Abstract

Iron sulfide scale deposition occurs throughout the Kaybob South field in Central Alberta. The scale typically forms directly on the tubular walls and generally results in corrosion of the metal beneath the scale crystallites. Under normal conditions, iron sulfide is soluble in hydrochloric acid. However, in sour gas and condensate environments, an asphaltene binder forms over the crystallites to limit, if not completely eliminate, the efficiency of chemical removal treatments. In such cases, the scale can only be effectively removed by mechanical means. A number of different methods of scale removal have been attempted, with an inconsistent record of success. For example, on some wells HCl Blends (7%–22% HCl) with xylene have been successful on the first attempt, on other wells this mixture required several attempts, and in some cases this technique resulted in no success. Mechanical options to resolve the problem include a tubing replacement (with an average cost of $150K - $550K CDN depending on completion complexity) or a milling treatment. The milling process, using coiled tubing, a motor and a mill, is costly and carries an element of risk. Well work overs are completed as required in this area on a regular basis - typically one per year.

This case study will define and present the Blaster scale removal system that has been developed to clean tubulars. The techniques have been continually improved and modified since their introduction in January 1998. Amoco Canada participated in the beta test of the system where several modifications to the process were made. Nineteen wells have been completed as part of the test program, the results of which has added valuable data to this case study.

This paper will demonstrate that coiled tubing technology has provided an effective, efficient and economic method of scale removal.

Introduction

The Kaybob field in Central Alberta, Canada was discovered in the early1960's and comprises several producing formations most of which are aquifer driven. To date there are 36 wells producing from the Beaver Hill Lake formation, where the reservoir pressure within the field has remained at 2150to 2450 psi (15,000 to 17,000 kPa) due to the influx of an aquifer. Reservoir temperature is 230 degrees F (110 degrees C) and the H2S content is 15–20%. Accelerated production decline in this area can result from water influx in to the gas pay zone, a reduction in gas permeability, and flow restrictions within the production tubulars resulting from iron sulfide scale build-up.

The accumulation of iron sulfide deposits on wellbore tubulars and downhole equipment is a potential problem for many wells in the South Kaybob region. Under conditions of sufficient salinity, pressure, and H2S concentration, ironsulfide scale precipitates onto completion tubulars and equipment. The growth of deposits can significantly lower production rates due to an increase in friction pressure caused by the uneven scale surface and a reduction in the tubing cross-sectional (flow) area. Over time, excessive accumulation may restrict production or prevent access to the lower wellbore intervals.

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