Functions of drilling fluid

A properly designed and maintained drilling fluid performs essential functions during well construction such as transporting cuttings to the surface, preventing well-control issues and wellbore stability, minimizing formation damage, cooling and lubricating the drillstring and providing information about the wellbore.

Transport cuttings to surface

Transporting drilled cuttings to the surface is the most basic function of drilling fluid. To accomplish this, the fluid should have adequate suspension properties to help ensure that cuttings and commercially added solids, such as barite weighing material, do not settle during static intervals. The fluid should have the correct chemical properties to help prevent or minimize the dispersion of drilled solids, so that these can be removed efficiently at the surface. Otherwise, these solids can disintegrate into ultrafine particles that can damage the producing zone, and impede drilling efficiency.

Prevent well-control issues

The column of drilling fluid in the well exerts hydrostatic pressure on the wellbore. Under normal drilling conditions, this pressure should balance or exceed the natural formation pressure to help prevent an influx of gas or other formation fluids. As the formation pressures increase, the density of the drilling fluid is increased to help maintain a safe margin and prevent “kicks” or “blowouts.” However, if the density of the fluid becomes too heavy, the formation can break down. If drilling fluid is lost in the resultant fractures, a reduction of hydrostatic pressure occurs. This pressure reduction also can lead to an influx from a pressured formation. Therefore, maintaining the appropriate fluid density for the wellbore pressure regime is critical to safety and wellbore stability.

Preserve wellbore stability

Maintaining the optimal drilling-fluid density not only helps contain formation pressures, but also helps prevent hole collapse and shale destabilization. The wellbore should be free of obstructions and tight spots, so that the drillstring can be moved freely in and out of the hole (tripping). After a hole section has been drilled to the planned depth, the wellbore should remain stable under static conditions while casing is run to the bottom, and cemented. The drilling-fluid program should indicate the density and physicochemical properties most likely to provide the best results for a given interval.

Minimize formation damage

Drilling operations expose the producing formation to the drilling fluid, and any solids and chemicals contained in that fluid. Some invasion of fluid filtrate and/or fine solids into the formation is inevitable. However, this invasion, and the potential for damage to the formation, can be minimized with careful fluid design that is based on testing performed with cored samples of the formation of interest. Formation damage also can be curtailed by expert management of downhole hydraulics using accurate modeling software, as well as by the selection of a specially designed “drill-in” fluid, such as the systems that typically are implemented while drilling horizontal wells.

Cool and lubricate the drillstring

The bit and drillstring rotate at relatively high revolutions per minute (rev/min) all or part of the time during actual drilling operations. The circulation of drilling fluid through the drillstring and up the wellbore annular space helps reduce friction. and cool the drillstring. The drilling fluid also provides a degree of lubricity to aid the movement of the drillpipe and bottomhole assembly (BHA) through angles that are created intentionally by directional drilling and/or through tight spots that can result from swelling shale. Oil-based fluids (OBFs) and synthetic-based fluids (SBFs) offer a high degree of lubricity, and, for this reason, are the preferred fluid types for high-angle directional wells. Some water-based polymer systems also provide lubricity approaching that of the oil- and synthetic-based systems.

Provide information about the wellbore

Because drilling fluid is in constant contact with the wellbore, it reveals substantial information about the formations being drilled, and serves as a conduit for much data collected downhole by tools located on the drillstring and through wireline-logging operations performed when the drillstring is out of the hole. The drilling fluid’s ability to preserve the cuttings as they travel up the annulus directly affects the quality of analysis that can be performed on the cuttings. These cuttings serve as a primary indicator of the physical and chemical condition of the drilling fluid. An optimized drilling-fluid system that helps produce a stable, in-gauge wellbore can enhance the quality of the data transmitted by downhole measurement and logging tools as well as by wireline tools.

Minimize risk to personnel, the environment, and drilling equipment

Drilling fluids require daily testing, and continuous monitoring by specially trained personnel. The safety hazards associated with handling of any type of fluid are clearly indicated in the fluid’s documentation. Drilling fluids also are closely scrutinized by worldwide regulatory agencies to help ensure that the formulations in use comply with regulations established to protect both natural and human communities where drilling takes place. At the rigsite, the equipment used to pump or process fluid is checked constantly for signs of wear from abrasion or chemical corrosion. Elastomers used in blowout-prevention equipment are tested for compatibility with the proposed drilling-fluid system to ensure that safety is not compromised.

The upper hole sections typically are drilled with low-density water-based fluids (WBFs). Depending on formation types, downhole temperatures, directional-drilling plans, and other factors, the operator might switch to an OBF or SBF at a predetermined point in the drilling process. High-performance WBFs also are available to meet a variety of drilling challenges.

Depending on the location of the well, the drilling-fluid system can be exposed to:

• Saltwater flows
• Influxes of carbon dioxide and hydrogen sulfide
• Solids buildup
• Oil or gas influxes
• Extreme temperatures at both ends of the scale
• All of these

Contamination also comes from contact with the spacers and cement slurries used to permanently install casing, and in the course of displacing from one drilling-fluid system to another.

The drilling-fluid specialists who prepare drilling-fluid programs should be aware of the operational and environmental challenges posed by any well. Working closely with the operator, the specialist (who typically is supported by technical experts and a research staff) can plan for the scope of conditions that are likely to be encountered, and generate a program that is both safe and cost-effective. The planning stage usually includes the identification of specific performance objectives and the means by which success will be measured.

Throughout the well-construction process, the drilling-fluid personnel assigned to the operation maintain:

• Accurate records of test results
• Fluid volumes
• Drilling events
• Product inventory
• Actions related to achieving environmental compliance

The standard drilling-mud report reflects the type of information the drilling-fluid personnel (often called “mud engineers”) provide at the rig site on a daily basis. These reports, often computer-generated and stored in a database, and the post-well analysis performed at the conclusion of the well serve as reference materials for future wells in the same area or wells that present similar challenges.

See also

PEH:Drilling_Fluids

Noteworthy papers in OnePetro

J. Griffith, Halliburton Energy Services, Inc.; S. O. Osisanya, University Of Oklahoma: Effect of Drilling Fluid Filter Cake Thickness And Permeability On Cement Slurry Fluid Loss, 99-13-15, http://dx.doi.org/10.2118/99-13-15

B. Jensen, J.E. Paulsen, A. Saasen, Statoil ASA; O.I. Prebensen, H. Balzer, M-I/Swaco Norge AS: Application of Water Based Drilling Fluid - Total Fluid Management, 87103-MS, http://dx.doi.org/10.2118/87103-MS