Wellbore failure is best simulated in the laboratory by subjecting a model hole to simulated in-situ stresses. This can be routinely done on available core by performing a hole collapse test. This test uses a hollow cylinder with an outer diameter of 1.5 inches and a hole diameter of 0.5 inches. The jacketed sample is subjected to increasing pressure on the outer surface until collapse of the hole occurs. This provides a direct measure of the stress required to cause hole failure in that particular formation interval. To predict hole stability and sanding, a method is required for applying this measured strength to the downhole situation. Equations are necessary to account for the fact that the downhole stress situation is more complex than that in the hole collapse test. These equations must include the three-dimensional nature of rock strength. A three-dimensional analytic model is presented that is applicable to both stability and sanding of open wellbores. This model uses a Modified Lade failure criterion which properly accounts for the effect of the intermediate principal stress on rock failure. The Modified Lade rock strength parameters are extracted from the measured hole collapse strength and are then combined with the in situ stresses and formation pore pressure using one of two sets of equations: The first set determines required mud weight for stability while drilling, while the second set determines allowable drawdown pressure for prevention of sand production.
Openhole Stability and Sanding Predictions by 3D Extrapolation from Hole Collapse Tests
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Ewy, R.T., Ray, P., Bovberg, C.A., Norman, P.D., and H.E. Goodman. "Openhole Stability and Sanding Predictions by 3D Extrapolation from Hole Collapse Tests." Paper presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, October 1999. doi: https://doi.org/10.2118/56592-MS
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