whereas the time delay method produced reliable estimates. The results for the man‐made fracture demonstrate the capability of resolving compliance values that are much smaller than those of natural fractures. This may open the perspective of using this technique to distinguish between natural and man‐made fractures. Key Points: Effect of background heterogeneity of the rock mass on fracture compliance estimation Compliance estimation of macroscopic fractures from standard, production‐type full‐waveform sonic data Fracture compliance estimation of man‐made and natural fractures from pre and post mini‐frac hydraulic stimulation