Anales AFA - XVI Meeting on Recent Advances of Physics of Fluids and its Applications 62-65

FLOW AND DEPOSITION OF DIFFERENT PROPPANTS CARRIED BY FLUIDS IN A

SCALED VERTICAL FRACTURE

C. Gracia1, M. Baldini2y M. E. Fernández*1

1Departamento de Ingeniería Mecánica, Facultad Regional La Plata, Universidad Tecnológica Nacional,

Av. 60 Esq. 124 - 1900 - La Plata - Argentina.

2YPF Tecnología S.A.

Av. Del Petróleo s/n - 1900 - La Plata - Argentina.

Recibido: 11/09/21; Aceptado: 08/12/21

Hydraulic fracturing is a technique used to stimulate the production of conventional and unconventional hydrocarbons,

being this type of resource a strategic part of Argentina’s energy reserve. The procedure consists in injecting ﬂuids at

high pressure into the wellbore to create fractures in the formation that later act as highly conductive paths through

which hydrocarbons can ﬂow. Since releasing the pressure of fracturing ﬂuids causes the fracture to close, proppant

(granular materials) is pumped together with fracturing ﬂuids. For that reason, the way proppant is transported and

deposited into the formation determines the future conductivity of the fracture.

We present experimental results on the transport and settling of particles carried by water in a narrow vertical fracture

scaled from typical ﬁeld conditions. We discuss some basic features of the dynamics of the settlement of the proppant

dune and the ﬁnal placement for different types of proppant. The effect of the chosen material on the proppant transport

is signiﬁcant, yielding a much deeper placement of the dune when in lower density materials are used.

Keywords: hydraulic fracture, proppant transport, ﬂuids.

https://doi.org/10.31527/analesafa.2022.ﬂuidos.62 ISSN 1850-1168 (online)

I. INTRODUCTION

In recent years, many oil companies have directed their

efforts towards developing unconventional reservoirs. The

challenges encountered to guarantee a proﬁtable operation

in these reservoirs lead the industry to devote a signiﬁcant

amount resources to optimize processes such as hydrau-

lic fracturing, a vital technique stimulating production. Hy-

draulic fracturing consist in the injection of ﬂuids, along

with granular materials (called proppants), into the for-

mation to induce or enhance existing fractures and open

high conductivity channels connecting the formation and

the wellbore [1].

Proppants ﬁll the fracture and support the closing pres-

sure, guaranteeing the fracture conductivity during produc-

tion. Although fracturing techniques have evolved, there is

still opportunities to increase efﬁciency. Many of these op-

portunities are related with the way in which proppants are

transported and deposited into the fracture.

There are a number of experimental studies that have

considered laboratory scale slots to model the transport of

proppant in a planar fracture. Kern et al. [2] performed

some of the ﬁrst slot ﬂow experiments using vertical Ple-

xiglass walls. In their work, a fracturing ﬂuid (water and

20/40 mesh sand) was pumped at a ﬂow rate that allowed

to reach ﬂuid velocities as high as 1.5 m/s inside the sca-

led fracture. Among other conclusions, these authors found

that: (1) at high pumping rates the proppant is fully washed

out from the cell and (2) at lower pumping rates the prop-

pant settles and a dune grows in the cell. However, this work

* mfernandez@frlp.utn.edu.ar

did not specify the exact conﬁguration used for the ﬂuid to

drain from the fracture. One may speculate that the end sec-

tion of the cell was a fully open slot as high as the cell itself,

connected to the drain system.

After Kern et al. pioneering study, there were several

authors who used the same methodology to study the trans-

port of proppant [3-5]. In all cases, low ﬂuid velocities in the

cell were used (below 0.1 m/s). In general, all these studies

reported the formation of a dune inside the cell. Liu [6] and

Fernández [7,8] presented devices designed to match the

ﬂuid velocities (and Reynolds numbers) attained in the ﬁeld,

which requires large positive displacement pumps. These

experiments, designed following a scaling, are closer than

previous setups to ﬁeld fractures. An important conclusion

from these studies is that the ﬂow develops large eddies that

erode the proppant from the initial part of the cell, leaving

the propped region disconnected from the wellbore [6]. In

all cases, experiments were conducted by leaving the end

of the fracture open to a drainage tank or by allowing the

ﬂuid to exit though a number of oriﬁces that later connect

to the drainage via a set of collectors. In general, a section

of a few centimeters at the bottom part of the draining side

of the slot was left sealed to create a small barrier that helps

the to stabilize a proppant dune inside the cell.

In this work, we use a scaled laboratory slot [7]. The slot

is placed vertically and it has smooth walls. The main goal

of this work is to elucidate whether the characteristics of

the material have an inﬂuence on the way proppant depo-

sit. The proppant pack is of vital importance for the future

conductivity of the well. We consider a number of different

proppant materials and analyze the effect on the ﬁnal dune

settled after treatment.

Section II presents our experimental setup for the scaled

vertical slot. The results on the ﬁnal area propped by the

settled proppant for different types are detailed in section

III. We draw our conclusions in section IV.

II. EXPERIMENTAL SETUP

The experimental setup has been described in detail in

Fernandez et al. [7]. Here we only describe the main fea-

tures. The cell that mimics a fracture is made of a stainless

steal frame 1.6 m long and 0.8 m high. Two acrylic plates

ﬁt in the frame leaving a 6.0 mm gap between them. In the

current study, we have used a smooth and tortuous slot. An

elastomer band is used as gasket to seal the contact between

the frame and the acrylic plates to prevent leakages. Two

metal matrices with bolts and nuts are used to press the two

acrylic plates against the frame and deform the elastomeric

seal. Moreover, these matrices provide additional support to

prevent the cell from deforming under pressure during the

experiments. Two perforations (6.0 mm in diameter) on one

side of the frame (right side in all images and plots) act as

inlets for the ﬂuid. On the opposite side the frame has 49

perforations of 6.0 mm distributed along the entire height

of the cell to act as ﬂuid outlets. For each group of seven

consecutive outlet perforations a prismatic collector welded

to the stainless steal frame collects the ﬂuid that exists from

the cell to direct it into the treatment and drainage system.

These collectors can be opened or closed at will by ma-

nual valves installed after each collector. The inlet side of

the stainless steel frame is welded to a 2.0 in (50.8 mm)

stainless steel tube that serves as casing of the simulated

wellbore. Fig. 1shows the experimental setup.

FIG. 1: Experimental setup: (1) Peristaltic pump and ﬂow damper,

(2) fresh water tank, (3) blender, (4) ﬂow meter, (5) cell, (6) pres-

sure gages (and purge valves), (7) supporting table, (8) drainage.

The full dimensional analysis used to scale these experi-

ments to a reference ﬁeld fracture is detailed in Fernandez et

al. [7]. Brieﬂy, we consider a reference a vertical ﬁeld frac-

ture with a half wing length of 80 m , an height of 40 m and

and a thickness of 6 mm. A closed fracture after completion

is typically narrower, however, during pumping, the fractu-

re is open and 6 mm is a reasonable estimation [3]. Since

the sand used in the experiments is similar to the one used

in real operations, the width of the scaled fracture cannot

be different from the ﬁeld fracture to avoid clogging. The-

refore, the cell is 6 mm wide. The other two dimensions are

scaled by 1/50, which leads to a cell 1.6 m long and 0.8 m

height. We assume that this fracture is connected to a verti-

cal wellbore at two active perforation clusters. Each cluster

is represented by a single injection point in the laboratory

cell. For a pumping rate of 60 BPM1each half wing takes

30 BPM, and each cluster takes 15 BPM. Inside the ﬁeld

fracture the average speed of the ﬂuid is 0.33 m/s. We set

our laboratory pumping rate in the range 60 to 100 l/min.

This yields a mean velocity in the cell in the range of 0.2

to 0.34 m/s, compatible with the ﬁeld operation (≈35 −60

BPM). This scaling ensures that the Reynolds number for

the proppant particles and for the fracture itself is conser-

ved (see Fernandez et al. [7]). The Reynolds number for the

perforations is not conserved, however, both ﬁeld and labo-

ratory perforations remain in the highly turbulent regime.

The dimensional scaling also implies that times are reduced

also by a factor 50 in the laboratory cell.

The mixture (proppant + water) is injected into the cell

through the casing and inlet perforations using a peristaltic

pump (Verderﬂex, Dura 45). Due to the high ﬂow rate requi-

red to pump through the small perforations, the pressure in

the casing increases up to 12 kg/cm2. We use a ﬂow damper

after the pump to deliver a more continuous ﬂow rate. The

pressure inside the acrylic cell is always below 0.2 kg/cm2.

The ﬂuid is prepared in a mixer that stirs continuously fresh

water and proppant. We use proppant according to API stan-

dard [9]: (a) premium sand, 30/70 mesh - 0.40 mm , bulk

density 1600 kg/m3;(b) glass spheres, 18 mesh - 1 mm, bulk

density 1500 kg/m3and (c) ceramic, 20/40 mesh - 0.63 mm,

bulk density 1800 kg/m3at a concentration of 0.5 kg/l. The

ﬂow rate is measured by a ﬂow meter installed just before

the casing. A pressure transducer provides a measure of the

pressure inside the cell.

To obtain rough walls in the cell, another set of acrylic

plates was machined as described in Basiuk et al. [10]. Due

to the grooves machined on the fracture walls the cell looses

transparency. However, the refractive index of the acrylic is

close to the refractive index of water. As a result, the prop-

pant inside the cell can be observed clearly when the cell

is ﬁlled with water. The cell is recorded during each expe-

riment via a digital camera (Optronix CR3000). The frame

rate is set to 120 fps with Full HD resolution. After pum-

ping is stopped and proppant has fully settled, an image is

taken at 4032×3024 pixel resolution. To illuminate the cell

we used two LED reﬂectors (22500 lumens) and light diffu-

sers to achieve a uniform illumination on the entire surface

of the cell.

We have carried out experiments for three types of prop-

pant, being compatible with ﬁeld operations: premium sand,

glass spheres and ceramic. In all cases we use the same con-

centration (0.5 Kg/l) and ﬂow rate (61 l/min). The tests were

repeated giving the same dune proﬁle in both cases.

1barrels per minute

Gracia et al. / Anales AFA - XVI Meeting on Recent Advances of Physics of Fluids and its Applications 62-65 63

III. RESULTS

The ﬂow rate used was 61.0 l/min, corresponding to 40

BPM in a ﬁeld operation [7]. The total duration of the pum-

ping is from 83 s, which corresponds to 69 min in the ﬁeld.

Once the mixture has been pumped into the cell, the pum-

ping is stopped and the proppant that still remains in the

cell is allowed to settle down. The photograph of each ﬁnal

dune is used to extract the proﬁle of the dune.

Fig. 2shows the ﬁnal deposited dune. The lower right pa-

nel of Fig. 2shows the extracted dune proﬁles for compari-

son. As we can see, the higher the density of the material,

the larger the ﬁnal deposited dune in the cell. The large vor-

tices in the cell tend to wash the ﬁrst half of the cell length

in the case of glass spheres, leaving only a small heap next

to the casing thanks to a low velocity region in the lower

right side of the cell. In the case of sand, the surface cove-

red by the material is greater than that in the case of glass

spheres. This is due to the higher material density. The ﬁ-

nal deposition occurs evenly forming a layer throughout the

length of the cell. Lastly, when ceramic proppant is used the

dune has a greater amount of settled material, again due to

its higher density. In all cases we can see that a similar dune

height is obtained close to the casing of the slot. It is impor-

tant to mention that the small heap (approximately 20 cm

high) observed for all materials corresponds to a height of

10 m in the ﬁeld.

To quantify the dune shape and position, we have mea-

sured the dune high (x) every 5 cm distance from the frac-

ture. Table 1summarizes the results. The data conﬁrm that

the mean dune heights for sand and glass are essentially

the same. The same goes for the maximum dune heights

X. Regarding the distribution of the mean and maximum

dune heights, it does not seem to be any pattern of simila-

rities. In the case of ceramic proppant, the maximum du-

ne height is around 23% higher than that the one obtained

with glass and sand. For the mean dune height, the average

is 37% higher than the corresponding mean dune height for

glass and sand. It is important to mention that the distribu-

tion proﬁle of sand and ceramic are similar, corresponding

the latter to slightly higher dune heights all across the cell

length, which is a consequence of the higher material den-

sity that allows a larger portion of the inyected material to

remain into the cell.

Despite the different dune shapes and positions observed

in the experiments, one key parameter to assess the quality

of an operation is the total area of the fracture effectively

covered by the deposited dune. As it can be expected, the

lower the density of the material, the lower the area of the

dune. The ceramic agent lead to a dune area 34% greater

than the one obtained with sand and glass proppant, which

are similar. However, we need to mention that the proppant

that left the cell through the exit perforations is effectively

covering deeper parts of the fracture in a real operation. The

values reported here are in fact an indication of the area co-

vered in the ﬁrst 80m of the fracture length. When looking

the propant distribution along the cell, we can observe that

in the ﬁrst third of its length, the glass proppant has an accu-

mulated area greater than the corresponding for sand. This

trend is reversed in the second third of the cell and then

TABLE 1: The columns show the partial height data of the sand,

glass and ceramic proppants respectively.

equalized in the last third. This is consistent with the strong

vortices that are generated in the center of the fracture and

prevent a deposition of the material [8].

An important characteristic of the experiment that we ha-

ve mentioned is the vorticity of the ﬂuid. Although it is not

the purpose of this work to explain this phenomenon, which

is already explained in Fernandez et al. [8], we want to high-

light its relevance. Fig. 3shows the velocity ﬁeld vector map

for a particular frame. In this ﬁgure you can see the trajec-

tory of the ﬂuid particles and quantify the value of the ve-

locity components. In this way, the average speed in every

region of the cell and for each frame can be obtained. It can

be seen that the ﬂow pattern is very complex with ﬂows and

counter-ﬂows throughout the entire cell, displaying strong

vortexes, particularly close to the inlets. We note that there

exist some foci of high ﬂuid velocities rather than a smooth

velocity proﬁle across the cell. This is in agreement with re-

cent simulations of a similar system (see Baldini et al. [11]).

IV. CONCLUSIONS

We have observed that the proppant injected into a na-

rrow cell, after an initial sweep with fresh water, settles in

Gracia et al. / Anales AFA - XVI Meeting on Recent Advances of Physics of Fluids and its Applications 62-65 64

FIG. 2: Images of the ﬁnal dune for different types of proppant. From left to right: glass, sand and ceramic. (Bottom-right) Proﬁle of

the ﬁnal dune in each image. The corresponding horizontal lines are the mean heights h.

FIG. 3: Velocity ﬁeld for a test using premium sand mesh 30/70.

an asymmetric stationary dune.

Lower density materials allow deeper placement into the

fracture, even beyond the limits of the experimental setup.

However, the dune height may be compromised. If sand is

used, a more homogeneous distribution can be obtained.

This could favor the connection between the fracture and

the well. When ceramic proppant is used, we obtain a com-

plete ﬁlling inside the cell and a higher percentage of occu-

pied area within the fracture is obtained. This packing of the

proppant is of vital importance to obtain a highly conduc-

tive channel through which hydrocarbons can ﬂow during

the production.

The use of low pumping rates (40 BPM) and larger prop-

pant particle sizes lead to a rather asymmetric dune with

low slopes next to the inlets. These dunes ﬁll tightly the re-

gion next to the injection points which may be beneﬁcial to

prevent arching after the fracture closure. These are usual

strategies followed in the ﬁeld and our results support their

rationale.

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