Effective Bradenhead Pressure and Mitigation Protocols for Reducing Stray Gas Risk in Groundwater

Hydrocarbon gas migration through uncemented annuli and small channels in cemented casing poses a risk that, under the right circumstances, may allow stray hydrocarbon gases to invade shallow groundwater. These gases migrate vertically and preferentially accumulate in the annular spaces between surface and production casing long before groundwater can become significantly impacted. For this reason, bradenhead pressure monitoring in conjunction with systematic sampling and analysis are effective risk mitigation tools.

An effective bradenhead pressure monitoring program includes the following: (a) defining a minimum 9 square mile area ahead of drilling; (b) compiling and mapping depths of surface casing and underlying uncemented annuli intervals; (c) monitoring bradenhead pressures in wells throughout the area; (d) identifying wells where bradenhead pressure builds up to significant pressures within a 24-hour period following venting; (e) collecting paired bradenhead and production gas samples in all wells found to have both initially elevated bradenhead pressures and significant recurring bradenhead pressures; (f) analyzing paired gas samples for gas composition and stable isotopes; (g) planning and implementing a cement squeeze or well abandonment program to shut off the source of bradenhead gas in wells; (h) planning and implementing a drilling and mud logging program for the first new well development well drilled in the 9 square mile area; (i) collecting mud gas samples and analyzing selected mud gas samples for gas composition and stable isotopes; (j) identifying and contouring a shallow formation boundary near or below the average depth of surface casing in the area; and (k) continuing periodic bradenhead pressure and groundwater monitoring following drilling. This approach is particularly effective in areas where historic drilling practices have left annular intervals of production casing uncemented. Such protocols also facilitate identifying point sources of stray hydrocarbon gases discovered in response to water well complaints and pre-drill water well sampling.