ABSTRACT
Cryogenic fracturing using liquid nitrogen can achieve extremely low temperatures and very strong thermally induced stresses. Grundmann et al. (1998) treated a Devonian shale well with liquid nitrogen and observed an initial production rate 8% higher than the rate in a nearby offset well that had undergone traditional fracturing with nitrogen gas. Unfortunately, subsequent production information was unavailable because the well had to be shut in for logistical reasons. To further advance the study of cryogen fluids on hydrocarbon producing formations, McDaniel et al. (1997) conducted simple laboratory studies where coal samples were immersed in cryogenic nitrogen. The coal samples experienced significant shrinkage and fracturing into smaller cubicle units, with the creation of microfractures orthogonal to the surface exposed to the cold fluid. The researchers found that repeated exposure cycles to the cryogen caused the coal to break into smaller and smaller pieces, or become rubblized. After 3 cycles of exposing the coal to liquid nitrogen and allowing the coal to ambient temperatures again, the coal was reduced to grain size particles. McDaniel et al. (1997) also conducted field experiments by re-stimulating four coal-bed methane (CBM) wells
one tight sandstone well with liquid The wells were retrofitted with stainless steel surface piping, manifolds, and wellhead component to prevent thermal contraction problems. A free hanging fiberglass tubing was used to inject the liquid nitrogen without compromising the casing integrity. The results were mixed: All 5 wells showed promising re-stimulating initial production rates 10-20 times the before-re-stimulation production average; however, those rates quickly dwindled. The CBM wells showed sustained 6 month re-stimulation production increase of 0-45%. The tight sand well initially had higher flow rates for 2 months after re-stimulating, but then had a 65% loss in production from pre restimulation performance. It is believed that the initial success in re-stimulating these wells was not that new fractures were created but that the damage from the gel filter cake of previous fracturing treatments were greatly reduced.
