A temperature-sensitive plugging material composed of shape memory polymer and self-made gel

Polymers with shape memory property and gel with resistance to high temperature can be both used for plugging formation, but they also have shortcomings. Nevertheless, it is possible to mix shape memory polymer(SMP) particles in the gel solution and use formation temperature to stimulate them to complete morphological transformation to achieve the purpose of plugging. This mutual combination method can help them reduce the shortcomings between each other. Here, a series of SMP with different glass transition temperatures and self-made gel were prepared. SMP needs to be prepared into particles to meet the dimensional requirements of plugging materials, but it can also retain shape memory property and pressure bearing capacity. The composition and thermal properties of both SMP and gel were characterized by FTIR and DMA. The plugging performance and mechanism of the composite of SMP and gel were systematically studied. The results show that the addition of SMP could improve the mechanical strength in system, and the viscosity gel can help stabilize the bridging structure formed by SMP particles at cracks. This composite of SMP and gel is expected to be a potential plugging material.


Introduction
The problem of leakage is inevitable during the process of oilfield development. According to different rate of leakage, it can be divided into seepage loss(<1.6 m 3 h −1 ), partial loss(<1.6-80 m 3 h −1 ), and total loss (>80 m 3 h −1 ) [1,2]. When leakage occurs in the wellbore, it will lead to the continuous leakage of drilling fluid into the formation, resulting in a large amount of raw material cost loss, and serious leakage will directly cause construction interruption [3,4]. Similarly, the loss of oil flow will also cause a decline in production, with formation damage, when leakage problem occurs in the range of oil reservoir [5,6]. Therefore, in order to avoid the subsequent expensive cost loss and dangerous events, the plugging operation is a direct and effective treatment method [7][8][9]. At present, commonly, the plugging is achieved by adding fiber materials(mineral fibers, glass fiber), granular materials(water-swellable hydrogel, polymer particles) and different plugging materials(colloid composite cement slurry, quick-setting cement) into the injected fluid to plugging the loss layer [10][11][12]. Of course, these methods also have some shortcomings or lack of adaptability. For example, some fiber materials become soft and lost bridging plugging function during the high temperature environment, when the leakage location in the deep formation [13,14]. Moreover, the plugging materials in the near-wellbore zone need to withstand high fluid impact force and mutual extrusion stress between plugging materials and rock wall. Serious loss of circulation accidents would happen, if the mechanical strength of material is insufficient [15,16]. In addition, building a plugging structure similar to the size of leakage point is important, since too large of too small will cause damage to the plugging structure. Hence, high temperature environment, fluid impact force and the plugging structure size would impact the successful rate of occlusion [17][18][19].
Currently, as the development of oilfield, the deficiency of conventional plugging agent becomes more and more obvious [20,21]. Thus, a variety of new plugging agents have been continuously developed and applied to oilfield, including gel polymers, different types of fiber materials, and so on. Shape memory polymer(SMP), with its unique shape memory characteristic, has shown outstanding practicability in many fields, while in the oil field, the application range of SMP is still small [22][23][24]. Using this feature of deformation caused by external stimulation, the risk of structural failure due to improper size of plugging agent can be reduced in the process of plugging [8,25,26]. And compared with gel or some fiber agents, the bridge structure made of SMP would be provided with higher strength. Of course, the effectiveness of SMP maybe limited when leakage occurs at the small fracture. Gel could be a proper choice when dealing with the leakage of small fracture, since gel can fully enter the interior of fractures, and low pressure from fluid impact [27]. Therefore, we hope to apply the advantages of SMP and gel to seal the leakage [28,29]. The high temperature of formation can act on the SMP and pregel at the same time. SMP will deform under high temperature to make its size more suitable for the fracture size [30][31][32]. The pregel will also gradually dehydrate and condense to form a three-dimensional grid structure. The polymer can help the gel increase the plugging strength, and the gel can carry the polymer to deeper formation.
In this study, we synthesized a SMP and a gel both with great characteristics, and applied them to self-made fracture plugging experiment simulating crack. Through controlling the proportion of raw materials, the change of glass transition temperature for SMP can be achieved. And we optimized the polymer and crosslinking agent to form a cross-linking system suitable for high temperature. These two materials are combined to form a composite plugging agent, and the comprehensive performance of the plugging agent is evaluated through a variety of experimental studies. The results show that the plugging agent shows excellent plugging ability for sealing fractures.

Preparation of gel
Slowly add a certain amount of polymer particles into the magnetic stirrer, and completely dissolve the polymer through stirring for a certain time to obtain the polymer mother liquid. Then a certain amount of phenolic crosslinking agent, aldehyde crosslinking agent and stabilizer are added to the polymer mother liquid under stirring, and the polymer gel solution is obtained after dissolution. Finally, the mixture was transferred to a hightemperature and high-pressure reactor, and then it was placed in a drying oven (120°C). After 12 h, gel was obtained. The prepared SMP compositions are given in table 1.

Preparation of SMP
Pour epoxy resin and into a three-necked flask, and stir at 200 rpm until the liquid becomes uniform. During the mixing process, the vacuum pump is continuously used to extract gas to ensure that there are no bubbles in the liquid. After that, the liquid is poured into the polytetrafluoroethylene mold for heating and molding. The whole heating and curing process includes two stages: the first stage is carried out at 80°C for 3 h to ensure the completion of mercaptan-epoxy click reaction, and the second stage is started at 110°C for 1 h, followed by 1h at 140°C to complete epoxy homopolymerization and cool to room temperature. The specimens were made into rectangles by Teflon mode.

Product characterization
FT-IR of SMP and gel were recorded on Nicolet750 FTIR Spectrometer, with scanning scope from 500 to 4000 cm −1 .

Thermal properties
Dynamic mechanical analysis was performed to measure the changes of mechanical properties of SMP under different thermal-mechanical conditions using TA Q800, equipped with heating rate was 3°C min −1 from 20 to 200°C. And the gelation time and strength of gel were measured at different temperatures.

Fracture sealing experiments
Within pressure range of 0-30 MPa and temperature range of 20-120°C, the self-made device was used to analyze effectiveness of mixture of SMP and gel in plugging fractures. By connecting a pressure transmitter and special software, all data changes could be recorded and displayed in graphs. The fracture shape was wedge and formed by two steel plates, with 3 mm × 2 mm inlet width, outlet width, respectively. The fracture simulation unit is shown in figure 1.

Results and discussion
3.1. FTIR spectrum of synthesized SMP and gel FTIR was performed to distinguish whether the required functional group change occur, since the performance of polymer would be greatly affected without suitable chemical reaction. Through curing reaction under high temperature, the epoxy group in E-51 and MDA reacts with the thiol group in PMP. Relative reaction mechanism of 'epoxy-thiol' has been researched before, as shown in figure 2, which includes nucleophilic addition reaction between thiol and epoxy groups as well as homopolymerization of epoxy groups [33][34][35].
Great growth of bond appeared around 3500 cm −1 , which represents the stretching vibrations of -OH groups in network. Nucleophilic ring opening of epoxy groups and thiol groups generated hydroxyl during the curing process, and the intermolecular hydrogen bonds enhance the compatibility between epoxy resin and PMP [36].
The disappearance of both bonds at 918 cm −1 (epoxy group) and 2567 cm −1 (thiol group) indicate the relatively complete crosslink during curing process between epoxy resin and curing agent. In order to improve the temperature resistance of gel, we introduced two chemical structures(M1, M2) into the self-made polymer, as shown in the figure 4. As shown in figure 3, the stretching vibration peak at 1635 cm −1 and 1543 cm −1 is ascribed to the C=O from amide group, and peak at 1043 cm −1 is attributed to the skeleton vibration peak of sulfonyl(O=S=O) in M1. Moreover, peak at 1301 cm −1 , characteristic of C-N stretching signal expected for M2 is measured [37]. All suggest that the functional groups that we expected to improve temperature resistance of gel are retained in the synthesized gel. 3.2. Thermal properties 3.2.1. SMP The synthesized polymers are prepared for sealing fractures or leakages in high temperature formation environment, so relevant thermal properties have also been evaluated, shown in figure 5. According to the TGA result, the curve has a small range of fluctuations before 250°C, the state is basically stable, and the mass loss is about 0.8%. The mass loss in this stage is caused by the evaporation of some volatile components due to temperature rise. The main weight-loss stage occurs in the range of 300-400°C, and the proportion of mass loss   accounts for the majority of the mass loss curve, and the mass loss is 84.4%. The main reason is that the molecular structure of the epoxy resin polymer is damaged by high temperature, the main chain and long chain parts break and break into small chain segments, and most of the chemical bonds break and gradually generate CO/CO 2 and gasification [38,39]. In the final decomposition stage, the TGA curve did not show some changes significantly, and the residue was carbon skeleton decomposed at a very slow rate. The above conclusion indicates that the synthesized SMP can be applied to the working environment of 0°C-250°C.
Due to the different temperature in different formations, in order to better seal fractures in formations, it is necessary to adjust the ratio of SMP raw materials to match its working temperature with the temperature at the fractures. For example, when the temperature at the fractures is 100°C, adjust the ratio to make the glass transition temperature(T g ) reach about 100°C, so that the memory characteristics of SMP can be used to make it deform and achieve the plugging effect. And the determination of glass transition temperature of SMP could be accomplished through DMA experiments. We prepared polymers with four different T g at 50.4°C, 86.2°C, 101.6°C and 108.5°C, respectively, as shown in figure 6.

Gel
As shown in figure 7, there is a significant difference in the influence curve of temperature on gel strength around 170°C. Before 170°C, the gel strength increases as temperature adding, from 370 Pa to 740 Pa. When the temperature exceeds 170°C, the gel strength becomes weaker, with a minimum of only 110 Pa. With temperature rising constantly, the chemical bonds and chemical structures in the gel system are destroyed. There  is no longer a strong force connection between the molecules, and the tightly crosslinked network structure cannot be maintained [40]. Water molecules are released from it, and the gel exhibits thermal degradation, becoming a fluid, losing strength and viscoelasticity.

Shape memory property
In this paper, these two polymers could be used as plugging agent is due to their common feature, that is, they can change shape under appropriate temperature. SMP material can return to original shape, while gel solution gradually lose its fluidity and become an elastomer. SMP helps to increase the strength of gel, and gel in turn helps to consolidate SMP at the leakage point, increasing the stability of the bridge structure.
Since the plugging polymer activates the deformation characteristics through the stimulation of temperature, the relevant factors affecting the deformation process, including external temperature, raw material composition and plugging agent particle size, were systematically studied through experiments. Shape recovery performance were investigated by U-type test, and the relationship between recovery time and recovery angle of different samples at T g , T g +10°C, T g +20°C shown in figure 7. Nearly, all samples can recover from U-type to original shape(rectangle) with in 100 s, which suggested great shape memory property based on networks formed by soft and hard segments. And, with the increase of temperature, the recovery speed increases significantly. Since the hard segments store external stress during memorizing temporary shape, which are responsible for shape recovery degree. The components in SMP also have an important influence on deformation. As exhibited in figure 8, the overall recovery time increases with MDA content adding. The extension of recovery process is due to the increase of crosslinked density, which means more external energy is needed to make molecular chains in frozen state become active. Due to the addition of MDA content, the recovery rate got changed, whereas entire recovery trend had occurred with no significant change. The recovery  process could be divided into three stages with a 'S' shaped curve. In the first stage, stimulated by external energy, molecular chains of soft segments gradually begin to move. Hence from the appearance, the samples start to transform to their original shape within a slower rate. In the second stage, recovery rates get more quickly on account of more active chains and sufficient energy supply. Finally, the constrained force stored in hard segments has been exhausted, and samples become to their original appearance in a slow rate. In addition, recovery process became more quickly as temperature elevating since higher temperature provide more energy for molecular chains being active, thereby achieving shape transformation early.
In order to apply SMP to seal lost circulation, prepared SMP also needs to be granulated to facilitate transportation from wellbore to formation. When the shape of SMP is relatively complete or its volume is large, the shape memory performance is great, but after being damaged to a certain extent, the shape memory property would be affected. Because the crushing and granulation process will reduce the shape memory property of SMP, we control the particle size through different sieving mesh pores, and study the effect of particle size on shape memory property.
As shown in figures 9 and 10, the expansion rate of SMP particles with different sizes is different. The expansion rate of 10-20 mesh particles is 35%-55%, the expansion rate of 40-60 mesh particles is more than 20%, and the expansion rate of 80 mesh particles is lower (>15%). The expansibility of particles is actually due to the difference in deformation response time caused by the difference in stress release and storage between the soft and hard segments within the polymer. When SMP is crushed and granulated, some structure of SMP is damaged, and a small part of its performance is lost. However, such expansibility can still ensure the shape expansion of the plugging agent particles in the high temperature environment of the reservoir, Then the physical plugging of adaptive pore throat size is completed.
Various solid particles in the ground will shear and compress the plugging agent material under the flow and transportation of formation fluid. When the compressive capacity of the plugging agent is insufficient, the bridging plugging structure formed by the plugging agent material at the hole throat will be unstable and damaged, and then the plugging ability will be lost. Therefore, we studied the pressure bearing capacity of SMP particles at different temperatures, and expressed it by the particle size degradation rate. The particle size degradation rate is defined as: Where R is particle size degradation rate, D 0 and D 1 are particle size of SMP before and after compression. It can be seen from figure 11 that the particle degradation rate of SMP particles at 25°C is 7.7% and 8.9%, while at high temperature (>T g ), the particle degradation rate is greatly reduced, only 1.3% and 1.8%. Because, the prepared SMP material has some brittle characteristics at room temperature, and it will undergo some brittle fracture under high pressure, resulting in particle size reduction. However, at high temperature, the soft segment in molecular structure gradually changes from frozen state to stretched active state, and the polymer macroscopically changes from brittle glass state to high elastic state, and the compressive capacity is greatly improved, so the particles are less damaged under high pressure. Therefore, in the high-temperature environment of the formation, SMP particles have a certain pressure bearing capacity and shape memory ability, which is the basis for becoming plugging material.

Plugging performance of SMP and gel
The self-made fracture plugging experiment was applied to evaluate the performance of SMP & gel to seal simulated cracks. The prepared SMP particles could be transported by gel solution under pressure. Subsequently, the opening was sealed and placed at 100°C for 12 h, followed by plugging experiment. The plugging ability is estimated by the lost circulation volume and break pressure. We prepared SMP with different particle sizes to study the compatibility of SMP particle size with crack size. The plugging materials composed of SMP and gel will gradually accumulate at a certain location along with fluid migration in this wedge-shaped fracture, forming a sealing structure. This wedge shaped artificial crack has a 3 × 2 mm opening, as show in figure 12. So we defined a physical value μ, which is the ratio of the particle size to the fracture opening at the location. Subsequently, the relationship between lost circulation volume, break pressure and μ is plotted as shown in the figure 13 and figure 14. Figure 13 shows the plugging effect of mixture of gel and SMP, with different particles. In the test, the break pressure is very small(<3MPa) and lost circulation volume is large(>100 ml), which indicates that too small particles could not form an effective sealing layer. As the μ value increases, the plugging break pressure gradually increases, and the lost circulation volume decreases, indicating that the plugging effect becomes better, and the compatibility between particles and fractures increases. When the μ value is 0.92, the plugging effect is the best. However, when r value increases, the plugging effect decreases. Although the gap between big particles is filled with gel, it is difficult to squeeze the big particles tightly. The main role of gel is to help SMP particles consolidate in the cracks. The strength of gel itself is not high, and it is easy to be washed away by high-pressure fluid.  Furthermore, by comparing the sealing conditions under 25°C and 100°C, it can be found that temperature has a significant impact on the prepared fracture plugging agent. At 100°C, the break pressure is greater, and the lost circulation volume is relatively low (23 ml), indicating that the sealing effect is good. Under high temperature conditions, the compressed SMP particles will be stimulated by high temperature and gradually expand. In the process of expansion and entanglement, the SMP particles and gel will combine more closely and form a more stable physical plugging layer.

Plugging mechanism of SMP and gel
The assumed plugging mechanism can be illustrated in the figure 15. At room temperature, SMP particles and gel solution would be transported to the front of cracks by fluid. With increasing temperature, SMP particles begin to deform and expand, forming a bridging and blocking structure before the cracks. Meanwhile, the gel solution filled between the particles and the rock matrix will shrink and gradually lose its fluidity, helping to stabilize the bridging structure of the SMP. SMP particles with irregular appearance expand under the stimulation of high temperature. This will result in smaller gaps between each SMP particles and rock matrix, making the plugged hole throat more compact. In this process, gel solution with a certain viscosity can help migrate SMP particles into smaller formation crevices for plugging. On the other hand, SMP particles with higher mechanical strength can increase the plugging strength of the gel. The mixed plugging agent composed of these two materials can make up for each other's shortcomings.

Conclusion
From the above study, the following conclusions could be draw: (1) A series of SMP materials based on epoxy resin, curing agent PMP and modifier MDA was prepared. And the shape memory temperature can be adjusted by changing the ratio of material. In addition, the lab also made a gel that can be used together with SMP as a plugging agent for formation plugging.
(2) When the mass ratio of MDA to epoxy resin is 1:9, 2:8,3:7, the shape memory temperature is 86.2°C, 101.6°C and 108.5°C, respectively. The SMP also has good shape memory performance and pressure bearing ability after being prepared into particles. The expansion rate of particles could reach 55%, and the particle degradation rates at room temperature and high temperature are 7.7% and 1.3%, respectively.
(3) Shape recovery performance were investigated by U-type test, which suggested great shape memory property based on networks formed by soft and hard segments. Since the hard segments store external stress during memorizing temporary shape, which are responsible for shape recovery degree. The extension of recovery process is due to the increase of crosslinked density, which means more external energy is needed to make molecular chains in frozen state become active. This is also the reason why SMP can be transported to the deep without deformation occurring in shallow locations.
(4) Plugging experiments show that plugging agent composed of SMP and gel has great effect. They all undergo morphological changes in high-temperature environments, which is more conducive to plugging cracks, and they can complement each other. SMP can increase the strength of the system, while gel can increase the stability of the system in cracks.

Data availability statement
The data cannot be made publicly available upon publication because no suitable repository exists for hosting data in this field of study. The data that support the findings of this study are available upon reasonable request from the authors. Figure 15. Schematic diagram of plugging mechanism for SMP particles and gel.