Editing OR/15/066 Engineering considerations

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==Hydraulic fracture fluid==
 
==Hydraulic fracture fluid==
Hydraulic fracture fluid plays a vital role in the formation of hydraulic fractures. Fisher et al. (2004)<ref name="Fisher 2004"></ref> examined microseismic monitoring results and found that hydraulic fractures propagate in both horizontal and vertical directions in complex patterns rather than single symmetric patterns. They also noted that a larger volume of fracturing fluid leads to a wider area swept by microseismic events and a higher gas yield. This suggests that a limit can be imposed on fracture propagation based on the volume of fluid injected. It may be theoretically possible to create a pressure that could overcome geological stresses so that a fracture could grow vertically to shallow depths or even the surface. However, this is not feasibly practical. During fluid injection a certain amount of leak-off is experienced, this is caused by fluid flowing into the shale gas unit or entering natural fractures and is pressure dependent. Different shale types will result in variations in leak-off. In order to create such an enormous hydraulic pressure that a fracture would propagate significant distances there would become a point where injection rate would equal leak-off and therefore the fracture could simply not grow any further (King, 2010<ref name="King 2010">King, G E. (2010). Thirty Years of Gas Shale Fracturing: What Have We Learned? Society of Petroleum Engineers.</ref>; Fisher & Warpinski, 2012<ref name="Fisher 2012"  >Fisher, K, and Warpinski, N R. (2012). Hydraulic-Fracture-Height Growth: Real Data, ''Society of Petroleum Engineers'', doi:10.2118/145949-PA.</ref>; Mair et al., 2012<ref name="Mair 2012">Mair, R, Bickle, M, Goodman, D, Koppelman, B, Roberts, J, Selley, R, Shipton, Z,  Thomas, H, Walker, A, Woods, E, and Younger, P L. (2012). Shale Gas Extraction in the UK: a Review of Hydraulic Fracturing. ''Royal Society and Royal Academy of Engineering'', London, pp.76.</ref>).
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Hydraulic fracture fluid plays a vital role in the formation of hydraulic fractures. Fisher et al. (2004)<ref name="Fisher 2004">Fisher, M K, Heinze, J R, Harris, C D, Davidson, B M, Wright, C A, and Dunn, K P. (2004). Optimizing horizontal completion techniques in the Barnett shale using microseismic  fracture mapping, ''2004 SPE meeting'', Houston, Texas, USA, Expanded  Abstracts, SPE90051.</ref> examined microseismic monitoring results and found that hydraulic fractures propagate in both horizontal and vertical directions in complex patterns rather than single symmetric patterns. They also noted that a larger volume of fracturing fluid leads to a wider area swept by microseismic events and a higher gas yield. This suggests that a limit can be imposed on fracture propagation based on the volume of fluid injected. It may be theoretically possible to create a pressure that could overcome geological stresses so that a fracture could grow vertically to shallow depths or even the surface. However, this is not feasibly practical. During fluid injection a certain amount of leak-off is experienced, this is caused by fluid flowing into the shale gas unit or entering natural fractures and is pressure dependent. Different shale types will result in variations in leak-off. In order to create such an enormous hydraulic pressure that a fracture would propagate significant distances there would become a point where injection rate would equal leak-off and therefore the fracture could simply not grow any further (King, 2010<ref name="King 2010">King, G E. (2010). Thirty Years of Gas Shale Fracturing: What Have We Learned? Society of Petroleum Engineers.</ref>; Fisher & Warpinski, 2012<ref name="Fisher 2012"  >Fisher, K, and Warpinski, N R. (2012). Hydraulic-Fracture-Height Growth: Real Data, ''Society of Petroleum Engineers'', doi:10.2118/145949-PA.</ref>; Mair et al., 2012<ref name="Mair 2012">Mair, R, Bickle, M, Goodman, D, Koppelman, B, Roberts, J, Selley, R, Shipton, Z,  Thomas, H, Walker, A, Woods, E, and Younger, P L. (2012). Shale Gas Extraction in the UK: a Review of Hydraulic Fracturing. ''Royal Society and Royal Academy of Engineering'', London, pp.76.</ref>).
  
 
Flewelling et al. (2013)<ref name="Flewelling 2013">Flewelling, S A, Tymchak, M P, and Warpinski, N. (2013). Hydraulic fracture height limits and fault interactions in tight oil and gas formations. ''Geophys. Res. Lett''., 40, pp.3602–3606, doi:10.1002/grl.50707.</ref> performed a fracture height study based on a simple energy balance. In order to hydraulically fracture shale, energy is needed to (1) counteract the least principal stress; (2) displace and open the walls of the fracture; (3) propagate the fracture at the fracture tip; and (4) counteract energy dissipation due to fluid viscosity and leak-off of fluid pressure. Flewelling et al. compared end-member situations for given pore fluid pressure, Young’s modulus, and fracture aspect ratio with data from 1,754 individual shale gas and tight rock conventional wells. This showed that the maximum fracture height is linked to the volume of the hydraulic fluid injected. All microseismic data showed the maximum observed fracture length was 600 metres, with the majority of heights much less than this.
 
Flewelling et al. (2013)<ref name="Flewelling 2013">Flewelling, S A, Tymchak, M P, and Warpinski, N. (2013). Hydraulic fracture height limits and fault interactions in tight oil and gas formations. ''Geophys. Res. Lett''., 40, pp.3602–3606, doi:10.1002/grl.50707.</ref> performed a fracture height study based on a simple energy balance. In order to hydraulically fracture shale, energy is needed to (1) counteract the least principal stress; (2) displace and open the walls of the fracture; (3) propagate the fracture at the fracture tip; and (4) counteract energy dissipation due to fluid viscosity and leak-off of fluid pressure. Flewelling et al. compared end-member situations for given pore fluid pressure, Young’s modulus, and fracture aspect ratio with data from 1,754 individual shale gas and tight rock conventional wells. This showed that the maximum fracture height is linked to the volume of the hydraulic fluid injected. All microseismic data showed the maximum observed fracture length was 600 metres, with the majority of heights much less than this.

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