Abstract

Kaybob South field first discovered in 1960's is located in Central Alberta. This field is an aquifer driven maturing gas reservoir with H2S concentrations up to 20%. The Iron Sulfide scale deposition coupled with a time layered asphaltenes affect overall production and increase risk during well intervention. The scale forms directly on the tubular walls and generally results in corrosion of 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. Numerous removal techniques have been attempted in the past with a limited degree of successes. For example, on some wells HCL blends (7% - 22% HCL) with xylene have initially been successful in creating a pilot hole to bottom in a single attempt. Over time this treatment was only successful over several attempts with 1.25" hole being achieved in the well to bottom. Treatments also involved Milling, Acoustic cleaning, rotating jetting all with a very limited degree of success. After the previous treatments failed a workover was the only remaining alternative. With any of the operations above an increased degree of risk and HSE issues are relevant in this maturing reservoir. Scale removal in this field is not uncommon to be required on a yearly basis.

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 to date with valuable data being captured. This paper will demonstrate that coiled tubing has provided an effective, efficient and economic method of scale removal.

Introduction

The Kaybob field in Central Alberta, Canada comprises of several producing formations mostly aquifer driven. In the Beaver Hill Lake formation there are 36 producing wells. The production tubulars consist of 3.5" to 2.875" permanent completions with a reservoir pressure of 2150Psi to 2450 Psi. Reservoir temperature is 230 F with an H2S content up to 20% (15% - 20%). Accelerated production decline in this area can result from water influx into the gas pay zone, a reduction in gas permeability, and flow restrictions within the production tubulars resulting from the iron sulfide scale build-up. The time-layered build up of scales in the Kaybob region is a potential problem along with the increasing risks of the current removal techniques on the market. Return on investment along with the decreasing production in this maturing reservoir can prevent feasible solutions from being performed. Growth of deposits will lower production rates due to increased friction pressure caused by uneven surface roughness along with decreased crosssectional (flow) area. Over time scale build-up prevents flow completely and prevents access to the lower wellbore intervals.

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