THE HARSH ENVIRONMENT OF LANDFILLS inevitably limits the effective life of gas extraction wells. But it is possible to postpone the inevitable, according to William M. Held, director of landfill gas (LFG) services for Mahwah, N.J.-based EMCON/OWT Inc., a solid waste consulting and construction company specializing in landfills and LFG.
By managing five common problems with gas extraction wells, landfill owners and operators can lengthen average well performance and control costs, he says.
Rising water levels: Rising water levels reduce the flow of LFG through gravel-packs and pipe perforations. In some cases, water can rise above the level of perforations in the piping and cut-off gas flow. To diagnose the problem, Held suggests evaluating changes in gas flow and wellhead vacuum over short periods implying a water surge, and comparing current water levels with as-built levels using a water meter.
A simple pump may solve water problems in a well with 6-inch or larger diameter pipes. One pumping session may reduce water levels permanently. In others, pumps must run continuously. This technique has proven less effective in pipes smaller than 6 inches in diameter, Held says.
Gravel pack clogging: Wet, acidic gas flows can cause the pieces of a lime-based gravel pack to fuse, cutting off gas flow into a well. While over-drilling may repair the problem, a replacement well may offer the only solution. Clogs also may develop when void spaces in lime-based or nonlime-based gravel fill with fine particles or biological agents. In such cases, it is possible to seal a portion of the perforated length of the well with a plug and inject water or chemicals into the pipe below the plug. Water pressure or chemical reactions may open the void spaces.
Silt build-up: Because a gravel pack is not an ideal filter, gas flowing into well pipes often carries fine, silt-like particles through the perforations and into the piping. When silt builds up inside a pipe, it reduces the gas flow. To solve this, use a pipe or hose to flush the well with water. Water pressure will agitate the silt and bring it to the surface. The technique requires gradually moving the water source deeper into the well until the silt filters through the perforations along the length of the pipe.
Slot and perforation clogs: Bacteria and fine particles that build-up on the edges of pipe perforations can reduce or inhibit gas flows from wells. Additionally, heat occasionally causes perforations in high-density polyethylene (HDPE) piping to swell shut. These problems are difficult to fix because slots often clog after other problems develop in the well's surrounding gravel pack. According to Held, hydro jetting, sonar jetting and acid treatments are somewhat but not entirely effective in clearing clogged perforations.
Pinched or collapsed pipes: Because waste settling occurs irregularly in landfills, the process can affect gas collection systems by causing pipes to shift and angle. Sometimes, pipes will pinch and collapse. To confirm the problem, Held recommends sounding a well with a tape to verify that the pipe is not clear to its installed depth. Driving a tapered sleeve into a partially pinched pipe can sometimes repair the problem. But it is not possible to fix a completely pinched or collapsed pipe.
Replacement wells: Given the cost of replacing a well, Held recommends trying appropriate remedies first. “Applied every couple of years, these techniques can help insure compliance and maintain energy performance,” he says.
However, it is important to recognize that remedies don't always work. While a replacement well will cost as much as a new well, it does not cost as much as an old well that failed too soon.
A version of this paper was presented by William Held at the 2003 Waste Tech Landfill Conference.