Selecting a Drilling Fluid
Authors: Bleier Roger 
Published: December 2007

" Although acceptable substitutes have been developed for some additives, the search for adequate replacements for environmentally objectionable systemscontinues to be the highest research priority in drilling priority in drillingfluids."

Introduction
Selection of the proper drilling fluid is important to the success of adrilling operation. No fluid is suitable for all situations. Fluids withdifferent base liquids, different dominating cations in the aqueous phase, different chemical additives, or broadly diverse physical characteristics havedifferent behaviors, physical characteristics have different behaviors, makingfor a large menu of choices. The names of drilling fluid types typically areformed by stringing together one or more of the descriptors in Table 1. Thestring is as long as necessary for the desired degree of precision (e.g., seawater lignosulfonate mud, saturated salt mud, potassium mud, relaxed-fluid-loss oil mud, gypsum potassium mud, relaxed-fluid-loss oil mud, gypsum mud, and polymer mud). Drilling-fluid selection can requireconsideration of numerous factors. The most important are safety, evaporitezones, high temperatures and pressures, environment, loss zones, shaleproblems, well trajectory, and economics. Taking these one at a time, and inorder, will in most cases lead to a proper choice of drilling fluid. properchoice of drilling fluid.

FLUID-SELECTION FACTORS
Safety Issues (Well Control, Gas Hydrates, H2S). Safety is paramount. Thefluid must be able to carry the mud weight required to control the well, andcirculating, surge, and swab pressures must not be excessive. Speed and easewith which a mud will accept weighting materials can be important in kicksituations. Although most mud types are satisfactory in these respects, at mudweights above about 15 Ibm/gal, polymer muds will need some dispersant andpolymer muds will need some dispersant and oil muds will need oil/water ratioshigher than 50/50. Gas hydrates can interfere mechanically with well-controloperations in deepwater drilling. Salt muds are currently the fluids of choicein deep water because high salinity tends to suppress hydrate formation. Theeffect of mud composition in preventing hydrates is the subject of ongoingresearch in several laboratories. The safest fluids for drilling H2S-bearingzones contain at least 10 Ibm/bbl excess lime plus a sulfide scavenger. Limeand oil muds plus a sulfide scavenger. Lime and oil muds are the only two mudtypes that are truly compatible with this requirement.
Evaporite Zones. Massive evaporite zones tend to leach out excessively intowaterbased drilling fluids unless the fluids are presaturated with theevaporite before the presaturated with the evaporite before the zone isdrilled. Gypsum muds are a natural choice for massive anhydrite sections. Thicksalt sections require a saturated salt mud or an oil mud. A properly formulatedoil mud is a good choice for any evaporite zone.
High Temperatures and Pressures. Hightemperature gelation andfluid-loss-control problems occur for most water-based mud problems occur formost water-based mud types at downhole temperatures of 250 to 350 degrees F. Specially formulated water-based muds are stable to 400 to 450 degrees F at mudweights up to 18 Ibm/gal. Oil muds are a viable, and sometimes less costly, alternative at these elevated temperatures and pressures.

Environmental Considerations.
Permit restrictions can limit the choice ofmud type. Environmental considerations are many and varied, depending on thewell location. They have led to avoidance of oil muds in some areas, salt mudsin others, and high-pH or chromium-treated muds in still others. Drilling-fluidbioassay tests are useful for assessing the toxicity of special additives. Although acceptable substitutes have been developed for some additives, thesearch for adequate replacements for environmentally objectionable systemscontinues to be the highest research priority in drilling fluids.

Severe Loss Zones.
Lost-circulation materials are added to muds to seal offthief zones when mud losses occur downhole. Some zones may be known in advanceas being prone to losses and difficult to seal. If it is prone to losses anddifficult to seal. If it is anticipated that large volumes of mud may be lostin an interval, the mud type should be simple and inexpensive if possible. Thistends to rule out oil muds and heavily treated water-based muds. Insubnormal-pressured intervals, aerated fluids may merit consideration.

Shale Problems.
Shale formations can swell, disperse, or slough into thehole.
JPT
P. 832