Impact of Perlite on the Properties and Stability of Water-Based Mud in Elevated-Temperature Applications.

Barite settling is one of the common drilling fluid issues encountered while drilling deep wells. In this study, the effect of perlite on the properties and stability of water-based drilling fluid was investigated. Perlite is an inexpensive additive used in different industrial applications such as bricks, concrete, thermal insulators, sludge absorbents, fillers, tiles, ruminants, and poultry. Perlite additive was also introduced to the oil industry in drilling applications as an effective fluid loss control agent to reduce the drilling fluid invasion into the formations. Perlite was added to the drilling fluid in various concentrations, ranging between 0 and 3.0 lb/bbl. The sag test was performed to assess the drilling fluid's stability under dynamic and static conditions at a temperature of 120/250 °F. Then, the impact of perlite on the properties of drilling fluid was assessed by measuring the density and pH at room temperature. While the rheological, viscoelastic, and filtration properties were evaluated at 250 °F. This study showed that an increase in perlite concentration, from 0 to 3 lb/bbl, slightly reduced the pH of the drilling mud; however, all of the values were within the acceptable pH range (9-11). In contrast, this concentration of perlite had an immeasurable impact on drilling fluid density. Perlite enhanced the drilling fluid's homogeneity and stability by reducing the dynamic and static sag factors, and 3.0 lb/bbl perlite was adequate to eliminate barite sag at a temperature up to 250 °F. Perlite was found to be effective in improving the rheological and viscoelastic properties. A significant enhancement of filtration properties was observed by the reduction in filtrate volume and filter cake thickness by 64 and 31%, respectively.

Introduction

Drilling fluids play a vital role in the success and total cost of drilling applications. Drilling fluids are introduced into the wellbore to serve many functions such as hole cleaning, controlling the well pressure, maintaining the wellbore stability by forming a filter cake on the wall of the well, and lubricating and cooling the drilling bit.[2−4] Therefore, the drilling fluid is designed by choosing the appropriate additives to maintain its properties throughout drilling operations and fulfill these functions.[5,6] Many parameters should be considered in mud design and additive selection such as lithology, temperature, pressure, drilling cost, and other encountered issues, such as lost circulation, wellbore stability, and well control issues.[7,8] In the past decades, the increased demand for energy pushed the oil industry and geothermal industry toward deep and unconventional drilling to unlock the energy resources.[9−11] Drilling in such environments increased the urge for special drilling fluid additives that can withstand the harsh conditions encountered downhole such as high temperatures and high pressures.[5] The degradation of polymeric additives,[12−15] flocculation and swelling of bentonite muds,[16,17] and solids sag[18,19] are different forms of drilling fluid instability induced by the high downhole temperature.

Solids sag, or barite sag, is the separation of solid particles from the liquid phase. Solids sag is experienced with weighted drilling fluids, where weighting agents are present in the drilling fluid formulation to increase the mud weight and suppress the high formation pressure.[20−23] Many parameters contribute to the sagging phenomenon, such as drilling fluid properties, weighting agents, particle size, downhole conditions, time, pipe rotation, well inclination, and well geometry.[24−26] The sagging phenomenon mechanism is that the solid particles separate from the liquid phase and start to accumulate downhole, causing the density of the drilling fluid column to vary with the depth (Figure ). Consequently, the mud weight becomes less than the formation pressure in the upper parts of the drilled section, which may cause a severe well control issue.[27,28] The solid particles, accumulated in the lower part of the well, increase the mud weight and equivalent circulating density (ECD). This increase may induce fractures in the formation leading to partial or total loss circulation, especially when the mud window is narrow.[29,30] Moreover, these accumulated solids interfere with drilling and completion operations and cause pipe sticking.[31−33]

Figure 1

Occurrence and complications of the solids sag phenomenon.

Many techniques were introduced in previous studies to mitigate the solids sag and other fluid stability issues. These methods can be classified into three main categories: modify or replace the weighting agent with more stable material or a combination of weight materials,[5,6,26,34,35] add antisagging agents,[20,36−41] and implement sound techniques for early detection of the sag phenomenon and prepare rig crew for such situations.[33] Lab measurements of the sag tendency are conducted to evaluate the effectiveness of these methods, such as sag tests that rely on the real monitoring of mud density using sag cells, flow loops, and viscometers.[27,31,36,42−45] Monitoring of fluid density can be achieved by direct measurements, nuclear magnetic resonance (NMR) and ultrasonic technique,[25] and light scattering technique.[46] Another sag detection method is to monitor the rheological and viscoelastic behaviors of drilling fluids.[18,24,47−50]

Some of the proposed solutions to the sag issue have limitations or challenging to apply in real-field applications due to availability, cost, technical issues, or environmental concerns. Therefore, the need for more feasible and advanced solutions still exists. Perlite is an amorphous volcanic rock having a high water content.[51] It has the ability to expand 6–17 times its volume when exposed to high temperatures.[52] Perlite is used in different industrial applications such as bricks,[53] concrete,[54−57] thermal insulators,[51] sludge absorbents,[58] fillers,[51] tiles,[1] ruminants, and poultry.[59] Perlite additive was also introduced to the oil industry in drilling applications as a fluid loss control agent to reduce the drilling fluid invasion into the formations.[52] This study evaluates the effectiveness of perlite to enhance water-based mud’s properties and stability and solve the solids sag issue in elevated-temperature drilling applications. Perlite was added to the drilling fluid in various concentrations, and the sag tendency was evaluated using dynamic and static sag tests. The performance of the drilling mud, with and without perlite additive, was compared, considering the pH, density, rheological and viscoelastic behaviors, and filtration properties.

No previous studies were conducted to investigate the effect of perlite additive on the drilling fluid stability in elevated-temperature applications to the authors’ best knowledge. Therefore, this work’s novelty is that it evaluates and introduces a new solution to the solids sag issue using an inexpensive additive to safely and efficiently drill oil and gas wells.

Material and Methods

The impact of perlite additive on the properties and stability of drilling mud was investigated as follows:

Perlite and barite powders were characterized by determining the elemental composition, particle size distribution, and morphology.

Several drilling mud samples were prepared by varying the concentration of perlite (0–3.0 lb/bbl).

The density and pH of the mud samples were measured at room temperature.

The sag tendency was evaluated for all mud samples using static and dynamic sag tests.

To verify the optimum perlite concentration, a complete evaluation was performed on the mud samples by measuring filtration, viscoelastic, and rheological properties.

The mud sample’s performance with the optimum concentration was compared to the base drilling mud considering all measured properties. Figure summarizes the experimental procedure followed in conducting this study.

Figure 2

Experimental procedure.

Material Characterization

The elemental compositions of both perlite and barite were determined by a micro-X-ray fluorescence (micro-XRF) method. The particle size distribution analysis was performed on dry samples using the laser diffraction technique, while the morphologies of perlite and barite were studied using scanning electron microscopy (SEM).

Drilling Fluid Preparation

The base drilling fluid was prepared by mixing the drilling fluid additives following the formulation shown in Table . First, soda ash was added to the base fluid (water) to maintain water hardness. Defoamer was added to prevent foam formation, while potassium hydroxide was used to control the pH of mud. The mud viscosity was maintained using bentonite and xanthan gum polymer. Then, starch, regular polyanionic cellulose (PAC-R), and calcium carbonate were used to enhance the filtration properties of the mud. Potassium chloride was added as a clay stabilizer, and barite was used to increase the mud density. All of the additives were mixed at room temperature using a three-speed mixer. The order and time of the mixing process are described in Table . Following the same procedure, several drilling fluid samples were prepared by adding perlite in various concentrations (0–3.0 lb/bbl). Afterward, the impact of perlite on the mud properties was studied.

Table 1

Drilling Fluid Formulation (1 bbl of Drilling Fluid)

component	quantity	mixing time, min	function
water	0.7 bbl		base fluid
defoamer	0.08 lb	1	antifoam agent
soda ash	0.5 lb	1	maintain calcium concentration
potassium hydroxide	0.5 lb	1	pH control
bentonite	4 lb	10	viscosifier
xanthan gum	1.5 lb	20	viscosifier
starch	6 lb	10	fluid loss control
PAC-R	1 lb	10	fluid loss control
potassium chloride	20 lb	10	clay stabilization
calcium carbonate	5 lb	10	lost circulation material
barite	350 lb	10	weighting agent
perlite	0–3 lb	10	antisagging agent

Sag Test

The effect of perlite on drilling fluid stability was studied by conducting a series of sag experiments, static and dynamic. An aging cell setup was used to perform the static sag test at 250 °F and 500 psi (Figure a). The maximum testing temperature was set at 250 °F due to the temperature limitation of the polymeric additives in the mud formulation, while the pressure was applied to prevent fluid evaporation. The inclination angle was varied from 0 to 45° to simulate vertical and inclined wells. The experiments were carried on for 24 h, and the sag tendency was measured for all samples using the densities of top and bottom fluids. The sag factor was calculated using eq . The higher the sag factor, the higher the sag tendency, while a sag factor ranging between 0.5 and 0.53 is considered acceptable.[5,6,44,49]Dynamic sag tendency was measured at standard conditions (120 °F and atmospheric pressure) using the viscometer sag shoe test, VSST (Figure b). Experiments were conducted by running the viscometer at 100 RPM for 30 min. Two fluid samples (10 mL each) were taken from the collection well in the sag shoe before and after the test. Using the weight of both samples (Wbefore and Wafter, in g), the dynamic sag tendency was calculated using eq . Stable fluids have a VSST equal to or less than one, while a VSST higher than one indicates solids sag.[42] The experimental conditions for sag tests are summarized in Table .

Figure 3

Sag test apparatus: (a) static and (b) dynamic. (Reprinted with permission from Elkatatny[20] and Basfar et al.[39])

Table 2

Experimental Conditions for Sag Tests

parameter	static sag	dynamic sag
fluid volume	190 cm3	170 cm3
pressure	500 psi	14.73 psi
temperature	250 °F	120 °F
inclination	vertical/inclined 45°
test duration	24 h	30 min

Rheological and Viscoelastic Behaviors

The effect of perlite on the rheological behavior of the mud was evaluated by measuring the gel strength, plastic viscosity (PV), and yield point (YP). The experiments were conducted using an OFITE viscometer (model 130-77). The measurements were performed at 250 °F, and a pressure of 1000 psi was applied to prevent fluid evaporation. The viscometer readings at 300 and 600 RPM were used to calculate the plastic viscosity and yield point, while the viscometer reading at 3 RPM was used to obtain the gel strength after 10 s, 10 min, and 30 min.

Then, an Anton Paar rheometer was used to study the viscoelastic behavior of the mud samples by conducting oscillatory tests. The oscillatory tests were performed at 250 °F and 1000 psi. First, the amplitude sweep test was run to obtain the linear viscoelastic range by running the rheometer at a fixed frequency (10 rad/s) and variable shear stress. A fresh sample was then used to conduct the frequency sweep test, where a constant strain value was applied, and the angular frequency was varied. The applied strain value should be taken from the linear viscoelastic range. The effect of perlite additive on the viscoelastic behavior of the drilling fluid was studied using the data obtained from the oscillatory test, loss modulus (G″), and storage modulus (G′).

Filtration Experiments

A series of filtration tests were performed to study the impact of perlite additive on water-based mud filtration properties. Filtration tests were conducted at 250 °F and 300 psi differential pressure using a high-pressure, high-temperature filter press. The filtration was performed using a 10 μm ceramic filter disk, and the experiments were run for 30 min. The filtration performance of mud samples was compared using the filtrate volume and the thickness and weight of the formed filter cake. The experimental parameters of the filtration tests are summarized in Table .

Table 3

Experimental Conditions for Filtration Experiments

parameter	description
mud volume	350 cm3
temperature	250 °F
differential pressure	300 psi
filtration time	30 min
filtration medium	10 μm ceramic disk

Results and Discussions

From the elemental composition analysis (Figure ), the barite sample mostly consists of 77.5 wt % barite; 17.7 wt % sulfur; and some traces of iron (1.4 wt %), silicon (2.8 wt %), and potassium (0.6 wt %). In contrast, the perlite sample is rich in silicon (59.6 wt %), potassium (25.6 wt %), and aluminum (10.3 wt %), and it consists of small traces of iron (2.0 wt %) and calcium (1.8 wt %). From SEM microscopy (Figure ), barite particles have an angular to subangular irregular shape that varies in size with a normal particle size distribution. The barite sample showed an average particle size (D50) of 18 μm, a D10 of 3.9 μm, and a D90 of 54.9 μm (Figure ). In contrast, the perlite sample showed platy to subplaty shape with a larger particle size than barite. Perlite particles exhibited an average particle size of 46.7 μm, a D10 of 15.6 μm, and a D90 of 92.5 μm.

Figure 4

Elemental compositions of barite and perlite using micro-XRF.

Figure 5

Morphologies of (a) barite and (b) perlite (SEM).

Figure 6

Particle size distribution of barite and perlite.

Several drilling fluid samples were prepared in the laboratory by varying the perlite concentration from 0 to 3.0 lb/bbl. After fluid preparation, the effect of perlite on drilling fluid density and pH was studied. The base drilling fluid, without perlite, showed a density of 14.7 ppg and a pH of 11.2. Figure shows that the perlite slightly reduced the pH as the concentration increased to reach a minimum of 9.75 at 3.0 lb/bbl of perlite concentration. The reduction in pH can be attributed to the low pH value of perlite additive, compared with the base mud. Perlite typically has a pH value ranging between 6 and 8.[60] However, this slight reduction in pH does not impact the drilling fluid performance because the pH value is still within the recommended pH range (9–11) according to oilfields’ drilling practices. Moreover, the pH can always be adjusted by adding small concentrations of pH control additives such as caustic soda, potassium hydroxide, and lime. In contrast, the impact of perlite on the drilling fluid density was immeasurable by laboratory equipment because the concentrations added to the drilling fluid samples were very low. All of the fluid samples had a density of 14.7 ppg (Figure ).

Figure 7

Effect of perlite additive on mud density and pH.

The effect of perlite concentration on sag tendency at different conditions, static and dynamic, is shown in Figures and 9. For static conditions, the sag tendency was measured at vertical and inclined (45°) conditions. The base drilling fluid, without perlite, exhibited a high sag tendency at both dynamic and static conditions. The static sag factor varied from 0.57 to 0.58 for vertical and inclined conditions, exceeding the acceptable sag factor range (0.50–0.53) as per field practices.[5,6,44,49] The sag factor in inclined conditions was always greater than the sag factor in vertical conditions because the sag tendency is accelerated at inclined conditions.[61] Similarly, the base fluid showed a high potential for dynamic solids sag with a high dynamic sag factor of 2.3, where it should be equal to or less than 1.0 for successful and safe drilling operations.[42] Adding perlite to the drilling fluid improved fluid stability by reducing the sag tendency under both dynamic and static conditions. As perlite concentration was increased, the sag factor was reduced. Adding perlite with 3.0 lb/bbl concentration successfully brought the sag factor to the safe zone with static and dynamic sag factors around 0.5 and 0.2, respectively. Thus, solids sag is unlikely to occur under these conditions. The improvement in mud homogeneity and stability is attributed to the colloidal interactions between perlite particles and water. In clay minerals, the colloidal activities are highly dependent on specific surface and surface charge.[3] Perlite has a high specific surface due to the platy shape of its particles (Figure b). Moreover, like smectite clays, perlite has the ability to absorb water, causing an expansion in the crystal lattice 6–17 times its volume.[52] This swelling significantly increases the specific surface, thus increasing the colloidal activity and its impact on the mud properties.[3]

Figure 8

Impact of perlite additive on static sag tendency.

Figure 9

Impact of perlite additive on dynamic sag tendency.

Rheological and Viscoelastic behaviors

The rheological and viscoelastic behaviors of the drilling fluid with and without perlite (0 and 3.0 lb/bbl) were studied at 250 °F by measuring the rheological and viscoelastic properties. Such properties are gel strength, yield point, plastic viscosity, storage modulus, and loss modulus. Adding 3.0 lb/bbl perlite increased the yield point of the mud by 70%, from 24 to 41 lb/100 ft2, while the plastic viscosity was slightly reduced from 18 to 16 cP (Figure a). The base drilling fluid exhibited an unstable gel structure at elevated-temperature conditions. The gel strength started with 13 lb/100 ft2 after 10 s and decreased with time to reach 7 lb/100 ft2 after 30 min of static gel time, confirming the sagging phenomenon (Figure b). Conversely, perlite improved the gel structure at elevated temperatures by forming a stronger gel with gel strengths of 21, 23, 25 lb/100 ft2 after 10 s, 10 min, and 30 min, respectively. The improvement in gel strength and yield point is caused by an increase in colloidal activity induced by the perlite platelets swelled when exposed to water.[3] This increase of yield point and gel strength indicates the enhancement in the mud capability to keep the solid particles in suspension and reduce the sag tendency at dynamic and static conditions, confirming the sag test results.[26,34,35] Another advantage of using perlite with high-density mud is enhancing yield point to plastic viscosity ratio (YP/PV). Perlite additive increased the YP/PV significantly from 1.31 to 2.58 to fall within the recommended range as per drilling practices (1.5–3). This increase in YP/PV improves the hole cleaning efficiency, drilling fluid stability, wellbore hydraulics, and other important drilling parameters.[38,47,62] Lower values of YP/PV would trigger stability issues such as solids sag, while higher values would cause mud flocculation and coagulation.[47]

Figure 10

Impact of perlite on rheological properties of drilling mud at 250 °F: (a) yield point and plastic viscosity and (b) gel strength.

Figure compares the viscoelastic behavior of 0 and 3.0 lb/bbl of perlite concentrations at 250 °F. The base mud sample below 2% strain exhibited a linear viscoelastic range where the loss modulus is less than the storage modulus. The mud in this range of strain behaves more like viscoelastic solids. After exceeding this range of strain, the gel started to break, and the mud behaved like liquids. In contrast, the sample with 3.0 lb/bbl perlite showed a broader linear viscoelastic range until the strain reached around 10%. Therefore, perlite increased the strength and stability of the gel structure, indicating a better suspension capability in static conditions.[26,50] The rheological and viscoelastic behaviors support the sag tests’ findings that perlite improves the homogeneity, stability, and suspension capability of the drilling fluid. This makes perlite a good additive to be used in elevated-temperature drilling applications. However, this work is a qualitative study to prove the effectiveness of perlite. More research studies should be performed to optimize the concentration and mixing procedure and extend the application for higher temperature conditions.

Figure 11

Impact of perlite on the viscoelastic behavior of drilling mud at 250 °F (perlite sample showed a broader linear viscoelastic range and thus stronger gel structure).

Filtration Performance

The impact of perlite on filtration performance was investigated at 250 °F and 300 psi differential pressure using an HPHT filter press apparatus. For the base drilling fluid sample, the filtrate volume was increasing rapidly until 15 min of filtration time to reach around 6 cm3; then, the fluid filtrate invasion started to cease with a total filtrate volume of 6.7 cm3. Conversely, perlite reduced the filtrate volume to 2.4 cm3 (by 64%), and the filter cake was built faster (Figure ). Figure shows the photographs and SEM images of the formed filter cake after the filtration experiments. Perlite particles formed a more compact filter cake, with a thickness of around 2.7 mm, while a 4 mm filter cake was formed with the base drilling fluid (0 lb/bbl perlite). This improvement in the filtration properties is attributed to the plugging mechanism due to the platy shape of perlite particles, as shown in the SEM images (Figure ). The jagged edges of perlite particles interlocked the solid particles that, in turn, clogged the pore space of the filtration disk and filter cake, preventing further solid and fluid filtrate invasion.[52] Moreover, the increase in mud properties caused by perlite can be another factor in enhancing filtration performance. As reported in a previous study, the fluid losses decrease significantly as the yield point increases.[63] As shown in the SEM images in Figure , fewer vugs and pores were observed with the 3.0 lb/bbl perlite, indicating a less porosity and more compacted filter cake than the base fluid. The reduction in filtrate volume observed with perlite helps minimize the formation of damage induced by filtrate and solids invasion.[64−66] Simultaneously, the thinner filter cake resulting from perlite addition decreases the possibility of differential sticking.[67] Moreover, thinner filter cake makes the filter cake removal process easier, eliminating further complications to cementing and casing operations[68] and minimizing the nonproductive time (NPT).[69] The filtration results also support the findings of the previous study[52] conducted by Bageri et al., studying the effect of perlite particles on the filtration properties.

Figure 12

Effect of perlite on the filtrate volume.

Figure 13

Formed filter cake: (a) base fluid (0 lb/bbl perlite) and (b) 3.0 lb/bbl perlite.

Summary and Conclusions

In this study, perlite additive was added to the drilling fluid in various concentrations, 0.0–3.0 lb/bbl. The influence of perlite additive on water-based mud’s properties and stability was investigated at elevated temperatures (250 °F). Based on the obtained results, the following conclusions can be made:

The base drilling fluid (0.0 lb/bbl perlite) exhibited poor stability with high static and dynamic sag factors. The value of static and dynamic sag factors exceeded the acceptable values (0.53 and 1.0), indicating the high potential of solids sag. While perlite enhanced the stability of the mud by reducing the static and dynamic sag factors, perlite at 3.0 lb/bbl concentration was enough to bring the sag factors to the acceptable range.

Perlite slightly reduced the pH of the drilling fluid from 11.2 to 9.75; however, all of the values were still within the acceptable range of pH (9–11) according to the field practices. In contrast, adding this perlite concentration to the drilling fluid had an immeasurable impact on the mud density.

Perlite enhanced the rheological behavior at 250 °F by increasing the yield point by 70%, while the plastic viscosity was slightly decreased by 11%, increasing the yield point to plastic viscosity ratio (YP/PV) from 1.31 to 2.58. This increase brought the YP/PV to the recommended range, 1.5–3.0, as per the industry practices, improving the drilling fluid stability and hole cleaning efficiency.

Perlite significantly improved the filtration performance of mud. The filtrate volume was reduced by 64%, and a thinner and more compacted filter cake was formed (30% less thickness than the base fluid). This improvement in the filtration performance minimizes the formation of damage induced by drilling fluid invasion into producing formations.

Perlite was proved effective in improving the drilling fluid performance at elevated temperatures; however, this work is a qualitative study rather than a quantitative study. Thus, preliminary research should be performed to determine the optimum perlite concentration before field implementation to account for any changes in drilling fluid formulation and downhole conditions.