The International Information Center for Geotechnical Engineers

Historical, Theoretical and Practical Perspectives on Hydraulic Fracturing - Mechanics of Hydraulic Fracturing

Practical Considerations

Although HF has been a common industrial method for several decades, it still faces a number of issues when the method is used in practice. Many of these issues are fairly new to the industry and are a result of the advent of new technology, or the application of HF in more complex environments. The following list is a list of some of the most prevalent:

  • premature screen-outs and near wellbore tortuosity
  • high treating pressures and multiple fractures
  • role of perforation practices
  • fracturing of arbitrarily oriented wellbores

This section will summarize the central concerns of each of these issues.

Premature screen-outs—A screen-out occurs when the pressure required to hydraulically fracture the formation exceeds the design pressures of the wellbore or wellhead equipment. This causes severe operational disruption, and typically requires cessation of pumping and cleaning of the wellbore before the operation can be resumed. The umbrella term for the cause of screen-outs is “near wellbore tortuosity,” which can variously arise from deviatoric stress, the presence of natural fractures, or the creation of complex fracture patterns during casing perforation. It essentially originates from fracture reorientation and multiple fracturing. Large fluid volumes and low proppant concentrations can be a solution in some situations, but can cause other problems in certain cases. Another solution is the emplacement of near-wellbore proppant slugs. See Cleary et al., 1993 for further reading.

High treating pressures – The analysis of net or excess pressure during HF treatments can be used as a diagnostic tool for petroleum engineers to evaluate fracture growth patterns. In many cases, these tests indicate that high treating pressures are caused in part by high near wellbore friction (i.e. the simultaneous propagation of multiple fractures). Multiple fractures can propagate in areas where preexisting fractures are present. Sometimes this can be controlled by changing the properties of the fracturing fluid. For further reading, see Davidson et al., 1993 and Narendran and Cleary, 1983.

Role of perforation practices – After a borehole is drilled and cemented, the length of the borehole adjacent to the producing unit will be perforated with perforating guns. The orientation of these perforations will determine the hydraulic fracture initiation sites and orientations. Two generic fracture sites are possible: at the base of the perforations and at the intersection of the plane normal to the minimum far-field stress that passes through the wellbore axis and the wellbore surface. Variations in pore-pressure buildup, reservoir and fracture fluid properties and fluid injection rates are all important parameters to constrain. See Behrmann and Eberl, 1991 for further reading.

Fracturing of arbitrarily oriented wellbores – As can be surmised from much of the above discussion, the planar, well-behaved single fracture that theory predicts is a gross oversimplification in practice. Especially when wellbore orientation is not optimally aligned in the stress field, phenomena including fracture turning, twisting, and linking can all be observed. Methods have been developed to determine the final number and orientation of such fractures. Relevant parameters include pump rates, fluid viscosity, perforation aperatures, induced fracture hydraulic conductivity, etc. Refer to Weng, 1993 for a comprehensive analytical study of the subject.

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