The total crude oil production from the Soldado field is pumped to one facility on land. The gas from the production is separated offshore and any residual gas is removed via a gas booth at the facility. The crude oil is then pumped through a free water knockout vessel and into settling tanks for separation by gravity. The gravity separation process is aided by an emulsion breaker to increase the rate of separation. The water from the settling tanks is bled into a water treatment system consisting of pits, gas flotation units and corrugated plate interceptors. The oil remaining in the settling tanks when its content is verified to be <2%BSW is pumped to the refinery for processing. The overall process is shown below:
However the water bled into the water treatment system usually carries with it a substantial amount of oil which if recaptured could be further treated to become saleable crude (<2% BSW) and also if removed from the water could reduce further treatment required to ensure that the water meets levels for environmental compliance. As a result the oil that floats to the top of the pits are skimmed off and collected and transferred to a collection tank. This oil is called slop oil. The slop oil is then further treated in a heater treater which heats the slop oil up to 180-200 °F and another emulsion breaker is applied to aid in the rate of separation. The slop oil recovered is pumped to another collection tank and allowed to settle until it is of a saleable quality of <2% BSW. It is then pumped to the refinery for processing. The slop oil treatment process is shown in Figure 2.
The rate of crude dehydration via these processes becomes slow enough that the crude must be removed to another facility where it is allowed to settle by gravity for longer periods. However there are cases where the crude and water will not separate even after periods of several years. The operations unit termed this crude ‘stored slop oil’ as it was impossible to separate via existing methods and had to be stored. Tests were conducted to ascertain if emulsion breakers and or heat could break the emulsion present but no chemical solution was found.
Analysis of the fluids indicated the presence of an emulsion layer that would have formed as a result of an agglomeration of paraffin, asphaltenes and other fine solids in the crude that was of API gravity <20. The Process Optimization unit decided to implement a solution in the field that combined the use of heat and centrifugal forces as it was felt that the heat would cause the paraffin deposits to melt and the centrifugal forces would separate the fine solids out of the crude oil and so the emulsion would be broken allowing the water and oil to separate. The solution was implemented contractually such that the contractor was only paid for a result. So payment was tied to the volume of saleable crude obtained. During the period of treatment an average recovery rate of 42% saleable crude was obtained from the ‘stored slop oil’ and the rate of treatment ranged from 600 to 800 bpd.
Statement of Theory and Definitions
It is known theory that strong emulsions can be formed between oil and water and there are certain chemicals and substances than can either break these emulsions (demulsifiers/emulsion breakers) or stabilized them (emulsifiers). This works by changing the hydrophilic-lipophilic balance. Crude that is transferred to a refinery needs to be low in water content as close to zero as possible.
The presence of water in the crude to be transferred to the refinery poses the following concerns:
- Increased risk of internal corrosion in pipelines being used to transfer crude oil to refinery.
- The higher presence of sediments and water in the crude also increase the likelihood of proliferation of sulphate reducing bacteria which can further increase internal corrosion rates.
- Cost to pump/transfer water which is of no value.
- Increased cost and time to feed crude to the distillation columns at the refinery as desalting process is more difficult and longer. This can also lead to increased water entering the distillation columns leading to corrosion and damage to distillation plates.
The crude dehydration facility currently has a target of transferring crude oil of a water content <2%BSW.
The established process at the facility is to use gravity separation aided by washing effect and chemical treatment. The slop oil recovered from the water treatment system being further treated by application of heat and additional chemical treatment.
Stored Slop Oil
However at the facility over the years there were batches of crude oil that was so difficult to separate the oil and water that it had to be sent to another facility for storage until a solution could be found. The emulsion could not be broken effectively with gravity separation and chemical treatment alone. Heat could assist the treatment but it was found to not be entirely effective. This was identified in the operations as ‘stored slop oil’.
Gravity Separation and Stoke’s Law
Separation by gravity is in general defined by Stoke’s Law. This also holds for the gravity separation that takes place in a centrifugal separator, the droplet separation rate is defined by:
This is the method used to ascertain the effectiveness of emulsion breaker chemicals with or without the addition of heat by placing oil and chemicals in a bottle (usually 100 ml) and agitation them and then allowing the content of the bottle to settle over time. The bottle testing technique utilized in this case was one developed by the company’s in-house laboratory and established effectiveness over a 24 or 48 hours period. Different dosages of chemicals were utilized and a range of temperatures were utilized by applying heat to the bottles via a water bath.
The rate of water drop out from the oil layer, the %BSW in the oil layer and the visual appearance of the emulsion layer and water clarity were the main components used to determine effectiveness of the heat and chemical treatments during the bottle testing.
Description and Application of Equipment and Processes
The process skid that was utilized to successfully treat the stored slop oil had the components as illustrated in Figure 3. The stored slop oil was pumped via an additional pump that was set up at the storage facility and the stored slop oil was transferred as required to Slop Oil Tank #1 and Slop Oil Tank#2.
The stored slop oil was pumped from the storage facility to slop oil tank#1 or #2. Then the slop oil was pumped through the heat exchanger located on the steam boiler skid. Heat was transferred to the slop oil from the steam generated in the boiler via the spiral heat exchanger.
The spiral type heat exchanger (shown in Figure 4) had a single channel and wound centered oriented in a horizontal position. So the fluid was fully turbulent at a much lower velocity than in straight tube heat exchangers, and each fluid travels at constant velocity throughout the whole unit.
This removed any likelihood of dead spots and stagnation. Solids were thus kept in suspension, and the heat transfer surfaces were kept clean by the scrubbing action of the spiraling flow.
The slop oil was heated from ambient temperature up to 90° C in one pass. The boiler and the heat exchanger units were insulated to minimize heat loss and for HSE protection.
Once the slop had been heated up it was forwarded to the decanter which had a two phase separator, i.e. solids were removed from the liquid. The decanter centrifuge had a slender cylindrical/conical bowl with a relatively large length/diameter ratio (refer to Figure 5). The screw conveyor fitted inside the bowl aided with continuous removal of separated solids. The range for the bowl speeds were 1000-3250 rpm where the developed “G: force was between 300 and 3000 G. Process liquid was fed into the cylindrical section, where it formed a layer (the pond) around the wall. The thickness of this layer was established by a series of discharge weirs at the end of the cylindrical section, and through which the clarified liquid was decanted by centrifugal force. Solids being heavier collected at the bowl wall, from where they were continuously removed by the screw conveyor and transported “up” the conical section, referred to as the “beach”, and out through the discharge ports at the narrow end (see Figure 6).
The liquid from the decanter buffer tank was then pumped to the disc stack which was a three phase separator.
The three-phase centrifuge was the final step in the treatment process where oil, water and small quantities of sediment were removed. The disc-stack centrifuge was a high efficiency mechanical separator which was used to break the stable emulsions as well as to ‘polish’ the oil phase giving a high quality, high value clean oil product. Saleable crude of as low as 0.1% BSW was achieved. The disc-stack centrifuge was especially effective at removing the ultrafine solid particles that became entrapped in the oily sludge emulsions. The three-phase centrifuge fed pump took its suction from the decanter effluent tank and pumped the liquid through a plate heat exchanger where the temperature was increased to around 95°C.
The clean oil was pumped out from the centrifuge using an internal impeller into a buffer tank. Then it was pumped to either clean oil storage tank. The separated water was pumped out from the centrifuge using an internal impeller to the pits. The solids were intermittently ejected to a tank below the disc-stack and then pumped to the pits. During start-up and solids ejection the disc stack required a hot water supply, this was supplied from a tank and pump located next to the disc-stack skid, cold water was heated up using a steam hose coming from the boiler. The fuel utilized for the entire process was diesel and this was provided by the operator.
Presentation of Data and Results
The unit was affected by the amount of solids present in the inlet streams to the centrifuges. At times the unit had to be stopped when solids present were 7%BSW. It was found that when the solids content was higher reducing the throughput of the unit assisted the ability to obtain saleable crude outlet of the disc-stack.
- The process skid utilized was able to effectively treat stored slop oil by combining heat and centrifugal forces. This was something that could not be achieved previously with the use of chemicals, heat or a combination of chemicals and heat.
- The process skid utilized was able to effectively treat the stored slop oil at a throughput that ranged from 600 to 800 bpd and an average recovery rate of saleable crude (<2%BSW) of 42% of was achieved.
- The volume of solids present in the stored slop oil affected the throughput of both centrifuges. The decanter was utilized to remove large solids and had to be stopped for cleaning when >3%BSW was found in the inlet slop oil. The disc-stack centrifuge was used for removing finer solids and was able to effectively do so without the use of additional emulsion breaker chemicals.
- Based on the amount of heavy and fine solids entering the system the throughput of the unit had to be reduced to get the saleable quality crude of <2%BSW.
- The disc-stack centrifuge is a unit that requires training and knowledge or use. It also requires expert maintenance and repairs due to damage that can occur over time due to the speed of fine solids hitting the surface of the discs.
- During operation of the process skid the operations crew had to stop and change the gravity disc in the disc stack to achieve effective separation (crude <2% BSW). It was also found that the gravity disc had to be adjusted to manipulate the oil-water interface in the disc stack. It was found that the ideal location for the oil-water interface in order to achieve optimum oil/water separation was should be just between the top disc and the rest of the ‘disc-stack’.
- If the interface is outside the top disc, whenever the disc stack makes a discharge (output stream) the water seal will break causing oil to exit through the water outlet.
- The boiler water quality like with all boilers had to be monitored effectively and the water was treated onsite however at times the unit had to be taken down for repairs due to the fact that the water supply in this case was from municipal sources and had fluctuating inlet quality that would not have been catered for when setting up the chemical treatment rates. A lesson learnt from this was to have more effective controls to adjust chemical treatment as the inlet water quality changed or to find a source of water supply that had a more consistent quality.
- The spiral type heat exchanger was an effective configuration for handling high solids content in the stored slop oil as there were no failures associated with its use and the stored slop oil was known to contain up to 7%BSW this would have caused fouling in other heat exchanger arrangements.
- After this successful exercise the process skid was again utilized to treat stored slop oil. However the disc-stack centrifuge had to be repaired as the impact of the solids on the discs and the speed of rotation caused damage to the formation of the stack. During this repair stage the unit was upgraded to have a smaller footprint and logic controls were added to be able to optimize the unit more effectively.