GB1579628A - Method of establishing the maximum production rate for withdrawing landfill gas - Google Patents

Description

(54) METHOD OF ESTABLISHING THE MAXIMUM PRODUCTION RATE FOR WITHDRAWING LANDFILL GAS (71) We, RESERVE SYNTHETIC FUELS, INC., of 1602 Monrovia Street, Newport Beach, California 92663, United States of America, a corporation organised and existing under the laws of the State of Delaware, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: Decomposition of the refuse within a sanitary landfill produces landfill gas which contains methane. In some landfills the methane concentration is sufficient to warrant recovery of the landfill gas so that the methane can be utilized.In some instances, the methane is separated from the other constitutents of the landfill gas by an adsorption process, and in other instances, the methane is used without removal of the impurity.

To recover the landfill gas, a well is sunk in the landfill. The chemical reaction witin the landfill creates a pressure greater than atmospheric so that the landfill gas migrates into the well. However, to augment this natural flow, a pump can be used to reduce the pressure in the well below ambient.

From the standpoint of maximizing production of landfill gas, it would appear desirable to withdraw the landfill gas from the well at the maximum rate at which the pump is capable of operating. However, if this is done, air may be drawn into the landfill through the surface of the landfill.

Air entry into the landfill is acceptable if the object of the well is simply to remove the methane to avoid the hazard presented by its combustible properties. However, if the purpose of the well is to recover the methane for use, air entry into the landfill is totally intolerable because the oxygen poisons the micro-organisms which are essential to the production of methane by the landfill. Accordingly, air entry must be avoided.

One attempt to avoid air entry involves withdrawing the landfill gas and making periodic analyses of its composition. If the gas has a predetermined oxygen content, the withdrawal rate is reduced in an effort to eliminate the oxygen in subsequently taken gas samples. The primary problem with this method is that it indicates air entry only after air has entered and flowed through portions of the landfill to the well. In other words, no indication of a problem is provided until the oxygen poisoning is well underway. There is also a lag between taking corrective action following oxygen detection and discontinuance of air entry into the landfill. For these reasons, the compositional analysis method is inadequate to avoid the oxygen poisoning problem.

Thus, a primary problem in recovering the methane from landfills is the selection of a withdrawal rate which is commercially feasible and which avoids oxygen poisoning.

A related problem is determining the spacing between the adjacent wells in the landfill. Proper well spacing is important to assure that all zones of the landfill are subjected to the influence of a well without causing air entry.

The present invention enables the maximum allowable withdrawal rate from a well without causing air entry into the landfill to be determined. This determination can be made in a testing phase which occurs before the well is used for production. In addition, proper well spacing can be determined in advance. Finally, as a safety measure, the present invention provides an early warning system for the detection of air entry into the landfill during production. Accordingly, if air entry should occur during production, an indication thereof is provided well before the oxygen flows through the landfill to the well.

When the landfill is not influenced by man-made devices such as pumps, the landfill gas is at a static pressure. The static pressure is normally slightly greater than ambient pressure. Thus, under static conditions the pressure differential betwen the landfill and the atmosphere tends to cause the landfill gas to escape to the atmosphere.

The present invention provides a testing program in which the maximum withdrawal rate is a function of the withdrawal rate when the pressure at a selected region in the landfill approximates ambient pressure. So long as the pressure with the landfill at the selected region does not drop below atmospheric pressure, there is no pressure differential which would cause air entry into the landfill.

The testing program of this invention provides a method of establishing a maximum production rate at which landfill gas can be removed from a landfill without drawing air from the atmosphere into the landfill in such quantities as to destroy the methane-making ability of the landfill, said method comprising:: withdrawing landfill gas from a well in the land fill at a first withdrawal rate during a first period; detecting the pressure at a region of the landfill outside the said well during said first period withdrawing landfill gas from the well at a second withdrawal rate during a second period; detecting the pressure in a region of the landfill outside the said well during said second period; establishing a relationship between the respective withdrawal rates and the detected pressures, and determining as the maximum production rate the withdrawal rate which, according to said relationship, would exist when said pressure is at approximately zero gauge pressure. In other words, the maximum withdrawal rate is the withdrawal rate which will produce a pressure at the selected region which is approximately equal to atmospheric pressure.This same maximum withdrawal rate can also be used for other wells in the landfill it is assumed that the landfill is relatively homogeneous. If the withdrawal rate were to exceed the maximum withdrawal rate, the pressure at the selected region would drop below atmospheric pressure and the differential pressure across the surface of the landfill would tend to draw air into the landfill.

To obtain maximum methane production, the withdrawal rate should equal the maximum allowable withdrawal rate. The use of a withdrawal rate less than the maximum allowable rate provides a safety factor, but to the extent that a safety factor is provided, production of the landfill gas is reduced.

The selected region at which the pressure is detected is preferably a region near the surface of the landfill. It is permissible for regions of the landfill spaced substantially from the landfill surface to be at pressures less than atmospheric so long as there is a layer or boundary at the top of the landfill which is at least at atmospheric pressure. In order to permit the lowest possible pressure to be used within the landfill, and hence obtain the maximum withdrawal rate, the selected region is preferably just beneath the cover material which is characteristically applied over the top of the refuse in the landfill. With this arrangement, the upper peripheral region of the refuse material in the landfill provides a pressure barrier which isolates the interior regions of the landfill from the atmosphere.

Horizontally, the selected region should be adjacent or relatively close to the well.

The reason for this is that the zone of the landfill surface immediately adjacent the well is most likely to be at the lowest pressure. If the selected region were spaced horizontally a substantial distance from the well and were maintained at zero gauge pressure, a pressure less than atmospheric may exist just beneath the cover material closely adjacent the well in which event the pressure barrier would not exist over the full surface of the landfill.

The selected region may be the same or a different region for each of the different withdrawal rates. In addition, the selected region may include a plurality of different zones at which pressure readings are taken.

For example, the zones may be at different depths in the landfill. By taking pressure readings at different depths in the landfill, the accuracy of the pressure data taken can be checked.

To further provide analytical data regarding the landfill, it is often desirable to provide a second well in the landfill spaced from the first well. The probes are then installed in a desired pattern and at the desired depths intermediate the two wells.

The steps described above for determining the maximum allowable withdrawal rate may be performed to determine such rate for the second well.

A second phase of the testing operation is determining the well spacing for a given production rate. To accomplish this, the pressure in the landfill at horizontally spaced regions is determined during the first and second withdrawal periods. The regions at which pressure readings are taken for the purposes of determining well spacing are preferably immediately below the cover material of the landfill. The undetected pressures during the first period are then utilized to establish the location closest to the well at which static pressure exists during the first period. The pressures detected during the second period are similarly utilized to determine the location closest to the well at which static pressure exists during the second period.These locations and withdrawal rates are then utilized to establish a relationship between the shortest distance from the well at which static pressure exists and withdrawal rate. For accuracy, it is preferred to have at least three of such points. The resulting curve provides an indication of the zone of influence of the well for a given production and rate. The zone of influence and well spacing can be determined for the maximum allowable withdrawal rate.

If the two wells have mutually exclusive zones of influence or if the two wells are to be operated nonsimultaneously, then each well can be operated at up to its maximum allowable withdrawal rate. However, if the wells are to be operated simultaneously and have overlapping zones of influence, an appropriate adjustment can be made. For example, an empirical adjustment can be made which reduces the maximum allowable withdrawal rate for a given well spacing.

After testing, the landfill is used for production. As a safety measure, it may be desirable to monitor the pressure at a selected region within the landfill during production. Such pressure monitoring can be continuous or intermittent. If the pressure at the selected region drops below ambient, the withdrawal rate can be reduced, i.e., the pressure in the well can be increased to allow the pressure at the selected region to build up to re-establish the pressure boundary. If this system is used; the magnitude of pressure in the well can be controlled in accprdance with the pressure at the selected region. The selected region for pressure measurement is preferably the same as, or similar to, the region selected for determining the maximum allowable withdrawal rate.

In the accompanying drawings: Figure I is a top plan view of a sanitary landfill of the type from which landfill gas including methane can be extracted.

Figure 2 is a fragmentary sectional view taken generally along line 2-2 of Figure 1.

Figure 3 is a sectional view illustrating a preferred form of well.

figure 4 is a top plan view of a preferred probe layout.

Figure 5 is a sectional view taken generally along line 5-5 of Figure 4 and illustrating a preferred probe arrangement.

Figure 6 is a plot of average probe pressure versus withdrawal rate.

Figure 7 is a plot of average cover probe pressure versus distance from the well for various withdrawal rates.

Figure 8 is a plot of well spacing versus production rate for both single well and dual well production.

Figures 1 and 2 show a sanitary landfill 11 of the type in which anaerobic decomposition produces a landfill gas which includes methane. The landfill 11 has been formed in the usual manner by depositing refuse 13 in a cavity 15 in the earth 17. The landfill 11 also includes a layer of cover material 19, such as dirt, deposited on top of the upper surface 21 of the refuse 13. Although the refuse 13 appears homogeneous in Figure 2, it should be understood that it is ordinarily not homogeneous, and it may contain several intermediate cover layers of dirt intermediate the upper surface 21 and the bottom 23 of the landfill 11. The testing program of this invention is applicable to the cavity type landfill 11 as well as other types of landfills such as the anticlinal landfill.

Assuming that it has been established that the landfill 11 contains a sufficient quantity of methane to make its withdrawal commercially feasible, then a testing program is carried out on the landfill in order to determine various factors such as the maximum allowable withdrawal rate and the spacing between production wells. The first step in the testing program is to locate a test area 25. The first step in the testing program is to locate a test area 25. The test area 25 is preferably in the form of a circle and is spaced inwardly from the periphery 27 of the landfill 11. For example, the test area 25 may have a 500-foot diameter and be located a minimum of 250 feet from the periphery 27. If the landfill 11 is too small to provide for these dimensions, the diameter of the test area 25 and the clearance between the test area of the periphery 27 can be proportionately reduced.For example the diameter of the test area 25 might be reduced to 400 feet and the clearance would then be corrrespondingly reduced to 200 feet. Of course, other dimensions may be utilized, and the dimensions given above are merely illustrative of dimensions which have been found satisfactory.

Production wells 29 and 31 are then drilled in the landfill 11 at diametrically opposed locations on the perimeter on the test area 25. Thus, the wells 29 and 31 are spaced apart 500 feet in the specific embodiment described. The wells 29 and 31 may be identical, and it is assumed for purposes of describing this particular embodiment that the wells are identical.

Figure 3 shows by way of example one construction for each of the wells 29 and 31.

Because the wells 29 and 31 are identical, only the well 29 is shown in Figure 3.

The well 29 includes a bore 33 which may have a diameter of 24 inches to 36 inches.

The bore 33 preferably extends almost to the bottom 23 of the landfill. A suitable number of pipe sections 35 interconnected by couplings 37 are provided within the bore 33. The lowermost of the pipe sections 35 is coupled to a perforated pipe section 39 by a slip joint 41 which allows relative axial movement between the perforated pipe section 39 and the lowermost pipe section 35. The slip joint 41 may be of conventional design. The perforated pipe section 39 has a series of perforations 43, and it is centered in the bore 33 by centralizers 45 which are mounted on the perforated pipe section 39 in any suitable manner such as by clamps 47.

The bottom of the perforated pipe section 39 is closed by a cap 49 which rests on a layer of gravel 51 at the bottom of the bore 33. The pipe sections 35 and 39 may be, for example, constructed of a suitable plastic material such as polyvinyl chloride.

The bore 33 is packed with gravel 53 for most of the length of the perforated pipe section 39. A seal 55 is provided adjacent the upper end of the perforated pipe section 39 to seal the perforations 43 from the upper regions of the bore 33. In the embodiment illustrated, the seal 55 includes a layer of dirt 57 and a layer of concrete 59. Virgin soil backfill 61 is provided in the bore 33 above the seal 55. The uppermost of the pipe sections 35 is cou coupled to a pump 63 through a butterfly valve 65 and a pipe 67. A suitable flow indicating device 68, which may include a pitot tube is coupled to the pipe 67 between the valve 65 and the ump 63.

Four rows 69, 71, 73, and 75 of probes (Figure 4) are installed between the wells 29 and 31. The number and depth of the probes and the probe pattern can all be widely varied depending upon the results desired.

In the embodiment illustrated, the rows 69, 71, 73, and 75 of probes are equally spaced and may be spaced, for example, approximately 10 feet apart. Probes 69a, 71a, 73a, and 75a of the rows 69, 71, 73, and 75, respectively, are closest to the well 29 and constitute a series arranged in a transversely extending line. Similarly, the remaining probes of the four rows of probes are arranged in transversely extending lines. In the embodiment illustrated the series of probes 69a, 71a, 73a, 75a are spaced 10 feet from a reference line 77 which extends thrugh the well 29 tangent to the perimeter of the test area 25. By way of example, the next four series of probes may be spaced from the reference line 77 distances of 25 feet, 50 feet, 100 feet, and 250 feet, respectively. These distances are not critical, but merely illustrative of one suitable probe layout.

Similarly, probes 69b, 71b, 73b and 75b are closest to the well 31. The distances between a reference line 79 drawn tangent to the perimeter of the test area 25 at the well 31 and the five series of probes closest to the well 31 may be identical to that described to that described above with the well 29.

The probes are preferably installed at different depths in the landfill. As shown in Figure 5, the probes 69a-75a are installed at progressively increasing depths. Although various arrangements are possible, the cross section shown in Figure 5 may be assumed to be typical of the embodiment illustrated, i.e. all probes of the row 69 are at the same depth, all probes of the row 71 are at the same depth, etc. Except for length, each of the probes may be identical, and accordingly only the probe 69a is described in detail, and corresponding reference numerals are utilized to designate corresponding parts.

The probe 69a is placed in a bore 81 which extends through the cover material 19. The bore 81 has a bottom 83 which, for example, may lie two feet below the cover material 19. The probe 69a includes a pipe 85 having a cap 87 on its upper end. The pipe 85 extends from above the cover material 19 to the bottom 83. The region of the pipe 85 below the cover material 19 contains a large number of perforations 89. Preferably the portion of the pipe 85 which has the perforations 89 is relatively short, e.g. two feet long, in order to isolate the vertical location at which pressure readings are to be taken. A porous material such as gravel 91 fills the region of the bore 81 around the pipe 85 below the cover material 19. The top of the bore 81 is sealed off by a plate 93 which may be in the form of a concrete grout seal.The pipe 85 may be small diameter pipe constructed of a suitable plastic material such as polyvinyl chloride.

The probe 69a is a cover probe because it communicates with the region of the landfill 11 immediately beneath the cover material 19. A pressure indicator 94 which may be a manometer type device communicates with the interior of the pipe 85 and provides an indication of the gauge pressure of the landfill gas just beneath the cover material 19 contiguous the probe 69a. Gauge pressure is the pressure relative to existing barometric pressure and not relative to standard atmospheric pressure.

The probe 71a is longer than the probe 69a. For example, the probe 71a may extend to a depth equal to 20% of the depth of the well 29. The probes 73a and 75a may extend to depths equal to 50% and 75%, respectively, of the depth of the well 29. Each of the probes is installed in its own bore 81 and the length of the pipe 85 which has the perforations 89 may be identical for each of the probes.

The wells 29 and 31 and the probes may be installed in any desired sequence. With the wells and probes installed, static pressures are detected by the indicators 94 in all the probes and in the wells 29 and 31. These pressures can be detected continuously or intermittently for a sufficient period to accurately determine the static pressure for the wells 29 and 31 and for each of the probes. For example, pressure readings may be taken at the indicators 94 at four hour intervals for 24 hours. In addition to detecting the pressure, the temperature within the probes and the wells may also be taken as well as ambient pressure and temperature readings.

Next, three withdrawal rates are empirically selected. For example, the withdrawal rates may be 100 cubic feet per minute (cfm), 300 cfm, and 500 cmf. The pump 63 for the well 29 is started and operated at a speed to withdraw landfill gas from the well 29 at 100 cfm. The pump for the well 31 is shut down during the entire test of the well 29. Pumping of the well 29 progressively reduces the pressure in the probes. Ultimately, the pressures in the probes stablize, i.e. the pressure does not fluctuate significantly as the withdrawal of landfill gas continues. Landfill gas is withdrawn at the 100 cfm rate for an interval after pressure stabilization in the probes occurs. The pressures in all of the probes as reflected by the indicators 94 are monitored either continuously or intermittently both before and after pressure stabilization occurs.The speed of the pump is manually adjusted as necessary to maintain the desired flow rate.

Of particular importance is the pressure in the cover probe 69a after pressure stabilization occurs. After pressure stabilization occurs, an average stable pressure is determined for the cover probe 69a. This may be established, for example, by averaging the pressure readings taken after pressure stabilization occurs.

The pressure at the cover probe 69a is of particular importance because it is the location at which air entry would most likely first occur. The reason is that it is the closest of the cover probes to the perforations 43 of the well 29. For this reason, during operation of the well 29 and the pump 63, the cover probe 69a is more likely to be at a lower pressure than any of the other cover probes 69. because the other cover probes are displaced a greater distance from the perforations 43. However, if for any reason any of the other cover probes 69 are at a lower pressure than the cover probe 69a, then such other cover probe would be selected as the critical cover probe, and its average pressure would be determined and utilized as described herein below.However, if a cover probe has a lower pressure than the cover probe 69a, it may be the result of the landfill providing a relatively unrestricted channel to such probe. In some instances this channel can be destroyed to make the cover probe 69a have the lowest pressure of the cover probes 69.

After an average stable pressure has been determined for the critical cover probe (assumed herein to be the cover probe 69a) for the 100 cfm withdrawal rate, the pump 63 is shut down and the landfill 11 is allowed to return to the static pressure conditions determined before the pump 63 was first used. This may require, for example, a 48-hour period.

After the landfill 11 returns to static pressure conditions, the pump is operated at the 300 cfm withdrawal rate, and the procedure described above is repeated. Thereafter the landfill 11 is again permitted to return to static pressure conditions, and then the pump is operated at the 500 cfm withdrawal rate, and the procedure discussed above is repeated.

Utilizing the data obtained, a plot as shown in Figure 6 can be made. In Figure 6, the points A, B, and C represent the average stable pressure in inches of water at the cover probe 69a for the withdrawal rates of 100 cfm, 300 cfm, and 500 cfm, respectively. The points A, B, and C serve to establish a relationship between pressure at the cover probe 69a and withdrawal rate. In Figure 6, this relationship is represented by a line 95. This relationship is then utilized to establish the withdrawal rate at which pressure at the probe 69a would be zero gauge pressure, i.e. equal to atmospheric pressure.

In the example shown in Figure 6, the pressure at the probe 69a would be zero gauge at a withdrawal rate of approximately 460 cfm.

460 cfm for this example then becomes the maximum allowable withdrawal rate because higher withdrawal rates create a negative pressure at the cover probe 69a. This negative pressure, i.e. pressure less than atmospheric, creates a pressure differential tending to draw air into the landfill 11 whereas no such pressure differential exists at zero gauge pressure.

It should be noted that whether or not a pressure differential exists may be the result of changes in barometric pressure. However, because the landfill responds relatively slowly to small pressure differentials and because the selection of the maximum allowable withdrawal rate is based upon average pressures, it has been found that the selection of a maximum allowable withdrawal rate in this manner is most satisfactory.

In the specific embodiment described, the relationship between withdrawal rate and pressure at the probe 69a is expressed in the form of the line 95 on a graph. However, it should be understood that this relationship can be reasonably approximated without plotting it. Moreover, the withdrawal rate at which pressure at the probe 69a is approximately zero gauge could be established through trial and error methods or through a combination of trial and error and interpolation of trial and error results.

The average stable pressure at any of the other probes may also be plotted on the graph shown in Figure 6, if desired. For example, it may be useful to plot the average stable pressures in probe 71a in Figure 6. This may be useful if one of the points A, B or C in Figure 6 is erratic. The deeper probes 73 and 75 are useful in determining zone of influence of the well 29 for different withdrawal rates.

To obtain an indication of the zone of influence to initiate a determination as to well spacing, the plot of Figure 7 can be made. Figure 7 shows average cover probe pressure, i.e. at the probes 69 versus distance from the well for the three selected withdrawal rates. In addition, Figure 7 shows the static pressure at the cover probes 69 at various distances from the well 29. The three cfm curves intersect the static pressure line at points D, E and F, respectively.

Figure 8 is a plot of well spacing versus withdrawal rate. The points D, E and F from Figure 7 are plotted in Figure 8 as shown to establish a relationship in the form of a line 97 between well spacing and withdrawal rate.

If the wells 29 and 31 are to be operated simultaneously and if their zones of influence overlap, then the maximum allowable withdrawal rate for a given well spacing should be reduced to compcnsate for the effects of interaction between the adjacent wells. This may be accomplished by utilizing the relationship shown by the line 99 in Figure 8. Specifically, the withdrawal rate for simultaneous dual well operation has been reduced 25%, and this is represented by the line 99 in Figure 8. The 256S, reduction factor is illustrative of a reduction which is believed to be satisfactory. Utilising the line 99 for the maximum allowable rate of 460 cfm, the wells should be spaced apart at least approximately 315 feet.If desired, the test procedure described above for the well 29 could be repeated for the well 31 while the pump 63 for the well 29 is shut down.

This would result in the plots of Figures 6-8 being made for the well 31. In this event, the more conservative figures for the two wells, for well spacing and maximum allowable withdrawal rate could be used for the wells 29 and 31 and for any other wells drillcd in the landfill 11.

After the testing program is completed, the wells 29 and 31 are used for production of landfill gas. In addition, other wells may be drilled within the landfill 11. It is reasonable to assume that the results of the test, i.e. maximum allowable withdrawal rates and well spacing for the wells 29 and 31 are applicable to other wells in the landfill 11.

Wells operated at production rates estblished in accordance with this invention should have no problem with air entry into the landfill 11 through the cover material 19.

However, the present invention provides an early warning system for air entry. This is accomplished by monitoring the pressure in the cover probes 69 adjacent one or more operating wells. For example, the pressure in the cover probes 69 and particularly in the cover probe 69a can be monitored during production with the well 29. If the pressure at any of the cover probes 69 drops below zero gauge pressure for a predetermined period of time or by a predetermined amount, the withdrawal rate can be correspondingly reduced to eliminate the pressure differential across the cover material 19 which may tend to force air into the landfill.

Of course, slight negative pressures in the cover probes 69 or negative pressures in the cover probes 69 which exists for only a short period of time would not result in air entry because of the slow response of the air and the landfill 11 to pressure differentials of this kind. The speed of the pump 63 can be controlled manually or automatically in response to pressure conditions in the cover probe 69 which would warrant a change in the withdrawal rate. For example, pump speed should be reduced in response to the pressure in the cover probe 69a dropping a predetermined amount below ambient, such as .25 in H2O or dropping below ambient for a period of 48 hours.

WHAT WE CLAIM IS: 1. A method of establishing a maximum production rate at which landfill gas can be removed from a landfill without drawing air from the atmosphere into the landfill in such quantities as to destroy the methane-making ability of the landfill, said method comprising: withdrawing landfill gas from a well in the landfill at a first withdrawal rate during a first period; detecting the pressure at a region of the landfill outside the said well during said first period; withdrawing landfill gas from the well at a second withdrawal rate during a second period; detecting the pressure in a region of the landfill outside the said well during said second period; establishing a relationship between the respective withdrawal rates and the detected pressures and determining as the maximum production rate the withdrawal rate which, according to said relationship, would exist when said pressure is at approximately zero gauge pressure.

2. A method as defined in claim 1 where

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. can be reasonably approximated without plotting it. Moreover, the withdrawal rate at which pressure at the probe 69a is approximately zero gauge could be established through trial and error methods or through a combination of trial and error and interpolation of trial and error results. The average stable pressure at any of the other probes may also be plotted on the graph shown in Figure 6, if desired. For example, it may be useful to plot the average stable pressures in probe 71a in Figure 6. This may be useful if one of the points A, B or C in Figure 6 is erratic. The deeper probes 73 and 75 are useful in determining zone of influence of the well 29 for different withdrawal rates. To obtain an indication of the zone of influence to initiate a determination as to well spacing, the plot of Figure 7 can be made. Figure 7 shows average cover probe pressure, i.e. at the probes 69 versus distance from the well for the three selected withdrawal rates. In addition, Figure 7 shows the static pressure at the cover probes 69 at various distances from the well 29. The three cfm curves intersect the static pressure line at points D, E and F, respectively. Figure 8 is a plot of well spacing versus withdrawal rate. The points D, E and F from Figure 7 are plotted in Figure 8 as shown to establish a relationship in the form of a line 97 between well spacing and withdrawal rate. If the wells 29 and 31 are to be operated simultaneously and if their zones of influence overlap, then the maximum allowable withdrawal rate for a given well spacing should be reduced to compcnsate for the effects of interaction between the adjacent wells. This may be accomplished by utilizing the relationship shown by the line 99 in Figure 8. Specifically, the withdrawal rate for simultaneous dual well operation has been reduced 25%, and this is represented by the line 99 in Figure 8. The 256S, reduction factor is illustrative of a reduction which is believed to be satisfactory. Utilising the line 99 for the maximum allowable rate of 460 cfm, the wells should be spaced apart at least approximately 315 feet.If desired, the test procedure described above for the well 29 could be repeated for the well 31 while the pump 63 for the well 29 is shut down. This would result in the plots of Figures 6-8 being made for the well 31. In this event, the more conservative figures for the two wells, for well spacing and maximum allowable withdrawal rate could be used for the wells 29 and 31 and for any other wells drillcd in the landfill 11. After the testing program is completed, the wells 29 and 31 are used for production of landfill gas. In addition, other wells may be drilled within the landfill 11. It is reasonable to assume that the results of the test, i.e. maximum allowable withdrawal rates and well spacing for the wells 29 and 31 are applicable to other wells in the landfill 11. Wells operated at production rates estblished in accordance with this invention should have no problem with air entry into the landfill 11 through the cover material 19. However, the present invention provides an early warning system for air entry. This is accomplished by monitoring the pressure in the cover probes 69 adjacent one or more operating wells. For example, the pressure in the cover probes 69 and particularly in the cover probe 69a can be monitored during production with the well 29. If the pressure at any of the cover probes 69 drops below zero gauge pressure for a predetermined period of time or by a predetermined amount, the withdrawal rate can be correspondingly reduced to eliminate the pressure differential across the cover material 19 which may tend to force air into the landfill. Of course, slight negative pressures in the cover probes 69 or negative pressures in the cover probes 69 which exists for only a short period of time would not result in air entry because of the slow response of the air and the landfill 11 to pressure differentials of this kind. The speed of the pump 63 can be controlled manually or automatically in response to pressure conditions in the cover probe 69 which would warrant a change in the withdrawal rate. For example, pump speed should be reduced in response to the pressure in the cover probe 69a dropping a predetermined amount below ambient, such as .25 in H2O or dropping below ambient for a period of 48 hours. WHAT WE CLAIM IS:

1. A method of establishing a maximum production rate at which landfill gas can be removed from a landfill without drawing air from the atmosphere into the landfill in such quantities as to destroy the methane-making ability of the landfill, said method comprising: withdrawing landfill gas from a well in the landfill at a first withdrawal rate during a first period; detecting the pressure at a region of the landfill outside the said well during said first period; withdrawing landfill gas from the well at a second withdrawal rate during a second period; detecting the pressure in a region of the landfill outside the said well during said second period; establishing a relationship between the respective withdrawal rates and the detected pressures and determining as the maximum production rate the withdrawal rate which, according to said relationship, would exist when said pressure is at approximately zero gauge pressure.

2. A method as defined in claim 1 where

the landfill includes a cover material and a cover probe is inserted into the landfill adjacent said well, and the pressure is detected at said cover probe.

3. A method as defined in claim 1 or 2 wherein the pressure detected during said first period is monitored to obtain an average pressure for a portion of said first period and said average pressure is used in establishing said relationship.

4. A method as defined in claim 3 wherein the withdrawal of landfill gas during said first period causes the detected pressure to reduce progressively to a relatively stable value and, said portion of said first period is taken after said relatively stable value has been reached.

5. A method as defined in any preceding claim wherein the pressure at the region where pressure is to be detected during said second period is reduced during said first period and an interval is provided between said first and second periods which is of sufficient duration to allow the pressure at such region to return to substantially static pressure.

6. A method of withdrawing landfill gas from a landfill, incorporating a method as defined in any preceding claim including withdrawing landfill gas from a location in said landfill during a production period at substantially no more than said maximum production rate whereby the likelihood of air entry into the landfill is minimized.

7. A method as defined in claim 6 which includes monitoring the pressure at a region adjacent said location during at least a portion of said production period and controlling the withdrawal rate from said well in accordance with the monitored pressure substantially to prevent drawing air into the landfill during said production period.

8. A method as defined in claim 7 wherein the landfill includes a layer of cover material, said region adjacent said location is adjacent said cover material, and the withdrawal rate from said well is controlled so that the pressure at such region does not drop below atmospheric pressure.

9. A method as defined in claim 6, 7 or 8 wherein a plurality of probes is arranged in a pattern in the landfill; and the pressure in at least a first group of said probes is detected during said first period of withdrawal of gas.

10. A method as defined in claim 9 wherein said first group of said probes is provided at a first depth within the landfill, a second group of probes is provided at a greater depth than said first group and a third group of probes is provided at a greater depth than said second group, said method including measuring a characteristic of the landfill gas in said second and third groups of probes.

11. A method of establishing a maximum production rate for removal of landfill gas from a landfill, substantially as hereinbefore described.

12. A method of withdrawing gas from a landfill, substantially as hereinbefore described.