Scale control in subsea horizontal wells is recognised as a particular technical and economic challenge, especially if effective chemical placement cannot be achieved through conventional bullhead squeeze treatments. When long horizontal or highly deviated wells produce from areas of high permeability contrast and high wellbore crossflow, coil tubing operations can often be the only manner in which effective chemical placement can be guaranteed. The costs associated with such treatments are extremely expensive and often prohibitive when compared with conventional bullhead operations. Such aspects were recognized as a particular risk for successful chemical treatments in the Draugen field, operated by Norske Shell. The Draugen field is located in the environmentally sensitive Haltenbank area of the Norwegian Continental Shelf, and produces oil from 6 horizontal (>1,000 ft.) wells completed with pre-packed resin screens to achieve effective sand exclusion. A moderate barium sulphate scaling potential was anticipated following sea water breakthrough requiring squeeze treatments. In order to improve chemical placement in this field and reduce the requirement for coil tubing interventions for squeeze treatment, work was initiated to investigate the potential use of viscosified fluids to improve downhole chemical placement.

A range of viscosifying agents was selected following detailed literature review based upon the rheology that was expected to lead to improved chemical placement. This included common water control additives such as xanthan and HEC (hydroxyethylcellulose) biopolymers and formulated water in oil (WIO) or oil in water (OIW) emulsions. Various formulations were selected that would either remain stable (higher viscosity) or to break (to low viscosity) at reservoir temperature. It should be noted, however, that viscosities were selected far below those normally applied in water shut off or multi-stage chemical diversion treatments. A series of formation damage core flood experiments has been conducted using analogue cores to ensure that the various formulations were non damaging, allowing rapid recovery to pre-treatment effective oil permeability. In addition, more sophisticated core flooding equipment was designed to examine chemical placement and recovery in zones of different permeability.

In summary, this paper presents a literature review describing the properties of different viscosifying agents and their potential applicability for improving bullhead scale inhibitor squeeze treatments. Core flood studies on viscosifying agents with selected rheological properties will then be presented to demonstrate the potential advantages of such applications for improved chemical placement by bullhead application in horizontal wells.

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