Existence and Flow Behavior of Gas at Low Saturation in U.S. Gulf Coast Geopressured Brine Zones

Abstract

The first geopressured brine well tested for the U.S. DOE produced gas and brine at a ratio far above the solution ratio for the gas in that brine. Potential explanations for that behavior are examined in this paper. Introduction The geopressured brine formations along the U.S. gulf coast contain an extremely large potential resource of heat and dissolved methane. These highly faulted formations are broken into thousands of individual reservoirs. Can these be exploited commercially for heat and methane? To help determine the answer to this question, the U.S. DOE is funding the conduct of sampling and flow tests in selected geopressured reservoirs. The purpose of these tests is to obtain sufficient data on the feasibility of commercial exploitation. The first well tested under the DOE program was Edna Delcambre No. 1 in Vermillion Parish, LA, in June 1977. This well had been a conventional gas producer from deep geopressured zones. It was plugged back and tested in two shallower zones by OHRW Engineering for the DOE. A very interesting and potentially very important phenomenon occurred during flow testing of this well. The well had been expected to produce geopressured twine, probably saturated with methane. However, both Sands 1 and 3 (Figs. 1 and 2) produced gas (mainly methane) at a gas/liquid ratio (GLR) far above the solution ratio. Only about 20 to 24 scf of produced gas could be dissolved in each barrel of produced water in laboratory recombination studies. However, both sands produced gas and water at ratios up to 50 to 60 cu ft/bbl for extended periods. A number of explanations have been advanced to explain this behavior. In the case of Sand 3, the streak of high resistivity rock near the top (the interval 12,873 to 12,876 ft in Fig. 1) is almost certainly gas bearing. Running of the TDT log and analysis of this log and the resistivity log has confirmed this first interpretation and has indicated that this interval is probably both tight and gas bearing. A more complete discussion of the behavior during flow testing is given in this paper. The behavior of Sand 1 is more puzzling. There is no evidence from logs that the sand is gas bearing. One possible explanation is that gas production is from the gas cap that exists updip of the top of the completion interval in Sand 1 (Fig. 3). Production of water could have "coned" the gas down into the well. It is also possible that gas from Sands 2 or 3 (Fig. 1) reached Sand 1 because of a poor cement job in the cemented liner that exists in the interval between Sands 1 and 3. Practice has shown that it is very difficult to obtain a perfect cement job when landing and cementing a liner at these depths. Rogers and Randolph believed the source of the gas produced in Sand 1 is from a prior blowout in a nearby well. Still another intriguing explanation of the gas production from Sand 1 is that gas exists throughout Sand 1 but at a very low saturation of about 2 to 5%. At such low saturation, the gas would not flow when there was no pressure gradient. However, when the well was placed on production and a pressure gradient established, it has been postulated that this gas might flow. This last explanation is examined in this paper. JPT P. 2214＾