Acid-stimulated well returns processed without shut-in, disposal problems

Jan. 1, 1999
Avoiding chemistry upsets in equipment Fluids returned from offshore acid stimulations present a complex challenge to the offshore separation process. When introduced to the normal production stream during flowback, these stimulation fluids can cause the equipment and chemistry capabilities of the processing system to be exceeded. The resulting system upset commonly causes expensive production losses and water disposal problems.

Avoiding chemistry upsets in equipment

Fluids returned from offshore acid stimulations present a complex challenge to the offshore separation process. When introduced to the normal production stream during flowback, these stimulation fluids can cause the equipment and chemistry capabilities of the processing system to be exceeded. The resulting system upset commonly causes expensive production losses and water disposal problems.

A new treatment program recently employed on a Gulf of Mexico platform avoided this production upset and allowed operations to continue during flowback. It allowed the recovery of oil from the test separator without any facility upsets and eliminated the need for injecting out-of-compliance fluids into a disposal well.

The operator of the large platform had selected three wells for stimulation. Using traditional methods, each well would have been individually acidized and unloaded before starting the next acid job. Each treatment would have incurred additional expenses including boat-trip charges and rig-up/rig-down charges. Even then, past experience showed that processing returns from just one acid stimulation caused significant operational difficulties for the platform's production system.

In order to avoid system upsets, all produced fluid would be pumped down a disposal well (instead of being sold) until the pH of the returns reached a level that allowed processing without upsetting oil BS&W (basic sediment and water) and overboard water quality. In addition, using the disposal well would require the expense of renting a downhole pump.

Processing upset avoided

A three-well stimulation package was pumped by BJ Services. The company's Unichem division then handled the return fluids using its Straight-Flowtrademark process. The returns were flowed into a test separator through a group test manifold. A combination of special Straight-Flow chemical products were fed into the group line ahead of the separator.

As a result, oil leaving the separator was maintained at less than 0.2% BS&W during the flowback.

Water leaving the test separator contained an average of 1,000-2,000 ppm oil and grease ahead of the processing vessel. Leaving the process vessel, the water contained an average of 200 ppm total oil and grease. Final polishing resulted in overboard water with a maximum measured level of 16 ppm oil and grease.

A total of 585 bbl oil were processed through the test separator, and 645 bbl of water were discharged overboard. The skimmings from the process vessel and the spent regenerant were transferred to the wet oil storage tank and were processed without incident. When the pH of the fluids reached 5.0, the wells were returned to the bulk production manifold.

By using the new process, the producer was able to simultaneously perform stimulations on the three wells, which resulted in increased production with no system upsets, downtime, or disposal costs. There were no processing problems or out of compliance oil and water. And the expense of two additional boat trips and a disposal well pump were avoided.

Equipment, chemistry

Two of the most important processes in the production of oil and gas offshore are the dehydration of produced liquids to meet pipeline specifications and the treatment of the separated water to meet overboard discharge requirements, both of which require properly functioning equipment and appropriately designed and applied chemical programs.

Most production processing problems can be attributed to upset conditions in the operation of the liquid dehydration and water processing systems. While equipment and chemical programs designed to process normal production fluids can tolerate moderate production changes without a system upset, certain types of well treatments introduce foreign components into the produced fluids. These components are outside of the system's processing ability and can cause a complete shutdown of a platform's production.

Acid stimulation can cause significant upsets to the normal oil and water treatment process. The chemicals used in acid packages include corrosion inhibitors, anti-sludging agents, iron control additives, surfactants, mutual solvents, and solids control components. These additives, as well as formation fines and the low pH of the returned fluids, can disrupt the liquid dehydration program and result in oil that will not meet pipeline BS&W specifications. They can also upset the water treatment system, causing overboard water quality problems.

The historical method of handling the well returns from acid treatments has been to segregate these fluids from the bulk production on the platform, or producing the fluids at a low enough rate that the upset could be tolerated. The fluids, if separated, are either injected into a disposal well or stored on the platform or a barge until they can be processed.

Both options present problems. Emulsified fluids can damage the disposal well, and storage is often limited or unavailable on most platforms. Barges and portable vessels can be economically prohibitive. And gradual introduction of these fluids can cause protracted upsets in the production processing systems. Stacked fluids and excessive slop oil generation can severely limit the stimulation schedule of other wells, which reduces the potential productivity and profitability of the field.

In recent years, alternative methods of treating the problematic fluids associated with acid stimulation programs have been developed. One popular approach involves filtration to treat the water phase of the returned fluids.

These filtration methods use Granular Activated Carbon (GAC), clay, and synthetic polymeric or natural fiber absorbents to remove the organic contaminants from the water prior to discharge overboard. However, this filtration process requires the water to be free of emulsion in order to be economical. In addition, the spent filtration media can pose a significant disposal cost and liability issue for the producer.

Flow process

The Straight-Flow process is a two-step system that first resolves the emulsion from the acid-stimulated well, and then processes the water to address overboard discharge requirements. A specially formulated series of products developed to treat emulsions from acid stimulation treatments is fed at the wellhead or, for multiple well stimulation, at a common group line. The emulsion is resolved in the test separator vessel, and the oil phase frequently is recovered at less than 1% total BS&W. Typically the oil can be sent directly to the sales line or combined with the platform total production ahead of the bulk treater.

Extensive pre-testing is performed on produced fluids from the candidate well using simulated acid stimulation packages in order to choose the proper treatment chemicals. Simulated skimmings and spent regenerant are tested to ensure compatibility with current treatment programs.

The water removed from the test separator is further processed using a combination of mechanical and chemical systems. Additional chemicals are fed to the water outlet of the separator, and the water is flowed into a specially designed process vessel. In this vessel, reverse emulsified oil, free oil, and solids are allowed to separate and are then skimmed to the waste oil handling system on the platform.

The water from the processing vessel is essentially free of insoluble oil contamination but still cannot be discharged due to soluble organic compounds originating from the acid additive package. The pH of this water is typically very low, less than 1, and will cause upsets in the water treatment system if returned to the bulk water stream on the platform.

To remove water soluble components and achieve standard Gulf of Mexico overboard water quality of 29 mg/l oil and grease, a final polishing vessel is used. This vessel contains a proprietary media designed to remove organic compounds similar in operation to GAC. Unlike GAC, this media can be regenerated on site and reused indefinitely.

The polishing vessel is a twin tower design where one side is alternately in operation mode while the other side is in regeneration mode. During regeneration, the media is flushed with a proprietary regenerant which removes trapped organic material from the media and restores it for reuse.

The spent regenerant is sent to the platform's waste oil handling system and can be processed with the crude oil or sent directly to the sales line. The regenerant is an oil-soluble mixture and does not contain components that are usually banned from the crude oil sales line.

The produced fluids from the acidized well are returned to the normal system when the acid treatment has been recovered and processed. Typically, the pH of the recovered water is monitored, and when the pH rises above 5, the fluids can be turned back into the system without upsets.

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