The ultimate goal of zonal isolation during well construction is to achieve complete coverage of every string of casing/liner, to help ensure competent isolation to protect casing strings, the environment, and natural aquifers in a cost-effective manner (Halliburton 2012). Typically, water-based muds (WBMs) are used to drill the surface through intermediate sections of the well (surface and intermediate). WBMs are preferred near the surface because of their lower cost and minimal environmental impact. When drilling the zone of interest, however, WBM is often substituted with oil-based mud (OBM). During this process, if cement is not brought back to surface, the operator can either leave OBM in the annulus above the top of cement (TOC) or displace it with a different fluid (usually WBM). Because OBM can be expensive, leaving it in the annulus increases the cost of the well. Particularly, increased oil prices and/or constrained budgets make reclaiming and recycling an expensive commodity like OBM worth pursuing (Duckworth 2011). WBM is a cost-effective fluid that can be used to displace the OBM from the annulus; however, WBM's may lead to costly casing corrosion and do not have the potential to consolidate or exhibit bonding above the TOC, which is desired.
This paper presents use of a novel and innovative efficiency fluid system that uses a microparticulate material with a high surface area to economically displace OBM from the annulus while effectively cleaning the wellbore. The rheology and density of the fluid can be designed to remain stable, therefore preventing settling of the microparticulates during placement. This efficiency fluid can be placed in the wellbore at volumes and costs similar to WBM's, developing casing and formation bonding over time. This bonding behavior results from the material consolidating after it has been placed, reducing the possibility of casing corrosion. By designing the tunable rheology and density of the system, improved displacement efficiency should result, leading to improved bond logs. It can be mixed on-the-fly with normal equipment, removing the need for batch mixing, which provides convenience to the service company and/or operator. It can be foamed to densities down to 8.0 lbm/gal and has been used up to 16.0 lbm/gal. Additionally, the system contains no hazardous materials and can be made with reused water, helping reduce cost and environmental impact. Case histories in which the system helped achieve optimum displacement and promoted increased protection and isolation behind the casing string from top to bottom are presented using both small and large volume example jobs.
Introduction
Wellbore preparation is fundamental to achieving optimum cement placement and bonding for zonal isolation integrity. For decades, research and development resources have focused on gaining an understanding of the problems and issues that can lead to incomplete cement placement and result in inadequate zonal isolation. As early as 1928, it was determined that many cementing failures were caused by inefficient wellbore fluid displacement, resulting in wellbore fluids channeling in the cement column (Halliburton 1996). These uncemented wellbore fluid channels provide a permeable conduit for reservoir fluids to migrate, causing loss of production and/or corroded casing over time.
