For oil & gas well pads not connected to pipelines, custody transfer of the produced oil and produced water occurs via truck transport. The trucks load oil from holding tanks and require accurate measurements for the custody transfer from producer. Similarly, this is true for the produced water in their holding tanks sent off for disposal.
In a Flow Control article, API 18.2: Ensure Safety While Providing Better Level and Volume Measurement Options, Emerson’s Tom Wienke shares effective measurement technologies for this application.
The forerunner to API 18.2 is API 18.1: Measurement Procedures for Crude Oil Gathered from Small Tanks by Truck. This standard only permitted manual measurements of this custody transfer process. Tom explained the manual steps:
Step 1: The truck driver climbs to the top of the tank and opens a thief hatch. A tape measure is lowered into the tank to determine the oil level. This first measurement is called the opening gauge and establishes the baseline measurement.
Step 2: The next step is to lower in a temperature sensor to read the oil temperature, which is used to support a density calculation. A temperature reading should be taken at three depths due to stratification, with the three readings averaged.
Step 3: A collector is dropped in to capture a sample. Specific gravity can be measured using a hydrometer. Some of the sample goes into a centrifuge for the “grind out” to determine how much water and solid material is mixed in. The measurements from this sample are applied to the entire lot.
Step 4: At this point, if the truck driver is satisfied with the quality, the oil can be pumped into the truck. When the transfer is over, the driver takes a second level measurement (closing gauge) with the tape to calculate the volume transferred.
He highlighted the hazards of this manual approach:
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