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Experimental study and finite element modeling of cryogenic fracturing in unconventional reservoirs
Alqahtani, Naif Bandar
Alqahtani, Naif Bandar
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2015
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Cryogenic fracturing rests on the idea that a very cold liquid can induce a fracture when brought into contact with a formation rock under reservoir conditions. In the context of cryogenic fracturing, the cryogen exists in the gaseous phase at standard conditions but takes a liquid form at low temperatures. The mechanism for fracture creation consists of a severe thermal shock that the cryogen imparts to the rock when the rock volume near the cryogen/rock interface rapidly cools and contracts, as the rock loses its heat to the cryogen. The rock thus fails in tension, with cracks developing orthogonally to the cryogen/rock interface. Three experimental setups were designed and built: frozen block tests, cryogenic fracturing stimulation without confining stress conditions, and cryogenic fracturing stimulation with confining stress conditions using a tri-axial loading system. The cryogenic fracturing experimental tests and their analyses were carried out on concrete, tight sandstone, and Niobrara shale samples using submersion, borehole testing of unconfined specimens, and tri-axial tests with and without borehole pressurization. These tests indicate different fracturing patterns. The contributions provided by this research will assist the industry in many aspects, including, demonstrating how fractures are created by cryogenic fracturing and propagate through homogenous and heterogeneous reservoir rocks in a laboratory setting. Through 3D finite element modeling of these laboratory experiments, predictions of stress behaviors around the wellbore generated in laboratory settings by cryogenic stimulation are made and analyzed. The combination of these laboratory experiments and 3D finite element models provide insight into the potential ability of cryogenics to stimulate unconventional reservoirs. The results show that the liquid nitrogen fracturing stimulation has an effect on permeability especially for the tested shale samples. In addition, tests showed that the breakdown pressure was reduced for all samples in this study. Liquid nitrogen has almost no formation damage potential to the reservoir rock whereas other hydraulic fracturing fluids have damage potential with some of them being highly damaging to the formation. In addition, this technology offers the advantage of minimizing or eliminating the need for water in hydraulic fracturing which helps to minimize the water consumption in oil and gas industry. Finally, liquid nitrogen is a non-reactive and non-toxic chemical making this fracturing method an environmentally friendly technology.
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