Initiation of Hydraulic Fractures in Natural Sandstones

Initiation of Hydraulic Fractures in Natural Sandstones

Lhomme, T.P.Y.

Currie, P.K. (promotor)

Civil Engineering and Geosciences

2005-04-04

Hydraulic fracturing is a stimulation technique commonly used for the enhancement of hydrocarbon reservoir recovery. Controlling the initiation of a hydraulic fracture from the open-hole section of a well without zone isolation requires an in-depth understanding of the factors which have a decisive effect on the onset and early growth of a fluid-driven crack. The objective of this research is the identification of the key aspects of hydraulic fracture initiation in a natural sandstone. It also aims at understanding how these different aspects combine and result in specific propagation regimes, depending on the experimental conditions. We propose a conceptual model for the initiation of a fluid-driven crack in a permeable, elastic quasi-brittle material exhibiting post failure tension softening in a cohesive zone. The influence of the finite borehole radius is introduced with appropriate kernels in the integral equation of elasticity. The model accounts for viscous fluid flow in the fracture according to the Lubrication equation, allows for the existence of a potential fluid lag, and fluid losses from the fracture and the borehole injection zone. Compressibility effects are introduced with a coupling between the fluid injection rate at the fracture inlet and the fluid pressure variations in the borehole. A complete set of scaling laws is established for the problem of a radial fracture driven by a viscous fluid in a permeable elastic semi-brittle material, in the case of non negligible compressibility effects influencing the effective fluid flow rate at the fracture inlet. Propagation regimes are identified for the simplified case of a fracture for which the borehole radius, the fluid lag and the process zone can be neglected. The scaling laws developed for this simplified problem highlight the nature of the mechanisms that control the fracture response during the fluid injection. The effect of the finite borehole radius can be introduced in the scaling through one single parameter with the meaning of a dimensionless borehole radius. If the negligible fluid lag and process zone are valid assumptions, the scaling laws provide a powerful tool for the design of experiments at laboratory scale. The introduction of a fluid lag and of a process zone for the description of semi-brittle failure and the creation of new fracture faces considerably increase the complexity of the scaling operations. Independent considerations about the scaling of the lag and the process zone are developed from the conclusions of independent studies and are mentioned in this thesis, but a unified formulation for the complete problem is yet to be achieved. We conducted experiments in model blocks of natural sandstones with various viscous fluids and injection rates. The experimental setup allows for the real time collection of observations concerning the treatment pressure, the fracture inlet opening and the deformation of the test block. A state of the art acoustic monitoring system is used for the active imaging of the crack growth during the fracturing experiment. The signature of the fracture plane is monitored continuously and the fracture opening assessed for several points of the fracture plane. Experimental evidence suggests that a non-negligible process zone has already developed at breakdown. The breakdown of a small scale fluid-driven fracture must be described as a complex process that depends on the fluid rheology, material microstructure and the fluid diffusion from the fracture toward the adjacent pore space. The injection parameters also have a major impact upon the early phase of the fracture growth and the post-breakdown fracture response. The appearance of multiple pressure peaks associated with a step-by-step fracture growth during the injection of the fluid is related to compressibility effects and high leakoff rates in the permeable sandstone. The comparison of the modeling results with experimental evidence collected in the laboratory indicates that some of the experimental observations cannot be understood within the framework of elastic brittle material failure. The existence of a sizeable process zone with specific hydraulic properties has to be invoked to reproduce the experimental fracture response. The impact of the process zone upon the viscous fluid flow and the mechanical response of the fracture need to be addressed as fundamental aspects of the problem of fracture initiation in natural sandstones.

hydraulic fracturing
fracture initiation
experiments
natural sandstone
scaling laws

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doctoral thesis

(c) 2005 T.P.Y. Lhomme