Finding Optimal Hydraulic-Fracture Angles in Productivity-Maximized Shale Well Design
- Adam Wilson (JPT Special Publications Editor)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- March 2015
- Document Type
- Journal Paper
- 104 - 108
- 2015. Society of Petroleum Engineers
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- 168 since 2007
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This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 170965, “Optimal Hydraulic-Fracture Angle in Productivity-Maximized Shale Well Design,” by Nadav Sorek, SPE, Jose A. Moreno, Ryan Rice, Guofan Luo, and Christine Ehlig-Economides, SPE, Texas A&M University, prepared for the 2014 SPE Annual Technical Conference and Exhibition, Amsterdam, 27–29 October. The paper has not been peer reviewed.
In general, hydraulic fractures propagate perpendicular to the horizontal-well axis whenever drilling is parallel to the minimum principal-stress plane. However, operators frequently drill horizontal wells parallel to lease boundaries, resulting in slanted hydraulic-fracture planes at angles less than 90° from the well axis. This study provides a model for the inclined fracture case. It applies and further extends the unified-fracture-design approach for rectangular drainage areas, relating the dimensionless proppant number to the maximum productivity index in pseudosteady-state conditions.
Industry experience suggests that horizontal shale-gas development is enhanced by drilling in the direction parallel to the local minimum principal horizontal stress. Because US mineral leases frequently are rectangular areas with north/south and east/west boundaries, operators often drill parallel to the lease boundaries, prioritizing well saturation over optimum fracture-length propagation. This practice leads to the creation of hydraulic-fracture planes that are slanted at an angle less than 90° with respect to the well axis.
Extensive research has investigated the effects of angled fractures on well productivity. Theoretical and empirical data have been used to address the effect of wellbore azimuth on well performance in the Marcellus shale. Results showed that, for each degree a well was suboptimal to minimum horizontal stress, the estimated ultimate recovery decreased by 7.25 Mscf per foot of effective lateral length.
Unified-fracture-design methodology indicates the hydraulic- fracturing treatment design that maximizes well productivity for any set of reservoir and proppant properties and a given injected-proppant mass. This methodology introduces the concept of proppant number, which describes the weighted ratio of propped-fracture volume to a square reservoir volume.
This study applies and further extends the unified-fracture-design approach to show that, for any given set of reservoir and proppant properties along with a given proppant mass, as long as the created fractures define the same stimulated rock volume, there exists a well direction resulting in maximized well productivity that is not parallel to the minimum-stress direction. First, a correlation is established between the proppant number and the optimum drainage-area aspect ratio. Then, this correlation is used to express what the optimum fracture angle is for a specific proppant number.
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