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Description
The effectiveness of bentonite clay as a wellbore plugging material often depends upon its penetration into near-borehole cracks associated with the drilling process. Here we present research aimed at understanding and maximizing the ability of clay materials to plug near-borehole cracks. A device was constructed such that the borehole is represented by a cylindrical chamber, and a near-borehole crack is represented by a slot adjacent to the center chamber. The experiments consist of placing bentonite clay pellets into the center chamber and filling the entire cavity with distilled water or brine so that the pellets hydrate and swell, thereby intruding into the slot. Results indicate that the bentonite clay pellets do not fully plug the slot. We propose a model where the intrusion length into the slot is limited by: 1) the free swelling potential intrinsic to the system comprised of the bentonite pellets and the hydrating fluid, and 2) the resisting shear force along the walls of the slot. This model accounts for the fact that narrow slots have a smaller volume for the clay to fill than wider slots, but wider slots present less resistive force to clay intrusion. These two limiting mechanisms work against each other, leading to a non-monotonic relationship between slot width and intrusion length. Specifically, the results show a linear increase of the intrusion length with the slot width for narrow slots where the limit is the shear strength resisting intrusion and scaling in proportion to the slot width over which the driving swelling pressure is applied. For wide slots the intrusion length is controlled by available swelling volume and scales inversely with the volume of the slot, and hence also inversely with the slot width. The experiments also show that increasing the salinity of the solution leads to a dramatic decrease in the clay expansion, evidenced here by the substantially smaller intrusion length when the pore fluid contains 10 g/L NaCl. We conclude that there exists a range of length to width ratios of near wellbore cracks that will be effectively plugged, and where a suitable plugging criterion is determined by a competition between availability of volume and shear resistance to clay intrusion.
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