Examining Our Assumptions – Have Oversimplifications Jeopardized Our Ability to Design Optimal Fracture Treatments?

In the design of hydraulic fractures, it is necessary to make simplifying assumptions. Fifty years ago, our industry was mathematically obliged to describe fractures as simple, planar structures when attempting to predict fracture geometry and optimize treatments. Although computing tools have improved, as an industry we remain incapable of fully describing the complexity of the fracture, reservoir, and fluid flow regimes. Generally, we make some or all of the following assumptions:

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  Simple, planar, bi-wing fractures

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  Completely vertical fractures with perfect connection to the wellbore

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  Flow capacity that is reasonably described by published conductivity data

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  Predictable fracture width providing dependable hydraulic continuity (lateral and vertical continuity)

To forecast production from these fractures, we frequently make the additional assumptions:

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  Reservoir is laterally homogeneous

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  Modest/no barriers to vertical flow in formation (simplified description of layering compared to reality)

However, we must recognize that all of these assumptions are imperfect. This paper will investigate the evidence suggesting that fractures are often subject to:

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  Complicated flow regimes

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  Complicated geometry

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  Irregular frac faces

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  Imperfect proppant distribution

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  Imperfect hydraulic continuity

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  Imperfect wellbore-to-fracture connection

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  Residual gel damage, possibly including complete plugging or fracture occlusion

Additionally, reservoirs are known to contain flow barriers that amplify the need for fractures to provide hydraulic continuity in both vertical and lateral extent.

The paper appendix tabulates the results from more than 200 published field studies in which fracture design was altered to improve production. Frequently the field results cannot be explained with our simplistic assumptions. This paper will list the design changes successfully implemented to accommodate real-world complexities that are not described in simplistic models or conventional rules of thumb. Field examples from a variety of reservoir and completion types [tight gas, modest perm oil, coalbed methane, low rate shallow gas, annular gravel packs] will be provided to demonstrate where the field results differ from expectations, and what adjustments are necessary to history-match the results.