Consequences of Fluid Lag in Three-Dimensional Hydraulic Fractures

Abstract

SUMMARYResearch investigations on three-dimensional (3-D) rectangular hydraulic fracture configurations with varying degrees of fluid lag are reported. This paper demonstrates that a 3-D fracture model coupled with fluid lag (a small region of reduced pressure) at the fracture tip can predict very large excess pressure measurements for hydraulic fracture processes. Predictions of fracture propagation based on critical stress intensity factors are extremely sensitive to the pressure profile at the tip of a propagating fracture. This strong sensitivity to the pressure profile at the tip of a hydraulic fracture is more strongly pronounced in 3-D models versus 2-D models because 3-D fractures are clamped at the top and bottom, and pressures in the 3-D fractures that are far removed from the fracture tip have little effect on the stress intensity factor at the fracture tip. This rationale for the excess pressure mechanism is in marked contrast to the crack tip process damage zone assumptions and attendant high rock fracture toughness value hypotheses advanced in the literature. A comparison with field data is presented to illustrate the proposed fracture fluid pressure sensitivity phenomenon. This paper does not attempt to calculate the length of the fluid lag region in a propagating fracture but instead attempts to show that the pressure profile at the tip of the propagating fracture plays a major role in fracture propagation, and this role is magnified in 3-D models.

Magmatic origin of hydrothermal response to earthquake swarms: Constraints from heat flow and geochemical data

“…It is also not sensitive to details of pressure distribution inside the dike. In the case K Ic = 10 MPa√m, which is conventionally used for hydraulic fractures of similar dimensions [ Shlyapobersky , 1985; Advani et al , 1997], the widest dike opening (i.e., its width at the base) was 1.43 m. This gives a dike aspect ratio (opening/length) of 2.4 × 10 −3 , consistent with that for fractures in elastic material (∼10 −3 ). Therefore, the mechanics of inflation of a thin magma lens suggests that the location of the microearthquakes (Figure 1b) is fully consistent with dike emplacement initiated at the west tip of the AMC.…”

Effects of mechanical layering on hydrofracture emplacement and fluid transport in reservoirs

“…However, in nature the tip of a hydrofracture commonly propagates ahead of its fluid front (Valko and Economides, 1995;Economides and Nolte, 2000). This is demonstrated by hydraulic fracture experiments and models which indicate that the fluid front normally lags behind the hydrofracture tip (Warpinski, 1985;Advani et al, 1997;Yew, 1997;Garagash and Detournay, 2000). We have run other numerical models where the part of the hydrofracture close to its tip is without any loading, whereas the hydrofracture was allowed to open along an internal spring of low stiffness (e.g., Brenner andGudmundsson, 2002, 2004a,b;Gudmundsson et al, 2002).…”

Hydraulic fracturing and its peculiarities

“…They suffer the limits of an analytical approach, in particular the inability to represent an evolutionary problem in a domain with a real complexity. An analysis of solid and fluid behaviour near the crack tip can be found in [10,11]. Boone and Ingraffea [12] present a numerical model in the context of linear fracture mechanics which allows for fluid leakage in the medium surrounding the fracture and assumes a moving crack depending on the applied loads and material properties.…”