Preparing an existing crude oil processing system for a major fluid composition change

5 Jun 2014

By Steven Williams, Process Consultant at Maxoil Solutions

Having a deep understanding of key flow assurance issues is vital to the design and successful operation of a process plant. As well pads are expanded and the process fluid composition changes significantly, existing plants are often upgraded without characterising the new fluids and appreciating their full impact on the process. 

Maxoil was recently involved in two-stage capacity study for a remote onshore oil and gas processing facility. Initially commissioned more than 30 years ago, the installation was designed for light oil, low water cut and solids-free service. 

The primary objective of the study was to determine the viability of using the existing facilities to process the planned increase in viscous oil production against the option of building a new $4billion processing facility. 

At present the installation processes fluids near the opposite end of the separating spectrum in the form of viscous oil, high water cut and high solids loading. The facilities have had many upgrades and expansions since start-up but none have adequately addressed the current processing problems associated with viscous oil production. 

Stage 1 

A holistic engineering assessment of the oil, produced water and sand removal systems and a full review of the flow assurance and production chemistry aspects from wellhead to plant outlet was conducted. The gas system and flowline limitations were also considered. 

The major processing problem, due to the fluid compositional change, was poor oil/water separation deemed to be caused by a combination of: 

  • Solids issues
  • Low Arrival Temperature
  • Emulsion formation negatively impacting water removal from the oil phase
  • Sub-optimal vessel internals configuration
  • Lack of inter-stage heating due to out of commission heat exchangers, which caused emulsion pads to build up in the downstream Dehydrators. 

The combined effect of the above issues led to incidents of high water carryover in the oil stream exiting the Slug Catchers (Primary Separators). Due to the absence of water draw-off facilities on the intermediate separators the high water content oil reached and overloaded the downstream Dehydrator causing BS&W excursions.   

The increasing solids production resulted in a significant rise in operating costs, associated with vessel clean-outs and pump repairs. This was primarily due to the unsuitable online solids removal system which was unable to eject solids from the main process vessels. The accumulation of solids and reduction of residence time in vessels meant solids were exiting with the separated oil and water outlet streams causing erosional damage of the downstream pumps.   

A number of improvement and further study work recommendations were made including performing the Stage 2 study.

Stage 2

In order to verify the Stage 1 observations and justify future plant upgrades it was necessary to obtain conclusive plant data by carrying out the following onsite activities:

  • Perform Process Diagnostics
  • System mapping
  • Wellhead fluid sampling

The Stage 2 work verified all but one of the issues identified in Stage 1. Specifically it was found the Slug Catcher oil residence times were less than half the theoretical value, caused primarily by blocked vessel internals. The only exception was that fluid analysis and separability investigations showed that increasing the fluid arrival temperature did not significantly improve oil/water separation. The requirement to increase the system inlet temperature was therefore ruled out. 

A significant step change in solids particle size distribution and type was discovered with considerably more fines (<210microns) and sticky-type solids being produced compared with the previously produced larger sand particles which settled out in the upstream vessels. Had this data been available at the design stage of the recently installed online sand removal systems, the need for fluidising capabilities would have been identified. This emphasised the importance of obtaining fluid data. The newly generated solids data can be used to further develop and improve the solids handling strategy for the facility.

It was established that the existing facilities were suitable for processing additional oil by employing relatively low cost system and equipment modifications, particularly compared to the option of building a new facility. The required modifications included:

  • Remove the blocked coalescer and foam breaker internals to increase vessel residence time.
  • Install water draw-offs on the intermediate separators to improve system water removal capacity.
  • Install a hybrid sand jet and sand cyclone removal system to allow regular online sand removal from the vessels.

The holistic approach taken ensured all system design and operational aspects had been captured, considered and included in the recommended upgrade design solution. Through this process the operator has gained a high level of confidence that the proposed solution would provide the production facilities with a robust means of processing additional viscous crude oil. Most importantly, the upgrade recommendations were founded on actual plant performance and fluid characterisation data rather than an assumption-based theoretical evaluation process. 

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