Experiments to Better Understand Pennsylvania's Gas Migration Problem
Iannacchione, Anthony; Vandenbossche, Julie; Janssen, Don
Arguably one of the greatest threats to environmental protection during development of the shale gas plays in Pennsylvania is integrity of the wellbores, especially in the upper portion of the drill hole where groundwater aquifers are prevalent. As such, the PA Dept. of Environmental Protection (PA DEP) requires significant engineering controls for the wellbore in deep Marcellus gas wells. One focus of these controls is on the performance of cements used to isolate the fluids and gases contained within the wellbore from the surrounding rock formations. PA regulations require oil and gas well operators to prevent the migration of gas or other fluids into sources of fresh groundwater (§ 78.81) (a)(2)).
For the purposes of this discussion, gas migration is synonymous with combustible gas flowing into an annular space during primary cementing operations. This gas has the potential to alter the physical properties of the cement. More precisely, formation gas has the ability to "invade" the cement when it is transitioning from a liquid to a gelled state.
A potential for improvement in the development of the Marcellus shale gas play exists and centers around an enhanced understanding of how gas migration manifests itself under different cement placement conditions. Currently we are only able to indirectly measure the success of these efforts, i.e. no gas flow/pressure at the well head, no complaints of contamination from nearby wells and springs, etc. Geophysical logging methods are improving but they aren't currently capable of producing definitive information about the existence of annular pathways in the cement.
Since the collection of this information under field conditions is quite difficult to collect, a laboratory test chamber capable of simulating down hole conditions has been developed. It is our aim to generate information on how the cement in the wellbore annulus is being affected by gas migration. This is accomplished by simulating in-situ conditions within a test chamber under a wide range of temperatures and pressures. Gas flows are injected through a simulated sandstone material into the cement as it cures. After each test, a series of forensic analysis are performed to map the migration paths of the injected gas within the cement.
Once a better understanding of these processes is achieved, additional engineering solutions will insure that gas isn't entering annular pathways in the wellbore and permitting cross-strata communication.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013