Hydromechanical Investigations on the Self-propping Potential of Fractures in Tight Sandstones

Abstract

The hydromechanical properties of single self-propping fractures under stress are of fundamental interest for fractured-rock hydrology and a large number of geotechnical applications. This experimental study investigates fracture closure and hydraulic aperture changes of displaced tensile fractures, aligned tensile fractures, and saw-cut fractures for two types of sandstone (i.e., Flechtinger and Fontainebleau) with contrasting mechanical properties, cycling confining pressure between 5 and 30 MPa. Emphasis is placed on how surface roughness, fracture wall offset, and the mechanical properties of the contact asperities affect the self-propping potential of these fractures under normal stress. A relative fracture wall displacement can significantly increase fracture aperture and hydraulic conductivity, but the degree of increase strongly depends on the fracture surface roughness. For smooth fractures, surface roughness remains scale-independent as long as the fracture area is larger than a roll-off wavelength and thus any further displacement does not affect fracture aperture. For rough tensile fractures, these are self-affine over a larger scale so that an incremental fracture wall offset likely leads to an increase in fracture aperture. X-ray microtomography of the fractures indicates that the contact area ratio of the tensile fractures after the confining pressure cycle inversely correlates with the fracture wall offset yielding values in the range of about 3–25%, depending, first, on the respective surface roughness and, second, on the strength of the asperities in contact. Moreover, the contact asperities mainly occur isolated and tend to be preferentially oriented in the direction perpendicular to the fracture wall displacement which, in turn, may induce flow anisotropy. This, overall, implies that relatively harder sedimentary rocks have a higher self-propping potential for sustainable fluid flow through fractures in comparison to relatively soft rocks when specific conditions regarding surface roughness and fracture wall offset are met.

Laboratory Study of the Influence of Fluid Rheology on the Characteristics of Created Hydraulic Fracture

“…The peak asperity height R p , the average roughness R m , and the root-mean-square roughness R rms , where z i is the height of the ith point and z a -the mean height of the elevation plane, which is discretized by n points, were obtained (Table 6). The statistical parameters provide a direct comparison between samples, aiding to distinguish the different surface topographies quantitatively [45]. For this part of the study, we applied statistical expressions directly t 3D surface data (Equations ( 1)-( 3)).…”

“…The peak asperity height 𝑅 , the avera 𝑅 , and the root-mean-square roughness 𝑅 , where 𝑧 is the height of the 𝑧 -the mean height of the elevation plane, which is discretized by n po tained (Table 6). The statistical parameters provide a direct comparison betw aiding to distinguish the different surface topographies quantitatively [45].…”

Experimental study of fatigue load effect on the hydraulic fracturing behavior of granite

“…Therefore, in specimen XA4, multiple microcracks distribute around the borehole, the propagation direction of hydraulic fracture deviates, and the average width of hydraulic fractures is smaller than the voxel size. Cheng and Milsch 47 investigated fracture closure and hydraulic width changes of sandstone under cyclic confining pressure between 5 and 30 MPa. The test results reveal that the hydraulic width decreased with increasing cyclic confining pressure.…”

Special Issue on Rock Fracturing and Fault Activation: Experiments and Models

“…While they found that cyclic injection with restricted pressure could reach similar displacements with lower slip rates as compared to monotonic injection, they also observed fast unstable slip after a certain displacement was reached. Cheng and Milsch (2021) performed an experimental study on fracture closure of aligned and displaced tensile fractures as well as saw-cut fractures of soft Flechtingen Sandstone and hard Fontainebleau sandstone under confining pressures between 5 and 30 MPa. They found that depending on fracture surface roughness, fracture displacement can significantly enhance fracture aperture, and that rough fractures, strong asperities, and a certain fracture offset are required for fractures under stress to stay open to fluid flow.…”