Massive hydraulic fractures are projected to play a major role in the recovery of the tremendous reserves of gas tied up in the tight gas sands of the Deep Basin. One of the major problems in designing fractures in such formations is the uncertain vertical extent of these fractures: fracture containment.
The present paper presents a procedure for predicting the vertical extent of fractures in multi-layered formations with varying material properties and tectonic stresses_ The numerical procedure uses the finite element technique for therock deformation calculations and emclays special high order, crack-tip elements to improve the accuracy of stress intensity calculations. Furthermore, it makes use of the powerful numerical technique of static condensation to reduce computer memory and computation time. The elastic deformation calculation can be coupled to a fluid flow model to predict dynamic fracture growth.
Massive hydraulic fracturing (HHF) has become an important technique to improve the productivity of tight gas sands such as are found in the Deep Basin of northern Alberta and British Columbia. Fractures of more than 1000 m in length have been produced in low-permeability, low-porosity gas-bearing formations to provide a high conductivity path of communication between the formation and the production well. Such MHF treatments can transform an uneconomic field into a viable gas producer.
A key uncertainty that has been identified as an important consideration in MHF operations is the question of fracture containment (c. f. Warpinski et al. (1), Simonson et al. (2), Hanson et al. (3)). It is essential that fractures be contained largely to the pay zone since break-out of the fracture into overlying or underlying formations can have serious consequences on the effectiveness of MHF operations. Some of the obvious consequences include:
If only a small portion of the fracture surface is in contact with the pay zone the result may be an uneconomic well that could be economic if properly stimulated.
The fracture could penetrate from the tight zone identified for stimulation into an adjacent high permeability gas zone. In this case, production tests may lead one to over-estimate the resources and productivity of the tight zone.
Penetration of the fracture into a water-bearing zone can lead to irreparable water damage to the formation.
Typically, when an MHF operation is designed, it is assumed that the fracture will be contained from above and below by horizontal discontinuities in the strata. For instance, limestone or shale zones will be assumed as confining strata. In particular, classical fracture mechanics theory (c.f. Simonson, et al. (2)) suggests that if the bounding stratum has a higher modulus of elasticity than the pay zone, the fracture will not penetrate the stratum as evidenced by the decrease in stress intensity factor as the crack approaches the interface. This tendency of the stress intensity factor to decrease implies that the pressure within the fracture must increase and ultimately approach infinity in order to make the fracture tip approach the interface. In practice, fracture containment by this mechanism is by no means guaranteed.