It would be nice to think that hydraulic fractures are one of the engineering marvels of the world, exposing vast tracts of underground formation to manmade cracks that aid hydrocarbons in their journey to the surface.

Relying on this assumption, however, has some serious drawbacks.

In a presentation during the recent Unconventional Resources Technology Conference in Denver, Consultants M.C. Vincent and M.R. Besler outlined the results of a recent study that shows a growing body of evidence suggesting that fractures lose their effectiveness over time. Noting that the study represents “a previously underappreciated concept,” the authors discussed their findings during one of the technical sessions.

The study is the result of the sometimes dramatic production declines that accompany shale development. On the surface, it might be assumed that these declines are due to insufficient reservoir quality or possibly poor engineering. But the authors outlined several circumstances that, when looked at in combination, point to the declining effectiveness of the fractures themselves as a key root cause.

For example, infill drilling often exposes the wellbore to microseismic swarms that indicate this part of the reservoir is still untapped, meaning that the parent wells have not depleted the rock volume. Additionally, restimulation is often successful, indicating that the original stimulation has lost its effectiveness.

Also, many fields are sensitive to the depth at which the lateral is landed. “If fractures were highly conductive durable vertical fractures that penetrated the entire pay interval, well productivity would not be significantly affected by the specific lateral depth,” they noted. And in some wells managing the drawdown has a positive effect on EUR, hinting that normal operations accelerate fracture degradation.

In laboratory testing, considerable degradation of conductivity has been demonstrated despite attempts to ensure that proppant packs are arranged widely with optimal packing.

These and many other observations led the authors to raise awareness of the situation and to suggest some best practices. Among these it is noted that dividing a fracture treatment into more stages increases production in most plays in contrast to production models that predict that more, smaller stages hurt long-term productivity. Curable resins have proven useful in wells with proppant flowback issues, as have uncoated ceramic proppants.

Slickwater fracs and high proppant concentrations have shown some benefit in wells with discontinuous proppant packs after closure. And traceable proppants can aid in diagnosing proppant location issues.

Applying these findings in the Bakken, the authors suggest that the following steps can improve fracture conductivity:
• Using high-conductivity ceramic proppants;
• Introducing a high-concentration proppant slug at the end of each stage;
• Flushing stages only to the top perforation; and
• Aggressive quality control of water, fluid rheology, breakers, pH, scale inhibition, and proppants.

Despite these efforts, this approach is described as an “operational compromise” to increase the likelihood of maintaining durable fractures.

Overall, however, the Bakken attempts did seem to result in superior production based on two years of production data. “There is not yet production evidence that closely spaced stages are causing competition detrimental to production in the first two years,” the authors note in their abstract. “On the contrary, [an] increased number of initiation points improves the extent of connection between the wellbore and fractures, which appears to improve sustained production.”