Frac Sand Characteristics and Requirements
Frac sand is the predominant
proppant used in oil & gas wells. The American Petroleum Institute sets
standards for frac sand. It has to be >99% sand or quartz. It should be in
one of four “mesh” sizes. Sphericity and roundness should be at least 0.6. This
is to increase permeability to fluids. It needs to have a crush resistance of
4000 to 6000 psi. A single shale well can use as much as 10,000 tons of frac
sand (from 2014 data). Frac sand grain sizes range from 0.1 mm to over 2 mm.
The most common range is 0.4 mm to 0.8 mm. In the U.S., most frac sand, about
75% (from 2014 data), is mined in the state of Wisconsin.
Ceramic Proppants
Ceramic proppants have a
higher crush strength than conventional frac sand. They can be used effectively
in deeper, hotter, and higher-pressure wells, including enhanced geothermal
wells. They are also easier to conduct into the fractures than frac sand. Crush
strength for ceramic proppants is up to 10,000 psi. Uniformity in size, shape,
and sphericity allows ceramic proppants to be conducted into the induced
fractures better than less uniform frac sand. Ceramic proppants are also more
stable and less likely to react chemically or thermally than frac sand. The
reactions can lead to deposits on the sand, which occlude porosity over time
through a process known as diagenesis. However, ceramic proppants are more
expensive than frac sand and resin-coated frac sand.
Ceramic proppants are
available in three density grades: lightweight low density, intermediate
density, and high density. Most ceramic proppants are made from bauxite, kaolin
(aka China Clay), or a blend of both materials. Magnesium silicate and fly ash
may also be used. Various additives may also be included. The alumina content
of ceramic proppant correlates with pellet strength and density.
Manufacturing ceramic
proppants requires five steps: crushing, pelletizing, drying, sintering, and
cooling. Sintering at 2700-2900 degrees F gives the proppants the hardness and
crush strength needed.
Resin-Coated Proppants
Any proppant, including sand,
ceramics, and petcoke-based proppants may be coated with resin. Resin coating
gives better crush strength and increased conductivity into the fractures. It
also results in less flowback of crushed fines, which lower the amount of
proppant that stays in place to keep the induced fractures propped open.
Flowback fines can also cause problems with pumps and surface equipment.
Reducing flowback fines is the key goal and function of resin-coated proppants.
The most commonly used resins
for coating are epoxy resins, furan, polyesters, vinyl esters, and
polyurethane. Epoxy resins are the most commonly used for proppants due to
their strength and chemical stability.
A 2021 paper in Petroleum Science found that a certain epoxy resin-coated (ERC) proppant performed particularly well against other proppants.
“Compared with uncoated ceramic proppants, the epoxy
resin-coated (ERC) proppant has a high self-suspension ability, nearly 16 times
that of the uncoated proppants. Besides, the hydrophobic property and the
liquid conductivity of the ERC proppant increased by 83.8% and 16.71%,
respectively, compared with the uncoated proppants. In summary, this novel ERC
proppant provides new insights into the design of functional proppants, which
are expected to be applied to oil and gas production.”
Lightweight Proppants
Lightweight proppants
decrease the amount of energy required for proppant transport. This is very
important when proppant transport is hindered by low permeability and friction
losses in the ultra-long laterals, which are becoming more common in the oil
and gas industry.
One metric that distinguishes
lightweight proppants from other proppants is settling
speed, as shown below from a 2021 comprehensive analysis of ultra-lightweight
proppants published in Petroleum. The table shows different types of
lightweight proppants. These include petroleum coke proppants, hollow ceramic
beads, amended walnut hulls, and many others.
Below are the advantages of
ultra-light-weight (ULW) proppants from the 2021 paper:
Petroleum Coke-Based Proppants
I didn’t get to read Hart
Energy’s articles about Exxon’s use of petcoke proppants, but I did find an
AI-generated analysis of them, which I reproduce below.
“Petroleum coke (petcoke) is a carbon-rich byproduct of
oil refining, and it’s being used as a proppant in hydraulic fracturing
(fracking) operations. Petcoke’s low density and potential for increased
fracture conductivity are making it an attractive alternative to traditional
sand proppants, especially in areas where sand is difficult to transport or
when a lighter proppant is needed to reach the ends of long laterals.”
“Advantages of Petcoke as a Proppant:
“Low Density: Petcoke has a lower density than sand,
which means it can be suspended and transported farther into fractures using
lower-viscosity fracturing fluids.”
“Potential for Increased Conductivity: The structure
of petcoke, with its roughness and uneven surface, can help support the
fracture walls at multiple points, reducing embedment and potentially leading
to higher fracture conductivity and prolonged gas flow.
“Cost-Effectiveness: ExxonMobil is manufacturing its
own petcoke proppant in its refineries, which helps to lower the cost compared
to other proppants.”
“Increased Recovery: Early results from Exxon’s
pilot program using petcoke show a 15% increase in estimated ultimate recovery
(EUR) in some wells.”
“Considerations:
“Shape and Roundness: Petcoke
grains tend to have a lower sphericity and roundness compared to sand or
ceramic proppants, which can affect their packing density and behavior in the
fracture.”
“Ash Content: The ash content of petcoke can vary
depending on the type and source.”
“Strength: While petcoke can be strong enough to
prop fractures, it’s important to consider its strength characteristics in
different conditions.”
“Environmental Concerns: Petcoke can contain sulfur
and other contaminants, which need to be considered during handling, storage,
and disposal.”
Blast Furnace Coke Proppants for Coal Bed Methane Frac Jobs
A pair of papers, one from January 2023 and one from December 2024, address the value of coke proppants for coal bed methane wells. The problem to be overcome is known as embedment, where the proppant grains damage fracture walls, leading to blocked fractures. It is common in soft-rock formations such as coal.
The 2023 paper tested
different types of coke as proppants. It was found that the coke made from coal
in blast furnaces provided the best ultra-lightweight proppant for decreasing
the embedment phenomenon. The 2024 paper reiterated that modified blast furnace
coke is the most effective proppant for reducing embedment, with other
advantages listed below, followed by the highlights and abstract to the 2024
paper.
“First, the coke roughness supports the fracture wall at
multiple points and reduces the embedment effect, which ensures prolonged
proppant conductivity within the fracture network, facilitating sustained gas
flow and maximizing production rates. Additionally, the low density of
coke-based proppants enables their suspension in low-viscosity fracturing
fluids, thereby reducing pumping pressures, minimizing water consumption, and
improving well cleaning efficiency.”
Proppants for Enhanced Geothermal Wells
Proppants are a key concern
for developing successful enhanced geothermal (EGS) wells, especially where
reservoir temperatures are high. Hotter rocks have lower permeability, so there
is a need to increase permeability to increase flow rates. Early EGS wells did
not use proppant, and as a result, the flow rates were not sufficient. There
are ongoing arguments about whether proppant is needed in hot rocks, as some
think pumping water below closure pressures allows for “self-propping” and
others think that pumping at higher pressures can lead to “hydro-propping,” but
recent tests indicate that the use of proppant will be needed. Fervo Energy’s
Project Red in Nevada was hydraulically fractured in 2023. The well pair
achieved the highest circulation rate ever recorded in an EGS project, despite
starting with a very tight initial permeability using a shale-style multistage
plug and perf fracturing treatment, with conventional proppant. Those wells are
now producing into the power grid. Fervo’s FORGE project tried some fracs near
larger faults without proppant, and they failed to get the flow rates needed.
The propped wells perform much better. However, there are some potential
concerns with proppant in geothermal wells. One is that surface infrastructure
cannot tolerate any sand or proppant returning to the surface. There is also
concern with minerals in the proppants speeding up reactions with mineral-rich
water, which can lead to plugging porosity.
A February 2025 paper presented at the 50th Workshop Proceedings on Geothermal Reservoir Engineering at Stanford University, compared different proppants under room temperature conditions and under simulated geothermal conditions of high temperatures (300 °C) for up to 30 days.
The results suggest that a mixture of
proppants could offer the best results. They also note that the low cost of
petcoke proppants can be advantageous. The abstract and conclusion of the
paper are given below:
ABSTRACT
“Recent advancements in hydraulic fracturing technology
have significantly improved the extraction of hydrocarbons and geothermal
energy from unconventional reservoirs. This study evaluates the performance of
various proppant materials, including petroleum coke based proppants (PC),
high-transported ultra-low-density ceramic proppants (LDC), and resin-coated
ceramic proppants (RC), focusing on their crush resistance and packing strength
under geothermal conditions. Utilizing the Standard procedure from API ISO
13503-2, we assessed both wet and dry proppants across two mesh sizes (10/35
and 35/60) and subjected wet samples to high temperatures (300 °C) for up to 30
days to simulate geothermal conditions.”
“The room temperature test results reveal that petroleum
coke-based proppants, while cost-effective, exhibit lower strength compared to
ceramic proppants. Whereas the resin-coated ceramic exhibited very high stress
tolerance. Notably, the crush resistance and packing strength of proppants
decreased with increasing temperature, with variations observed based on
proppant type and size. Specifically, petroleum coke-based proppants did not
show a significant decrease in strength after exposure to high temperatures.
Conversely, ceramic-based proppants experienced a noticeable reduction in
strength under the same conditions. In mixed proppant tests, blending PC with
LDC or RC at various ratios showed a significant reduction in fines generation
and improved mechanical performance, particularly at a closure pressure of 5000
psi, which is representative of geothermal reservoir conditions. These findings
provide valuable insights into optimizing proppant selection and mixing for
enhanced performance in both unconventional reservoirs and geothermal
applications. The study highlights the importance of tailored proppant
strategies to balance cost, strength, and efficiency, contributing to the
development of more effective and economical fracturing solutions.”
CONCLUSION
“This study provides a comprehensive evaluation of the
mechanical properties of various proppants under simulated downhole conditions.
The findings demonstrate that while PC proppants are economically advantageous,
they exhibit lower strength compared to ceramic-based alternatives. Among the
ceramic proppants, RC proppants showed superior stress tolerance.”
“Temperature has a significant impact on proppant
performance. The crush resistance and packing strength of all proppants
decreased with rising temperatures; however, the extent of degradation was
notably different between proppant types. PC proppants retained their strength
even after exposure to high temperatures, whereas ceramic-based proppants
experienced a substantial reduction in strength. The results underscore the
importance of selecting appropriate proppants based on both economic
considerations and performance requirements. For applications involving high
temperatures, such as in geothermal systems, PC proppants might offer a more
stable and cost-effective solution compared to ceramic proppants, which show
more significant strength reduction under similar conditions. Further research
and field testing are recommended to optimize proppant selection and mixing
strategies for enhanced efficiency and cost-effectiveness in various fracturing
scenarios.”
“The SEM-EDX analysis of the proppants before and after
heat exposure displayed noticeable changes in the surface structure.
Microcracks were more prevalent in the proppant exposed to heat, and the
presence of gas release pores was observed, suggesting potential alterations in
the proppant's properties due to high-temperature conditions. The findings will
contribute to the selection and optimization of proppant materials for
efficient and sustainable development of unconventional oil and gas reservoirs,
as well as geothermal fields.”
Another study presented at
the same conference in 2023 showed that coated proppants under simulated
geothermal conditions of 320 degrees C produced more fines than non-coated
proppants. Results are summarized below.
“The results show that the heated proppant in EGS
conditions has a lower approximate packing strength and yields a higher fines
percentage, indicating reduced conductivity after its exposure to high
temperature fluid for 14 days. The capability of polymer coating, encapsulating
and trapping fines within polymer coating, is reduced by coating degradation in
high temperatures fluid, resulting in significant increase in fines percentage
compared to non-coated proppants.”
Proppant Load as a Metric: Proppant Used Per Frac Stage
Correlates with Higher Fracture Conductivity and Higher Well Production
Along with pumping rates, the
amount of proppant loaded into each frac stage provides an important method
that correlates very well with better well production, indicating better
fracture connectivity. As frac jobs got bigger with higher pumping rates and
higher proppant loads, production rates climbed. Getting the most proppant
emplaced to keep the fractures open is the goal of hydraulic fracturing, and
work is ongoing to optimize the process.
References:
The
Critical Role of Proppant in Enhanced Geothermal Systems. Mark McClure, Rohan
Irvin, and John McGrath. Petroleum Connection. March 27, 2024. The
Critical Role of Proppant in Enhanced Geothermal Systems – Petroleum Connection
Selection
and Testing of Proppants for EGS. Sunghyun Ko, Ahmad Ghassemi, and Matt
Uddenberg. PROCEEDINGS,
48th Workshop on Geothermal Reservoir Engineering. Stanford University,
Stanford, California, February 6-8, 2023. SGP-TR-224. Selection
and Testing of Proppants for EGS
When
Fracturing for Geothermal, Is Proppant Really Necessary? Stephen Rassenfross. Journal
of Petroleum Technology. March 16, 2023. When
Fracturing for Geothermal, Is Proppant Really Necessary?
Exxon’s
Custom, Lightweight Proppant Boosts Permian EURs by 15%. Chris Matthews. Hart
Energy. December 16, 2024. Exxon’s
Custom, Lightweight Proppant Boosts Permian EURs by 15% | Hart Energy
Crush
Resistance and Packing Strength of Candidate Proppant for Enhanced Geothermal
Systems. Sree Sujon Sutradhor and Ahmad Ghassemi. PROCEEDINGS, 50th Workshop on
Geothermal Reservoir Engineering. Stanford University, Stanford, California,
February 10-12, 2025. SGP-TR-229. Crush
Resistance and Packing Strength of Candidate Proppant for Enhanced Geothermal
Systems
Resin-coated
petroleum coke as proppant particulate material and methods related thereto.
Patent. Canada. CA3173741A1
- Resin-coated petroleum coke as proppant particulate material and methods
related thereto - Google Patents
Assessment
of the Suitability of Coke Material for Proppants in the Hydraulic Fracturing
of Coals. Tomasz Suponik, Krzysztof Labus, and Rafał Morga. Materials 2023,
16(11),4083. Assessment of
the Suitability of Coke Material for Proppants in the Hydraulic Fracturing of
Coals
Coke-based
proppant for coalbed methane technology. Rafał Morga, Krzysztof Labus, and Tomasz
Suponik. International Journal of Coal Geology. Volume 295, 4 December 2024,
104647. Coke-based
proppant for coalbed methane technology - ScienceDirect
Ceramic
Proppants: A Specialized Alternative to Frac Sand. Carrie Carlson and Alex
Ebben. Feeco International. Ceramic
Proppants: A Specialized Alternative to Frac Sand
A
comprehensive review on proppant technologies. Feng Liang, Mohammed Sayed, Ghaithan
A. Al-Muntasheri, Frank F. Chang, and Leiming Li. Petroleum. Volume 2, Issue 1,
March 2016, Pages 26-39. A comprehensive review on proppant
technologies - ScienceDirect
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comprehensive review of ultralow-weight proppant technology. Review.
Yong-Cun Feng, Cheng-Yun
Ma, Jin-Gen Deng, Xiao-Rong Li, Ming-Ming Chu, Cheng Hui & Yu-Yang Luo. Petroleum
Science. Volume 18, pages 807–826, February 21, 2021. A
comprehensive review of ultralow-weight proppant technology | Petroleum Science
Hydrophobic
epoxy resin coated proppants with ultra-high self-suspension ability and
enhanced liquid conductivity.Fan Fan, Feng-Xia Li, Shou-Ceng Tian, Mao Sheng, Waleed
Khan, Ai-Ping Shi, Yang Zhou, and Quan Xu. Petroleum Science. Volume 18, Issue
6, 15 December 2021, Pages 1753-1759. Hydrophobic
epoxy resin coated proppants with ultra-high self-suspension ability and
enhanced liquid conductivity - ScienceDirect
Resin
Coated Proppants. Ammat Technology Company. Resin Coated Proppants |
AMMAT Technology | United States
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Sand Mining. Alyssa Schmid. June 19, 2014. PPT - Frac Sand Mining
PowerPoint Presentation, free download - ID:1986801
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