WELL INTEGRITY IN COLORADO'S DENVER-JULESBURG BASIN: CONVENTIONAL VS. UNCONVENTIONAL WELLS AND IMPLICATIONS FOR STRAY GAS MIGRATION

Groundwater data collected by the Colorado Oil and Gas Conservation Commission in the Denver-Julesburg (DJ) Basin between 1998-2014 identify 42 water wells impacted by thermogenic stray gas. Recent studies suggest that faulty oil and gas wells are a primary pathway for thermogenic stray gas between the deep subsurface and shallow freshwater aquifers. Colorado regulations allow operators to construct wells with production casings that are not completely cemented to the ground surface. This leaves an uncemented annulus along the wellbore that stray hydrocarbons enter if a well develops an integrity issue. These hydrocarbons migrate vertically and build pressure at the wellhead beneath the valve that seals the annulus between the surface and production casings. We refer to this pressure as surface casing pressure (SfCP). Here, we analyze SfCP records from the COGCC’s online database for oil and gas wells installed in the Wattenberg Testing Zone (WTZ), a sub-region of the DJ Basin. Deviated and horizontal wells developed SfCP greater than the COGCC’s SfCP limit of concern (1034 kPag) more frequently than vertical wells, which indicates that newer unconventional oil and gas wells develop integrity issues at a higher rate than older vertical wells. If SfCP exceeds the pressure of the formation fluid at the bottom of the surface casing, the gas in the annulus dips below the bottom of the surface casing and a stray gas migration incident is potentially induced. We define critical SfCP as the pressure required to induce a stray gas migration incident. Fewer oil and gas wells in the WTZ have developed SfCP ≥ critical. Deviated wells develop the highest rate of SfCP ≥ critical followed by vertical and horizontal wells. Legacy wells installed with surface casings shallower than the deepest drinking water aquifer pose the greatest risk for stray gas migration. In the WTZ, 41 vertical and 5 deviated legacy wells exhibited SfCP ≥ critical. Thus, newer unconventional wells develop SfCP at a greater rate than older conventional wells but pose a lower risk for stray gas migration because of improved well construction practices.