Experiments versus theory for the initiation and propagation of radial hydraulic fractures in low‐permeability materials

Abstract

We compare numerical predictions of the initiation and propagation of radial fluid‐driven fractures with laboratory experiments performed in different low‐permeability materials (PMMA, cement). In particular, we choose experiments where the time evolution of several quantities (fracture width, radius, and wellbore pressure) was accurately measured and for which the material and injection parameters were known precisely. Via a dimensional analysis, we discuss in detail the different physical phenomena governing the initiation and early stage of growth of radial hydraulic fractures from a notched wellbore. The scaling analysis notably clarifies the occurrence of different regimes of propagation depending on the injection rate, system compliance, material parameters, wellbore, and initial notch sizes. In particular, the comparisons presented here provide a clear evidence of the difference between the wellbore pressure at which a fracture initiates and the maximum pressure recorded during a test (also known as the breakdown pressure). The scaling analysis identifies the dimensionless numbers governing the strong fluid‐solid effects at the early stage of growth, which are responsible for the continuous increase of the wellbore pressure after the initiation of the fracture. Our analysis provides a simple way to quantify these early time effects for any given laboratory or field configuration. The good agreement between theoretical predictions and experiments also validates the current state of the art hydraulic fracture mechanics models, at least for the simple fracture geometry investigated here.

Fluid Injection Experiments in Shale at Elevated Confining Pressures: Determination of Flaw Sizes From Mechanical Experiments

“…Each of these studies found a linear increase in breakdown pressure with confining pressure, and rapid, uncontrolled breakdown once the peak injection pressure was reached. Zoback et al (1977), Bunger and Detournay (2008), Stanchits et al (2015), and Lecampion et al (2017) found that for experiments injecting higher-viscosity fluids, the breakdown pressure can be higher than the fracture initiation pressure, in agreement with the models summarized by Detournay (2016).…”

Solution for a plane strain rough‐walled hydraulic fracture driven by turbulent fluid through impermeable rock

“…It is also important to note that the compressibility of gas or supercritical CO 2 may effect early‐time crack propagation. However, for the intermediate/long time, compressibility is expected to have a negligible impact on the solution …”

“…However, for the intermediate/long time, compressibility is expected to have a negligible impact on the solution. 52 The first comparison between laminar and turbulent flows is for the normalized opening. Figure 5 illustrates that the difference between normalized opening along the crack is relatively subtle; the scaled turbulent crack width is just slightly narrower.…”

Explicit versus implicit front advancing schemes for the simulation of hydraulic fracture growth

“…Different confining stresses are applied in three layers with the upper layer having a larger confining stress layer of 11 MPa, the lower layer having a confining stress of 5 MPa, and the layer in the middle (of 50 mm height) that contains the injection point having a confining stress of 7 MPa (see Figure ). The others parameters are as follows: Due to a rather large injection line, the effective injection rate entering the fracture varies with time due to fluid compressibility effects during the initial pressurization phase (see Lecampion et al for discussion). The evolution of the injection rate into the fracture can be approximated here by three constant injection steps: 0.0009 mL/s for the first 31 seconds, 0.0065 mL/s for the next 120 seconds, and 0.0023 mL/s for the rest of the experiment.…”