Continuous Monitoring For Oil And Grease In Oil Field Waters
Authors: Gafford R.D. , Scott T.F. 
Published: December 2007

It is conventional wisdom that oil nd water do not mix. But, oil field operators know well that such is not the case. In fact, a great deal of effort is spent in oil field operations to separate mixtures of oil and water.

Although petroleum engineers may often think of water as a contaminant in the product, it is necessary, ultimately to take the opposite view and consider oil as a contaminant in water. From this point, of view, the degree of separation varies in importance, depending mainly on the ultimate fate of the water phase. In many cases, the seperation is sufficiently important to justify the use of a continuous on-line oil content analyzer to monitor the seperation process.

Water which is separated from petroleum products may be disposed of as a waste product, products may be disposed of as a waste product, or it may be processed and reinjected into the substrata. If reinjected, the objectives of the reinjection program and, therefore, the quality requirements, will vary from one location to another. If the water is disposed of as waste, it is usually necessary to meet quality standards specified by environmental protection authorities. These standards almost protection authorities. These standards almost invariably limit the allowable concentration of oil and grease in the water. Of course, other quality criteria apply, but it is an absolute certainty that oil field waste waters will be subject to special attention with respect to their petroleum content.

Recovered water may be reinjected for at least two reasons. Certainly the most common is to enhance secondary recovery operations. In this type of operation, older oilfields are flooded with water or injected with steam under pressure, thereby recovering additional petroleum pressure, thereby recovering additional petroleum from the production wells which would otherwise not be produced. We have encountered three different types of oil field operations involving reinjection of water. The first concerns an oil field having 500 or so producing wells and an equal or perhaps larger number of water injection wells. In this field, water is injected at a rate of about 200 to 210 thousand barrels per day (BPD) with a product flow of 125 thousand per day (BPD) with a product flow of 125 thousand BPD yielding 25 thousand BPD of oil and 100 thousand BPD of water.

In this secondary recovery operation, the principle incentive for limiting the oil content principle incentive for limiting the oil content of reinjected water is to conserve the product. Since 210,000 BPD of water are injected, if the oil content of the injected water were 1,000 ppm (0.1%) it would result in 210 barrels of ppm (0.1%) it would result in 210 barrels of oil being pumped back into the ground every day. If the concentration were consistently held below 100 ppm, only 21 barrels per day would be lost. (Although only 100 thousand BPD of water is recovered from this field, the balance of the 210 thousand BPD is oilfield water from another producer).

The second and third operations involve steam injection. They are similar to each other except that in one process a portion of the water is used to scrub sulfur from oil-fired boiler stacks.

In one of these operations, produced oil and water, at approximately 190 degrees F, are separated in a Wemco API separator. The water leaving the Wemco is monitored continuously to determine the effectiveness of the separators operation and adjustments are made to keep the oil concentration below a predetermined level. The water then flows through a softening process and is reheated for reinjection into the oil field. A portion of the water is contacted with fly ash and is used to scrub stack gases containing sulfur. Small concentrations of oil in the produced water cause the fly ash to agglomerate in the scrubbers causing premature scrubber maintenance and boiler shut down.

The second steam injection process does not use production water for scrubber operation. Produced water flows through a Wemco separator Produced water flows through a Wemco separator and is then clarified. Dispersed oil is skimmed from the top of the clarifier.