Drilling-Fluids Displacement and Cased-Hole Cleaning
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 99104, "Displacement of Drilling Fluids and Cased-Hole Cleaning: What Is Sufficient Cleaning?," by E. Berg, SPE, S. Sedberg, and H. Kaarigstad, SPE, BJ Services, and T.H. Omland, SPE, and K. Svanes, Statoil, prepared for the 2006 IADC/SPE Drilling Conference, Miami, Florida, 21–23 February.
The full-length paper reviews the results from drilling-fluids-displacement and cased-hole-cleaning operations performed on 158 wells on the Norwegian continental shelf during the past 6 years. These wells cover a variety of different completion scenarios where water-based and oil-based drilling fluids were used. The paper discusses different casing-cleaning requirements vs. well-completion scenarios and displacement techniques.

Introduction
Displacing the particle-laden drilling fluid with clear fluids when entering the completion phase is common procedure in drilling and completion operations. A drilling engineer will emphasize the importance of well control and fluid capabilities, while a completion engineer will focus on the ability to run the completion equipment into the well and operate it successfully during production. A reservoir or production engineer is concerned about the exposed formations and the effect that drilling and completion fluids can have on formation productivity. All these aspects are important to achievement of the optimum well design. Development of solutions to meet these demands is important not only from a technical and financial standpoint, but also from an environmental point of view. The ability to reduce environmental exposure and waste generation represents a cost and efficiency improvement.

Techniques and Solutions
Well-completion solutions are chosen on the basis of the following:Production strategy.Reservoir properties and quality.Well design.Well lifetime.Re-entry abilities.
The most common mechanical completion solutions are the following:Cased hole.Preperforated liner.Standalone screen.Gravel pack.
Cased holes typically are cleaned underbalanced before perforating. Preperforated liners normally are installed in the drilling fluid and are cleaned with the open hole exposed. Standalone screens can be installed in drilling fluids, low-solids oil-based drilling fluid, brine, or brine-based kill fluid. Displacement of drilling fluid to brine in the production casing can take place before or after installing standalone screens. A safety valve can be installed on top of the liner/screen to isolate the formation below and improve the displacement conditions with regard to pump-rate and pressure limitations.
In gravel-pack operations, the well can be cleaned and displaced from drilling fluid to brine before or after installing the gravel-pack screens. In some cases, the production casing above the gravel pack is cleaned after the gravel-pack operation is finished, before installing production tubing.
Smart completions normally are installed in a cased hole. The liner is perforated before installing the inner string with packers between the perforated zones to select and control production from different formations. Because of the small tolerances in these wells, a high degree of cleanliness is required.