Effect of Shear Thickening Fluid on the Hydraulic Fracturing Behavior

Abstract

Listen

Translate article

ABSTRACT: Hydraulic fracturing nucleates fractures along the orientation of maximum stress in the normal plane to the borehole. Meanwhile, standard hydraulic fracturing cannot nucleate several fractures in different directions from a borehole and achieve permeability enhancement in another direction other than maximum stress. We conducted the hydrofracturing experiment using shear thickening fluid (STF) as a fracturing fluid in the uniaxial loading condition. We expected that STF could affect the behavior of fracturing since STF has the ability to plug the flow path by changing its viscosity. Hydraulic fracturing occurred at around 38 MPa, which was significantly higher than the tensile strength of the granite. Simultaneously, borehole pressure dropped, and a number of AEs were observed as standard fracturing phenomena. However, sub-consequently, borehole pressure built up again, and then we observed several repeating breakdowns accompanied by AEs. Surprisingly, from the visual observations of the specimen after dismounting from the apparatus, we observed several fractures that propagated in different directions from the borehole. So, the rock was fractured in multi-direction by the effect of STF. 1. INTRODUCTION Hydraulic fracturing has been used widely to generate fractures in intact or low-permeable rock formations for permeability enhancement in various subsurface resource development fields. A new fracture is nucleated along the orientation of maximum stress in the normal plane to the borehole since fracturing to the direction of maximum stress requires minimum energy to nucleate new fractures and open them. Nucleated fractures drastically increase the permeability around the borehole, especially in the direction of maximum stress, so that the energy fluid exploitation will be improved. Hydraulic fracturing needs high borehole pressure to yield the tensile strength and stress concentration around the borehole, (equation) where Pb is breakdown pressure, Sh is minimum horizontal stress, SH is maximum horizontal stress, Pp is borehole pressure, and St is the tensile strength of the rock mass. Note that Eq. (1) is the case of vertical wells. As soon as the hydraulic fracture is nucleated, the borehole pressure drops immediately due to the simultaneous increase of space. This pressure drop is regarded as evidence of fracture nucleation, and the maximum borehole pressure is recorded as breakdown pressure (Pb). After the first breakdown, borehole pressure never increases as high as Pb in general because significant permeability has been achieved by hydraulic fracturing, and the borehole cannot be sealed anymore. Pore pressure is used for the extension of fracture. Meanwhile, in the laboratory hydraulic fracturing, nucleated fracture often reaches the edge of the specimen. In such cases, borehole pressure would not build up significantly since the other side of the fracture is connected to open air.