The forces of supply and demand are having a positive impact on gas prices in the 1990’s, and the producer cannot afford to waste production through a faulty meter, or a meter that is not appropriate for conditions at each well site. There is no question that Production gas measurement is one of the most difficult and costly areas of gas measurement for the independent producer as well as the Gas utility. Through proper design and meter selection, accuracy and reliability can become the rule for increased profit. Your biggest thief is inaccurate measurement, which has a hand In your pocket 24 hours a day.

GAS MEASUREMENT

Traditionally, natural gas is metered through one of two different physical principles: either by positive displacement or by inference. Meters which use the positive displacement principle are of the diaphragm and rotary types, while gas volume is "inferred" through either orifice meters or turbine meters. The meter is one of the most valuable pieces of equipment at the well site. Each well producing gas should have its own meter, if for no other reason than good financial practices. Profit can only be made on the quantity of gas that is accurately measured. Gas that has leaked to the atmosphere is gone forever. It cannot be retrieved and sold. Gas that has passed through an inaccurate meter is not lost to nature but can only be counted as lost profit. In some areas where the gas is classified as tight sand gas, the selling price is in excess of $2.50 per thousand cubic feet. Do you realize, at $2.50 a thousand a three cubic foot an hour leak, a few bubbles in a mud puddle, is a lost profit of approximately $70.00 a year. This is just one three cubic foot leak, how many of these leaks exist at a well site or in the gathering lines? A three cubic foot an hour leak can hardly be heard. Just think how much gas is leaking through the ones you can hear. A gas leak has a hand in your "profit pocket" every day. Your biggest thief is inaccurate measurement. A 1% measurement error of a meter registering 10000M cubic feet per month can cost you $3000.00 a year in lost profits. This is the cost of a complete measuring system that will even correct for gas temperature variations. Your gas measurement system costs must be compared to your profit increases and reduced maintenance costs. With today’s tighter economic conditions in the production industry, gas sales may be the factor keeping some producers profitable, and the gas meter is his cash register. Accurate measurement not only provides economic pay backs, but is also an invaluable aide in trouble shooting measurement disputes and production problems. Several types of metering devices are available and each of these will be discussed in this paper. Before proceeding with a discussion of the different types of metering devices we need to look at the protection required for a meter being used in a production application. There is no gas meter made that was designed to measure crude oil, frac sand, pipe scale or brine. All of these may be present on a producing well, and the gas meter needs protected from these elements. All meters should have a positive shut off drip. Be it a diaphragm, rotary or orifice meter, the accuracy of each will be adversely affected, when flooded with crude oil or brine. Separators are not foolproof devices and occasionally have been known to fail, sending oil into the gas line. A positive shut off drip will prevent a costly repair by interrupting the flow before it has a chance to enter the meter. There are a number of these devices commercially available and one should be on the inlet side of the meter, stopping crude oil or brine contamination. All meters should have an inlet strainer. Pipe scale or frac sand will damage the edge of an orifice plate. lock up a rotary meter, wear the edge of a turbine blade or wear out the bushings and valves of a diaphragm meter. Strainers can be conical shaped, sandwiched between two flanges. Or as simple as a fine mesh screen attached to a pipe plug, and inserted into a tee in the gas line. What ever type of strainer is used, care should be taken to ensure that the screen mesh or hole size is small enough to block particles that may be of sufficient size to cause meter damage. All meters should have a pressure relief valve. The pressure rating of any type of meter should never be exceeded. Some times the attitude is, if a meter is rated at 200 PSI then the factory must test to 400 PSI. This can be a very dangerous assumption. If the producer exceeds the manufacturer’s pressure rating he has assumed a great deal of personal liability. When a system is over pressured the meter can change into a fragmented bomb. After determining our operating pressure we must estimate our maximum flow rate per hour at line conditions. This information will determine the size of the meter. It is also helpful to know the minimum flow rate at line conditions. This information will determine the type of meter to be used. By knowing the maximum operating pressure, the maximum and minimum flow rates at line conditions, we can now select a meter that will accurately measure the gas. All meters should have a flow restriction. Any type of metering device will be damaged when its flow rate capabilities have been exceeded. These surges in flow rate may be caused by the way the well is produced, broken lines downstream of the meter, or improper start up of the meter. A flow restriction is relatively inexpensive and can be a simple as a drilled plate, welded inside of a pipe nipple. The restricting hole should be sized to pass the maximum flow rate of the meter being protected and should always be located on the downstream side of the meter. Most meters should have a heater. All metering devices except for the diaphragm meter require a heater when operating in winter conditions. On an orifice meter the orifice plate flanges and taps should be heated to prevent the plate and differential lines from freezing off. Rotary and turbine meters can be completely enclosed in a heater assembly to prevent condensation from freezing, inside the meter body. The heater used should be of the catalytic type, which is safe for use around natural gas, and should be left on all year. Commercially available prefabricated production meter sets are available to handle all of the above requirements. Every piece of equipment used on the sets is designed to eliminate a problem which can occur in the measurement of production gas.

The most important factor for good measurement is the selection of the proper gas meter. Like any piece of equipment we must be sure it is right for the job. Meters cover flow rates from a few cubic feet an hour to over ten million cubic feet per hour. Most meters are designed for a limited pressure range. When selecting the proper meter a few guide lines must be followed. Keep in mind that meters are priced by maximum capacity and pressure rating. The higher the capacity or the pressure the higher the price. The question we must answer is which is the most economical meter system to use to meet our maximum conditions for flow and pressure. Yet still maintain accuracy at the minimum conditions. The least expensive is not always the most economical. The meter we select must do a god job for a long period of time. Good measurement is the backbone of the industry. There is no such thing as a maintenance free meter. Some require more attention than others. However, they should all be checked at least twice a year, depending on the volume of gas produced. The only way to know if the meter is accurate is to check it against a known standard. The use of the Deduct Meter has no place in the modern day measuring system. The practice of using a deduct meter was acceptable when the price of natural gas did not warrant a substantial financial outlay for an accurate measuring system. The deduct system is where there is one or more producing wells, more, than there are meters. The total production is measured and compared to the buyers purchase meter. The total reading from the metered wells is subtracted from the purchase meter reading. The difference in the reading is the amount, the unmetered well is said to have produced. The Diaphragm meter is the oldest positive displacement device used by the industry. The diaphragm meter has remained basically unchanged in operation. Only the materials of construction have been updated to keep pace with technology. The main advantage of this type of meter would be its excellent range ability. It can provide accurate measurement down to 1/200th of its rated maximum capacity. If the producing well has a wide varying production rate, the diaphragm meter may be a logical selection. The installation of this type of meter is relatively simple since the inlet or outlet piping configuration has little effect on its accuracy. However, the meter needs to be mounted in a level position with adequate support beneath it, to alleviate pipe strain. The meter should also be slightly elevated from ground level to allow easy access to the drain plugs. The Rotary Meter is lightweight accurate metering device that has been used successfully in many production applications. With years of proven reliability and accuracy. A rotary meter have a published rangeability of 10:1, meaning that if it is rated at a maximum flow rate of 3,000 CFH, it can accurately record flow rates down to 300 CFH. Field experience has shown that at elevated pressure this range ability can be increased, although very little is published to verify this relationship. When installing a rotary meter it should always be mounted so as to accommodate a top inlet configuration. This will allow the meter to be basically self-cleaning by allowing contaminates to pass through the meter body, instead of accumulating inside. Inlet straightening is not considered important, however care should be taken to prevent pipe strain, which might distort the meter body, adversely affecting its accuracy. Turbine meters do have applications for production measurement. This type of meter can provide reliability and accuracy over a varying flow rate range. It should be noted that it has been documented that the turbine meters rangeability will greatly increase as the gas pressure increases. Installation requirements for a turbine meter are more exacting than for a diaphragm or rotary meter. This type of meter is basically a velocity sensing device. Protrusions into the gas stream such as temperature probes or abrupt piping changes can adversely affect this meters accuracy. Rule of thumb on turbine installation is 10 pipe diameters of straight pipe on the inlet side and 5 pipe diameters on the outlet side. With better nose cones being installed in the meter body, this requirement may be waived. This does not mean you can use a 2" pipe and a reducing flange to install a 4" meter. This will cause jetting of the gas into the turbine meter, which will indicate a higher velocity. The Orifice Meter is the oldest type of metering device available, and is widely used in the production industry. Since it is not currently possible to field verify the accuracy of an orifice meter, its installation and maintenance requirements become critical. The installation and maintenance requirements for an orifice meter are covered in detail in AGA Report #3 and anyone utilizing orifice measurement should have a copy. The greatest drawback to orifice measurement is the lack of field verifiable accuracy, which can causes endless problems in custody transfer disputes. When discrepancy between the volume of gas a producer claims to have delivered, and the amount the purchaser claims to have bought arises, it can only be settled if the accuracy of the measurement equipment can be verified. When trying to settle a dispute it is evident that the equipment with the verifiable accuracy will always come out ahead.

CONCLUSION

• Quality measurement is the rule in today’s tighter economic environment.

• Gas is a precious natural resource that must not be lost or wasted.

• Ask questions and get good measurement for Profit.