Integrating rock ductility with fracture propagation mechanics for hydraulic fracture design

Abstract

The invention relates to the calculation of parameters to inform hydraulic stimulation of non-conventional hydrocarbon-bearing rock formations, such as shales. Unlike conventional formations, non-conventional formations tend to display elastic-plastic behavior and have stress-strain characteristics which with substantial non-linear regions. A parameter which has been termed Elastic Index (EI) is proposed, together with a demonstration of how this parameter, when coupled with principles of fracture mechanics, may be used to extract meaningful calculated or estimated values for e.g.; total required volume of fracturing fluid; treating pressure; fracturing fluid viscosity; proppant size; and proppant concentration.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a non-provisional application which claims benefit under 35 USC §119(e) of and priority to U.S. Provisional Application Ser. No. 61 / 828,368 filed 29 May 2013, entitled “INTEGRATING ROCK DUCTILITY WITH FRACTURE PROPAGATION MECHANICS FOR HYDRAULIC FRACTURE DESIGN,” which is incorporated by reference herein in its entirety.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT[0002]None.FIELD OF THE INVENTION[0003]This invention relates to hydraulic stimulation (“fracking”) in shale and other unconventional subterranean hydrocarbon reservoirs.BACKGROUND OF THE INVENTION[0004]Shale and other non-conventional formations such as tight gas, mudstone, siltstone and marl systems are becoming an increasingly important source of hydrocarbon resources. Such unconventional resources, however, present challenges not only in their extraction but also in the analysis of their properties in order to design strategies for drill...

AI Technical Summary

Benefits of technology

The patent text describes a method for calculating parameters for hydraulic fracturing in non-conventional hydrocarbon reservoirs. The method involves taking a core sample from the reservoir and performing a tri-axial compressive test on it. The resulting data is then used to determine the axial yield stress and failure stress. An elastic index value is then calculated by combining these stresses. The elastic index value is used to estimate the fracturing fluid viscosity, total required volume of fracturing fluid, treating pressure, proppant size, and proppant concentration. The method takes into account the non-linear behavior of the stress-strain curve and can provide a more accurate estimate of the fracture volume and the amount of fracturing fluid needed for the fracturing process.

Problems solved by technology

Such unconventional resources, however, present challenges not only in their extraction but also in the analysis of their properties in order to design strategies for drilling and treatment such as hydraulic stimulation (“fracking”).

Calculation of the above parameters has proven problematic in shale and other non-conventional rock, which appears to have geomechanical properties which differ substantially from conventional rock.

Shale and other non-conventional rock, however, do not always behave in this way and often show a considerable degree of plastic behavior.

This will generally produce incorrect results and often a large amount of trial and error is also involved.

However, the brittleness approach has been shown to provide inaccurate predictions for shale properties and the “frackability” of non-conventional reservoirs.

This may lead to a dependence of apparent fracture toughness on confining pressure for mode-I cracks (Perkins and Krech, 1966; Abou-Sayed, 1977; Schmidt and Huddle, 1977; Atkinson and Meredith, 1987; Thallak et al., 1993).

However, there is to date no straightforward way of predicting the required fracture pressure, density and volume of fluid or size and concentration of proppant for an unconventional, e.g. shale, reservoir.

Getting these parameters wrong can have serious consequences for field operations.

A common issue encountered in hydraulic fracturing is called a ‘screen out’, which occurs when the formation does not break-down at the expected pumping pressure.

The frac sand collects in the wellbore causing serious logistics issues; the problems arise because the sand is never properly placed in the formation i.e the hydraulic fracture operation fails.

This causes delays and extra expense and, in addition, the complete hydraulic fracture product (fluid, sand, manpower and horsepower) is wasted on an unsuccessful attempt.

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