Conventional drilling and completion fluids containing weighting solids or hydrocarbons or halide brines can create problems with hydraulics, well control, well integrity and well productivity in HPHT operations. The negative influence of conventional fluids on drilling and completion operations can be sufficiently serious to compromise safety and degrade the economics of challenging HPHT field developments.
Formate brines have been developed specifically to provide improved drilling and completion fluids that are free of the troublesome components found in conventional fluids and therefore better suited to meet the needs of oil companies involved in difficult HPHT well constructions.
Formate brines have been successfully used as reservoir drill-in, completion, workover and suspension fluids in more than 130 HPHT well construction operations over the past 10 years. These applications have included 100 cases in which high down-hole pressures have necessitated the use of cesium formate brines for well control purposes.
Some 15 applications of cesium formate brines to date have been HPHT reservoir drill-in operations in high angle wells where operators considered that conventional fluids could create a safety risk and adversely effect project economics. We review the published information on the field performance of the cesium formate brines in HPHT applications, and conclude that the novel benefits of the technology that were first promised some 15 years ago during the early product development phase have now been fully validated.
The objective of the drilling and completion process is to safely deliver high quality wells that are optimized in terms of providing shareholder value:
Best well productivity at lowest drawdown
Best well integrity and longest structural lifetime
Lowest well construction cost
Lowest environmental impact and liability exposure
Best reservoir information capture
The choice of drilling and completion fluid used in a well construction operation has a critical influence on the extent to which an operator can meet this objective. In particular the fluid's performance will play a significant part in determining whether or not an operator meets its key performance indicator targets in the following areas:
Time to drill and complete
Well control and safety incidents
Well lifetime and maintenance costs
Well productivity index
Waste management costs
Logging capability and interpretation
Environmental footprint and impact
Exposure to liability (short- and long-term)
The drilling fluid chosen for the upper well sections must offer a host of functionalities:
Ability to maintain the integrity of weak rocks
Ability to minimize fluid loss into permeable rocks
Ability to provide stable well control
Ability to efficiently transfer hydraulic power
Ability to move cuttings to the surface
Provide steel/steel and steel/rock lubricity
Provide protection against all forms of corrosion
Allow formation evaluation
Pose little or no hazard to rig personnel
Have little or no adverse effect on the environment
Have little or no adverse effect on elastomers
If the drilling fluid is to be used in reservoir sections without further intervention it must cause minimal change to the native permeability of the reservoir rock in the near wellbore area. The drilling fluid filtrate must also be compatible with other filtrates that might leak-off from subsequent cementing and completion operations. A completion fluid should have the same overall properties as a reservoir drilling-in fluid and, ideally, should be the same fluid minus any drilled solids.