Custody Transfer: The Value Of Good Measurement And The Search For The Truth

Custody transfer measurement in the oil and gas business has been described many ways. It has been called an accuracy in measurement that both the buyer and seller can agree upon and it has been called the best that can be achieved to meet the contract conditions.

I like to call it, “The Search for the Truth.” Ever since petroleum has been bought and sold, better ways to measure and better accuracies have been sought. A big advancement was the pipe prover. Today, the American Petroleum Institute (API) requires an accuracy of 0.02% when compared to a standard such as National Institute of Standards and Technology (NIST) traceable Seraphin cans. If we want to put 0.02% accuracy into perspective, that is 6.45 teaspoons, or a little more than two tablespoons, of oil in one barrel. That is very good measurement and that is worst case.

Converted to dollars on a small 10,000 barrels per day (bpd) Custody Transfer Liquid Metering Skid, a 0.02% error is about two bpd. At the average price of oil today, say $50.00/bbl, that is $100 dollars per day. In a year’s time, that is $36,500. That is why we all strive to exceed the 0.02% required by API. We know and understand the value that increased accuracy delivers to our companies.

Accuracy of measurement is important when oil is selling at $100/bbl and profits are good, but it is even more important when oil is at $50/bbl and the margins are tight. One lost barrel becomes a much larger percentage of the profit.

Welker Flow Measurement Systems (WFMS) has a patent pending on a new design bi-directional prover called the WFMS SCS Prover™. The SCS stands for straight calibrated section prover. With the SCS Prover, accuracies of 0.002% to 0.007% have been achieved regularly. The calibrated section is in one straight piece of pipe which eliminates the elbows and flanges in the calibrated section, the cause of most of the inaccuracies and problems in other bi-directional provers.

What Is Accuracy?

Accuracy is what we are after when we zero in on a target. Any good meter like any good rifle is repeatable. If every shot is within a small group, but not at the center of the target, we make an adjustment to the sights or to the scope to bring the pattern to the center of the target. Now our rifle is repeatable and accurate. It is the same with meters. The more repeatable they are, the more accurate we can make them.

A precise meter that is repeatable also needs to be zeroed in for accuracy. For this we use a “pipe prover.” This is a device whose volume between two switches has been checked and verified to a known volume. The length of pipe between the detector switches is determined by the volume needed to ensure good repeatability. A sphere is then inflated to a diameter larger than the inside of the pipe and placed in the prover. The two switches installed in the pipe detect when the ball reaches the beginning and end of the known volume in the pipe.

Normally, the meter prover volume between the switches is calibrated against a device called a Seraphin can whose precise volume is traceable to NIST. Before we can put the meter prover in service, it must be calibrated against a known volume such as these Seraphin cans. The prover must also conform with the API Manual of Petroleum Measurement Standards, Chapter 4 - Proving Systems, Section 2. The volume must match the calibrated Seraphin can’s volume three consecutive times at different flow rates to within 0.02% (0.0002).