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DRILLING FLUID

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RHEOLOGY Rheo = Flow

Logy = To Study

Rheology is the science of studying ow.

Rheology is de ned as hysics of the ow and the defo!"ation of "atte!.

Rheology desc!i#e the #eha$io! of uids in "otion.

Rheology !elate di!ectly to #o!ehole sta#ility and how e%ecti$ely the#o!ehole is cleaned.

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Rheology a!e not e&act sciences' #ut a!e #ased u on "athe"atical "odelsthat closely desc!i#e the !heology of the uid and do not confo!" e&actlyto any of the "odels.

(onse)uently' di%e!ent "ethods a!e used to calculate !heology.

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FLUID DEFORMATIONRheology is the study of the defo!"ation of all fo!"s of "atte!.

The defo!"ation of a uid can si" ly #e desc!i#ed #y two a!allel latesse a!ated #y so"e distance as shown in Figu!e.

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SHEAR STRESS*n a lied fo!ce +F,' acting o$e! an a!ea +*,' causes the laye!s to slide astone anothe!.

The!e is a !esistance' o! f!ictional d!ag' fo!ce that o oses the "o$e"entof these lates. This !esistance o! d!ag fo!ce is called shear stress

The unit of shea! st!ess is of l#f /00 ft1 o! as Dynes c"1 .

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SHEAR RATE The di%e!ence in the $elocities #etween two laye!s of uid di$ided #y thedistance #etween the two laye!s is called the shea! !ate + 3 ,.

This is e& !essed in sec4/ +!eci !ocal seconds,.

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VISCOSITY 5iscosity is the !e !esentation of a uid6s inte!nal !esistance to ow'de ned as the !atio of shea! st!ess to shea! !ate.

5iscosity is e& !essed in oise.

* oise is a $e!y la!ge nu"#e! and the!efo!e' $iscosity is ty ically !e o!tedin centi oise +/00 centi oise = / oise,.

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FLUID TYPES The !elationshi #etween shea! st!ess + - , and shea! !ate + 3 , defines the

ow #eha$io! of a uid.

Fo! so"e uids' the !elationshi is linea!. If the shea! !ate is dou#led' then theshea! st!ess will also dou#le.

These uids a!e called Newtonian uids. 7&a" les of Newtonian uids includewate!' alcohols' and light oils.

5e!y few d!illing uids fall into the Newtonian catego!y.

Fluids which ha$e ow cha!acte!istics such that the shea! st!ess does notinc!ease in di!ect !o o!tion to the shea! !ate a!e called non4Newtonian uids.

8ost d!illing uids a!e of this ty e.

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NEWTONIAN FLUIDSNewtonian uids a!e those in which the $iscosity !e"ains constant fo! allshea! !ates !o$iding te" e!atu!e and !essu!e conditions !e"ainconstant.

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NON-NEWTONIAN FLUIDSNon4Newtonian uids ha$e $iscosities that de end on "easu!ed shea!!ates fo! a gi$en te" e!atu!e and !essu!e condition.

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FLOW REGIMES The!e a!e th!ee #asic ty es of ow !egi"es.

i. La"ina!

ii. Tu!#ulent

iii. T!ansitional

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1. Laminar fo. La"ina! ow occu!s at low4to4"ode!ate shea! !ates when laye!s of uid"o$e ast each othe! in an o!de!ly fashion with a "ini"u" of "i&ing.

. This "otion is a!allel to the walls of the channel th!ough which the uid is"o$ing.

. F!iction #etween the uid and the channel walls is lowest fo! this ty e ofow.

. Gene!ally' la"ina! ow is the !efe!!ed annulus ow !o le #ecause it!esults in less !essu!e loss and !educes hole e!osion.

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!. T"r#"$%n& fo. Tu!#ulent ow occu!s when the uid is constantly swi!ling as it "o$esth!ough the ow channel.

. 9!essu!e losses within a ci!culating syste" inc!ease as the deg!ee oftu!#ulence inc!eases.

.(o""on ow !egi"e in no!"al d!illing o e!ations' #ut would a ea! "o!ein i e !athe! than in annuli

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'. Tran(i&ion F$o. T!ansitional ow is a ow !egi"e whe!e the uid "otion is neithe!co" letely la"ina! no! co" letely tu!#ulent.

. The T!ansitional Flow Regi"e uid is in a t!ansitional state f!o" one ow!egi"e to the othe!

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DETERMINATION OFFLOW REGIME

Flow !egi"es can #e dete!"ined #y Reynolds nu"#e!.

Reynolds nu"#e! ta:es into conside!ation the #asic facto!s of i e ow ; i edia"ete!' a$e!age uid $elocity' uid density' and uid $iscosity.

Reynolds nu"#e! is de ned as<

whe!e'V = a$e!age uid $elocity

D = i e dia"ete!

= fluid density

> ) uid $iscosity

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F$o T*+% R%*no$,( N"m#%r

La"ina! ?= 1000

T!ansitional @etween 1000 ; A000

Tu!#ulent B= A000