Patent Application
20070256831
1. Field of the Invention
The present invention relates to controlling and maintaining a desired quality of a gas by controlling the producing pressure of a group of wells in a gathering system.
2. Description of Related Art
Currently, a well tender monitors the individual quality of a gas, adjusts the amount of volume of gas from the individual well, and then has to adjust the pressure. This practice is time consuming and can cause the well tender to have to constantly move back and forth between the two areas or locations.
There are compressors which are capable of self-regulating the amount of pressure being produced, however, these compressors are not being utilized to maintain the quality of gas produced from a group of gases.
Subterraneous formations often contain desirable materials which can be used for many applications. Therefore, there is a need to remove the desirable materials from the subterranean formation. The subterranean formations often extend horizontally over many thousands of feet, and are often very shallow in depth.
Coal is a desirable material contained in subterranean formation. Methane is also contained in the subterranean formation. When the coal its mined methane gas is released. When the coal is mined, air is introduced into the methane gas (or into the mine), to provide adequate ventilation/atmosphere for the coal miners.
Wells are drilled into the subterranean formation in order to obtain the methane gas for commercial production. There can be hundreds of wells spread throughout a mining formation.
The methane gas produced by the wells is used for various endeavors, including sales to commercial energy suppliers. Wells of this nature (mining related in both active and sealed mines), typically have very low producing pressures as the main seam of coal has been removed, thus leaving a void. This void causes a strain on the upper strata which leads to breakage and fracturing from the void extending, in some instances, all the way to the surface of the ground above. This fracturing of the upper strata creates pathways for gases in the upper coal seams (seams above mined seams) to be released. Since the reservoir pressures of the fractured coal seams can reach well into the hundreds of pounds, it is common practice to provide an alternate exit for the gas other than down into the mine. This is done for safety reasons, as Methane gas has an explosive range of 5% to 15% by volume meothane to air concentration. A well is drilled through all the seamus of coal so that the gas can be produced on the surface for sale, as well as to prevent the gas from escaping into the mine via the fractures. Especially in active mining areas, the pressures on both the surface and inside the mine are very close to the same.
Each of these wells is connected to a central pipeline. As stated, the producing pressures of the well are typically low, so measures must be taken to compress the gas to higher pressures for sale. The more a gas is compressed, the less space it takes up and the smaller the pipeline can be used to transport the product. Since often times it is not economical to compress the gas produced from each well at each individual well location, a network or a grouping of several wells may be gathered into one central location and compressed.
After the well is compressed is often directed to a transmission line where it sold to commercial energy suppliers. The energy suppliers often require the gas to have a minimum BTU level.
Very slight changes in a producing pressure of a well that produces gas from both activate and sealed gob area of a coal mine can have a very large effect on the both the volume and the quality of the gas produced. Sealed gob wells, for example, normally do not produce more than 0.5 psig (pounds per square inch of gas) of pressure. An example of the relationship of well quality as a function produced pressure would read as follows: A high quality well can produce at 0.25 psig and then become a low quality well when the pressure is pulled down to 0 psig. A well may produce 100 MCF/Day at 0 psig at 950 BTU (British Thermal Units). That same well under a vacuum and could produce at 150 MCF/Day at −1 psig and 750 BTU. In many gas fields 750 BTU is not considered pipeline quality, thus it would better to produce the lower volume at the higher BTU value.
In a natural gas (CBM) gathering system here multiple producing wells are feeding into a closed system, a change in flow rate/producing pressure at any single well location will have an effect on the producing pressure/volume/BTU of the other wells on the closed system if the blower/compressor is not adjusted accordingly to compensate for the change. This becomes a problem when the quality (BTU) of each given well is a function of the producing pressure (whether vacuum or positive) applied to the wellhead, and when the closed gathering system has a low number of wells with lower volumes. It is very advantageous to keep a precise and constant suction pressure on each well location in the closed system regardless of how much volume is produced from each well bore.
Assume a gathering system having ten wells with each well producing 100 MCF/Day produces a total of 1,000 MCF/Day. If five of the wells produce an energy content of 990 BTU and five of the wells produce 850 the total quality of the system would be 920 BTU. If one of the ten wells' quality deteriorates to only produce 750 BTU, then this will lower the total quality the system. In order to optimize the production of the system, a well tender may opt to cut this well off or close it from production into the gathering system. The compressor station will still run 1,000 MCF/Day, and it will simply make up the volume from the remaining 9 wells in the gathering system. The well tender would then have to run back to the compressor and reduce the throughput of the compressor to compensate for the field change (slut-in of the one of the wells that only produce 750 BTU). If the well tender does not reduce the throughput of the compressor, the added volume that the 9 remaining wells produce will cause them to reduce their quality and could cause the entire system to drop below acceptable BTU levels.
Additionally, a well tender could go to a well after reducing throughput of the compressor, and find that the BTU is 990, and want to add more volume from that particular well. He would first open the throttle valve (to further open the well into the pipeline), then run to the compressor station and increase the throughput of the machine, so that the rest of the wells on the gathering systems producing pressure would remain unchanged.
The present invention prevents field operators from running in circles adjusting wells then running to the feed station to adjust pressure.
This invention provides for a method for maintaining the quality of gas produced from a group of wells by maintaining a constant pressure. A plurality of wells are provided. Each well is capable of producing a different individual quality and individual volume of gas, both of which can change over time and because other factors such is atmospheric pressure changes (the weather/rise and fall of the atmospheric pressure). The individual wells are connected via a gathering line. The gathering line has a total quality of gas and a total volume of gas that results from the sum of the individual qualities and the individual volumes obtained from the plurality of wells. A compressor is connected to the gathering line and pulls/collects a volume of gas through the gathering line. The gas pulled/collected through the gathering line is obtained from the plurality of wells connected to the pipeline. A control device (such as a valve) controls the volume of gas flowing through the gathering line, and thereby controls the pressure of the gas in the gathering line as well as the pressure of gas exiting the compressor. The control device is set at one level to maintain a desired pressure. The individual volume of a particular well is adjusted to maximize the quality of gas produced. The adjustment of the volume produced from the individual well adjusts the total volume of gas available to the gathering line. The control device automatically adjusts in response to the change in the total volume of gas available to the gathering lines so that both the desired pressure and the desired quality of gas are maintained. If the pressure was not maintained, there would either be additional or less volume pulled from the remaining wells. When additional or less volume is pulled from the remaining wells, it can lower the quality of the individual wells.
The object of this invention is to maintain the quality of gas produced from a group of wells.
Maintaining the quality of gas produced from a group of wells—Keeping the total BTU of a group of gases above certain number, or keeping the total BTU of a group of gases within 200 BTUs of a desired number, preferably within 100 BTUs of a desired number, or most preferably, within 50 BTUs of a desired BTU.
Quality of gas—The energy content. For Example, British Thermal Units (“BTU”), is a unit for measuring heat quantity in the customary system of English units of measurement, equal to the amount of heat required to raise the temperature of one pound of water at its maximum density [which occurs at a temperature of 39.1 degrees Fahrenheit (° F.)] by 1° F. The BTU may also be defined for the temperature difference between 59° F. and 60° F. One BTU is approximately equivalent to the following: 251.9 calories; 778.26 foot-pounds; 1055 joules; 107.5 kilogram-meters; 0.0002928 kilowatt-hours. A pound (0.454 kilogram) of good coal when burned should yield 14,000 to 15,000 Btu; a pound of gasoline or other fuel oil, approximately 19,000 BTU.
Maintaining a constant pressure—keeping the pressure within a defined bandwidth.
A plurality of wells—More than 1 well.
Capable of producing a different individual quality and individual volume of gas—Each well could produce a different quality of gas at different flow rates.
Individual volume of gas—The volume of gas produced by a single well.
Individual quality of gas—The energy content value of the gas produced from a single well, for example, 950 BTU
A gathering line—A pipeline that individual wells are connected to. This pipeline carries the total amount of gas produced bu the individual wells.
Connected to the plurality of wells—Attached via a pipeline to the wells so that gas can flow from the wells.
Total quality—The energy content of the gas flowing through the gathering line.
A compressor—A device which changes the pressure of a volume of gas. It could be a blower. A machine used to supply air or other gases at an increased pressure.
Examples of these devices include:
A control device—A device which controls either the flow of gas through a pipeline, or the volume of gas into the pipeline. It could simply be a valve within the gathering line or within the recycle line of a compressor, variable frequency drive for the motor of the compressor, or loader valves for the compressor, or any other device or controller that changes the throughput of the machine. Examples of these devices included:
Setting the control device for a desired pressure—Inputting an optimum pressure value.
Desired pressure—A pressure when applied and remained constant to the gathering line, the quality and volume of gas at each individual well is said to be at an optimum level.
Adjusting the individual volume of a particular well—Allowing either more or less gas produced from the well into the pipeline. An example would be to close the valve completely.
Maximize the total quality of the gas being produced—Trying to achieve the highest BTU value.
Automatically adjusting the control device—The control device changes without a human being taking any action.
Shut in—A state or period in which an oil or gas well has unused capacity, such as when the well is manually closed.
Adjusting the volume of gas automatically by an automated—The volume of a particular gas changed without a human being taking any action. An Example of such a device can be seen in application Ser. No. 11/264,477.
The total volume of gas going through the compressor—The volume of gas taken from the gathering line and compressed into the transmission line.
“Programmable logic controller”—A programmable logic controller, PLC, or programmable controller is a small computer used for automation of real-world processes, such as control of machinery on factory assembly lines. The PLC usually uses a microprocessor. The program is usually created by a skilled technician at an industrial site, rather than a professional computer programmer. The program is stored in battery-backed memory. The main difference from other computers is the special input/output arrangements. These connect the PLC to a process' sensors and actuators, PLCs read limit switches dual-level devices, temperature indicators and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs drive any kind of electric motor, pneumatic or hydraulic cylinders or diaphragms, magnetic relays or solenoids. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a proprietary computer network that plugs into the PLC.
The pressure transmitter 22 is connected to a programmable logic controller (“PLC”) 24. The PLC uses the information from the pressure transmitter 22 to adjust control device 26. The control unit 26 can change the throughput of the compressor to either increase pressure or to reduce pressure. This all happens automatically.
If a field person monitoring well 2 finds that it is only producing 650 BTU, the field person may opt to shut in well 2. The control device 26, the PLC 24, and the pressure transmitter 22 work together to automatically keep the pressure in gathering line 16 constant. By keeping the pressure constant the best BTU/flow) rate is maintained for the group as their specific producing pressure will remain unchanged. Without the control device 26, the PLC 24 and the pressure transmitter 22 the suction pressure in the gathering line would increase (in the case of a vacuum state the vacuum would draw down deep, say −1 psig to −3 psig). The shortage of volume formerly produced from well 2 being closed, would be made up by Wells 4, 6, 8, 10, 12 and 14. The increased suction pressure and produced volume from wells 4, 6, 8, 10, 12, 14 could cause the BTU values being produced from those wells to drop, thus causing a drop in BTU value in the gathering line 16.
The above-described well system can be used in connection with the well system described in application Ser. No. 11/264,477 the disclosure of which is herein incorporated by reference. The shutting in of a single well in a gathering system having an automated system described in application Ser. No. 11/264,477, without the above described method, could cause the entire gather system to shut down.
Various changes could be made in the above construction and method without departing from the scope of the invention as defined in the claims below. It is intended that all matter contained in the above description, as shown in the accompanying drawings, shall be interpreted as illustrative and not as a limitation.
