Influence of Pore Fluid Pressure Diffusion on Hydraulic Fracturing.
Results of laboratory experimental study of the hydraulic fracture
initiation, its propagation and closing, re-fracturing, the fluid pore
pressure variations and related acoustic emission (AE) are considered.
The experimental setup differs from common equipment designed for
testing cores or cubic samples; it is designed for work with samples of
disk form with diameter 430 mm and height 66 mm, which allows us to
measure the pore pressure distributions along the sample boundary
together with passive and active acoustic emission measurements (Fig.1).
The experiments were conducted with artificial porous saturated samples
made from gypsum-cement mixture in agreement with simulation criteria.
The samples were created by filling the pressure chamber with the
gypsum-cement-water mixture. After drying, the samples were saturated by
gypsum water solution and loaded by three-axial stresses. To produce the
fracture, mineral oil was injected with the constant rate 0.3 sm3/sec
through the cased borehole. A set of experiments was conducted, in which
the main stress axis orientation was changed after the first hydraulic
fracture creation. Registration of the change in the amplitude of the
waves passing through the fracture allowed to determine the fracture
formations, its filling with fluid, the increase in the fracture opening
as the injection continued, and the fracture closing after the injection
was stopped. It was found that the influence of the fluid pore pressure
diffusion from the point of injection is of great significance: the
fracturing pressure was several megapascals higher than it should be
expected. The fracture closing pressure also occurred to be higher in
comparison with the case, when the pressure diffusion is neglected. It
was obtained, that the AE source positions are closely related with the
fracture position. It is remarkable, that we did not registered any AE
outside the fracture. Comparison that fact with the results of the pore
pressure measurements allows to suggest, that the pressure increase due
to the viscous fluid injection is not enough to induce AE pulses at some
distance from the borehole and the fracture. When the injection fluid
has the same viscosity as the fluid saturating the sample, the pressure
increase is sufficient to induce acoustic emission corresponding to the
pore pressure increase.