Emulsion-Based Fluid Used to Optimize Casing-Cleaning Operations

Abstract

The removal of drilling fluids from the cased well prior to installation of critical completion equipment often involves use of special fluids to ensure a complete removal of both the drilling fluid itself and particles attached to the casing wall. This is a critical operation where failure can lead to excessive complications in the subsequent completion operation with stuck packers or formation damage due to particle invasion into the formation as a consequence. Furthermore, these operations have the highest attention from the environmental authorities because casing cleaning operations are considered as highly polluting operations during drilling and completion of a well. Modern drilling fluids used in the North Sea region are designed to meet occupational hygiene requirements and environmental criteria. These drilling fluids are relatively difficult to remove from the casing surface. The current paper describes in detail a well cleaning fluid system designed to meet these technical objectives with special focus on reducing environmental impact. The casing cleaning fluid system is designed using a partly unstable emulsion, where the surfactants are partly dissolved in the water phase and partly used as emulsifiers. Since the emulsion is unstable, the individual components can be separated after the operation. This allows for re-use and recovery of the fluid components at the same time as the fluids has optimized the well cleaning efficiency. The removed base oil can be used to formulate new drilling fluid or re-used in casing cleaning fluid systems. The improvement of new designed fluid has been verified in both laboratory tests and North Sea field experiences. Field experience from approximately 30 operations show that the average operation time has been significantly reduced. The paper compares field experience obtained using different casing cleaning operation techniques. Introduction After finishing the drilling and cementing operations of an oil/gas well it is necessary to displace the drilling fluid system to a completion fluid system in the well. Well completion requires the use of a packer fluid in the annulus between the casing and the production tubing. Different well completion designs and reservoir conditions set limits on the content of particles in the packer fluids. The required cleanliness of this fluid can vary from several hundred Nephelometric Turbidity Units1 (NTU) down to 10–20 NTU. The displacement of a drilling fluid with a completion fluid is normally performed through several steps. First the drilling fluid is displaced by a casing cleaning fluid. In addition to act as a displacement fluid, the casing cleaning fluid shall remove, or dissolve the remaining oil based drilling fluid and possible debris remaining after the drilling operation. Normally, seawater follows the casing cleaning fluid. Finally the seawater in the well is displaced by the packer fluid. The complete displacement of a fluid by another by hydraulic forces, only, is difficult to achieve in an annulus2. To obtain an efficient displacement of the drilling fluid with a packer fluid, it is common to use different types of chemical additives in the cleaning fluids pumped between the drilling fluid and the packer fluid3. The function of these chemicals is to disperse and break different components of the drilling fluid as well as decrease the surface activity between the two fluids. If oil or synthetic based drilling fluids were used these chemicals would be oil dissolving soaps. If water based drilling fluids were used, it is possible to rely more on displacement efficiency alone. Soaps are not required, however, there may possibly be a need for dispersants in some cases.

Contamination of cement slurries with diesel-based drilling fluids in a shale gas well

“…OBMs is a non-aqueous fluid (NAF) system that offers great benefits to drilling through high lubricity and low friction coefficients, superior shale stabilization, excellent fluid loss control, ensure adequate cooling of the drill string, low bit-balling tendency and high associated rate-of-penetration (ROP), etc. (Darley and Grey, 1988;Mike et al, 2000;Berg et al, 2002;Nagarajan et al, 2010). But these benefits of OBMs do not extend to compatibility with cement, unfortunately.…”

Effective High-Density Wellbore Cleaning Fluids: Brine-Based and Solids-Free

“…In effect, these enhanced SOBM are significantly more difficult to break and displace from the wellbore than are traditional muds. As a result, new chemistries for chemical wash spacers have been developed and implemented in the field 6,7 .…”

Effective High-Density Wellbore Cleaning Fluids: Brine-Based and Solids-Free

“…In effect, these enhanced SOBM are significantly more difficult to break and displace from the wellbore than are traditional muds. As a result, new chemistries for chemical wash spacers have been developed and implemented in the field (Berry 2005;Berg et al 2002).…”