Abstract

Since 1995, refinery distillation tower plugging has been observed in some refineries processing Canadian sweet light crude. This plugging has resulted in numerous and costly unplanned refinery shutdowns and could potentially result in a devaluation of Canadian crude.

The Canadian Crude Quality Technical Association (CCQTA) formed a Phosphorus Project Group to investigate and manage this problem. The group includes representatives from refineries, pipeline companies, service companies, chemical producers, and testing laboratories. The key finding of the group was that residual phosphate esters used in fracturing operations are the primary cause of plugging.

This paper describes work done to understand and solve the problem through formulation of new phosphonate ester oil gellant chemistry. As described in CIM 95–861and SPE 75666,2 CO2-miscible, gelled hydrocarbon fracturing fluids offer superior gas well stimulation through a number of advantages, including avoidance of capillary pressure effects associated with water-based fluids. Therefore, simply not using oil-based fracturing fluids is not an acceptable solution from an upstream production standpoint.

The first part of the paper discusses reasons why phosphate-ester oil gellants cause tower plugging through the formation of volatile, phosphorus-containing compounds. This is also shown to be the cause of plugging observed at a gas plant processing returns after gelled-oil fracturing treatments.

A solution to this problem through formulation of a new phosphonate ester oil gellant is explained. Fluid properties of the new phosphonate ester system are compared to those of a conventional phosphate ester system.

Field test results from two fracturing treatments conducted with the new gellant are presented. Flowback fluids were analyzed for volatile phosphorus compounds at the wellhead and found to provide a 98% or better reduction in these plugging compounds. It was also observed that plugging emulsion formation at the gas plant processing returns did not occur after the phosphonate ester treatments as it had after conventional phosphate ester treatments. The reasons for this are investigated and explained.

Enhancements to the job design process2 applied during the two phosphonate ester oil gellant fracturing treatments are also described as well as the economic value added.

Introduction

Plugging of distillation trays was simultaneously detected in 1995 at three Canadian refineries:

  • Imperial Oil Strathcona, Edmonton

  • Petro-Canada, Oakville, Ontario

  • Chevron Burnaby, British Columbia

Investigations by the CCQTA Phosphorus Project Group into the source of fouling have been ongoing since 1995. Phosphate ester oil gellants have been identified as the source of the problem. This is currently the most prevalent oil gellant technology in use by service companies.

Additional fouling continued at one refinery. In the spring of 1998, tower fouling again occurred in all three refineries. Subsequent to a change in crude supply by Petro-Canada Oakville, the fouling problem disappeared at that refinery.

Since 1998, additional fouling has occurred in Canada. In 2002, United Refining Company in Pennsylvania detected fouling in their towers while processing Canadian sweet, light crude. The problem was discussed at the October 2002 meeting of the National Petroleum Refiners Association.

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