Drilling Fluids Selection

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II. DRILLING FLUID SELECTION:

Drilling Fluid Selection

Fluid types
Water Base Muds Invert Emulsion Muds

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Section two of this Course discusses in general the relationship applicability of water base and invert emulsion muds to the matters presented in Section one. Section two also includes a brief discussion of the Economics of Drilling Fluid Selection. The most important function of drilling fluids is to provide a stable borehole. Without that, nothing else is possible. Stability is a relative thing. The hole that is going to be drilled and cased off in one day does not need the same inhibition as one that will take a week to drill, and another week to log and run pipe on. Circulating and conditioning, reaming and wiping holes is unproductive time. Cheap muds can become very expensive when rig time becomes part of the mud cost.

II. DRILLING FLUID SELECTION: Fluid Types

WBM Advantages and Disadvantages

Higher frac pressure / frac gradient than SBM Good fracture sealing & healing ability Low gas solubility, promoting detection & handling of kicks Typically cheaper than SBM Perceived to be more environmentally friendly than SBM Rheology / gels / density not as strongly affected by Temperature & Pressure.
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More accretion and bitballing tendencies slower ROP Shale & borehole instability as a function of open-hole time Less lubricating than SBM More differential sticking potential (fluid loss control less tight) Hole cleaning in deviated wells not as good as SBM

II. DRILLING FLUID SELECTION: Fluid Types

Oil or Synthetic Base Muds

Excellent ROP Excellent shale & hole stability Thermally stable Excellent fluid loss control / low differential sticking tendencies Excellent lubricity & tubular wear characteristics No hydrate formation problems
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Low frac pressure & poor fracture healing ability Fluid Rheology strongly dependent on P,T Poor borehole stability when formations are fractured in-situ High gas solubility, complicating kick detection & handling Expensive if losses are heavy Compliance requirements of new NPDES permit

II. DRILLING FLUID SELECTION: Fluid Types

Shale Swell Testing

SHALE SWELLING TEST
BHP, Zamzama 2, Pakistan

80

70

60 PERCENT EXPANSION

D I WA T ER V E R S A D R IL D i s per s ed K+SILD R IL KC l \S ILD R IL KC l \P HPA

50

40

30

20

10

0 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 TIME (MINUTES)

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II. DRILLING FLUID SELECTION

Water Base Mud Systems

Dispersed / Deflocculated Non-Inhibitive
Freshwater / Lignosulfonate

Dispersed / Deflocculated - Inhibitive

Lime / GYP Seawater / Lignosulfonate

Non Dispersed Non-Inhibitive

Freshwater / Polymer

Non-Dispersed - Inhibitive
Seawater & NaCl / Polymer Muds KCl / Polymer Muds Glycol & Amine Additive / Polymer Muds
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Water base muds may be classified on the basis of the groupings shown in the slide. Fresh water / Lignosulfonate muds are probably the worst choice possible in a drilling fluid. Lime muds have applications where high mud weights, and moderate inhibition is required. Seawater / Lignosulfonate muds are only marginally better than Freshwater / Lignosulfonate. Freshwater / polymer muds can be run in certain areas of low reactive clays. They are low solids fluids if run correctly, but only provide a limited time for drilling and evaluation of a hole section. The nondispersed / inhibitive fluids are the peak of water base mud technology at the present time.

II. DRILLING FLUID SELECTION: Fluid Types

Inhibition in Water Base Mud

Inhibitive Additives:
Potassium Chloride GLYCOLS (High Molecular Weight) Amines

Inhibitive Systems:
KCl / PHPA Lime Muds Cationic Polymer Muds Calcium Chloride Muds Aluminum Complex Muds Formate Based Muds
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Water base muds may be formulated with inhibitive additives. Some of these work better than others. Usually these muds use combinations of polymers and inhibitors such as KCl to delay hydration and maintain hole stability for a long enough period for the hole to be drilled, evaluated and cased off. A number of inhibitive mud systems are on the market now. Some of them more in the realm of the experimental and exotic than a tried and true system. KCl / PHPH muds are probably the best all around proven inhibitive water base system. KCl may no longer be used offshore in the Gulf of Mexico. No current water base mud can provide total inhibition. This is only possible when water is eliminated as the continuous phase of the drilling fluid. Another limitation of water base muds is in ROP. Drilling rates with oil muds far surpass that of water base fluids.

II. DRILLING FLUID SELECTION: Fluid Types

Cuttings from GOM Using CaCl2 Mud in 4500 Water

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SPE 59186

II. DRILLING FLUID SELECTION

Invert Emulsion Mud Systems

Diesel Base Fluids Mineral Oil Base Fluids Synthetic Base Fluids Lime Based Emulsifiers Lime Free Emulsifier Package 100 % Oil Systems Balanced Activity Water Phase High Salinity Water Phase
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Several Invert Emulsion systems may also be defined on the basis of base fluid utilized, emulsifier package, and some specialty applications Invert Emulsion Drilling Fluids provide far superior performance to water base fluids. As shown previously, calcium chloride must be added to the water phase of an invert emulsion mud to provide an osmotic force that will prevent water being sucked from the mud into the cuttings and borehole wall. There is still some controversy about whether it is correct to merely balance the forces in the formation, or whether it is better to run a high enough WPS to exceed the hydrational forces. Unocal has found in our operations that higher water phase salinities are better. In general we run water phase salinities of above 30 % by weight CaCl2, or 300,000 ppm CaCl2. Applying a dehydrating force on the formation, particularly on the cuttings, allows for better hole cleaning and reduced hole problems.

II. DRILLING FLUID SELECTION

Hole Stabilization with Invert Emulsion Drilling Fluids

OIL IS THE CONTINUOUS PHASE IN THE MUD Calcium Chloride added to the water phase of the mud generates osmotic force and may be used to dehydrate formation clays.

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Invert Emulsion drilling fluids are far superior to any water base fluid for inhibition and ROP, and usually would be the fluid of choice for most Unocal operations.

II. DRILLING FLUID SELECTION

High Temperatures and Pressures

Normal water base muds have temperature limitations of + / - 300 F. High temperature water base muds (+/- 400 F) may be formulated using special additives. Invert emulsion drilling fluids may be formulated to be stable to temperatures in excess of 400F by increasing concentrations of standard products, and may be converted into higher temperature formulations during the course of a well as conditions dictate.

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High temperature drilling fluids require special formulations. Water base fluids require special products. Oil base fluids usually just require higher concentrations of normal emulsifiers and fluid loss agents.

II. DRILLING FLUID SELECTION: ECONOMICS

Economics
Drilling fluid cost should always be considered as part of the big picture of operational costs. Drilling faster wells is the objective. That is where the money may be saved.

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No decision on Drilling Fluid selection should be made without consideration of the Economics. Too often the cost of the fluid is the only consideration made, and a minimalist approach is used. This often leads to lost time and increased well costs due to mud related hole problems. Decisions on drilling fluids should never be based solely on the cost of the fluid, but on the value of the fluid to the overall operation. This is easy to see in a Deepwater operation where rig rates and daily operation costs are so high that the potential saving of a day or more on a well more than pays for the best mud and mud system obtainable. Where the decision becomes more difficult is at the other end of the scale, such as with land rigs drilling relatively shallow wells. Nevertheless, the costing exercise is relevant, and the effect to overall operating bottom line of spending more on mud should be examined.

II. DRILLING FLUID SELECTION: ECONOMICS

Economics
Price per Sack Price per Barrel Mud Price per Well Total Well Cost Total Well Cost per BOE/Day (production)

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II. DRILLING FLUID SELECTION: ECONOMICS

Ship Shoal Wells SBF vs WBM Ship Shoal Wells SBF vs WBM
0 2,000 4,000 6,000 8,000

DEPTH (MD)

12-1/4" WBM (M-2) 12-1/4" SBF (H-7) 12-1/4" SBF (H-8) 8-1/2" SBF (H-9) 12-1/4" SBF (H-10) 8-1/2" SBF (H-11) 8-1/2" SBF (H-12)

10,000 12,000 14,000 16,000 0 2 4 6 8 10 12 14 16 18 20

DAYS

22 24

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