Abstract:
The present invention relates to a method and apparatus for perforating a landfill gas well in situ. The invention allows for improved recovery of gas from a gas well without the danger of explosion.

Description:
FIELD OF THE INVENTION 
   The present invention relates to methods and devices for in situ perforation of landfill gas wells. 
   BACKGROUND OF THE INVENTION 
   The decomposition of waste in a landfill produces methane and other gaseous emissions. Landfill gas recovery wells are used to remove the gases from landfills. Removal of methane and other gases is both an environmental and a safety measure for preventing an accumulation of flammable gases. The gas wells typically consist of pipes made from PVC, high-density polyethylene (HDPE) and similar materials. The gas well&#39;s pipes are slotted or perforated to allow for recovery of the gases. However, over time the slots and perforations become clogged as a result of the formation of precipitates and biological films in the well. Consequently, the amount of gas recovered or produced from a well may decrease over time. 
   Another problem with the gas well piping is that it is often installed as the landfill lifts are created. Consequently, the top section of the pipe is not perforated because it must be extended over time as additional lifts are added to the landfill. 
   The current solution to these problems is to install a new gas well next to the existing, obsolete well. However, this is time consuming and expensive. Thus, a simple and inexpensive solution that allows retrofitting of an existing well to maintain the amount of gas produced over time is needed. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates to perforating devices useful for perforating an in situ landfill gas well to increase gas recovery. The perforating device consists of at least one perforator with at least one cutting edge. The perforating device also has a cable for lowering and raising the perforating device in the gas well. The perforating device further features cutting edges in the form of a drilling system and/or rotator to allow perforation of the gas well. Finally, the diameter of the perforating device is less than the inner diameter of the gas well. 
   The present invention also relates to methods for in-situ perforation of a landfill gas well to increase gas recovery. A perforating device is lowered into the gas well to a predetermined depth from the landfill surface until the perforating device is adjacent to a portion of the gas well to be perforated. Next, the perforating device is activated at the predetermined depth to perforate a portion of the gas well. After perforating the gas well at one or more predetermined depths, the perforating device is removed from the gas well. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of an embodiment of perforating devices of this invention; and 
       FIG. 2  is a cross-sectional representation of the embodiment illustrated in  FIG. 1 ; and 
       FIG. 3  is an alternative embodiment which illustrates the gas monitoring aspect of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention provides a perforating device and method for perforating a landfill gas well in situ. 
     FIGS. 1 and 2  illustrate an embodiment of a perforating device  10  used to perforate a landfill gas well  12  in situ. Typically, gas well  12  is constructed of PVC, high-density polyethylene (HDPE) or other similar materials. Gas well  12  typically has an inner wall  28  and an outer wall  38 . Gas well  12  has an effective inner diameter  32  ranging from about six to about twelve inches. Typically, effective inner diameter  32  is about eight inches. Perforating device  10  consists of at least one perforator  14  that has at least one cutting edge  16 . Perforator  14  is made from a material that is able to perforate landfill gas wells, such as steel, and typically weighs one to three pounds or more. Perforating device  10  has a diameter  18  that is less than effective inner diameter  32  of gas well  12 . For instance, diameter  18  of perforating device  10  is four inches, compared to an effective inner diameter  32  of eight inches. Preferably, diameter  18  of perforating device  10  ranges from four to six inches. Perforating device  10  also includes a cable  20 , which may be used to lower and raise perforating device  10  into and out of gas well  12 . Cable  20  has a first end  44  and a second end  42 . Cable  20  is attached at the first end  44  to a top surface  22  of perforator  14  by a connector. The second end  42  is kept at a landfill surface  30 . Cable  20 , in addition, may have a measuring mechanism for determining the distance from landfill surface  30  to perforating device  10  in gas well  12 . For example, the cable may be a hydraulic hose or a stainless steel cable that is graduated to measure length. However, a graduated stainless steel cable is preferable. Perforating device  10  may also include a power source, such as a hydraulically or pneumatically powered motor. Additionally, perforating device  10  has a rotator  24 . Rotator  24  may be any device capable of driving a cutting edge, but will typically consist of a gear and rotating shaft. Rotator  24  activates perforator  14  thus allowing cutting edges  16  to perforate a portion  26  of gas well  12 . 
   In another embodiment of the invention, perforating device  10  is a drilling system. The drilling system is hydraulically or pneumatically powered, and made from hardened steel or carbide. Preferably, the drilling system is hydraulic because a hydraulic drilling system can reduce the risks associated with the explosive nature of landfill gas. Furthermore, the drilling system may have adjustable settings. For example, the perforating device  10  may be configured to have four or six perforating drills, with each having a cutting edge  16  to perforate holes into the gas well&#39;s circumference. Adjustable settings allow a user of perforating device  10  to select a desired number of perforations to be perforated or drilled in gas well  12 . The adjustable setting is chosen before perforating device  10  is lowered into gas well  12 . The size of the drill bit used to perforate the well may be adjusted to adjust the size of the perforation in the gas well. Typically, a perforation will be approximately one half inch. However, the perforation size may vary in order to keep the perforation size smaller than the gas well&#39;s granular backfill material, thus preventing the backfill material from seeping through. 
   Another aspect of perforating device  10  is that it may be stabilizable. For example, perforating device  10  may have one or more retractable arms that extend outward to inner wall  28  of gas well  12 . This enhances the stability of perforating device  10  while it perforates gas well  12  by maintaining the position of perforating device  10  in gas well  12 . Moreover, stabilizing the perforating device  10  provides for easier removal of perforating device  10  from gas well  12 . 
   The present invention also provides a method of in-situ perforation of a landfill gas well. The method begins by lowering the perforating device  10  into gas well  12 . Gas well  12  generally should have a straight vertical orientation. However, often gas well  12  will not be vertical due to landfill forces that cause some misalignment. This misalignment typically results from extending gas wells to accommodate additional landfill lifts. This invention addresses this problem providing perforating device  10  with a short body, and a smaller diameter than the inner diameter  32  of the gas well. As a result, it is possible to lower the perforating device  10  to gas well depths beyond the misaligned areas. Perforating device  10  is lowered into gas well  12  to a predetermined depth  34  from landfill surface  30 . At predetermined depth  34 , perforating device  10  is adjacent to portion  26  of gas well  12 . Perforating device lowering is done either manually or automatically. Manual lowering is accomplished by manually lowering perforating device  10  into gas well  12  with cable  20 . Automatic lowering may be done with a power source, hydraulic or pneumatic, which may be used to power the lowering of perforating device  10  into gas well  12 . 
   Once perforating device  10  is located at a predetermined depth  34  from landfill surface  30 , perforating device  10  is activated. Perforating device  10  perforates portion  26  at predetermined depth  34 , which is adjacent to perforating device  10 . Perforation is accomplished by drilling or cutting system. Perforating device  10  may rotate vertically within gas well  12 , thus perforating gas well  12  in an up and down manner. 
   The step of positioning perforating device  10  at predetermined depth  34  from landfill surface  30  and then perforating gas well  12  may be done once or it may be repeated a plurality of times at various predetermined depths from landfill surface  30 . Perforations will be made each time the perforating device  10  is activated at the predetermined depth  34 . Typically, the perforations are done in six-inch increments throughout the gas well  12 . The ability to recover landfill gas is improved by maximizing the number of perforations in the gas well  12 . Perforating the gas well too close to the landfill surface  30  can contribute to air infiltration. Thus, perforations should be made approximately twenty feet from the landfill surface  30 . The steps of the present invention will be repeated until all desired portions of gas well  12  are perforated. After the gas well  12  is sufficiently perforated the perforating device is pulled from the gas well  12 , and back to the landfill surface  30 . The step of pulling perforating device  10  out of gas well  12  may be accomplished manually or automatically. In another embodiment, perforating device  10  is attached to a winch that powers pulling perforating device  10  out of gas well  12  and back to landfill surface  30 . 
     FIG. 3  illustrates another embodiment of the method of this invention that includes a step for maintaining the amount of methane gas in gas well  12  outside methane gas&#39;s explosivity range. Methane gas has an explosivity range of 5 to 15% by volume. This is a necessary safety precaution, which ensures the methane gas located in gas well  12  does not ignite to cause an explosion during the perforation steps. If while monitoring the amount of methane gas in the gas well it is found to be within methane gas&#39;s explosivity range, no perforating of the gas well should be done. For example, to get outside the explosivity range may involve introducing a sufficient amount of inert gas into gas well  12 . For instance, introducing nitrogen into gas well  12 . Monitoring the amount of methane gas inside gas well  12  may be accomplished in several ways. For example, it can be accomplished by monitoring the percent of methane gas in gas well  12  with a gas sensing device  36  that is connected to a sample tube  40 . Gas well sensing device  36  will be placed at landfill surface  30 . Sample tube  40  extends from landfill surface  30  down into gas well  12 , where sample tube  40  is mounted next to perforating device  10 . Attaching sample tube  40  to the cable  20  allows for easier monitoring of gas in the vicinity of the predetermined depth  34  in the gas well  12 . Thus, allowing monitoring of methane gas in the vicinity of predetermined depth  34 . Alternatively, gas sensing device  36  may be attached to perforating device  10 . Gas well sensing device  36  may be a gas well sensor for monitoring methane gas concentration that is known by those of ordinary skill in the art. One common gas well sensor is the GEM™500, which is manufactured by CES-Landtec. The GEM™500 is used to analyze gas content and determine flow from LFG collection wellheads. 
   Another embodiment of the method of this invention involves applying steps of the invention&#39;s method to a gas well that as become filled with water at the predetermined depth. In other words, the gas well contains water prior to lowering the perforating device into the gas well. The water in the gas well can prevent the extraction of gases from the gas well. The invention&#39;s method of perforating the gas well with a perforating device may be accomplished in a water filled portion of gas well  12  to create slots or perforations, for draining the water from the gas well. As a result, it becomes possible to recover gas from previously water filled gas wells. 
   The invention is now described in such full, clear, concise and exact terms as to enable any person skilled in the art to which it pertains, to make the same. It is to be understood that the foregoing describes preferred embodiments of the present invention and that modifications may be made therein without departing from the spirit or scope of the invention as set forth in the claims.