CA2005195A1 - Monitoring drilling mud - Google Patents

Abstract

ABSTRACT

MONITORING DRILLING MUD
In the rotary drilling of oil wells a drilling mud is used both to transport the cuttings up to the surface and to impose an hydrostatic pressure on the walls of the borehole. For these functions the mud must for example have an acceptable viscosity and density. It is therefore important to monitor the characteristics of the mud, and to keep them within certain limits. Only recently, however, has drilling practice recognised the importance of monitoring the mud's ionic composition. The various techniques proposed involve separation of the mud into liquid and solid portions, and analysis of these. Though they have have proven useful, yet there are a number of problems. For example, the separation has not always been easy, and the available techniques often may not satisfactorily remove the fines.
The invention suggests that these two problems, at least, can be overcome by the relatively simple expedient of first acidifying the mud sample, for acidification both causes the mud particles to flocculate, and so be more easily separated off, and causes the active fines to dissolve.
In a preferred embodiment hydrobromic acid is employed, together with tetramethylammonium bromide (a displacement agent enabling the mud solid's Cationic Exchange Capacity to be measured), the separation is by filtration, and the analysis is by ion chromatography - and the results are fed into a computer model that then calculates the original mud components.

Description

3~i MoNl~roRING ~RILLrNG MUD

Ihis invention relates to the monitoring of drilling m~d, and concerns in particular a method ~or monitoring changes in the chemical ccmposition of the mud, preferably by ion chr~matography ~t the rig site during dr.illing opeLations.
In the rckary drilling of wells, su.ch as h~dm czrton (oil an~ gas) wells, a mud is continuousl~ circulated frcm the surface down to the bottom of the hole being drilled and ~ack to the surface again. ~he mu~ -usually a fluid mixture of a clay such as bentonite suspended Ln a continuous phase such as water - has several functions. One of these is to transport the cuttings drilled by the drill bit up to the surface where they are æparated frcm the mud. For ~his purpcse the mud must be viscous enough to entrain the cuttings yet ~luid ~nough to pump. Another function is to impose an hydrostatic pressure on the walls of the borehole so as to avoid a collapse of the borehole and an Lnflux of gas or liquid from the formati~ns- being drilled. For this function the mud must be dense enough to resist formation pressure, yet not so dense that its pressure forces it deep into the formations, possibly fracturing them. It is therefore important to monitor the characteristics of the mud, and to keep them within certain limits. Weighting materials/ barite for example, are added to the mud to make it exert as much pressure as needed to contain the formation pressures. Clay is added to the mud so as to keep the drilled cuttings in suspension as they move up the hole. The clay also sheathes the wall of the hole (this thin layer of clay, called mud cake, forms a permeability barrier, and prevents ox reduces fluid loss). Numerous chemicals are available to give the mud the exact properties it needs to make it as easy as possible to drill the hole, and the importance of the mud, and the difficulties of controll mg its composition, can be further appreciated frcm the following additional comments.
Maintaim ng the stability of the borehole is one of the major problems encountered in drilling oil and gas wells. It has keen observed in the field that holes m shale sections freyuently go out of gauge, dNe to loss of material from the borehole wall. This material can become detached from the w~ll in the form of large fragments (cavings), which are normally carried to the surface by the circNlating mMd, just as the drille1 ~q~ .3 cuttLngs are. However, if ~he hole-cleaning C~paCity of the mud is insufficient, cavings collect on ledges, an~ may cau~e khe drill pipe ~o ~tick on pulling out of the hole. The necessity to re-drill through fill accumulated on the bottom of the hole durLng trips is anoth~r resulk of the process.
M~reover, ~egar~less of the effici~ncy of the cavLngs remr~val, in all cases there is ineNitably a gradual bulld-up of dispersed particles in the mNd, which particles axe too f.ine to be removed by the solids control equipment. This m~y give ri æ to a hos~ o~ secondary problems. For instance, the increased solids content slGws down the drilli~g rate, and, as drilled solids form a poor filter cc~ke, problems in controll m g the fluid 106s may cau æ differential sticking on Fermeable sands. In addition, there may be difficulty controlling ~he mud weight,leading to lost circNlation, and an unstable rheology.
In s~me circumstances, shales swell in contact with the mud in such a way that the well bore diameter decreases. In such cases, identified in the field by a need for frequent reaming, the well bore closes dcwn on to the drill string, and there is once again an increas~d risk of pipe sticking.
The various forms of hole instability resulting frcm the interaction between the drilling fluid and the subterran~an formations penetrated by the borehole are related to the hydxation and dispersion of the clay sediments.
It is known that during the drilling process the ionic composition of the drilling mud changes from its original ~ormulation. Ihese changes in compoeition are in part a m~asure of downhole prwesses which may be termed mud-rock interactions. An important example of mud-rock interactions is ion exchange between cations in the mud and in shale formations. Until recently drilling practice has not requirel the ionic oo~position of the mud to be monitored, so that the extent of these interactions has not been determined, and the composition of the drilling mud has not been acc~rately maintained. ~owever, in the Specification of our c~-pending Application for European Patent No: 88/301,856.6, we have described how impor~ant such a monitorir~ pro~es~ is, and how u~ful it can be~ In general, in that Specification we de~cribe~l a method for controlling the drilling of boreholes hy determ m ing the ionic ;~0~ 3~

compositions of the drilling m~ds anqvor drilled cuttings in order to monitor varicus chemlcal processes which occur in ~he well bores, eg salt water influxes, chan~es in the solubility of salts with changes m pH, and cation exchange pr3cesses involving th~ cations added to the wa~er-base mud (eg. pokassium, calcium) to stabilise sh21e sections.
The varicus general and preferred methods of ~he eælier invention have pr~ven useful in the control of drilling mud composition, and ye~ our fhrther research has indicated that thexe may n~ver*heless be a num~er of problems. For example, the æparation o~ the liquid part of the mud by filtration has not always been easy, for, dependant on the mud's type, its clay ccmponents often ~ulfil only too well ~heLr inte~ded purpose (to shea~he ~he well bore wall with th~ thin, imperme3ble ' ~ d cake" layer), and in jus~ the same way form an almost impenetrable layer in ~he filter apparatus, so reducing the flow of liquid therethraugh to almost nothing, even when filtering under several atmcspheres pressure. A~ain, the filter techniques may not always satisfactorily remove the fines ~the very small particles generated, for instance, by the drilling procedure), and if the fines in a ~ud sample dissolve on dilution of the sample, as will ca~onate fines, then this can seriously affect the ionic concentrations, and so give rise to significantly mislead~ng analytical results.
We have no~ found tha~ these two problems, at least, can bç ovexcome by the rel~tively simple expedient of reducing the pH of the mud sample before its further treatm~nt by filtration and analysis, for pH reduction brings two immediate improvements in the characteristics of the mud.
Firstl~, the m~nner in which the clay ccmponents of the mud are dispersed changes dramatically; the finel almost colloidal, mlld particles flooculate - that is, aggr~gate into large, cse, cl~ps - so that the resultir~
"1~" dispersion bec~es very much easier to filter. Secondly, the active fines - s~ecifically, those car~onate fines that would normally pass through the filter in solid, undissolved form, to cause misleadir~
analytical res~lts - are dissolved, ar~ the filtrate, upon dilution, provides correct ion chramatogaphic analyses.
In one a~pect, t;herefore, this invention prc~vides a ml3thod for the determination of the ionic components Oe a drilliny n~l, ln whic~ method:
a su~table sa~ple of the ~ud has it~ pH reduced, to ~locculate the clay cc~ponents thereof, and to solubilize any u~lis80:Lved active x~

ma~erials therein; and the resNltant prcduct is then separated into solid an~ liquid parts, and the liquld part is subjected to analysis to determine i~s ionic con~ent.
Ihe ionic components of a drillin~ mud ~ay be ions o~ many types, in many ~orms. m e principal ones o~ interest, however, are the potassium, sodium, calcium and magnesium cations, ~nd khe chloride, sulphate ~nd bromlde anions - ~n~ the carkonate and bicarbonate anicns (it is these lat~er that can cause difficulties, for in the presence o~ calcium and/or magnesium catio~s they can form undissociated, insoluble, calcium an~or magne~sium carbonate and bicarbonates). The matter i.s discussed further hereinafter in connection with the preferred method of analysis, ion chrcmatsgraphy.
m e method of the invention appears to be applicable to the determination of any variety of wa~er-based (as o~Fosed to oil-based) drilling mud. A typical water-based mud - and hereinafter referenc~s to mud are to ~ater~based mud, unless some other m~a m ng is clearly intended - is one that is essentially a suspension of a bentonite clay in water (usually sea wa~er, where the drilling takes place off shore) together with varicus additives for ~iscosity, pH and density control. For e~aNple, such a bentonite/sea water mud might contain the following components:

Seawater~dispersed Mud Ccmponent Function ~mcun~s ~1 bentoni~e primary viscosifier 36 XC-polymer viscosifier CMC low viscosity fluid loss control 10 CMC high viscosity viscosifier, fluid loss 2 chrome lignosulphate dispersant as req.
sodium hydroxide pH control 3 sodium carbona~e calcium control 0.9 barite mud density as req.
CMC is CarboxyMethyl Csllulose.
XC is a polysaccharide produced by the action of th~ plant pathocJen Yantbomcras Campestris on carbahydrates.

2 ~ #3~

Other c~mmon ~ o~ mud contain the following cc~ponen~s:~

Freshwater-dispersed Mud (Density-1,500 X~
Component Function . _ . (X~/n~l bentonite primary viscosifier 57 chrcme lignosulphate dispersant 9 lignite dispersank/thinner 6 sodium hydroxide pH con~rol 3 barite weightLng agent 600 Potassiu~Poly~r Inhibitive Mud (Density=1,500 Kg/m Component Function AmKun~
(~m bentonite primary viscosifier 45 C~C low viscosity fluid loss control 1.5 potassium hydroxide potassium~pH con~rol 4.5 XC-polymer sh21e inhibition 9 calcium hydroxide calcium contr~l 13 barite weightiny agent 600 m e methud of the invention starts, naturally, by ~aking a suitahle sample of mud. In principle this mud sample can be taken from anyw.here in the system, but in general it is convenient to sample the return mud twice - once after it has just emerged from the lore (and the cut~ings separated off) and again just before it is re-circulated back down into the well bore jafter an~ additive treatment). ~he first o~ these provides information about what is happening to ~he m~d dawn hole, whilst ~he second provides a check that the subsequent ~reatment did indeed res~ore the m~d to its optimum ccmp~sition. In practice, the first sample is conveniently taken immsdiately below the shale-shaker, and the second is taken either dcwn5tream from the active tank or in the flcw lLne to the drill pipe. Ihe matter will ke m~st clearly understood fram a consideration of Figure 1 of the ac ~ ing Drawings.
Figure 1 shows the mu~ circulation equipment. lhe ~ud 10 is conta ~ ~mud pit 12, called the ac~ive tank. A pump 14 ~raws up the mud fm m the pit through a pipe 16, and ~OrCPd the mud through the discharge line 18, khe stand pipe 20, the rotary hose 22 an~ khe swi~el 24. Ihe mud then flcws m to the kelly 26 and dcwn the borehole 28 in the drill pipe 30 an~ the drill collars 32. The mud reaches the boktom of the hole a~ the drill bit 34, and then flows up to the surface m the annulus 36 and m the mW~ return line 38. The mud then falls w sr a vibrating screen-like device 40, called a shale shaXer.
m e role of the shale ~haXer is to æparate fram the liquid ~hase of ~he m~d the cuttings drilled by the bit 34 and transported up in the annulus by the mud. ~he æparation is made by having the mL~ pass thrcu~h a screen which vibratesO ~he solids (called ~he cut~ings) whlch are larger than the me~h sizP of ~he screen don't pass throu~h the scr en, and are rejected either m a reserve pit (when the drilling rig is on land) or in a barge or the sea (when ~he drilllng operations are conducted offshore). The solid particles conta med'in ~he mud w'hich have a size smaller than the ~esh size of ~he screen pass thxcugh the screen, an~
therefore rema m in the mud. These fine solids comprise part of the w~ighting material added to the mud to reach a certain m~ density, as well as fine solids from the formations traversed by the borehole.
Rfter the shale shaker 40, the mud flows mto the solids control e~uipment, represent~d schematically by 42, thr~ugh the pipe 44. The solids control e~uipment 42 cGuld include a degasser, a desilter and a de~ander (these are not shown separately here). Then the mud falls into ~he pit 10 through the pipe 46. A mud-muxing hopper 48 is generally used to add solid ma~erials like clay and barite to the mud in the active tank.
In the practice of the invention mud samples shculd be taken from both the active tank 12 or preferably dcwnstre3m from a samplin~ tap on line 16 and from the pipe 44 between the shale shaker 40 and the solids control equipment 42. m e sampling shcNld be effected at known tim~s, and at frequencies relevant to thQ application concerned (for example, for general mud engineering, every eight hours or ~o may suffice, whPreas for lithological determination m~re frequent san~le~ - say, every 15 mLrmtes -may be required).
In the method of the invention, then, pre~erably t;wo s~les are taken, one o~ the mud as it i~ on exiting ~he well bor~, and before any treatment, and the other as i~ is just prior to being p~d into the well bore, and after any treakme~t. Frcqn a purely rne~anical point o~ view, each mlld sarnple is conveniently placed in a srnall sanple bottle ~uch that ;~ ~3~ t3~

~he n~d c~plete:ly fills the bottle (care is ta}~n to ensure thak no a~r space is p~nt in t~e sample bottle, t:hus m~n~nisiny ~y r~uction .in pH
caused 1~ ~he absorption o~ ca~Don dioxide fr~m the a~r~.
~ Iav~n~ obtained the mu~ sa~le, the first prcper ~tage ~n the method of t~e ~ ention is to reduce the FH of the san~le, so 2S to cause the clay components to ~loccula~e, and the undi~solve1 act.ive solid materials to dissolve.
In principle/ this reduction can be ef~eoted simply by adding an acid, but in practice ~hls may result in a seri.ous problem, for as the pH
drops to belcw 7 ~ to around 4 or 5, ~ay - the conversicn of ~he carbonates in the sample proceeds t ~ the dQsired bicarbcnates and then to carbonic acid, which immediately dissociates into water and gaseous ~ on dioxlde, and the latter bukbles off, and is lost to the system ~unless special precautions are taken). Consequently, the subsequent analytical stage records a carbo~te content less th~n the true carbonate con~ent of the mud sample be~ore p~ reduction - such a result would be misleading.
Accordingly, ~he pH re~uction is most conveniently carried out in one of two preferred ways. Firstly, it can be effected using an acid, so long as it is do~e in a "closed" system, from wnich the released carbon dioxide cannot escape. Once the acid has done its ~Jork, dissolving the fines, the pH is then raised tto around 7 - neutrality), and the still~dissolved carbon dioxide gas is converted back to bicarbonate, to give a correct analysis.
Alternatively, the pH reduction can be c æ ried out using a buffer solution that will naintaLn the pH at a level - arcund 8 to 9 - ~nere the c æbona~eY conversion proceeds only to the intermediate bicartcnate, and not through to carbonic acid (and gasecNs carbon dioxide). Unfortunately, using a "neutLal" buffer in this way is rather slow - the reaction kinetics for dissolution of solid carbonates are very much slower than using an acid proper, at a pH around 4 to ~ - but for some purposes this may be acceptable.
~ here an acid is employed it may be almost any acid, muneral or organic. Mineral acids are generally preferred, for not only are they cheaper but it may be desir0d to analyse the mud for oryanlc n~ater:ials in addition to the usual inorganic ions, so ~hat preferably no more of these ~0~ 3~

æ e added in the acidification. Howev~r, because use of the more cGmmon mineral acids - sulphuric and hydrochloric - will okscure the quantities of the ~o prLnciFal inorganic anions beiny determlned (sulpha~e and chloride3 it is best to emplo~ an acid with a scmewhat less confusing anion. m us, hydrobromic acid is the preferred acid.
Where a buffer solution is used ~mlch the same cGnsiderations of ccst, convem ence and resNl~s-cbscuration still apply. A pr~pitious kuffer is a conv ~ i~nal boratPJboric acid buffer (with a pH of ~bout 7.6); a possïbility is kri(hydroxymethyl)aminomethane (TRIS), together with sGme boric acid (whic~ has a pH of ~bout 8.2).
~ he amcunt of acid or buffer emplcyed is sufficient to cau æ both flocculation and solubilisation - and how much that is depends to some extent upon the original pH of the mud (and how much undissolv0d fines it carries). Ih general, however, muds will have pHs in the ~ange 9 to 12, and the desired degree of flocculation, and the certainty of fines dissolution, can be achieved by ~educing that to 6 to 8 (around neutrality) or slightly below. Of course, if the pH is reduced to a level at which ~here is a significant likelihood of any bicarbonates formed releasing their C02 (the loss of which from the sample would possibly result in an inaccurate analysis), then the reduction shculd be done in a closed syst~m.
The pH reduction causes dissolutio~ of any undissolved active - that is, basic - fines in the mud. These active fines will for the most part be carbonates, specifically calcium and magnesium carbonates, present as minute solid paxticles in a complex equilibrium with dissolved calcium and magnesium bicarbonate, and dissolved carbon dioxide (2) p~ reduction shifts the equilibrium such that all the carbonate salts dissolve to, give what is in effect dissociated calcium~magnesium bicarbonate.
Alt~hou~h the pH reduction will normall~ cause sufficient flocculation to give an acceptable impro~ement in filterability, further imprcvement can, if desired, be achieved by adding one or more of the many materials specifically known for their flocculating ability. A typical example of a class of these materials is that of the polymeric anions - for i~stance, partially h~drolysed polyacrylamides such a~ ~hose available from ~llied Colloid under the name~ MoeGNAFlnC and ZET~G (the~e are ccmmonly used ~or the remsval of undesirable solids from ~rinking water, or for the ~ 3~

preparation of solids-laden water for d~scharge in~o ~he envircnment).
Another useful class of flooculating agents is quaternary ammonium cc=pcLnds, such as tetrabutylammonium bromide. All these flocculants tend to be ~xtremely efficient, and can be used in very low concentrations -thus, aroun~ 10 3 Molar.
Hav m g reduced the mud's pH, causing flocculation of the clays anl dissolution of the fines, the resNltant material is separated into its solid an~ liquid components (with the various ions t~ ke determined be mg in ~he latter). Ihe separation m~y be effected by any of ~he usual wa~s -thus, by centrifuyLng, for example, to give a supernatant liquor and a solid residue but pressure filtration is generally more convenient (an~, b~cause of the initial flocculation, relatively easy). Hereinafter it is, for c~nvenience in referrin~ to the liquid portion, assumed that it is indeed a filtrate.
Ihe separated liquid portion of the mud is then subjected to analysis to determine the ionic constituents thereof, bcth as regards ~heir kind and as regards their quantity.
There are various ways this analysis could be performed, including a classical chemical analysis, but most conveniently it i8 carried out by the technique of ion chromatography. A major advantage o~ this technique is its ability to identify anion species, in contrast to most other techniques - eg, atcmic absorpkion spectro~coQy, flame emlssion photometry, or in~ucki~e~y coupled plasma [ICP]). Further advantages of an ion chroma~ography system are its sensitivity (resolution dcwn to about 1 part per billion), precision (better ~han 0.5% ba&ed o~ peak area~, and ability to dif~erentiate ionic species wi~h generally small interference effects. ~he principles of cperation and general use of ion chrcmatcgraphy are well kncwn.
~ n the pr~sent l~vention, a mud filtrate ion may be a "principal" ion and of interest for one or more of a nNmber of reasons. It may have a significant effect on m~d properties at any concentration, which is frequently the case when i~ is a delibera~e special additive ~o the mud;
ik might be one yiviny rise to potential environmental prcblems if discharg~d even at low conoentrations. A11 m~d filtrate ions of interesk cculd be assessed by ion chromatoyraphy, but are not necessarily so assessed. Thus, hydroyen and hydro~yl ion concen~rations can be provided ~2~0~3~

by pH n~YsLrem nt, and carkonate and bicarbonate ion concentrations can ~e deduced from the measured concentrations o~ okher ions. 0~ the principal mud filtrate ions present which are suitable for ion chrcmatography, n~t all neel to be measured, thcugh at least one cation concentration and at least one anion concentration are measured in this way. Typi~l principal mud filtrate ions for determination by ion chromatoyraphy are sDdium, potassium, calcium, magnesium, chloride, ~ulphate an~ carbcn~te.
At the locations where the samples are tak~n, the p~ and temperature of the mud are measured and logged by a ccmbined probe insertel into the mu~ stream. Each mud sample is then transferred t3 a separator, and the f;ltrate is injected In~o the three ion chramatography units simLltaneously to determine i~s anion, moncvalent cation and divalent cation contents. It may be necessary to dilute ~he filtrate by some suitable factor to ensure that the analyte ao~centration is m the optlmum range o~ the ion chrcmatography syst~m. ~hus, ~hile f~ssl-waber-oas3d filtrates will rarely require dilution, Eea-water-basei filtrates may need dilu~ing by a factor of 100. m e p~ of the filtrate (undiluted) is de ~ d at ambient temper~ture, and corrected to ~he value at the temperature recoxded at its sample point.
Once the analysis has been, ccmpleted, the results can be ~sed to calculate the quantities of ionic constituents in the mud. m e various stages involved æ e described in detail in the Specification of cur aforementioned EuroEean Applica~ion; a version thereof is usel in the Examples given hereinaftex.
In ~he invention of our aforementioned European Application it is suggested that the ~ud solids may be analysed instead of or in addition to the mud filtrate, and it is po mted out that the current practice in the oilfield on the analysis of the solid component of the mud is the detexmination of its cation exchange capacity (CEC) using the methylene blue test. m e main object of this test is to determ me the build-up of dispersed clay minerals frcm drilled shale whose particle size is too small for remcval by the drilling rig's solids control equipment. It is then stated that in accordance with ~hat invention, an ion chromatography system is used to prcvide an accurate measure of ~he OEC of the mud solids, and to identify the exchange cation~ of the clay m~inerals dur:~
the drillin~ process, and it propose~3 w3ing t~tramethyla~ononium b~ m~de to r~ 3~

displace the exchange cations in the ~ld solids. In the pres~ invention this l~ay be effected, as well ~ but mstead of carrying out ~ s displacement upon a separated sample of the mud solids it is very much preferred to effect it upon the whole n~d prior i~s separation mto liquid and solid portions. Thus, the present invention m~st preferably includes a stage in which an excess of a displacement agent is added to the n~d sample prior to its separation, whereby the cations carried ky ~he n~d solids as a re~ult of the ~ cation e ~ e capacity æe displac~di into solution, ~hereafter to be æparatedi off with the liguid portion. By determdnin7 how much of the displacement agenit remains free in the liquid, andi from a knowledge of how ~uch was added, there may be calculated the quantity of displacedi cations - and thus the cation exchanye capacity (CEC) of ~he mNd solids.
m e displacement agent to be used is an ion exchanger that is hi~hly selected ~y ~he cla~ in preferenoe to the ions initially absorbe1 ~hereon. However, the exchanger should nGt be too highly selected, else it will also be strongly absorbed onto the active sites in the ion chromatograph's ion exchange oolumn, and will be difficult (and slow) to elute out as the sample proceeds down the column durLng the analytical stage. Typical exchangers selected by clays æ e the ~uaternary ammonium salts (many of which are also exoellent flooculating agents), and the lower aIkyl salts, such as tetramethylammonium bromide, are particularly satisfac~ory for use as disp]acement agents.
m e addition of the displacement a~ent may be effected at any tLme, but - in order to avoid h~drolysis of the clay oontent of the mud oonfusing the results - pr~ferably not significantly after the mud sample is acidified. Ihe exchange cations associated with the mud solids are of course an ~ntegral part of the cations in the ~d system. The contribution CimS of ex~hange cation i in ~e ~d solids to the total con~ent of i in the ~ is gi~ren b~r ClmS ~ XiCEC~l - wa)dm where xi is the fraction o:e ~he cation exch~e capacity (OEC) of the mud solids occ~pied by cation i, wa is the weiyht fraction oE the water in the mud, and dm is the mud densit~r.

3~

Ihe met~hod of the ~n~ention ~prc~vides a determixlaticql of the ionic cc~onents of a drillir~ mud. ~ ~he de~ermined ion values may be seful as they stand, it is preferred to emplc~ ~ese values as a basis for a calculation of the moleular ~onents - t~hat is, the c~is, both dissociated and ur~issocia~d, an~l in l~th dissolv~d and ~lissolved form - ff~t w~re in the original nmd sa~ple at 1~e cond:itic~s of t~eratur~, press-~re, p~ and so c~n that were e~ ~en it was ~
Reactive ca ~ination of the pri ~ ionic ~ Q ~ leads to a distribution thRreof through a ranye of chemical species which includes free hydrated ions, aqueous complexes, ions bcur,d to clay surfaces, ar~
elements kcNnd into solid mlneral phases. PredictLng the concentrations of thes2 species from the analy æs requires a method of calculation based on mass balance, whereby the total concentration of each prLmary component is eguated to the sum of the concen~rations of that ccmponent bcund into each chemical species in existence. A ~econd æt of constraints is given by ~he well-established laws of classical thermodynamics, which govern the relationships between the concentrations of chemic~l species present at equilibrium. The essential features of such predictions involve establishing the e~uilibrium constants for all reacSions known to occur between the primary ccmponents, constructing ~he mass balance equations, and solving numerically for each species concentration. This yields output data containing the concentrations o~ all aqueous ionic species, of any minerals preæ nt, and of any ions bcund to clay elges and surfaces, and the pH. Details of the chemical and numeric techniques are given in F
Morel's & J Mor~an's "A numerical method for ccmputing equilibria in aqueous chemical s~stems", 1972, Ehv. Sci. and ~ nology, 6, 58. This type of calculation ~an be done by hand, iterating the cc~putations ~ntil any changes frc~ one iteration to the next are insignifican~. Hcwever, such a task is especially suit2d ~o a osmputer, and there are in fact available a number of oomputer m~dels that will produce a reasonably accNrate asssssment of the original mud ionic components based upon the ion analysis figures obtained from the ~ample, scme of which mGdels are designed to be extended by the inclusion o~ a~dition~. thermcd~n~tc data m a ~omplemntary database. One such recammended ~or this purpose is GEOCHEM (~ee G Sposito's ~ S V Mattigocl's "GECX~E3M: A camputer Program for the calcula~ion of Chemical Equilibria in Soil solutions and other Natural ~Z~J~r ~ 113~

Water Systems", 1980, Kexney Foundation of Soil Science, Universlty of California, Riv~rside). The mcdel ccmes fully equippe~ with all the ther~odynamic data. Okher models æ e:
MINEQL J C Westall, J L Zachary, & F M M Morel, '~r~2L~
A co~puter prcgram for the calculation of chemical equilibrium ccmposition of electr~lyte soluti~ns", 1976, Tech, Not2 18, Ralph M Parsons Iab., MIT, Cambridge EQ3NR T J Wolery, "EQ3NR: A cumputer program for g ~ ical aquecNs speciation-~olubility calc~lations~, 1983, user~8 Guide and Documenta~ion, UCRLr53414, Lawrence ~ivermore National Iaboratory, Livermore, Cali~ornia, US~
(both these re~uire their databases eq~ipping with e~uilibrium constants for ion exchange reactions, and some m~difications to the numerical tech mques the~ employ).
The numerical techniques u8ed in all these m~d~ls are roughly similar, and will yield similar predictions of speciation. ~he differences for the most part due simply to the use of slightly different activity coefficient expressions and thermodynamic data. A u~eful range of data can be obtained fr~m L V Benson ~ L S Teague, 1980, Lawrence BerkRley Lab., Univ. California, LBLr11448.
As explained in detail in our aforementioned e n Application, the assessment o~ the original m~d components kased upon the determined ion values is most conveniently m~de part of a larger sys~em that outputs recc~merdations as t~ how the actual, present, mud components should be modified to attain the optimum values for the conditions currently being encountered dcwn hole. More specifically, the nea~urament of ~he ionic composition of the mud filtrate is accompanied by a rig-site, compu~er-based interpretation giv m ~ continuous information on ~he ohemical co~position of the mud and the exkent of ~he mu~/formation interactions; this is associated with an advisory m~dule recommen~mg appropria~e changes in the mud formulakion.

~06)~ 3r-EX~

m e followmg Examples are now given, th~uyh by way of illustration only, to show details of various ~mbodim~nts of the invention.

Descripkion of ~enera1 Erocedure and Calculations for Whole Mud hnalysis Ihe mud samples used for whole mud analysis are ~aken from the circulating mud system of a drilling rig. Two sample ~ s in particular are identified: samples of mud from the active tank (or the ~1CW line ~etween the active ta~k and the drill pipe), and return mud samples ~aken from ~mmediately below th8 shale shaker but before the solids control equipmnt.
An accuratel~ kncwn volume of mud Vm is weighed (Mm) to determdne the density Pm of the mNd. A sample of the mud is dried to determine the uncorrected weight fraction solids content W' defined by M ~ Mw Wl =

where Mw i5 the weight of water lost on drying the mud to constant weight at a muni~um temperature of 105C, hut preferably oloser to 160C. The weight of remaining solid is composed of 'che true mud solids Ms and the weight Me of sal~s frcm the evaporated filtrate. Ihe weight of solu~e ~b in the filtrate will be determined by chemic~l anal~sis~
m e volume fraction Uw of water in the mud is given by ~w ~
W (2) Vm PcrVm and the volume fraction v~ of ~d solid~ is thus (l-vw). qhe a~Qrage density Ps of the mud solids i~

%~ 3~

Ps (3) Vs Vm - V~
where ~he volume of filtrate Vw as~ociated with the ~ud is M~pw.
Ihe density of pNre w~ter can ke used Ln the abcve calculations if ~he ionic strength of the filtrate is suffid entl~ low. When mud filtrates have a high ionQc strength, e.g., salt-saburated muds, the density (i.e., partial molar vol~me) of w~ter can be calculated.
Acidification and Di6placement An accNrately known weight of ~Nd i~ rem~ved from the buIk mud, and the pH me2sured; ~he pH of the m~d is assumed to be equal to ~hat of the solids-free f;ltrateO As explained hereinbefore, s~veral methods in accordanoe with the in~ention can be used to dissolve the insoluble salts (such as calcium carbonate) and place th~ exchange cations from the clay solids into solution.
Closed System If the m~d is analysed as a closed system - ie, not in equilibrium wi~h the carbon dioxide in the atm~sphere ~ then the pH o~ the mud can be lcwered to about p~I--5 by acidification wi~hout loss o~ carbon dioxide.
Ccmplete carbonate dissolution is achieved at this pH, and the carbon dioxide remains in solution. q~e cations on the clay solids can be placed into solution by ex~hange with a highly selected oation such as tetramethylammonium, which is added to the mNd as the bromide salt. Ihe acidification and ion exchange reaction for a clo6ed syst~m can therefore be achieved by treatment of the wh~le mud with a mixtNre of a mineral acid, such as hydrobromic acid, and tetrame~hylammonium brcmide; typical concentra~ion are 3xlO 3 and 5xlO 3 Molar, respectively.
The aocurately-known volume Vr of (Me)4NBr/HBr added to the m~d depends on the estimated ionic strength of the mNd filtrate and solids con~ent. A dilution fac~or D is defLned by Vw + Vr D = ------- (4) Vw and it is reccm_eoded that D shculd ke chwse~ to be ak lea~t 10 - and very preferably in the range 100 ko 500.
OPen Sys~l If the mud is to be analysed as an ~pen s~stem, where the acidification wcNld result m the loss of carbon di~xide to the atmcsphere, then an alternati~e te~hni~ue ~ust be applied. ~he insoluble ~Alts are dissolved in a bN~fer fix~d at ab~u~ pH--8, which ensures that the carbonate is con~erted to bicarbonate and remains in ~olution. The preferred buf~er is ~he twc-part sys~em boric acid plus tris(hydr~xyme~hyl)amlncmethane (this latter ccmponent is referred to generally as "TRIS"), whi~h, together with ~annit~l (hexahydroxy-hexane), constitutes a preferred eluent for use in the chemically~unsupp~essed ion chrcmatogr~phy technique. qhus, ~he insoluble salts in the mud are dissolved into an excess of the unsuppressed anion eluen~: the typical ccmposition of ~he eluent is 6x10-3 molar IRIS, 25x13-3 molar mannitol, and 8.5x10 3 molar boric acid, giving a buffer at pH=a~2.
The release of the exchanye cations frcm the clay solids into solution is effected by ~he addition of tetramethylammonium bromide.
Iypically, dilution factors of akout 500 æe required for this technique.
Separation Ihe solid and liquid phases in the tre~ted mNd are separate~ by either centrifugation or by a high pressure filtration device, e.g., the API (American P2troleum Institute~ filter press. q'he solid-liquid æparation is easier to perform in the treated mud since the volume fraction of solids is lcwer and the addition of acid/buffer has lowered the pH which causes ~he clay solids to flocculate. Further, the problem of the presence of carbonate and hydraxide fines in the filtrate after separation, particularly by centrifugation methods, is rem~ved.
Analysis m e treated mud filtrate is analysed for moncvalent and divalent cations and anions using three separate ion chrcmatcgraphy units. All of the anions and cations of interest except carbona~e can be determ m ed by the chemically suppressed techni~ue~ o~ lon ~hromatography. The measurement of carbonate concentration is deter~ined hy the che~ically unsuppressed m~thod of ion chromatography.
Ihe cation exchange capacity of the mud solid~q 1~ determ med by ~t)6) ccmparing the concentration of (Me)4N+ kefore and a~ter a~dition to the mud. A convenient way to mÆasure ~his difference is to subtract ~he chrcmat ~ phy of (Me)4N~ in the treated mud filtrate from tha~ o~ ~he imtial ~Me)4NBr/acid (or/kuf~er) mixture. The di~ference chr~matograph is accurately ~alibrated t~ giY~ the ~ e of (Me)4N+ by the clay solids; the exchange cations are unknown at this point.
m e ion chrcma ~ aphy analysis gi~es ~he total ~ sition o~
available ions m the mud system. For example, the tctal available calcium content of the m~d content consists of the calcium concentration of ~he filtrate (i~e., the calcium in solution), ~he calcium in ~he insoluble salts (e.g., calcium carbonate), and the exchange calcium ions on the clay surfaces. In general these three components will be in equilibrium with each other in the m~d system, and any change in the concentration of one ccmponent may lead to changes in the concentration of the other two. The distribution of ions between the three components can be altered by chemical additions to the m~d, either in the form of mud products at the surface or materials fram the borehole.
Mud Ccmponent Assessment Ihe chemical composition of the treated filtrate can therefore be thought of as co~sisting of three parts: the diluted mud filtrate, the dissolved salts from the mNd solids, and the axchange cations released by the excess of (Me)4N+ cations. A computer m~del is pre~erc~bly now used to partition the total ionic composition into ~he filtxate, precipitated solids and exchange cations. Firstly, the CEC is calculated fr~m the removal of (Me)4N+ ions, c~nd the value is reported c~s both moles of monovalent exchan~e sites per kilogram of dry mud solids (or milliequivalents per gram of ~ry solid) and moles of exchange sites per litre of treated filtrate.
Ihe ion concentrations determ m ed by the ion chroma~ography systems, with the exception of the added tPtramethyk~mmonium and br~mide ions, are mNltiplied by the dilution factor D to give their effective values i the original ~iltrate volume Vw. The EC of the mNd solids, measured c~s moles per litre o~ treated filtxate, i~ also ~ultipli~d by D to give the concentration of exchange sites Ln the orlginal ~lltrate volume. I~ese corrected concentrations are ~sed in the computer mcdel to give ~le equilibrium partition of the ions between ~iLtrate, exchanyer phase and 3~

1~

precipitate~ solids. qhe ~el is r-ln wi~ 1~he F~I fi~l to t:he value determi~ad on the ~ sa~1~ before txea~t and wit:h the ~ closed to the ~t:~e, i.e., th~ n~d is not in equilibxi~n with ~e car~on dioxide ~ ~he a~no~ere. qhe ou~ut fr~n the T~del ~s ~e c~r¢ration of ions ~n the f;ltrate (tog~her with t~he conce~r~tion of cc~[~l~e ~pecies, e~g., the ion pair NaS04 ), 1:he nature of ihe exc ~ e cations, and quantity o~ solid precipitat0d. Ihe preaipitated solids are a part of t~ mud 801ids, an~ æe only brcugh~ into solution by the lowerLng of the pH.
Ihe ~ud filtrate analysis allows the cor~eted solids content W to be determined fr~m W' using ~e TrS x Mw W = W' - = W ~ (5) ~ Pw wher~ TDS, the total dissolved solids in the filtrate, is TD6 = ~ciMim (6) where Ci is the concen~ration of ion i and Mim its molar mass.
Mud Solids Assessment Ihe whole mud analysis described abcve yiel~s a number of measurements which can be used to quantify the reactive components of the mud solids.
The CEC of the mu~ solids, expressed as moles of monovalent exchange sites per kilogram of dry mud solids, i5 a m~asure of the clay cont~nt of the ~ud. For a given reference m~d formulation, the CEC and the solids content W (largely consist m g of bentonite together wi~h a weigh~ing agent such as barite) are known. T'he measurement of W and the CEC will ~nable the build up of drilled clay solids to be diagnosed more accurately, and the acbual effici~ncy of any tr~at~en~ pro~ess used in an attempt to return the mud ~o specifica~ion can be mea~lred.
An estimate of th~ wei~ht fraction cla.~r content Wc (k~ ben~oni~
per 1~ of n~d) of the n~d solid~ which contrlbutes to the soli~3 content W
can be estimated frcqn ZC)g~ 3~

(OEC)Imld x W
Wc= ----_ (7) (cæ) l~ton:Lte ass~ni~ tha~ the CEC of the n~ is ~ue o~ly to ~he t?r~sence of mont~orillon~te .
I~e insol~le car~onate/hy~roxide c4rr~ of ff~e ~ ~olids can be dete~ fmm the m~del cc~ tions ~ic~ give ~e am~ of ~olid precipitated p~r un~t of origin~l fi~tr2~te vol~ne Vw. ~[he contributio~
l~p of these precipitated salts to the solids c~tent W is given ~y Uw x W
Wp = ~ MiP (~3) where MiP is the weight of salt i per unit ~olume of filtrate precipitated by the mcdel.
e remaining fraction of solids Wr, defined by Wr ~ W - WC ~ Wp (9) therefore represents the chemlcally unreactive solids in the mud, Ihese remainLng solids are mKst likely to be barite, added as a mud product, an~
silica fines produced by drilled sa~dstone formations. Ihe value of Wr can be compared to the weight fraction of barite Wb in the reference m~d formulation to give an approxlmate barite o~ntent.
m e change a w m the solids conten~ of the mud can be ccmpared with the measNred value of '\ Wc (estimat2d from Eqn. 7 with (CEC~mUd replaced by a (CEC)mUd) and ~ ~ to determine the origin of the solids. In dispersing shale sections ~ W i5 e~pected to be largely accoun~ed for by ~ Wc, which is measured by the increase in the CEC of the mud solids. ~he CEC of the a~ditional clay solids in the mud system can be determined by CEC = ~ CEC x (~ W - ~ Wp) x ~m (10) where the measurer[ents of bo~ the CEC an~ ,W are made on ~e ~ne mass of n~d M~o l'he estima~ion of the CEC of the added clay solids to ffle ~d ass~[~ that AWr is zero~
~ he ~hale di~;persion ~ich has been discussed to da~e has bee~
largely conc~ned with t~he dispersion of shales ~ic~ are beir~ drilled.
The direct ~ of clay sollds in t:he n~ bsr CE~C en ~les the dispersion of an cpen shale abcve the drilling proc2ss to be identified when non-~hale formations are beIng drilled. For example, ~he clay solids producel by the ~ sion of an open shale can be discri ~ ked from ~he carbonate fines produced dNring the drilling of a limestone secticn.
In lim~stone or dolcmite sections the mud solids will acquire Caoo3 or ~gCa(C03)2 fines which are measured by a~ Lncrease in ~ ~ . Ihe discrimmation of CaO03 fmm M~Ca (C03) 2 by chemical analysis will therefore allaw limestone and dolc~nite sections to be identified without recourse to the usual practice of calci~net~r.

Exam}?le 1: ~ole Mud Analysis - Buffer Treatment PrelLmm ary E~aration o~ a "syn~hetic" mud sample A representa~ive ex~ple of a weighted b ~tonite and fre~Tater ~a~
was m~de ul~ frc~n the following cc~Qonen~
De-ionised water 1000 g Sodium sulphate 0.178 g (1.25x~0 3 M) Sodium chloride 0.292 g (5xlO M) Calcite (calcium carbonate) 10 g (0.1 M) Bentonite (Montmorillonite) 77 g Barite (bari~n sulphate) 391 g Dispersa~ (lignosulphate) 1.45 g Ihe s~dium sulphate and chloride are added to the water to sim~late the effect of ions which originate from the bvreholes (the source of the "fresh" water), and these concentrations will serve to check the validity of the analysis of the various mNd treatments. Calcite is added to sim~late the contamination of the mud by drilled lim~stone formations, and the precipitation o~ calcium carbonate caused by the addition oP sodium bicarbonate ~to re~uce the calcium concen~r~tion in the ~iltrat~).
First, the ~odium sulphate and chloride, and the calcite, are dissolved/mixed with the ~later. The bentonite clay and the d.ispersant are then added, and the resultant su~pension is sheared for several hours to r~11'35 achieve ccmplete hydration o~ the clay. r~he pH o~ the mud is then adjusted to about pH 10 by the addition o~ scdium hydroxide. r~he barite is then added to increase the mud~ density to the required value, and the whole is thorcughly hom3genised, to give the desired synthetic freshwater m~d. m e mud density is 1.31 g/ml, and the solids content ie 31.6 wk%.
The to*al ionic composi~ion o~ the m~d, includin~ the ions in the filtrate, the calcike, an~ the exchange cations on the clay, i5:-Ion Molar Con ~ tration Na+ 17.7xlO
Ca++ 0.1 Cl 5.4x10-3 so4 3.9xlO 3 co3 0.102 CEC Bentonite 0.8x10 3 moles/g dry clay Exchange sodium conc. 0.062 Total sodium conc. 0.080 It will be apparent that the sodium, carbonate and sulphate concentrations are larger than the added co~centrations. This is due to the presence o~ impurities in the bentonite (analysis of an aqueous suspension of 77 g bent~nite showed it to co~tain the following ion molar concentrations: Na+=1.02xlO~2; Ca++=1.2xlO~4; Cl-=3.5x10-4;
S04 -2.6x10-3; C03--=2.6x10-3). ~he figures in the Table abc~e are part calculated (from a knowledge of what was used to make the mNd) and part measured (to determine the impNrities in the bent~nite).
Whole Mud Analysis 1) Cbmparative Fil~ration of Untreated Mu~
A sample of the mud, undiluted, was then filtered using an API
(American Petroleum Institute) filter press. Pressure was applied wi~h nitrogen at 100 psi ~6.8 bar). Ihe variation of the filtrate volume colle~*ed with time is shown by Line 1 in the graph of Figure 2 of the accompanying Drawings. It will be seen that despite the high pressure less than 5 ml were collected in the first 5 minutes, and that with m even as long as 30 minutes less than 10 ml had filtered through.
Two more samples of this mud were dlluted 10 and 50 tin~s respectively with water, and then filtered in the same way. The results are shcwn b~ Llnes 2 and 4 of ~he graph o~ Fi~ure 2. It will be apparent X~ 35 that dilution makes a considerable, and apparently beneficial, difference;
with the 10 tim2s dilution, over 15 and nearly 40 ml were collectel a~ter and 10 minutes respecti~ely, while with the 50 tlmes dilution nearly 50 ml were collecked within 10 munutes.
Hcwever~ analysis of the filtrates by ion chromatography showed anomalous results, thus:-Ion Molar Concentration (xlO-~
+ actual 0 times10 times 50 times Na 17.7 23. 34. 46.
Ca++ 100. 1.2 1.6 4.6 Cl 5.~ 5.4 4.8 4.9 SO4 3.9 5.S 5.7 5.4 oO3 102O 2.6 12. 22.
~ he chloride and sulphate figures are consistent with normal experimental errGr at these dilutions.
From th~ 10- and 50-times Pigures it is clear that the sodium and carbo~a~e concentrations have apparently increased. m is increase is probably due to two processes which are acting sim~ltaneously during dilution; dissolution of the simulated calcium carbonate fines in the mud, and ion exchange between the calcium rsleased into the filt~ate and the sodium ions on the bentonite. For example, in the 10-times dilution data the carbonate ooncentration has Lncreased by 9xlO 3 molar from ~he untreated ~ud filtrate while the calcium concentration has increased by only 0.4x10-3 molar. The appar~nt charge imbalance between the calcium and carbonate concen~rations (8.6x10-3 molar) is acccun~ed for, in part, by an increase in the &odium c~ncentration of 11x10-3 molar. However, in neither the 10 times nor the 50-~1mes dilution has all the known carbonate concentration been recovered; the dissolution has therefore only been partial. '~he other ion concentrations have remain~d largely the same. 'rhU8, although dilution of the mud increases the rate of filtration, it can lead to misleadLng analytical results.
2) Preparation (pH re ction ~ f he Mud A 10 ml sample o~ the synthetia mud de~cribed akc~e was placed in a suitable container, and about 90 ml o~ TRIS bu~er (6x10 3 M TRIS, 25xlO 3 M m~nni~ol and 8.5xlO 3 M boric acid) were added thereto. 'me whole was thoroughly m1xed, pro~uciny a 10-times dlluted dispersion of the 2~15~9~

mud (an~ of its solids); its pH was thus reduced to, and fixel at, 8.2.
Filtration A sample of t~e diluted, buffered mNd was then filterel using an AEI
filter press. The variation of the filtrate volume collec~ed with ti~e is shcwn by Line 3 in the ~raph of Figure 2 of the acs3mpanying DrawLngs. It will be seen that ovt~r 15 ml ~ere ~oll~cted in as little as 5 minutes, and that within 30 minu~es nearly 50 ml had filtered t~rough.
The filtrate ~as then subjected t~ ion chromatography. Ihe resNlts we~ as follcws (with the pure water dilution figures in brackPts):
Ion Molar Concentration (x10-) + actual 10 times 50 times500 times Na17.7 [23 ] 40.1 [34 ] NM. [46 ]75. [NM]
Ca~~+100. [ 1~2] 3.2 [ 1.6] NM. [ 4.6] 101. [NM]
Cl- _5.4 [ 5~4] 4.7 ~ 4.8] NM. [ 4.9] 23. [NM]
so43.9 [ 5.8] 5.5 [ 5.7] NM. [ 5.4] 8. [NM~
OO3102. [ 2.6] 14.1 [12. ~ NM. [22. ] 101. [NM]
(NM m~ans "not m~asured") i It is not clear why these results appear to shcw a considerable increase in sodium an~ chloride, and a significant increa æ in sulphate.
~ he dilution of the mud wi~h the buffer b~ a fact~r of 10, or even a factor of 50, ~ay not cause dissolution of all the "solid" caxbonate material therein. A~cordingly, to ensure full dissolution it is desirable to dilute with buffer by a ~actor of 500.

Example 2: Whole Mud Analysis - Acid Treatment Various samples of the synthetic mud prepared in Example 1 abcve were acidified with 10 3 molar hydrobr~mic acid. One sample was acidified/diluted by a actor of 10, another by a factor of 50.
The filtration results are shown in Figure ~ of the acoompanying Drawings; Line 5 shcws the 10-tImes results, Line 6 the 50-tLmes ones. It will be clear that acidification and dilution together greatly improved the filtxability of the mu~ ~amples ~ and that the combination was a considerable a~vance aver dilution (wl~h w~ter~ alone.
~ he ion chrcmatcgraphy results were as ~ollows:-ZCI~ 3S

Ion Molar Cbncentration (xl ac ~ 10 times 50 times Na+ 17.7 NM
Ca+~ 100. NM g5 Cl _ 5.4 NM NM
S04 3.9 NM NM
C03 102. NM 56 Frcm these results it is clear that even at as lit~le as 50-times dilution the use of acid to re~uce the pH is success~ul at dissolving 'ch~
~alcium carbonate, al~hough in the ~ (rather than closed) syst~m employed here the dissolved carbonate was largely c~nverted to gaseous carbon dioxide, and lost from t~e ~ystem. Perform m g the acidi~ication is a closed vessel, and subsequently raising the pH ayain, "trapæ" the cax~on dioxide, and gives a better result.
i Example 3: Determination o~ CEC
A freshwater mud (A) was formulated with the following composi~ion:
!

2.5 litres deionised water 0.01 g ~odium hydroxide (conc. - 1 x 10 4 molar) 0.731 g sodium chloride (conc. = 5 x 10 3 mol æ) 2.02 g sodium sulphate (conc. = 2.5 x 10-3 mol æ) 25.00 g calcium carbonate 192.5 g bentonite 980.0 ~ bari~e The above ~ormula~ion gives-total mud volume = 2.820 litresmeasured pH = 9.30 solids content = 32.20 wei~ht percent mud density = 1310 kg/m3 API fluid loss = 11.75 ml A second ~reshwater mud (B) WclS ~orm~lated with the ~ame concentrations of c~ll c~mponents c~s mud ~A) but withou~ sodium chloride c~nd sodium sulphate. API filtrates of both mN~s (A) and (B) wera c~nalysed Z6~ 35 by ion chrcmatography and the results shown in Table 1. The differences between the i~n concentrations in filtrate (A) and filtrate (B) are ~ue to the a~ded sodium, sulphate an~ chloride ions. Excellent agreement is seen between the measured dif~erences in ion concentrations and the added salt concentrations. Ians measured in the filtrate o~ the i (B) originate f~m the added i solids. All of the carbonate faund in the i filtratRs (A) an~ (B) originate frcm the impNrity salts in the bentonite.
Very little of the added calcite is soluble m either m~d and therefore does not appear Ln the i filtrate. The estImdted sodi~n carbonate content of ~he bentonite is 2.11%.
The ionic content of the i (A) an~ the cation exchange capacity of th~ mud solids is ncw dete~mlned ky the whole i analysis technique. Gne ml of i (A) is remcved from the bulk i and added to a kncwn volume of a tetraethylammonium br~mide solution of a Xncwn conce~rdtion. The addition of the quatern3ry ammoni~m salt both ion exchanges the clays in the i solids and flocculates the solids to facili~ate solid-liquid separation. m e. reaction mixture was diluted by the addition of a known volume of water, mixed thorcughly and fil~ered using a lcw pressure filtration method, eg, a syrLnge filte~. The filtrate was then analysed by ion c~romatography. In addition to the calibrations needed for each ion analysed by the ion chr~matograph, a separate calibration curve was constructed for the tetraethylammonium ion consisting of a plot of peak are æ as a function of known ion concentration. The decrease in the concentratio~ of tetrae~hylammonium ion in the filtrate gives a direct measure of the cation exchange capacity of ~he mud solids.
The results are ~ummarised in Table 2 which shows the measured uptake of tetraethylammonium ion and the m~asured concentration of all of ~he ions in the filtrate, including the ions rele~sed by the cation exchange process. The measured uptake of tetraethylammonium ion was 0.05 moles per litre of mNd oompared to a r~lease of sodium of 0.055 mole~ per litre of mud which corresponds to th~ Na+ content in the whole mud (A~ filtrate tTable 2) minus the Na~ conten~ in mud filtrate (A) (Table l). ~he removal of tetrae~hylammonium ions from ~he filt~e has been compensated by the relea æ of sodium ionLs into the ~iltrate.
Frcm the measured uptake of te~raethylammonium ion the OE C of the mud solids can be calculated. The CEC of the mud solids is therefore 0.05 moles per litre of mud or 50 milliequivalen~s (me~) per litre of mud. The S~3S

solids co~tent of the mNd is 32.2 weight percent or 422 grams per litre of mud or 0.422 g per ml o~ mud; ~he CEC per unit weight o~ solids is therefore 5O0 x 10 5 CEC = - ~ m~les/g = 0,12meq/g (11) 0.42~
Clearly the CEC of the ~d solids is du~ only to the pres2nce of bentonite ~68.2 gram per litre of mud) the CEC of the bentonite is therefore 5.0 x 10 5 BentonlteCEC = - - - moles/g = 0.73mg~y (12) 0.068 which is wi~hin the range 0.7-0.8 m~g usually fcNnd ~or commercial bento mtes. Another useful calculation i~ t~ use the masured CEC of the mud solids to ~alculate the bentonite content fr~m ~he known value of the CEC of ~he bentonite.

~ J~9 ~B'LE 1 ION AN~LYSIS OF A$I FILTRATE

ION ION OONCENTR~IION (moles/l of mud) _ _ . . .. .
~REs~arER MUD FIIIRAIE DI~E~LNcE
FILIRAIE (B) (A) - (B) (A) Na+ 4.73 x 10-2 3.64 x 10 2 1.0~ x 10-2 1.6 ~ 10 4 1.5 x 10 4 1 x 10 5 ~2+ 0 0 0 ca2+ 1.2 x 10 4 1.1 x 10 4 1.1 x 10 5 Cl 1.08 x 10 2 5.4 ~ 10 3 5.4 x 10 3 S042 4.8 x 10 3 2 x 10 3 2.7 x 10 3 C032 1.37 x 10 2 1.36 x 10 2 1 x 10 4 9~

'~BIE 2 IONS ~LYSIS OF ~IOLE ~JD (A) FII~E

ION OONC (moles/l o~ n~d) ~THYI~NIt~ 5.0 x 10-2 (tak~ ~p3 Na+ 0.1023 K~ 0 (too law to r~asu~) a2+ 2.8 x 10 4 M~2+ 0 (to~ law to meas~e) Cl 9.1 x 10 3 so4 4.5 x 10 3 C032- 1.58 x 10 2 .

Claims (16)

1) A method for the determination of the ionic components of a drilling mud, in which method:
a suitable sample of the mud has its pH reduced to flocculate the clay components thereof and to solubilize any undissolved active materials therein; and the resultant product is then separated into solid and liquid parts, and the liquid part is subjected to analysis to determine its ionic content.

2) A method as claimed in Claim 1, in which the mud additionally contains one or more viscosifier, dispersant, pH controller, calcium controller and/or density regulant. -

3) A method as claimed in any of the preceding Claims, in which the mud samples are taken just after the mud leaves the well bore and/or just before the mud is returned to the well bore.

4) A method as claimed in any of the preceding Claims, in which the pH is reduced by adding an acid or a buffer.

5) A method as claimed in Claim 4, m which the acid is a mineral acid, or the buffer is a quarternary ammonium buffer.

6) A method as claimed in Claim 5, in which the acid is hydrobromic acid, or the buffer is essentially tri(hydroxymethyl)aminomethane.

7) A method as claimed in any of the preceding Claims, in which the amount of acid employed is sufficient to cause the mud sample's pH to be reduced to from 6 to 8, or the buffer causes the pH to be reduced to from

8 to 9.
8) A method as claimed in any of the preceding Claims, in which improved flocculation can be achieved by adding one or more of an additional flocculant.

9) A method as claimed in Claim 8, in which there is used as an additional flocculant a quaternary ammonium compound.

10) A method as claimed in Claim 9, in which the quaternary ammonium compound is tetramethylammonium bromide.

11) A method as claimed in any of the preceding Claims, in which, after any dilution required, the separated liquid portion of the mud is subjected to analysis by an ion chromatographic technique to determine the ionic constituents thereof.

12) A method as claimed in any of the preceding Claims, in which, to provide a measure of the Cation Exchange Capacity of the mud solids, an excess of a displacement agent is added to the mud sample prior to its separation, whereby the cations carried by the mud solids as a result of their Cation Exchange Capacity are displaced into solution, thereafter to be separated off with the liquid portion, so that, by determining how much of the displacement, agent remains free in the liquid, and from a knowledge of how much was added, there may be calculated the quantity of displaced cations - and thus the Cation Exchange Capacity of the mud solids.

13) A method as claimed in Claim 12, in which the displacement agent is tetramethylammonium bromide.

14) A method as claimed in either of Claims 12 and 13, in which the addition of the displacement agent is effected at any time not significantly after the mud sample is acidified.

15) A method as claimed in any of the preceding Claims, in which the determined ion values are thereafter employed as a basis for a calculation of the molecular components that were in the original mud sample at the ambient conditions extant when and where it was taken.

16) A method as claimed in Claim 15, in which the calculation is performed by a computer suitably programmed to simulate a thermodynamically and chemically accurate model of the reactions involved.