Well Stimulation: A Mature Asset Case Study
Authors: Strong A.J. , Wright R.J. , Skelly G.J. 
Published: December 2007

Abstract
Scale dissolvers have long been in use in the oil industry as remedial treatments for the removal of scale deposits. The most commonly found scales in this industry are the carbonate and sulphate salts of calcium, barium and strontium which frequently occur in the perforated zone near the wellbore area. Due to the location of these scales it can be appreciated that considerable decreases in well production can result when this occurs (Ref 1,2).
The use of Scale Dissolvers in stimulation exercises associated with standard acid based treatments can improve the productivity index enabling mature assets to become more cost effective. With the average increase in production ranging from 30–50% the overall cost of these treatments can be paid back in days. This offers significant savings when compared to the greater costs associated with a complete well workover. The implications of these well treatments offer substantial advantages for mature fields. This paper discusses actual field data collected from several of such treatments performed on Amoco's North West Hutton platform.

Introduction
The North West Hutton field lies in the East Shetland Basin and is located within block 211/27a of the UK sector, lying about 100 miles NE of the Shetland Isles and 300 miles NNE of Aberdeen. The field lies in a water depth of 450 feet and the oil produced has an API gravity of 370 Amoco are the field operators with a percentage holding of 25.77%, with the other asset partners being Cieco 25.77%, Petrobras 28.46% and Mobil with 20% share of the N.W. Hutton development. The reservoir units are composed of Middle Jurassic Brent Group sandstone which is divided into 13 different layers. The N.W. Hutton oilfield provides some of the deepest oil production from the Brent Group sandstones with oil productivity coming from pay zones with TVD of 11,500 feet. Oil production in the field commenced in April 1983. In the absence of a natural aquifer support, water injection was initiated in 1984 with all wells starting life as producers being later recompleted as injectors once they had fallen below their economical limit. Reservoir data interpretations have indicated that there is in excess of 500 million stock tank barrels of oil in place and to date, more than 115 million barrels have been produced.

Background
When applied to mature assets, well stimulation techniques can often improve reservoir productivity. Any incremental production which can be obtained via remedial treatments is advantageous with respect to increased revenue and deferring the inevitable costs associated with decommissioning. A detailed study of individual well status on the N.W. Hutton was undertaken in 1995 with wells suitable for remedial actions selected. Beneficial treatment types considered are summarized below:

Mechanical
– Reservoir fracturing. – Re-perforating certain production layers. – Conservation of low producers to provide water injection support in other wells. – Milling/ Underreaming operations.

Chemical
– HF/HCL (Mud Acid) treatment programmes. – Combined Acid/Scale Dissolver treatments.

Water Chemistry
The formation water is of typical Brent type, with low (2.4%) salt content and moderate barium (84 mg/l). Produced water is currently &lt;20% formation water &gt;80% injection water) in the majority of producers.
P. 199^