PET E 367 Experiment #1: Yield of Bentonite and Sepiolite Clays Rheological Characterization Of Water-Base Drilling Fluids
Lukasz Nader
1088130
Group #5
January 23, 2008
Nancy Bjorndalen
780-450-5394
University of Alberta
Edmonton, AB
Dear Nancy Bjorndalen:
The purpose of this experiment was to determine how drilling fluid viscosity is affected by clay concentration (part1) and also to select the best rheological model describing the shear stress-shear rate relation of a water based drilling fluid system (part 2).
In part 1 of the experiment we used two different clays, bentonite and sepiolite, and mixed them in different percent by mass and either fresh water or salt water. With a rotational viscometer and mud balance we were able to determine and plot apparent viscosity versus clay concentration. Interpretation of the resulting graphs along with analysis of the theory provided in the lab manual enabled us to develop some interesting and appropriate findings.
Mud ViscositiesAPPARENT VISCOSITY, µApp (cp)
Fresh Water / 15000mg/L Brine
Clay Content (% weight) / Bentonite / Sepiolite / Bentonite / Sepiolite
4 / 7.5 / 8
7 / 27.25 / 38.25 / 6.5 / 48.5
10 / 51.5 / 83.5
· Bentonite and Sepiolite increase viscosity with increased clay concentration in fresh water.
· Sepiolite has similar viscosity effects in salt water and fresh water because of its high affinity for water. Sepiolite shows useful property of possessing shear strength and viscosity quite independent of electrolyte concentration.
· Bentonite fails to increase viscosity of the drilling fluid in salty formations
· The plots also allowed us to find the yield of bentonite and sepiolite from interpolation of figure two of the lab manual (Typical clay yield curves – Gray and Darley, 1988). A yield of 120 bbl/ton was found for bentonite and 140 bbl/ton for sepiolite.
In part two of the experiment we determinined the representative rheological models using two drilling fluids. Bentonite and Xantham Gum were used. Newtonian’s and non-newtonian’s rheologic models were compared to the experimental rheograms. Shear stress versus apparent viscosity was plotted. Outcome of the comparison found that bentonite displays characteristics consistent with Bigham Plastic Model. While Xantham Gum was found to display characteristics consistent with the Power model. Both of these rheological models represent Non-Newtonian fluids. Bentonite mud demonstrates a linear relationship that requires an initial force to allow flow. Xantham gum presents a non-linear relationship that does not require an initial force to flow.
This experiment seemed to display trends and results similar to the theory provided indicating that the procedures, equipment used, formulas used and calculations carried out where appropriate and efficient.
Sincerely,
Lukasz Nader
Theory, concepts, and objective of the experiment:
Part 1: Yield of Bentonite and Sepiolite Clays
Objective:
§ To determine the effect of clay concentration on drilling fluid viscosity for the two common commercially available clays, bentonite and sepiolite.
§ To analyze the effect of salt-contaminated fluid on viscosity and clay yield on bentonite and sepiolite.
Theory and concept:
The use of clays in drilling operations is very important because:
§ Clays represent the main component of drilling fluid.
§ Clays help to minimize the loss of drilling fluid into the formation.
§ Clays assist with contamination and wellbore stability problems.
§ Clays help to control formation pressure.
§ Clays assist with removing drilling cuttings from the wellbore.
One of the most used clays in the oil industry is montmorillonite clay. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral silica sheets sandwiching a central octahedral alumina sheet. The lattice is loosely bound with a cation such as sodium (bentonite) which will swell to ten times its initial volume when mixed with fresh water and calcium (sub-bentonite) which swells 2-4 times it original volume. The ability of the molecules to swell is caused by water entering between the 3 layered complex inducing expansion of the mineral structure. Sepiolite clay on the other hand is a crystalloid hydrous magnesium-aluminum silicate mineral present in needles, fibres, or fibrous clusters. It has very good colloidal properties such as high temperature endurance and salt/alkali resistance. Its higher affinity for water is the main reason that it is used in salt water systems to build viscosity.
Yield of clay is the ability of commercial clays to increase the viscosity of water and helps to grade different clays used in drilling fluids. It is representative of the “quality” of the clay and is equal to the number of barrels of 15 cp mud that can be made from one ton of dry clay. In this part of the experiment bentonite and sepiolite were used to build viscosity and plot of apparent viscosity versus clay content was made.
Part 2: Rheological Characterization of Water-Based Drilling Fluids
Objective:
§ To identify the best rheological model for the relation between shear rate and shear stress of a water based drilling fluid.
§ To determine the flow behaviour parameters of the rheological models.
Theory and concept:
Shear Stress is the force per unit area required to sustain a constant rate of fluid movement. Shear rate is velocity gradient measured across the diameter of a fluid-flow channel. It is the rate of change of velocity at which one layer of fluid passes over an adjacent layer. These values are found in the lab using a rotational viscometer. A rotational viscometer works by having the drilling fluid contained in the annular space between to concentric cylinders and outer cylinder is rotated at a constant rpm. The rotation in the fluid causes a torque on the bob which is displaced and gives a reading at that specific rotational velocity.
Figure 1: Relation between shear rate and shear stress.
The readings obtained are:
Shear stress in (lb/100 ft2)
τ = 1.067*Ө where Ө: dial reading corresponding to the applied shear rate.
Shear rate in (s-1)
γ = 1.703*N where N: viscometer rotating speed.
Apparent Viscosity (cp)
µ= θ600 /2
Plastic viscosity (cp)
µp= θ600 - θ300
With these parameters we can build a shear stress versus shear rate plot and determine to which type of model a certain drilling fluid belongs.
Rheological Models
Rheological models are based on relationship between shear rate and shear stress for a given drilling fluid. How we determine this is to run a standard test to measure the shear rate response to changes in shear stress.
1) Newtonian Model, τ = µγ
- The Newton model gives a linear relationship between shear rate and shear stress with the y-intercept equal to 0 from the shear stress vs. shear rate graph (examples of such fluids are water,gasoline). Viscosity for Newtonian fluid depends only on temperature and pressure ( independent of shear stress applied).
2) Bingham Plastic, τ = τo + µpγ
- The Bingham plastic model gives a linear relationship with the y-intercept not equal to 0 on a shear stress vs. shear rate graph. Some stress required to overcome mud’s gel structure to initiate movement.
τ = τo + µpγ
where τo: yield point (lbf/100ft²)
µp: plastic viscosity (cp); affected by size, shape, concentration of particles in mud system
µp= θ600 - θ300
τo = θ300 - µp
where,
θ600: viscometer reading at 600 rpm
θ300: viscometer reading at 300 rpm
3) Power Law, τ = K(γ)n
- The power law model gives a curved relationship and the shear stress vs. shear rate graph that the y intercept is equal to 0. ( do not require initial stress to start flow)
Where K: consistency index (lbf/100ft²)
n: flow behavior index ( n<1: pseudoplastic, shear thinning; n>1:dilitant, shear thickening)
n = 3.322*log(θ600/ θ300)
K = (510* θ300)/(511^ n)
Concept of Thixotropy:
Thixotropy is the property of non newtonian fluids to show a time-dependent change viscosity ; the longer the fluid undergo shear stress ; the lower its viscosity.
Experimental procedure:
Part 1:
1) Calibrate mud balance using fresh water.
2) Measure funnel viscosity of water at room temperature.
3) Mix desired drilling fluid sample and stir for 10 minutes at high speed.
4) Place drilling fluid sample in rotational viscometer and measure apparent viscosity at 600rpm (Apparent Viscosity = Ө600/2).
5) Measure density of drilling fluid using the mud balance and discard sample.
6) Repeat steps 3)-5) for 4%, 7%, 10% bentonite and sepiolite by weight in fresh water and salt water.
7) Plot apparent viscosity vs. clay content and density vs. clay content. Also, compute the yield of bentonite and sepiolite in fresh water (bbl per ton).
Equipment used: refer to the following diagrams.
Figure 3. Rotational Viscometer.
Part 2:
1) Place 350ml of water in a blender can.
2) Add 30 grams of bentonite to the water and mix at high speed for 5 minutes.
3) Record temperature of mud and measure its density using the mud balance.
4) Measure and record fann viscometer readings at 600, 300, 200, 100, and 6 rpm.
5) Find 10 sec. and 10 minute gel strength.
6) Repeat steps 1)-5) for 4 grams Xanthan Gum as a viscosifier.
Results and calculations:
Part 1: Yield of Bentonite and Sepiolite Clays.
Data:
Table 1: Water PropertiesDensity (ppg) / 8.4
Funnel Viscosity (sec/qt) / 27.72
Table 2: Mud Viscosities
APPARENT VISCOSITY, µApp (cp)
Fresh Water / 15000mg/L Brine
Clay Content (% weight) / Bentonite / Sepiolite / Bentonite / Sepiolite
4 / 7.5 / 8
7 / 27.25 / 38.25 / 6.5 / 48.5
10 / 51.5 / 83.5
Table 3: / Clay / Yield
% / bbl/ton
Fresh water & bentonite / 5.4 / 120
Fresh water & sepiolite / 4.4 / 140
Graph 1: Apparent Viscosity vs. Clay Content for Fresh Water Mud
Part 2: Rheological Characterization of Water-Based Drilling FluidsData:
Table 4: Bentonite SolutionTemperature (oC) / 29.5
Density (lb/gal) / 8.55
Fann Viscometer Speed, N (rpm) / Shear Rate, γ (s-1) / Shear Stress
(lb/100ft2)
600 / 1020 / 86.96
300 / 510 / 78.96
200 / 340 / 68.82
100 / 170 / 65.09
6 / 10 / 56.02
10 second gel strength(lbf/100ft²) / 47
10 minute gel strength(lbf/100ft²) / 77
Graph 4: Shear stress vs. Shear rate for Bentonite solution.
Table 5: Xanthan Gum (XG) SolutionTemperature (oC) / 29.2
Density (lb/gal) / 6.5
Table 5: Xanthan Gum Solution
Fann Viscometer Speed, N (rpm) / Shear Rate, γ (s-1) / Shear Stress600 / 1020 / 83.23
300 / 510 / 69.36
200 / 340 / 62.42
100 / 170 / 52.82
6 / 10 / 34.68
10 second gel strength(lbf/100ft²) / 27
10 minute gel strength(lbf/100ft²) / 30
Graph 5: Shear stress vs. Shear rate for Xanthan Gum solution.
Analysis and discussion:
Part 1: Yield of Bentonite and Sepiolite Clays.
In this portion of the laboratory we tried to determine the effect of clay concentration on drilling fluid viscosity for the two common commercially available clays, bentonite and sepiolite. We also analyzed the effect of salt-contaminated fluid on viscosity and clay yield on bentonite and sepiolite. Interpretation of the apparent viscosity curves gave some interesting and appropriate results. Graph 1 shows the effect of increasing clay concentration on the apparent viscosity of the resulting drilling fluid. In this graph it is noticeable that apparent viscosities of both bentonite and sepiolite seem to increase with increasing amount of clay with sepiolite increasing at a slightly higher rate. The plots also allowed us to find the yield of bentonite and sepiolite from interpolation of figure two of the lab manual (Typical clay yield curves – Gray and Darley, 1988). A yield of 120 bbl/ton was found for bentonite and 140 bbl/ton for sepiolite. The yield values are an indication of a commercial clays ability to increase the viscosity of water and as a result can be used to grade the different types of clay used. As indicated above the yield value for sepiolite was higher than that of bentonite and therefore indicates that sepiolite has a greater ability to increase the viscosity of fresh water. The funnel viscosity of water was also determined in this portion of the lab using the marshel funnel. We found a value of 27.72sec/qt which matches the expected value. The behaviour of bentonite and sepolite in brine solution seemed to be quite different as compared to performance in fresh water. Increasing bentonite clay concentrations appeared to be more effective in fresh water rather than in brine solution. In the salty solution we observe that a higher concentration of bentonite will not change the viscosity very much. There is a significant reduction in apparent viscosity for 7% weight clay content obtained in the experiment from 27.25 cp to 6.5 cp. It seems that adding salt to the bentonite holds back its ability to increase the viscosity. In fresh water, bentonite’s viscosity increased almost exponentially. Sepiolite’s apparent viscosity in the brine solution was recorded to be 48.5 cp as compared to 38.25 cp in the fresh water which is an indication that sepiolite’s apparent viscosity is almost unaffected in brine solution. These results are indicative of sepiolite high affinity to water which allows it to produce the same results in fresh water and salt water.
Part 2: Rheological Characterization of Water-Based Drilling Fluids
In this part of the laboratory we were asked to select the best rheological model describing shear stress-shear rate relation and find the rheological parameters of Bentonite and Xanthan Gum. To do this we used the fann viscometer at multiple rotational speeds. Plotting the shear stress versus shear rate gave a plot that we used to determine which rheological model each type of drilling fluid represented. Based on the figure given in the lab manual page 17; bentonite can be represented as a Bingham Plastic Fluid Model, and Xanthan Gum as a Power Law Fluid Model. Also regression value supports a rheological model that is closer to a value R2 = 1.00. The Xanthan Gum mud resembles the power law model with a regression value of 0.97. As for Bentonite the rheological model that resembles closely would be Bingham Plastic model. Bentonite has a regression value of 0.99.
Bentonite R2 values :Bingham Plastic: R2 = 0.99
Power: R2 = 0.77
Xanthan R2 values :
Bingham Plastic: R2 = 0.89
Power: R2 = 0.98
Sources of Error: