In a time when drilling efficiencies are significant, the advancement of conventional clear brine systems has led to both economic and technological breakthroughs. Clear brine fluids have been in the industry since the 1950's, and since inception, different base brines have been utilized to maintain different densities.
Calcium chloride is a great base fluid due to its natural density and highly lubricious nature. By adding calcium nitrate, densities have been increased to 1.6 sg (13.35 lb/gal), exceeding the density of potassium formate 1.57 sg (13.1 lb/gal). These calcium brine systems can be cost effective providing a reduction in cost for base fluid up to 90% as compared to formate or bromide systems. However, divalent brines are known for having excessively high corrosion rates and compatibility issues when used with polymers.
By focusing efforts on process control and product compatibility, low corrosion rates can be achieved in a brine system. After exhaustive autoclave testing, a corrosion package was developed and perfected in the field. Results of a 140-well dataset showed the calcium chloride system with a novel corrosion package yielded better results than formate brines employed in the same area. The exhaustive efforts in the field also led to determining a suitable flocculent polymer that can hydrate in the calcium chloride/calcium nitrate dual-brine system. This novel application of a polymer provided improvements in both corrosion and drilling performance.
This paper discusses the use of clear calcium chloride brine drilling fluids in drilling over 140 wells and the challenges overcome. It also details the equipment used and highlights the comprehensive technique for evaluating all aspects of the drilling fluid.
Clear brine fluids create density without the need to maintain viscosity solely to carry weight-material particulates as all the weight-material is totally dissolved. The fluid density is dictated by the amount and type of salt used. Each salt has a maximum concentration before it reaches saturation limiting the maximum density of the solution. Thermal expansion of water affects the solubility, crystallization point, and density of clear brine.