Friction Factors for Hydrated Bentonite Plugs
Authors: Towler Brian F. , Ehlers Guy C. 
Published: December 2007

Abstract
This research has investigated the potential for plugging oil and gas wells with hydrated bentonite. This process is already being used in seven states, including Wyoming, Colorado, and Montana, to plug seismic shot holes. We seek to extend this method to plugging oil and gas wells and if successful would reduce plugging costs greatly as well as providing a superior plug. A theory to predict the pressure that could be contained by a hydrated bentonite plug when inserted in a standard cased well-bore is proposed, which predicts a parabolic relationship between the height of the plug and the pressure that it can withstand.. Experiments conducted in the laboratory at plug heights of 3 feet to 10 feet are consistent with the theory and we have measured bentonite to casing friction coefficients between 0.5 to 1. Using this theory we can predict the pressure that could be withstood by any length of bentonite plug. This is currently being tested in an experimental well at the Rocky Mountain Oilfield Testing Center.

Introduction
Plugging oil and gas wells are usually governed by regulations which require that perforations in permeable zones be isolated with cement plugs. It usually requires a work-over rig to pump and set these plugs which represents a substantial plugging cost. The research presented here is directed towards a proposal to reduce these costs by using hydrated bentonite to isolate permeable zones. Bentonite can be placed at the appropriate point in the well-bore by setting a bridge plug with a slick line and dropping the bentonite onto the bridge plug. Alternatively it has been proposed to separate the plugs with loose gravel. If water is not already present, water can then be added which will hydrate and swell the bentonite until a tight seal is effected against the casing or even against an open hole. Besides the cheaper cost these hydrated bentonite plugs have another important advantage over concrete plugs. If the well-bore shifts for any reason (earthquakes or salt migration) a concrete plug will crack and lose its plugging integrity. However a hydrated bentonite plug has the ability to swell and thus heal any cracks that form from shifting formations. In this current research a theory to predict the pressure drop that can be withstood by a hydrated bentonite plug is developed and this theory is tested using laboratory measurements on the pressure to dislodge the hydrated bentonite plugs from Sch 40 casing of three different sizes. This allowed the calculation of plug-casing friction factors and will predict the containment pressure of different plug lengths in cased well-bores. Though the data has some scatter a clear trend is developed.

Previous Work
Since 1984 the Wyoming Oil and Gas Commission has pioneered the use of hydrated bentonite to plug seismic drill holes which had penetrated ground water. James recently described this program, which has been highly successful in plugging off the ground water. It also had the added benefit of containing the seismic detonation and preventing "blowback" (where the energy is not contained and moves up and out of the hole). In all the cases James investigated the hydrated bentonite plug remained exactly in place when containing the explosions of 5 to 50 lbs. It was successful in plugging wells varying in depth from 60 to 250 feet. He also reported that a hydration time of greater than 21 hours was sufficient to fully hydrate coarse ground Wyoming sodium bentonite.
Wheaton et al. described experiments in subsurface applications of bentonite in seismic holes in Montana. They concluded that bentonite was 40% of the cost of neat-cement but was superior in sealing holes where ground motion is a concern, because the bentonite tends to re-establish a seal after it is disturbed.
Ogden and Ruff measured the axial shear strength bentonite plugs in the annulus between two pipes, the inner one being steel casing and the outer one being PVC Sch 40 water drain pipe of 4 inch ID and 24 inches in length. The inner pipes ranged in diameter from 0.5 to 3 inches.
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