Capstone: The Sand That Fails

When the Sand Comes

A capstone. This section puts the whole course to work on one of the most common and costly failures a producing field faces, sand production. A weak reservoir sand, drawn on hard, shear-fails at the perforations and produces its own grains with the oil, eroding equipment, filling the wellbore, and eventually killing the well. Whether and when it sands is a geomechanics prediction, and it uses everything: the stress path that raises the effective stress as the field depletes (Section 10.1), the rock strength read from logs (Section 8.3), and the wellbore stress concentration at the perforation cavity (Part 6).

The physics assembles into a single criterion. At the perforation wall the effective tangential stress is roughly twice the far-field effective horizontal stress plus twice the drawdown, and the sand fails when that exceeds its strength. Two things push it toward failure: depletion, which raises the far-field effective stress along the stress path, and drawdown, the pressure the well pulls below the reservoir to produce. The onset is a line in depletion-drawdown space, and everything above it sands. For the canon weak sand, the critical drawdown starts near 9 MPa in the fresh field and falls to zero by about 28 MPa of depletion: late in life, the sand fails at any drawdown at all.

The Well Ages Into Failure

Read the envelope as a life story. Early, the operator can pull a healthy drawdown and stay in the safe region below the line. As the field depletes, the stress path lifts the effective stress, the onset line drops, and the safe drawdown shrinks. The same production rate that was safe at the start crosses into sanding as the reservoir pressure falls, without anything changing at the well. This is why sanding is a late-life problem, arriving on schedule as depletion accumulates, and why a sand-management plan is written from the stress model at the start, not diagnosed after the first sand appears. Drag the depletion and the drawdown and steer the well; the verdict flips from clean to sanding as you cross the line. The strength slider is the other lever: a stronger sand pushes the whole envelope up, more drawdown before failure, while a weaker one collapses it. The canon main sand at UCS 65 is competent and sands only under extreme conditions; the weak, friable interval modeled here is the one that fails, and knowing which sands are weak, from the log-derived strength of Section 8.3, is knowing where the sanding will start.

Managing It

The management options all read off this same map. Reduce the drawdown, choke the well back below the onset line, and the sand stays put, at the cost of rate. Complete with sand screens or gravel packs to hold the failed sand in place and produce through it. Consolidate the sand chemically near the wellbore to raise its strength. Or accept the sanding and handle it at the surface. Which choice makes sense depends on where the well sits on the envelope and how fast it is aging into failure, a geomechanics judgment made from the stress model, the strength profile, and the depletion forecast, the three things this course built. The final capstone turns from a well producing sand to a well being fractured near a neighbor: the frac hit.

References

• Willson, S. M., Moschovidis, Z. A., Cameron, J. R., & Palmer, I. D. (2002). New model for predicting the onset of sand production. SPE/ISRM Rock Mechanics Conference, SPE 78168.
• Veeken, C. A. M., Davies, D. R., Kenter, C. J., & Kooijman, A. P. (1991). Sand production prediction review: developing an integrated approach. SPE Annual Technical Conference, SPE 22792.
• Zoback, M. D. (2007). Reservoir Geomechanics (ch. 10). Cambridge University Press.