ABSTRACT

The present operative concept model for hydraulic fracturing (see Miskimins 2019) is shown to be physically impossible. The conceptualisation that underpins current practice is little changed from the time of its introduction by Hubbert and Willis (1957), and imagines a planar fracture opened within the rock mass by the injection of high-pressure fluid into the rock. Current practice typically uses very large volumes of water that exceed the void space created inside the notional fracture. The opening of the fracture causes new strains within the rocks that bound the fracture, with energy budget consequences. The excess water is often implied to go into pre-existing void spaces, also with energy consequences. Both of these explanations involve consequent energy changes that are more than an order of magnitude larger than the input energy amount, so they have to be impossible. Alternate process models, based on activation of natural fracture systems, require less energy, and so are deemed to be the likely reality

1. INTRODUCTION

This paper presents an approach that is based on assessing the budget of extensive (conserved) energy during a hydraulic fracture stimulation operation. The Appendix provides a brief discussion of extensive/intensive energy expressions, and how these relate to familiar state parameters. The rock mass possesses, pre-stimulation, some distribution of extensive energy forms: in the pore fluid, in the rock framework, and in the contained heat. During stimulation, the cooler and pressurised fluid that is pumped into the well affects the energy budget. The pre-treatment energy contained within the rocks, the energy effects in the rock that are associated with the injection of fluids, and any energy that is ‘activated’ from the prior state (e.g. seismicity, or changes in the potential energy related to changes in elevation), all must balance with the energy supplied by the injection fluids. The conservation of energy provides a means of assessing ideas and inferences about what actually takes place during the treatment.

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