X-Ray diffraction and clay mineralogy composition

methods. X-Ray Diffraction is a non-destructive technique that can identify the shale rock mineralogy composition and is one of the most precise methods to define clay composition.

Known the shale rock mineral composition and more specifically its clay content allow to better understand the type of hydration process a specific shale might experience and based on this customized the drilling fluid formulation (Deville, 2011).

The principle behind the X-Ray Diffraction theory is based on the Bragg’s Law (1912). X-rays are electromagnetic radiation of exactly the same nature as light but of very shorter wavelength (10^-10m = 1Å = 0.1 nm). Thus, X-rays are ideal to measure interatomic distances in crystal solids which are typical of that order of magnitude. When X-rays of known wavelength are directed to the sample, the crystal structure of the material can act as scattering center for the X-rays and generate constructive and destructive interference.

The interferences produce a diffracted ray that is then detected, processed, and counted (Cullity, 1978). Only the constructive interferences are of interest in the final process because the destructive interferences cancel each other. Figure 3.1 shows and scheme of the diffraction process.

Figure 3.1. X-ray theory based on Bragg’s law.

The scattering is related to a specific angle of diffraction based on the d-spacing between atoms in the crystal structure of the sample. The Bragg’s law describes the last relationship as:

𝑛𝜆 = 2𝑑𝑠𝑖𝑛𝜃 (5)

Where n is an integer, 𝜆 is the wavelength of the radiation, d is the lattice spacing between atomic layers in the crystal structure of the sample (in angstroms), and 𝜃 is the angle of diffraction.

To perform an XRD analysis a mechanical assembly called theta-theta goniometer system is used as shown in Figure 3.2. The system holds and maintains the sample horizontally while the anode x-ray source and the detector tube rotate at a rate - θ °/min and + θ °/min respectively. The angle between the X-ray source and the sample is defined as ω (incident angle) and the diffracted angle between the incident beam and the detector tube is defined as 2θ. Diffracted angle, 2θ, is always twice the incident angle, ω. (Rigaku, 1993)

Figure 3.2. XRD goniometer outline.

By scanning the sample through a range of 2θ angles, all possible diffraction directions of the lattice should be attained. Then, conversion of the diffraction peaks to

d-spacings with Bragg’s equation allows identification of each mineral because each mineral has a set of unique d-spacings (Moore & Reynolds, 1997).

 Shale sample preparation. The reliability of XRD results are based mainly on the quality of the sample preparation, any contamination during this step can affect the validity of the XRD results (Moore & Reynolds, 1997). For the purpose of this research, a semi-quantitative method described by George S. Austin (New Mexico Bureau of Miners &

Mineral Resources) and later modified by Dr. David Wronkiewicz at Missouri S&T was selected. The procedure produces a clay mounted sample were the single clay particles are oriented with their c-axis perpendicular to the glass slide. The latter will produce an intense basal refraction during the XRD analysis minimizing the refractions from other crystallographic directions. The preparation consisted of 7 steps described below:

1.) Thermally resistant glass sidles (600 ᵒ C) were selected and labeled for the XRD analysis.

2.) 50 grams of shale sample was weight and then crushed to a fine powder using a mortar and pestle.

3.) 25 grams of the shale powder in step 2 was added to a 100 ml beaker with deionized water and then mixed for 5 minutes. The clay mixed solution was then allowed to stand for 10 minutes.

4.) During the stand period, the coarse material should settle, and a clay decanted fraction should remain suspended. If flocculation is observed and the clay particles cannot remain suspended a longer period of settling time should be allowed until the water fraction is clear and can be poured off and new water added and remix.

5.) If after several attempts in step 4 clay still settling. Few drops of hexametaphosphate (CALGON) should be added to the solution and remixed. This product control clay flocculation allowing the clay particles to be dispersed in the solution and remain suspended after the coarse material has been settled.

6.) Once the clays particles could remain in suspension after the 10 min. stand period the clay sample was mounted. A pipette was used to withdraw clay-water solution from the top of the suspension and then transferred to the glass slide covering all of it avoiding any overflow.

7.) Clay samples mounted in the slides were then covered to protected them from dust contamination and allowed them to dry overnight at room temperature. Figure 3.3 shows the procedure for clay sample preparation.

Figure 3.3. XRD sample preparation.

 Test procedure. The shale XRD samples were analyzed 3 times under different conditions. Untreated (dry overnight), glycol exposed (24 hrs.) and heat treated at 550 ᵒC for 1 ½ hour (Figure 3.4). The three different analysis were run in sequence order. The equipment used for the XRD investigation was a PANalytical X’Pert Pro (MPD) part of the advanced material characterization laboratory (AMCL) of the Missouri S&T. The equipment has a Bragg-Brentano beam geometry and CuKα X-ray source. The X-ray tube was set to 40 kV accelerating voltage, and 45 mA heating amperage (Figure 3.4).

Figure 3.4. XRD samples at different conditions.

The following procedure explains the conditions and parameters for each type of test:

1.) Untreated scan: Initially, the dried overnight sample with no treatment was brought into θ:θ goniometer for XRD analysis. Two runs were performed. The first one was named rapid scan, and the analysis was from 2ᵒ to 40ᵒ 2θ at speed of 2ᵒ2θ/min.

Then, a low scan was run on the same sample from 24ᵒ to 26ᵒ 2θ at speed of

0.4ᵒ2θ/min. this one was designed for better differentiation between chlorite and kaolinite peaks.

2.) Glycol treatment scan: after the untreated scan, the same sample was put inside a vessel with ethylene glycol for a period of 24 hrs. to allow expandable clays layers to swell for better mineral peak identification. The rapid scan range was from 2ᵒ to 40ᵒ 2θ at speed of 2ᵒ2θ/min and the slow scan from 2ᵒ to 15ᵒ 2θ at speed of 0.4ᵒ2θ/min

3.) Heated scan: to better identification of kaolinite and chlorite peaks the previous glycol sample was heated at 550 ᵒ C for 1 ½ hour in the oven. This step helped to identify kaolinite and chlorite since the first become amorphous to X-rays and the peak tends to disappear (Moore & Reynolds, 1997). The rapid scan range was from 2ᵒ to 40ᵒ 2θ at speed of 2ᵒ2θ/min and the slow scan from 2ᵒ to 15ᵒ 2θ at speed of 0.4ᵒ2θ/min.

The qualitative identification process for each mineral and clay peak present in the XRD patterns begins by identifying the peak with the highest intensity. Once the mineral related to this peak is confirmed the second step is to identify the weaker peaks of the same mineral present in the XRD pattern. The same procedure is following for the identification of all minerals and clays present. To precisely identify the 2θ positions of each peak Fityk 0.9.8 software was used. Then the 2θ position where transform with Equation (5) to basal d-spaces and compare with the values corresponding to each mineral in the literature. the quantification process was performed with the Software associated with the XRD equipment.

3.1.1.2. Shale pore determination through SEM. Evaluate the mineralogical