shale analysis for mud engineers

7/31/2019 Shale Analysis for Mud Engineers

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2 D. Breeden, J. Shipman AADE-04-DF-HO-30instabilities of the crystal within itself. The lattice willhave to compensate for these instabilities by changingits d-spacing and, thus, altering the diffraction peaks.

Methods

Sample preparationThe method used is derived from the method

described by Schultz.3 The sample is washed using avariety of organic liquids and solvents. After this thesample is dried and hand ground so that it passesthrough a fine sieve. The powder is then groundmechanically for 15 minutes to achieve optimum grainsize.

Bulk analysisThis entails looking at a broad range of potential

angles of diffraction for the sample. Therefore the x-raydiffractometer is set to scan the sample starting at angle3.5o and ending at 70.0o at a constant speed. Thisallows the technician to see any possible minerals in thesample. It is vitally important that the powders

orientation be as random as possible so that eachmineral gets a fair representation of itself. A furtherdiscussion of this topic will be given later.

By using a spreadsheet and a programmed macrothe computer mathematically generates the results forthe minerals listed in Table 1. See Figure 4 for anexample of what is obtained when doing a bulk analysis.If other high intensity peaks appear that are notassociated with the minerals listed in Table 1 it is up tothe technician doing the analysis to identify those peaksmanually. The amounts of these extra minerals may beapproximated depending on the mineral. Some of themore common ones are listed in Table 2.

Oriented Clay SlidesAbout 2 grams of the powdered sample is mixed with

30mL of deionized water. This is stirred and thenallowed to stand so that the more dense particles sinkand the clay particles remain suspended in the upperlayer of water. A pipette is used to extract this claysolution and deposit it on to a frosted glass slide. Thewater is allowed to evaporate overnight; this is donebecause, in this case, the particles on the slide need tobe oriented.

The individual clays (kaolinite, illite, smectite andchlorite) are distinguished by the clay slide. For eachrun the range of 2 of interest is 3.5o-20o. The clay slideis manipulated and then scanned using XRD after eachstep of the process.

The clay slide will be scanned four times. First, theslide is run in the diffractometer after all the water hasevaporated. This initial run is referred to as the finessample, it can be thought of as the before snap-shot ofthe manipulations. The slide is then put in a desiccatorwith ethylene glycol in it and heated to 70oC. (A sparedesiccator is ideal for this because it allows the pool of

glycol to sit at the bottom and be partitioned from theslide resting on a screen above the glycol.) After abou2 hours the slide is briefly air dried and analyzed on thediffractometer again. This diffractogram is referred to asthe glycolated sample. The slide is then put in an ovenat 300oC for about an hour and a half. After cooling toroom temperature, the diffraction peaks are obtainedand this data is labeled 300oC. Similarly, the oven isheated to 550oC and the slide is heated at thistemperature for about 30-minutes. On cooling, the slideis run again, and this data is labeled 550oC. Theglycolation process, alone, distinguishes the claysmectite. The mixed-layer of illite and smectite isdetermined by the combined manipulations of glycolationand heating to 300oC. Kaolinite is determined byheating to 300oC. Chlorite is noted by its dramatic latticecollapse after heating to 550oC. Illite is distinguished byits ability to resist all of these manipulations.3 As withthe bulk analysis a macro is performed on the collecteddata according to some calculations briefly discussedbelow.

Semi-quantifying using Microsoft ExcelUsing a Microsoft Excel macro, the data, obtained by

the XRD software, is processed for interpretation. Achart of the minerals commonly found in shales and theirespective primary 2 values are listed in Tables 1 and2. The pure standards of the minerals in Table 1 areanalyzed in the same manner as the bulk samplesHere the highest intensity value is noted for all of thepure standards, see Figures 7-13. This value is used tosemi-quantitate the relative amount of each mineral inthe mixture. By taking the value at that 2 reading in thesample and dividing that by the value derived from thepure standard a good estimate of its percentage is given

For example, refer back to Figure 3 where the quartzpeak has a maximum at 6299 cps. A sample is run andgives a value of 1417 cps at 26.65o. Therefore theestimated quartz percentage is 1417/6299=22.5%Quartz, feldspar, calcite, dolomite and siderite aretreated in this direct manner. The clays (as a whole) andpyrite are mathematically manipulated more by factoringin an intensity factor.

Using a computer macro that compares thediffractogram of each step the clays are classified andsemi-quantified. Standards are utilized but only foidentification purposes, see Figures 14-17. An exampleof the final x-ray analysis process, including bulks andfines, is given in Figure 5. By looking at each graph frombottom to top one can see how the peaks alter aftereach application of the process. Based on thesechanging peaks the computer calculates the finavolumetric percentages. Both the bulk and fines data isdirectly interpreted into weight percents because thedensities of the minerals are about the same or becausean intensity factor is used.


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4 D. Breeden, J. Shipman AADE-04-DF-HO-30the leftover amount is placed in the total clay cell.This, also, can be seen in the pure non-clay standards,Figures 7-13. Again, it is up to the technologist toensure that the XRD data is interpreted correctly.

Mineral implications in the fieldBy using the preparation and interpretation

techniques discussed above the expectations of thatfield can be predicted. A brief account is given on thegeneral understanding of how each mineral yields anidea of what to expect and thus leads to modifyingdrilling fluids for the site.

Quartz and the feldspars are crucial to understandbecause, together, they are the most abundant mineralsencountered during drilling. Quartz is very hard(hardness of 7) and abrasive. Its crystals can increasethe porosity of the shale, allowing fluid to penetratefarther and faster than in similar low quartz shales. Thiscircumstance can destabilize the shale, leading todifficulties in drilling. Likewise, feldspars are hard (6 6.5) and are the primary matrix of shales. In otherwords, the shale will be more easily dispersed if the

feldspar content is low. Furthermore, the carbonates(calcite, dolomite and siderite) are of interest becauselike quartz they may cause additional porosity to theshale, but are significantly softer (3 - 4). Some of thecarbonates are also a potential source of low-levelcarbonate build-up in the mud. Similarly, pyrite is a hardmineral (6 6.5) that may impart some increasedporosity to the shale.

Beidellite and montmorillonite are known to be themost common and most well known smectites,respectively. It is generally accepted that vermiculitesbelong in a separate class from smectite due to differingCEC results (discussed below). As a group, vermiculites

are much more reactive than the smectites. Thesmectite concentration in shale is important. Being thatsmectites swell when hydrated, they can make theformation unstable. Smectite clays are most oftencontrolled these days with inhibitive water-based, oil, orsynthetic-based fluids. Furthermore, kaolinite is adispersive, non-swelling clay. It is commonly found indrilling wells in the North Sea and off the West Coast of

Africa. Kaolinite can be controlled most effectively withthe addition of a source of potassium. Which leads toillite, which is a dispersive, non-swelling clay that canalso be controlled by the addition of a source ofpotassium or some of the amine compounds, currently inuse today. Similarly, chlorite clays are dispersive, non-swelling materials that are generally not found insufficient quantities to need special attention. Typically,methods to control the kaolinitic and illitic clays willgenerally control the chlorite clays. Mixed layer clayscan be of several types: kaolin/smectite,chlorite/smectite, chlorite/vermiculite and illite/smectite.The most frequently found and therefore most importantof these is the illite/smectite mixed layer. When waterinteracts with this mixed layer the consequences

rendered are much more damaging than if it were puresmectite or illite. The illite allows the smectite to swelmore easily due to increased exposed surface area

Also, the mixed layer sets up a situation where the lessreactive clays act like ball bearings for the swelling claysto roll upon. Macroscopically, this results indisintegrating shale.

Cation Exchange Capacity (CEC)

CEC values are important due to the fact that claysvary in their reactivity. Some of the cations (positivelycharged atoms) of the clay are easily replaced with othepositively charged species. This phenomenon is drivenby the fact that the original cations are not as compatibleto the negatively charged site as the newly introducedpositively charged species. Thus, it is rationalized thathe exchangeability of the cations are directly related tothe reactivity of the shale. However, the CEC withinitself does not provide this information alone. Theindividual cations of the clays that are replaced and leftin solution indicate reactivity of the clay as well. Thesecations, also called exchangeable bases, primarily are

calcium, potassium, magnesium, and sodium. Foexample, shale with a high CEC value along with a highconcentration of calcium may not be as reactive as shalewith a lower CEC value and a high sodiumconcentration.

To summarize the reaction, the Mg2+, Ca2+, K+, and/oNa+ initially present in the clay are loosely bonded tonegatively charged sites within the clay. The CECreagent is a positively charged species in solution. Theclay and the CEC reagent are allowed to mix with oneanother for an extended period of time. During this timethe CEC reagent is strongly attracted to the negativelycharged sites. It is, therefore, drawn out of solution and

bonded to these sites. This in turn, drives the Mg2+

Ca2+, K+, and/or Na+ into solution. Either Ionchromatography (IC) or Atomic Absorption Spectroscopy(AA) can quantify these cations.

OGS has used several CEC proceduresMethylenblue (MBT) is desirable for the field due to itsfast results. However, it is known that MBT is not veryreliable when compared to the other methods.

Ammonium acetate is commonly used in deriving soiCEC values for agricultural applications. This procedureis indeed useful but has limitations. It consists of alengthy amount of time to complete. This method isregarded as an indirect method, which involves adistillation and titration to derive the CEC value. Thesecan be easily mishandled and/or misinterpreted whichwill lead to inaccurate CEC values. The ammonium lefover from the process is not easily taken out of thesolution and therefore interferes with the calculation ofthe cations, if IC is used to identify the cations. Thisparticular reagent replaces salt and carbonate cations aswell. This can increase the CEC value and rendeinaccurate cation counts.


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AADE-04-DF-HO-30 Shale Analysis for Mud Engineers 5OGS has found the cobalt(III)hexamine procedure to

be very useful.4 It does not demand as much time asthe ammonium acetate procedure. The CEC value isobtained directly by measuring the cobalt(III)hexamineconcentration by either AA or by UV/Visiblespectrophotometery. See Figure 18 for the completeXRD and CEC spreadsheet.

In the limited time OGS has been using the cobalt

procedure, we have found that it gives a higher CECthan does the ammonium procedure. Thecorresponding cations also increase an equal amount.

CEC data can indicate whether a clay mineral,particularly smectite, has a predisposition to cationretention or whether or not diffusion of charged oruncharged molecules can occur within the clay.5 In otherwords, if a shale has a high CEC it might be able to bealtered either by nature or by the application of a drillingfluid. This may be detrimental or useful to the drillingprocess depending on the alteration.

ConclusionsWith good organization skills an accurate account as

to what is included in a sample can be rendered. Beingable to wash and grind a sample effectively is of up-mostimportance. Preparing the powdered bulk sample to beeven and as random as possible is vital to the successof semi-quantitating the minerals. Patience is anecessity in preparing and manipulating the clay slidesto obtain accurate results. Utilizing CEC to betterunderstand these minerals is of utmost importance aswell. Having experienced and knowledgeable personnelexecuting the procedures is important to ensure that thebest possible analytes are created and that the XRD andCEC data is interpreted logically. Understanding theessential theories and applications behind XRD and

CEC will enable the reader to better recognize andunderstand these very useful applications. It alsoenables the individuals from the field to better interpretand appreciate the reports submitted by OGS LaboratoryInc. These factors in combination will, hopefully, providethe field personnel a better understanding of the site thatthey are considering or drilling.

AcknowledgmentsA special word of thanks to Dr. James K. Meen, Task

Leader of the Materials Characterization Facility at theTexas Center of Superconductivity and Applied Materialsof the University of Houston, for his advise.

Nomenclature

AA = Atomic Absorption SpectroscopyCEC = Cation Exchange Capacitycps = counts per secondIC = Ion ChromatographyMBT = Methylenblue TestSEM = Scattering Electron MicroscopeXRD = X-Ray Diffraction

References

1. Skoog, D. A., Holler, F. J., Nieman, T. A. Principles oInstrumental Analysis, 5th Ed.; Saunders CollegePublishing: Austin, 1998; 849.2. West A. R. Basic Solid State Chemistry, 2nd Ed.; JohnWiley & Sons, Ltd: New York, 1999; 480.3. Schultz L. G. Quantitative interpretation o

mineralogical composition from x-ray and chemical datafor the Pierre Shale, U. S. Geological Surveyprofessional paper 391-C, 1964.4. Ciesielski H, Sterckeman T. Determination of cationexchange capacity and exchangeable cations in soils bymeans of cobalt hexamine trichloride. Effects oexperimental conditions,Agronomie (1997) 17, 1-75. Meier, L. P., Kahr, G. Determination of the CationExchange Capacity of clay minerals based on theComplexes of the Copper (II) ion withTriethylenetetramine and TetraethylenepentamineClays and Clay Minerals, Vol. 47 No. 3, 386-388, 1999.

Additional Reading

1. Moore, M. M., Reynolds R. C. X-Ray Diffraction andthe Identification of Clay Minerals, 2nd Ed.; Oxford: NewYork, 1997; 378.2. Cullity, B. D. Elements of X-Ray Diffraction; AddisonWesley Publishing Co., Inc.: Reading, 1956; 514.


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6 D. Breeden, J. Shipman AADE-04-DF-HO-30

Table 1Primary shale minerals and their 2 values

Mineral 2 (degrees)Quartz 26.6Feldspar 27.5Calcite 29.6

Dolomite 31.0Siderite 31.8Pyrite 33.1Clays 19.9

34.661.9

Table 2Additional minerals common in drilling and their 2 Values

Mineral 2 (degrees)

Halite 31.7Barite 28.9Portlandite 34.0Gypsum 11.7Magnetite 35.5

Figure 1Simplified Diagram of the diffraction of x-rays by a crystal(Adapted from ref 1)

Figure 2Symbolic Schematic of XRD Experiment

(Adapted from ref 2)


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AADE-04-DF-HO-30 Shale Analysis for Mud Engineers 7Figure 3

XRD of pure quartz

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Results after a bulk analysis

X-Ray Diffraction Interpretation and Data

Project: NDFH 1978 Sample: 4/11/2003

Bulk Composition - wt%

Quartz 24Feldspar 3Calcite 1Dolomite 2Siderite 3Pyrite 0Total Clay 67

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8 D. Breeden, J. Shipman AADE-04-DF-HO-30Figure 5Example of Bulks and Fines


Project: NDFH 1978 Sample: 4/11/2003


Quartz 24Feldspar 3Calcite 1

Dolomite 2Siderite 3Pyrite 0Total Clay 67

Clay Composition - wt%

Kaolinite 23Chlorite 7Illite 31Smectite 12Mixed-layer 28Illite/smectite 25 / 75

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Left to right: Correct and Incorrect way bulk sample should be in sample holder

Figure 7

Quartz Standard

OGS Laboratory, Inc. - X-Ray Diffraction Interpretation and Data

Project: Quartz Sample: Quartz



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10 D. Breeden, J. Shipman AADE-04-DF-HO-30Figure 8Calcite Standard


Project: Calcite Sample: Calcite Standard


Quartz 0Feldspar 0Calcite 92Dolomite 0

Siderite 1Pyrite 0Total Clay 8

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Figure 9

Standard of Dolomite


Project: STANDARD Sample: DOLOMITE

Description:


Quartz 0Feldspar 1Calcite 1Dolomite 87Siderite 2

Pyrite 1Total Clay 8

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Microcline Feldspar Standard


Project: Microcline Sample: Microcline Feldspar




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Figure 11

Oligoclase Feldspar


Project: Oligoclase Sample: Oligoclase Feldspar




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12 D. Breeden, J. Shipman AADE-04-DF-HO-30Figure 12Standard of Pyrite

Figure 13

Standard of Siderite


Project: Pyrite Standard Sample: Pyrite



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Project: Siderite Standard Sample: Siderite



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Standard of Smectite


Project: Montmorillonite Sample: Sodium Montmorillonite






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14 D. Breeden, J. Shipman AADE-04-DF-HO-30Figure 15Standard of Kaolinite


Project: Kaolin Sample: Fisher Kaolin Standard






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Standard of Illite


Project: Illite Sample: Illite






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16 D. Breeden, J. Shipman AADE-04-DF-HO-30Figure 17Standard of Chlorite


Project: Chlorite Std Sample: 1/16/04

Description: Chlinochlore






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shale analysis for mud engineers

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