Abstract: Evaluation of Grouting Materials for Drill-hole Abandonment

John Wheaton, Jon Reiten, Steve Regele

Proper grouting of boreholes and wells is the principle step in avoiding adverse impacts to ground-water resources from drilling activities. Several bentonite and cement-based grouts were evaluated in this research. Bentonite produces a hydraulically competent, flexible hydraulic seal; whereas, cement produces a strong, rigid seal. Of the grouts tested, bentonite tends to re-establish a seal if it is disturbed, whereas cement-based grouts do not reseal. In seismic-shot holes, both low- and high-grade bentonite performed well, except in one low-grade plugged hole where a blow-out occurred.

In an unobstructed borehole at depths of as much as 2,000 feet, gravity-led bentonite chips appear to provide the best seal, being both hydraulically and physically strong. If obstacles such as centralizers on well casing preclude the use of chips, then a high-solids bentonite slurry provides an excellent flexible grout and can be installed with pump pressures of 200 psi or less. Slurries should contain at least 8 percent (preferably more) bentonite by volume. Volumetric (not weight based) measurement of solids content of abandonment slurries is appropriate.

Of the grouts tested, neat-cement is the most expensive technique at about $3.75/foot in a 6-inch hole. Bentonite-slurry grouts are about $2.65/foot and high-grade bentonite chips are about $1.60/foot. Low-grade bentonite chips are less expensive than high-grade chips, but the exact amount is difficult to determine because no commercial mines are currently selling low-grade products.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana