Chemicals applied in the experiments, such as KCl and methyl alcohol, were all of analytical-grade and purchased from Sinopharm, China. The quartz sand proppants with a size of 20/40 meshes were provided by Juxing Mining Products Plant. The coal fines with a size of 200 meshes were purchased from Zibo Mining Group C., Ltd (Zibo, China). The heterocyclic polymer was synthesized in our laboratory, and the monomer was purchased from Shanghai Xietong Co., Ltd.

The surface modifier is a methyl alcohol solution of heterocyclic polymer with the mass fraction of 25 wt %. The coating processes were carried out at 20°C. First, 20g of the proppants were put into the sand mixer (GJ-3S, Qingdao Jiaonan Analysis Instrument Co., Ltd). 0.8g of the surface modifier was added into the container. The rotary speed of the sand mixing equipment was 100 rmp and the mixing time was 2 minutes. Then the coated quartz sand proppants were removed out of the container, and were put into an evaporating dish for further process. At last, the modified proppants were put into an oven with temperature of 60°C for 2 hours to dry the proppants. After all of the procedures, the dry coated, non-sticky, and strewing quartz sand proppants were obtained.

The fracture conductivity was measured by the conductivity test instrument designed according to API standard RP 61-1989 “Recommended practices for evaluating short term proppant pack conductivity”, as shown in Fig. (1). In order to obtain more accurate results, each experiment was repeated three times, and the averages of the results were selected. The quartz sand proppants or SAPs were loaded onto the bottom plate at a concentration of 15kg/m². The closure stress was impose onto the cell. In the entire test, the temperature was 60°C and the injecting fluid was 2 wt % KCl solution. The conductivity of the proppant pack can be calculated through the formula, . In which, k is the permeability of proppant pack, μm²; W_f is the width of the proppant pack, cm; k W_f is the conductivity of proppant pack, μm²·cm; μ is the viscosity of the fluid at the test conditions, mPa·s; Q is the flow quantity, cm³/min; Δp is the differential pressure, kPa.

Fig. (1). Schematic layout of fracturing conductivity test instrument.

The maximum sand free flow rate is a common parameter to evaluate the proppant flowback control ability. The fracture conductivity instrument was employed to determine the proppant flowback rate under different closure stresses, and each test was conducted three times. The test method was as follows. The API Cell was loaded with self-aggregating proppants, the closure stress was applied onto the cell, and the stress and the temperature were adjusted to the specified conditions, the parameters of which were consistent with that of the fracture conductivity tests. The KCl solution was used as the formation fluids, flowing through the proppant pack. The closure stress was set and started to inject the KCl solution with an increase rate of the flow rate of 5cm³/min. If proppant particles were observed in the effluent, stop the tests, and the flow rate of the KCl solution was the maximum sand free flow rate under the specific closure stress.

Fine control tests were conducted in a special sand pack model, as shown in Fig. (2). From top to bottom, the model included a top plunger, a screen of 80-mesh, a layer of coal fines with a size range from 63μm to 75μm, a layer of self-aggregating proppant pack, the bottom screen and the bottom plunger. In order to simulate the migration of formation fines into the proppant pack, coal fines with the size of 200 meshes were packed after proppants with the size of 20/40 mesh. With this placement, the coal fines were allowed to migrate with the fluid and invade into the proppant pack easily. The whole flooding tests were carried out at 20°C. At the beginning of the tests, in order to saturate the sand pack, the KCl solution was injected from the top of the cell at the flow rate of 5 mL/min for 5 minutes. Then the flow rate was increased at a gradient of 5mL/min until the maximum rate of 100mL/min, and the sand pack was flushed for 10 min under this flow rate. At last, sands were removed out from the sand pack and observed by Smartzoom 5, Carl Zeiss AG, to study the fines control ability of the modified proppants from the microscopic view. For the contol tests, the layer of proppant pack was filled with untreated proppants.

Fig. (2). Schematic of the sand pack for fines control tests.

The self-aggregating property of SAPs was qualitatively analyzed by their status in the tube, as shown in Fig. (3). The 20g SAPs were poured into a centrifuge tube with the capacity of 100mL, and the tube was filled with the KCl solution. It can be seen that proppant column was formed and dropped down as a whole when the tube was turned upside down (Fig. 3, left). While the untreated proppants were still strewing sands and fell down to the bottom immediately (Fig. 3, right). The experimental phenomenon illustrates that the treated sands can aggregate to proppant column spontaneously. EM-30 of COXEM is used to study the microstructure of the proppant column, as shown in Fig. (4). The SEM pictures show that proppant particles are stacked together tightly, and stable structure is formed among the grains, with which the aggregated columns can bear large overburden stress.

Fig. (3). Photographs of (left) proppants treated by self-aggregating agent and (right) untreated proppants.

Fig. (4). SEM pictures of aggregating structure of SIAPs (left: 60×; right: 100×).

The heterocyclic polymer can be absorbed onto the proppants easily, because there are a large number of nitrogen atoms and fluorine atoms on the heterocyclic polymer both of which can form hydrogen bonds with the silicone hydroxyl on the surface of the quartz sands, as shown in Fig. (5). The processes of coating and aggregation are illustrated in Fig. (6). After the methyl alcohol is evaporated, the solid coating of the heterocyclic polymer is formed on the surface of the particles. When the surface modified proppants are put into the liquid environment, the softening effect of the water to the polymer changes the surface from the solid coating to flexible long-chain groups. The soften effect can be explained by the shapes and dispersion states of the polymer particles of heterocyclic polymer in distilled water, as shown in Fig. (7). At the beginning, the particles are all angular, while the edges disappear after two minutes’ contact with water. With the softening effect of the distilled water, the flexible long-chain groups of heterocyclic polymer were stretched into the distilled water. These particles float on the surface of the water and keep moving irregularly in the form of Brownian movement. If the twisting force of flexible long-chain groups of heterocyclic polymer is high enough to overcome the separation force of Brownian motion, the aggregations between the adjacent particles occur. Oppositely, if the former is smaller than the latter, the contact particles separate again, which can be explained by the broken filament between the particles as shown in Fig. (6c). When the hydration shell is thick enough to release more flexible long-chain groups, more and more particles contact with each other, as shown in Fig. (6d). Therefore, with regard to polymer coated proppants, when the two particles are close to each other, the flexible long-chain groups will come into contact, intertwine again and form a complete structure, leading to the macro performance that a sand column is formed.

Fig. (5). Schematic of the mechanism of self-aggregating proppants.

The maximum sand free flow rates of both SAPs and untreated proppants under different closure stresses are shown in Fig. (7). It can be found that the maximum sand free flow rate of SAPs is higher than that of untreated proppants under every closure stress. In other words, the self-aggregating proppants can reduce the proppant flowback rate effectively. Compared the trends of the two curves, it can be concluded that the higher, the closure stress, the bigger the gaps between them, indicating that the advantages of self-aggregating proppants in proppant flowback control are more obvious under high overburden pressure. That’s because the SAPs can reaggregate again if the proppant column is shocked to strewing sands. Some weakly bonded particles are washed off inevitably, if the fluid flow rate is too large. With the reaggregating property, new connection could be formed between the moving particles and the main part. This reaggregating property plays an important role in maintaining the morphology of the fracture and providing high conductivity for a long period.

Fig. (6). Pictures of the state of heterocyclic polymer particles in distilled water.

Fig. (7). Maximum sand flow rate at different closure stresses.

The effluents of the tests were collected and compared to study the fines migrations. The released liquor of untreated proppants was turbid, and coal fines were suspended in the liquid or absorbed on the bottle walls. While the effluents from sand pack model of self-aggregating proppant were transparent. The proppants were taken out and observed by a digital microscope (Smartzoom5, Carl Zeiss AG, Germany), as shown in Fig. (8). It can be found that both the untreated proppants and SAPs particles are moist. Meanwhile, masses of coal fines are captured onto the SAPs. However, the surfaces of the former are clean, and almost no fines are found.

Fig. (8). Pictures of (upper) untreated proppants and (lower) SAPs after fines control tests.

With this fines control ability, the formation particles peeled and carried by the fluid in hydrocarbon productions, can be absorbed and bonded on the proppant pack. The grains are less likely to move again, which will reduce the risk of seepage channel blockage by a large margin, maintaining a long term of sand-free production and keeping the fracture cleaning with a high conductivity after the hydraulic fracturing treatment.

The fracture conductivities of both untreated and self-aggregating proppants at different closure stresses are shown in Fig. (9). At beginning, the SAPs are 1.4 times the conductivity of the untreated proppants. That’s because the column of the self-aggregating proppants itself has a certain strength, which can sustain some overburden pressures, so the SAPs exhibit a bit higher conductivity than the other. With the increase of the closure stress, the conductivity of untreated proppants decreases rapidly. The conductivity is nearly 20 μm²·cm under the closure stress of 60 MPa, while the conductivity of SAPs is almost 70 μm²·cm which is 3.8 times larger than that of the former.

Fig. (9). Fracture conductivity at different closure stresses.

This experimental phenomenon can be explained from two aspects. On the one hand, when the fracture closure stress exceeds the strength of the proppants, the particles are crushed along the axial stress. The fines are released when the proppants are shattered (Fig. 10, upper) [2]. The released fines are carried by the fluid, which will block the pore throats and lead to the permeability reduction of the proppant pack, as well as the fracture conductivity. Meanwhile, the width of the fracture decreases because of the proppants crushing, which will aggravate the reduction of the conductivity of proppant pack. But for polymer coated proppants, according to the experimental data presented in our former research [32], the crush rates of the quartz sand at 52MPa (69MPa) are reduced from 3.69% (10.30%) to 1.81% (5.11%) after polymer coating. This means that less proppants crush, and relatively small amount of fines are released if crush occurs. The polymer coating, like a “Rubber Band”, has potential to encapsulate some of the fines into the polymer film (Fig. 10, lower). Therefore, SAPs can significantly reduce the emission proppants fines.

On the other hand, even though some fines of SAPs are released and washed away by the fluid, the migrated fines may be absorbed on the main structure because of the reaggregation property of the SAPs, as mentioned in 3.2. In the fracture conductivity tests, the screen at the outlet end was changed to a 60-mesh screen to collect the crushed fines under the closure stress of 60MPa, in order to study the anchoring effect of the SAPs. After 10 PV of the KCl solution was flushed with the flow rate of 5cm³/min, the collected discharged liquor was filtered, and the solid particles were dried. The mass of the particles collected from the untreated proppant pack was 3216mg, while 28mg of SAPs. In considering that the crush resistance of the SAPs was a bit higher than that of the untreated proppants, the exact masses of the released particles under the pressure of 60MPa of both the proppants should be figured out: the proppants in the API cell were taken out, and washed with distilled water and ethanol (in order to completely remove the polymer coating from the proppants) for 3 times; the solid particles, in other words, proppants fragments less than 60 meshes (250μm) were obtained by virtue of the 60-mesh screen. The total fragments of untreated proppants and SAPs were 4862mg and 1416mg, and 66.1% and 2.0% of the crushed fragments were flushed out with the fluid, meaning SAPs have excellent fragments migration control abilities, and then reducing the risk of blockage caused by particle migration and narrowing down of the fracture. Therefore, the increase of the pressure resistance, the prevention of the release of the fines, and the reabsorption of the fragments result in a stable fracture and high conductivity for a long time after hydraulic fracturing.

Fig. (10). Schematic of the status of (upper) untreated proppants and (lower) SAPs under high closure stresses.