Bentonite better than cement to seal wells

HYDRATED bentonite clay could be a safer, more cost-effective material for plugging and abandoning oil and gas wells than conventional cement, according to chemical engineers at the University of Queensland, Australia.

Brian Towler from Queensland’s Centre for Coal Seam Gas and School of Chemical Engineering and his team have reviewed a number of field trials using bentonite nodules in the US and Australia, all of which were successful.

Towler explained to The Chemical Engineer: “When [bentonite] is hydrated it is malleable and self-healing, which we have demonstrated many times in the lab. Cement, on the other hand is prone to shrinking and cracking. Once that happens it is no longer sealing. If bentonite is disturbed in any way it swells and re-heals itself, so it is a much more forgiving medium for sealing holes in the ground.”

Oil giant Chevron already markets a commercial bentonite well-sealing product called Zonite, made up of compressed sodium bentonite. The nodules are dropped or lowered into the well hole, which is then filled up with water. The clay then swells as it hydrates to seal the well.

In Coalinga, in central California, US, 19 wells were successfully sealed with Zonite in 2001, while in 2002, Chevron and partners Texaco, Apache and Phillips successfully plugged and tested 21 wells in West Texas. In New Mexico, Oklahoma and Wyoming, 25, six and eight wells have been plugged. In one trial in Barrow Island, Western Australia, in 2003, plugging a well with bentonite was estimated to cost A$16,500 (US$13,000), less than half the cost of using cement. It was also quicker, taking on average 39 working hours, compared to cement’s 60.

The uptake of bentonite as a well-sealing material, despite its demonstrable advantages, has been slow for one simple reason – the vast majority of oil and gas regulatory bodies around the world require all abandoned wells to be sealed with cement, specifically. Some regions of the US, such as California, Nebraska, New Mexico and Pennsylvania, allow the use of alternative materials following successful trials. Towler is now calling for changes to regulation to allow bentonite to be more widely adopted. He believes that if the regulators do change the rules, the use of bentonite will take off.

Towler’s research group is currently working to improve bentonite technology.

“We have developed new compressed shapes that make it easier to deploy in wells. We are working on ways to make it applicable to saline water, particularly sea-water. We are developing additives that enhance its use in particular applications,” he told The Chemical Engineer.