Oil Skimming Operation
SWDs receive waste water from oil and gas exploration and production activities. Waste water usually contains significant quantities of crude oil entrained in the fluid but the word “significant” can mean a large fraction or a barely perceptible trace. The amount of oil and the ability of the SWD operator to skim the oil will determine the economics of crude oil production from any given SWD. The process can be done many ways and may involve simple settling in the tank battery and enhanced settling by way of a “gunbarrel” separator or a centrifuge. The more common technologies used in the oil field are discussed below.
Skimmed oil sales add to the revenue of many SWDs around the country. Crude oil recovered from waste-water is sold to refiners at approximately 60% to 80% the local spot-price for crude oil; for example if the local spot-price of crude is $60/bbl, skimmed crude can be sold at prices from $36 to $48/bbl. It is discounted by purchasers because it has higher than normal amounts of water, silt, and other impurities and is not considered pipeline quality. SWD operators can run the skimmed oil through a “polisher” to remove impurities and price paid by refiners is at spot-price level but costs to the operator are higher. If a local market exists, the skimmed oil can be sold on a retail basis as boiler fuel oil, usually for a higher price than spot crude.
The simplest way to remove oil from the waste stream is allowing it to settle in the water tanks. Tanks are usually 500 gal and steel or fiber glass. Some operators use large tanks to replace several smaller tanks in the battery. The large, bolt-together tanks are 2,000 to 10,000 bbl in size; they can be placed at the start of the waste stream or at the end where it feeds the main injection pumps. In either case the waste-water has a residence time that is considerably longer than in a 500 bbl tank. Residency time determines oil separation. Operators who install large tanks report better-than-average crude oil recoveries but this is difficult to demonstrate due to the differences in the intake of various SWDs.
The next step for increasing oil recovery rates is the installation of a traditional gunbarrel (Figure 1 below) as marketed by numerous vendors. To increase separation, the gunbarrel is commonly a standard steel 500 bbl tank that contains inner plumbing to enhance separation. Emulsion (waste water from tank-trucks) enters at the top of the downcomer tube and enters the gunbarrel. As emulsion exits the downcomer tube, the oil droplets rise to the top of the water column and coalesce into an oil layer. The operator will draw de-oiled water from the bottom of the vessel and periodically will withdraw accumulated oil from the top of the vessel. The process is step-wise or continuous as dictated by waste-water deliveries and the size or number of gunbarrels at the SWD facility.
A variation is the horizontal gunbarrel as marketed by NOV and others. Manufacturers claim increased oil recovery due to increased residence-time in the separator.
Figure 1. Schematic of a gunbarrel tank.
The latest technology to enter the race for better oil recovery is the NOV Water-WolfTM, a skid-mounted, integrated unit that takes raw waste-water at the load-out point and removes sediment and entrained crude oil. The Water-Wolf employs zero-shear pumps to drive the emulsion through a three-phase hydro-cyclone separator that outlets water, crude oil, and sediment. On-board sensors and controls maintain peak efficiency at the pumps. Effluent goes directly into the main injection pumps. The unit has the capability to replace shearing pumps, tanks, and filters. This process is described in an on-line article http://www.nov.com/WaterWolf/media/English_DOR_article.pdf . The manufacturer has information on several onshore installs.
An additional tool installed by some operators is the oil centrifuge to high-grade the skimmed oil by removing excess water. The centrifuge is a continuous-flow process that subjects the crude oil from the gunbarrel to very high gravity fields to remove almost all the water and particulates. The result is pipeline quality crude oil that will fetch a higher price at the well-head.
Figure 2. Example of a water meter.
It would behoove the SWD operator to invest in a basic oil-in-water meter (Figure 2) that can be carried in the field to measure the crude oil content of the specific waste-water stream. In addition the operator can measure the changes in oil content throughout the SWD facility. The operator will also see how much oil is being pumped down-hole and therefore not being reclaimed. Such a meter could obviously be used on several SWDs.
There are several options for the SWD operator to enhance oil recovery. Costs of some options are high and can only be rationalized when oil content of waste-water is high. In any case, maximizing oil recovery will bolster the economics of the SWD.
Marian M. Langhus, PhD
Bruce G. Langhus, PhD