The requirements for zonal isolation in Conoco's Hutton field have greatly increased as production from the field continues. High differential pressures approaching 1500 psi between zones which are less than 15' apart have made obtaining effective isolation of paramount importance. Problems with zonal communication behind the production casing were experienced due to an incomplete cement sheath. High pressure water zones flowed through cement channels to a low pressured oil zone and reduced oil production. In addition, water injection did not enter the desired zone.
A review of the methods used for primary cementing was completed and recommendations were made to improve the quality of the cement job. These recommendations were then implemented on subsequent wells. These recommendations included practices for conditioning the hole for the casing, designing the cement slurry and spacer system, and running and cementing the casing.
A high priority was placed on preparation of the hole for casing. Drilling practices were addressed such as maximum preferred hole angle, circulation rates, properties of the drilling fluid and making a conditioning trip after logging.
The cement slurry design was addressed in concert with the mud and spacer systems. A new test was devised to design a spacer system suitable for water wetting and mud filter cake removal. Rheological, settling and thickening properties of the slurry were addressed.
Casing running and cementing practices were reviewed. Sandblasting the casing, casing centralisation, casing running speeds and slurry volumes were, engineered. Fluid properties and circulation rates were optimised prior to cementing, as well as the use of multiple wiper plugs for separation of the mud, spacers and cement. Selection of cement mixing equipment and onsite supervision were also addressed.
As a result of this work, substantial improvements in primary cementing have resulted in near perfect cement bonds and elimination of zonal communication in the Hutton field.