EXAMINATION OF FACTORS INFLUENCING METHANE CONCENTRATION WITHIN WILCOX AQUIFER IN NORTHWEST LOUISIANA

Recent studies relate the presence of methane within groundwater to the proximity of drilling/hydrofracturing activity associated with development of a shale gas play. However, for the Wilcox Aquifer in northwest Louisiana other factors appear to influence methane concentrations in groundwater more than proximity to drilling/hydrofracturing activity within the Haynesville gas shale play.

Seventy-five groundwater samples were analyzed for dissolved methane concentration and 447 samples were analyzed for head space methane concentration for wells in southern Caddo, southern Bossier and northern De Soto Parishes in the study which covers 153 township sections. Dissolved methane values were determined by gas chromatography-mass spectrometry; methane head space values were determined by a gas chromatography-flame ionization detector. Analysis shows that head space methane concentrations (HSM) appear to be related to dissolved methane concentrations by a ratio of approximately 4.3:1; hence variations in dissolved methane are implied by HSM in this discussion. The following parameters were considered for the more abundant HSM data: location, depth to sand, sand seam a well is screened in, proximity to lignite in the aquifer, and number of oil and gas wells in the same section as water wells: total, those spudded prior to 1950 and recent Haynesville shale gas play wells.

Results show significant differences of HSM values among the three parishes. The average HSM (AHSM) in Bossier Parish is 5 times that in Caddo Parish and 9 times that in De Soto Parish. Both differences have a statistical confidence of difference >99.9%, which is significant. Results also show that oil and gas exploration and production in the study exert little or no influence on AHSM in the study area. The AHSM in nine sections with 3-4 Haynesville wells is ~0.5 the AHSM in 74 sections that have no Haynesville wells. Furthermore, AHSM in 31 sections with 0-1 older oil and gas wells is ~2 times that of AHSM in 22 sections with more than 20 older oil and gas wells. However, where screen position is known AHSM is ~4 times greater in the lower sand than in the upper sand. AHSM for wells with lignite in the screen interval is ~2.5 times that for wells that lack lignite. These imply a stronger relationship between AHSM and aquifer geology than proximity to oil and gas activity.