Dynamic Fracture Propagation Behavior of Hydraulic Fracture
Branching in Shale Gas Reservoirs
Bing Hou, Mian Chen, Yan Jin
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum
(Beijing), Beijing, China
Abstract: In shale gas reservoirs, hydraulic fracture growth are usually more complex
than imagined due to interaction with natural fractures and branching during
propagation. This paper investigated the dynamic fracture mechanism during
hydraulic fracture branching, including natural fractures reactivation and hydraulic
fracture tip branches in gas shale formation. The impact of reservoir mechanical
properties on treatment design is also evaluated. The paper begins with an evaluation
of stress intensity factor around hydraulic fracture tip under fluid filled and multinatural fractures condition. It is found that natural fractures can be reactivated by the
triple reduction mechanism on fracture surface. Shear-slip impact on natural fracture
is much more effective than filtrate-inflation deformation. The critical pump rate for
hydraulic fracture reorientation is existed, and hydraulic fracture reorientation can be
effected by the geometry of natural fracture, the mechanical parameters on the
fracture surface, the viscosity of fracturing fluid and eventually the pump rate. The
critical pump rate increases as the natural fracture inclined from 0° to 180° with a
minimum value at a 90° inclination. The critical pump rate increases as natural fracture
length increases and rock elastic modulus decreases. Elastic strain energy accumulated
before hydraulic fracture propagating in the shale can usually be several times the
surface energy needed to promote a new fracture increment. Residual energy after a
new fracture surface generated can still provide enough energy for promoting other
fractures, and then a new branch may generate. The residual energy, which is
influenced by fracture geometry, surface energy characteristic, the elastic properties
of the material and the internal pressure on the fracture et al, is a critical factor
affecting the fracture branching results. Analysis results show that the longer the
fracture propagates, the easier the fracture branches. We can control the engineering
factors to create new fracture branches far away from the wellbore to stimulate more
rock volume and then produce more gas. The outcomes of this study finally explain
the hydraulic fracture complicated underneath, and give out critical engineering
parameters which could be controlled to create fracture networks. Understanding and
predicting hydraulic fracture branching behavior can be practically useful for fracturing
interval selection, treatment design and results evaluation.
Key words: dynamic fracture propagation, fracture branching, slippage, networks, rock
mechanics.