Minimising drilling costs is an ongoing concern, especially with global uncertainty and volatile oil prices. One area in which costs can be reduced is in the wellbore cleanup phase. Inefficient cleanup can result in large volumes of liquid waste that must be treated or disposed of, particularly if a non-aqueous drilling fluid (NADF) is used. In addition, completion fluid costs can be significant, especially where high-density brines are required for wellbore safety. Improved cleaning efficiency can lead to significant cost savings.
Wellbore cleanups in high-temperature, high-pressure (HTHP) wells can be particularly challenging. Not only is high-density brine required to maintain hydrostatic overbalance to control formation pressures, the brine is often at or close to its saturation point. The selection of surfactants that function effectively in high-salinity fluids can be very challenging. Extensive laboratory testing must be performed to ensure compatibility between the cleanup spacer and the drilling fluid. The performance of the cleanup is then assessed through a series of bottle and viscometer sleeve tests to ensure the solids and metal surfaces were rendered fully water-wet.
A significant challenge when designing cleanup spacers is balancing excellent performance of oily residues and solids removal with thermal and chemical stability. Microemulsion cleanup spacers have been used to perform efficient wellbore cleanups in deepwater applications, resulting in reduced volumes of waste fluids. This paper describes the field application of a high-density caesium formate brine microemulsion cleanup spacer to efficiently displace and remove a high-density barite and manganese tetraoxide-weighted NADF. Results show that microemulsions exhibit excellent displacement and cleanup performance of a challenging drilling fluid. Microemulsion technology results in a quicker cleanup with reduced waste volumes, thereby minimising costs.
The high value of caesium formate brine means that maximising the recovery of this valuable asset is essential to justify the economics of the cleanup process. This paper describes the transfer of microemulsion cleanup technology used in deepwater environments to HTHP applications.