This paper discusses the development of a novel fracture treatment technique which uses a nonreactive, buoyant diverter to control upward propagation of vertical fracture during the treatment. It is necessary to control such growth to prevent fracture penetration into undesirable zones, such as gas cap, water-bearing or nonproductive zones, and to promote maximum fracture extension within the producing interval.
Results of field evaluations indicate that the new treatment technique is successful. It offers a unique solution to preventing upward fracture growth that is independent of the formation properties and geologic stress environment.
References and illustrations at end of paper.
The ability to control vertical propagation of fractures has always been of considerable interest in hydraulic fracture treatment design. Uncontrolled vertical growth can lead to problematic production of water or gas. In addition, optimum lateral extension of the fracture cannot be achieved because the fluid, proppant and pumping energy are wasted in propagating the fracture out-of-zone.
Studies to identify parameters which are considered to be influential to the containment of hydraulic fractures are well documented. Most of the authors have concluded that in-situ minimum horizontal stress contrast between the pay zone and the barrier is the dominant factor in arresting or at least restricting vertical growth of fractures. Warpinski et al.1 found that a stress difference of 200 to 500 psi (1.4 to 3.5 MPa) between a basal ash-flow layer and an ash-fall tuff layer was necessary to contain the fracture within the ash-fall tuff. Teufel and Clark2 concluded that an increase of 700 psi (5 MPa) in horizontal stress was required for complete fracture containment in a number of limestones and sandstones. These experimentally derived stress differences serve only to indicate the magnitude of the stress contrast and thus should not be generalized.
Inherent properties of the interface al so contribute to fracture containment. Teufel and Clark2 suggested that if shear strength of the interface is small relative to the tensile strength and minimum horizontal compressive stress of the bounding layer, vertical fractures may turn into interfacial fractures at the interface and containment is, in effect, achieved. However, this type of containment may be very temporary. Hanson et al.12 showed experimentally that after the fracture is laterally displaced at the interface, it can still grow into the bounding layer if the frictional properties of the interface change. Daneshy3 found that at shallow depth, a weakly bonded interface may stop fracture growth. Others1,4,13,14 indicated that a stress discontinuity (i.e., fault) may terminate the fracture.
Contrast in material properties (elastic modulus, porosity, density, etc.) and pore pressure generally has only secondary effects on containment. Elastic modulus is more influential than the other material properties;1,5,13 however, it may only be able to reduce the width of the out-of-zone fracture, not stop it.
Treatment parameters which may influence fracture geometry are fluid viscosity and density, proppant concentration, pump rate and placement of perforations.
