WATER CONTENT TEST FOR EOR CRUDE SIMULATES DESALTER

R.B. Duke
Marathon Oil Co.
Littleton, Colo.

Crude oil produced from enhanced oil recovery (EOR) projects employing micellar/polymer flooding can require an alternative test method for water content to the ASTM centrifuge test, or grindout procedure.

The reason is that centrifuging cannot break the surfactant-stabilized emulsion. As an alternative, Marathon Oil Co. has developed a simulated desalter test (SDT) and necessary apparatus for the accurate evaluation of the quality of crude oil from such projects.

Oil quality parameters such as basic sediment and water (bs&w) values are used almost universally for determining the acceptability of crude oil into pipeline or refinery systems.

Crude oil from EOR projects employing micellar/polymer flooding may contain over 5% water, yet give a zero bs&w value using the ASTM grindout procedure.

SIMULATED DESALTER TEST (SDT)

The SDT simulates the desalting process, thereby determining whether EOR crude, after being emulsified with fresh water, can be electrostatically dehydrated to an acceptable water level in a period equivalent to the average residence time in the desalter.

To conduct the test, the crude, desalter water, and desalting chemical (in the same proportions used in the refinery) are emulsified by blending, and then demulsified in an apparatus to be described.

Passing or failing the SDT is based on the water content of the oil, determined by Karl Fischer titration.

Typical pipeline acceptance standards for the maximum amount of water remaining in the crude before and after the test are 0.40 and 0.70%, respectively. The most critical parameter is the after SDT value. This upper limit is impossible to meet if significant quantities of surfactant remain in the crude.

In addition to the quantitative aspects of the SDT, qualitative observations, such as the sharpness of the oil/water interface, the color and clarity of the water, and even the amount of coalesced water, may be recorded to better anticipate the quality of the forthcoming desalter separation. These observations are seldom used as passing or failing parameters, however.

SDT EQUIPMENT

Three basic pieces of equipment are necessary to conduct the SDT:

A blender for emulsifying the crude, desalter water, and desalting chemical
A simulated desalter testing apparatus (SDTA) for conducting the electrostatically accelerated coalescence step
A Karl Fischer coulometric titrator for determining the water content of the desalted crude.

The blender and titrator are available from a number of manufacturers. The construction of the SDTA is the subject of this article.

The lack of commercially available SDTAs is not meant to imply that such devices have not been built. Several proprietary versions have been constructed by oil field service companies. We were able to work with one of these prototypes prior to designing our apparatus.

SDTA COMPONENTS

Marathon's SDTA is shown in Fig. 1. The basic components include a constant temperature bath, a timer/controller, a high-voltage power supply, vessels for conducting the emulsifying and demulsifying steps (SDT tubes), a blender, mixing and electrode assemblies, and a carousel assembly.

Almost all of the components are readily available directly from laboratory equipment vendors; consequently, the amount of machine shop time required to assemble the device is minimal.

THE CAROUSEL

The heart of the SDTA, the carousel (Fig. 1), is constructed around a high-voltage slip ring (IEC Corp., IELHS) mounted to a nonrotating, linear-motion rod and bearing (Thompson TWN-8-BG). This allows the carousel to be raised and lowered. The slip ring allows the carousel to be rotated, which facilitates attachment and detachment of the electrode assemblies, as well as sampling.

A circular aluminum plate (9 in. OD) is attached to the slip ring, on which are mounted six high-voltage outlets (Alden 8101 FSP) and a corresponding number of adjustable-grip latches. The latches attach the electrode assemblies to the carousel by gripping them underneath the ground/attachment collar and form the external ground.

The vertical travel of the carousel is determined by lower and upper stop collars located on the linear-motion rod. The stop collars are adjusted to immerse the SDT tubes to the proper depth at one end of the traverse, and to hold them clear of the bath at the other.

A catch is provided to lock the carousel in the up position (SDT tubes out of bath) by engagement with the lower stop collar.

The carousel assembly is coupled to a pneumatic cylinder (Fig. 1) which allows it to fall gently (without manual intervention) until the upper stop collar is reached when the up position catch is released. This prevents damage to the carousel and breakage of the SDT tubes.

The frame of the SDTA is made entirely from standard unistrut parts. Leveling feet are provided at each of the four corners to properly position the apparatus above the bath.

SDT TUBES

The SDTA is designed so that all of the operations leading up to the Karl Fischer water analysis (i.e., mixing coalescing, and sampling are conducted in a single vessel. This eliminates the need to transfer the samples and minimizes the amount of equipment to clean after the test. This is important in field operations.

The SDT vessel is a calibrated, screw-capped, centrifuge tube (Wheaton 255623). The use of calibrated tubes eliminates the need for graduated cylinders for measuring the brine and oil. In addition, they provide a convenient expanded scale for measuring the water drop.

The SDT tubes are nonpressurized vessels, making it impossible to reach the normal operating temperatures of commercial desalters, which is about 240 F.

Although the lower temperatures result in slower separation rates as compared to a desalter, good correlations of oil quality (water content) with coalescing time (or voltage) can be established.

MIXER ASSEMBLY

The mixer assembly (Fig.2) is made by drilling a hole (0.5 in. OD) in a centrifuge tube cap and mounting the impeller (Eberbach 8730). Teflon-coated caps (Wheaton 240481) are used in the construction of both the mixer and electrode assemblies. They provide years of service with little or no maintenance.

ELECTRODE ASSEMBLY

An expanded view of the electrode assembly is shown in Fig. 2. All of the metallic parts (tubing, rods, collars, etc.) are fabricated from aluminum to minimize the weight. Like the mixer, the electrode is mounted in a Teflon-coated, centrifuge bottle cap for interchangeability on the SDT tubes.

To assemble the electrode (Fig. 2), the hot electrode conductor (0.156 in. OD), threaded at one end (6-32) to attach a high-voltage connector (Alden 8101 MA-6-32), is fitted with a Teflon, heatshrink sleeve. The conductor is then cemented with epoxy into the ground tube, and a ferrule is pressed into position where r the cap is to be mounted.

The electrode is inserted through a predrilled, Teflon-coated cap until the ferrule is flush with the inside of the cap (Fig. 2). Then a spacer (0.5 in, long x 0.625 in OD) is slid over the ground tube extending above the cap. To form the electrode and cap into a single unit, the ground/attachment collar (1.25 in. OD x 0.5 in. ID) is attached to the protruding ground tube while pressing the two parts together.

The electrode is completed by attaching the hot and ground collars (0.75 in. OD x 0.156 in. ID and 1.0 in. OD x 0.5 in. ID, respectively). The placement of the collars is such that when the electrode is screwed firmly onto an SDT tube, the top edge of the hot and ground collars are at the 6 ml and 120 ml levels, respectively. All rod or tubing extending beyond the collars after they are in place is removed. The electrode assembly has a small hole in the cap so that sampling can be conducted with a syringe and needle without having to remove the electrode from the hot SDT tube. This also serves as a vent to prevent the buildup of pressure in the tubes from degassing crudes. The slot in the ground collar of the electrode is aligned with the sampling hole in the cap to permit the syringe needle to be inserted without interference.

HIGH-VOLTAGE SOURCE

A variable power supply (Hipotronics HDA3), capable of delivering up to 3,000 y ac, is used to activate the electrodes (Fig. 1). It has an internal leakage detection system (adjustable between 500 m amps and 5 ma).

If the current exceeds the setting, an arcing lamp lights up and the arcing can be heard. In addition, the instrument automatically adjusts the voltage to below the leakage potential.

TIMER/CONTROLLER

The high-voltage source and an audible signaler (Fig. 1) are controlled by a programmable timer/controller (ChronTrol DL).

The audible signaler is programmed to beep once when the timing cycle is manually initiated; twice, 5 min later when the high-voltage source is activated; and three times, 20 min after the timing cycle is initiated, when the high voltage source is deactivated. The three-beep signal also informs the operator that it is time to begin sampling.

CONSTANT-TEMPERATURE BATH

The constant-temperature controller employed is a Lauda Model B-1 mounted in a Lauda C-20 bath (Fig. 1). Silicone oil is used as the heat transfer fluid.

PROCEDURE

The oil, desalter water, and desalting chemical are combined in an SDT tube in exactly the proportions used in the refinery (typically, 120 ml oil, 6 ml water, and 10 ppm chemical). A mixer assembly is then screwed tightly to the tube, and the mixture blended for 30 sec at medium speed. Mixing is done with the tube in the inverted position (Fig. 2) while the apparatus is held in position by hand.

When mixing is complete, the tube is removed from the blender, returned to the upright position, and the mixer assembly replaced with an electrode assembly. The resulting electrode/SDT tube assembly is then plugged into one of the high-voltage outlets on the carousel and attached (and grounded) to the carousel plate with the adjustable-grip latch.

The carousel is then lowered to immerse the SDT tubes in the oil bath (1801 F.) and the timing cycle is started. The fluid is allowed a 5 min preheat time, followed by a 15 min electrostatic coalescing period. The coalescing voltage is usually set at 1,000-1,200 y.

As soon as the demulsification cycle is complete (voltage off), sampling begins by withdrawing oil into a syringe and charging it directly into a Karl Fischer coulometric titrator.

After locking the carousel in its up position, sampling is performed with the SDT tubes still attached. A 100 m syringe is inserted into the SDT tube through the small hole in the electrode assembly cap (Fig. 2). The syringe needles are custom made to a length so that when inserted with the butt of the needle flush with the cap, the tip of the needle is halfway down into the oil phase. Large-gauge needles (20 gauge or less) are preferred, to allow rapid sampling of even the most viscous oils.

Enough syringes should be on hand to sample all of the experiments in progress immediately after the electrostatic coalescence period is complete.

This is necessary because it takes several minutes to complete a Karl Fischer titration, which means that a period of 20 min or more may elapse between the first and last titrations if six samples are being run simultaneously.

During this period, additional dehydration of the oil will occur in the SDT, even though the samples are out of the constant-temperature bath and the coalescing voltage is off.

ACKNOWLEDGMENTS

Appreciation is expressed to S.R. Smith who was instrumental in the design of the electrodes and to Marathon Oil Co. for permission to publish this article.