Exploration of Super-Gravity Rapid Dissolution Method of Polymer for Offshore Oil Repellent

Abstract

The long dissolution time and large dispensed volumes of oil repellent polymers in offshore oil fields lead to a great increase in the volume and number of dissolution and maturation tanks in the polymer formulation system. However, there is limited space and load-bearing capacity at the offshore platform and only a small space is available for the dispensing system. To further optimize the polymer dispensing system and reduce its floor space, the super-gravity technology may be considered as a way to speed up the dissolution of the polymer. The mechanism of super-gravity rapid dissolution was investigated by establishing mathematical models and with indoor experiments. The effects of filler pore size and super-gravity factor on polymer dissolution time and solution viscosity were investigated using the super-gravity rapid dissolution device, then combined with established graded forced stretching devices for field magnification experiments. The results indicated that the super-gravity method can substantially shorten the polymer dissolution time. The basic dissolution time of the polymer AP-P4 was shortened by 35 min compared with the conventional formulation method after use of the super-gravity rapid dissolution device. The optimal process conditions for the preparation of polymer solution by the super-gravity rapid dissolution device were selected as the optimal super-gravity factor range of 1031~1298.

1. Introduction

As one of the important methods to improve oil and gas recovery by chemical drive in tertiary oil recovery [1,2], the main mechanism of polymer drive technology relies on increasing the viscosity of the replacement phase and reducing the permeability of the reservoir to reduce the flow ratio [3,4], thus increasing the wave efficiency and improving the oil drive effect [5,6]. As a low cost-effective recovery method [7,8,9], it has been widely used in oil fields on land [10,11,12] and sea [13,14], and the effect of development is remarkable [15,16].

Due to the high viscosity of crude oil, strong formation shear capacity [2,17], and high mineralization of formation water in offshore fields such as Bohai [18], hydrophobic associative polymer AP-P4 can better meet the needs of viscosity building ability [19,20], shear resistance and salt tolerance under the conditions of offshore oilfield injection and polymerization than conventional linear polymers [21,22,23]. The polymer dispensing system used in onshore fields is no longer suitable due to the large volume of single well dispensing in offshore fields [24,25], the limited space and carrying capacity of offshore platforms [2,26], and the long dissolution time of polymers [27,28]. The introduction of hydrophobic associative polymers [29,30,31], which dissolve more slowly due to the introduction of hydrophobic groups [32,33], will inevitably lead to a significant increase in the volume and number of dissolution and maturation tanks in the polymer dispensing system [34,35], leaving little space for polymer dispensing equipment on the offshore platform [36,37], further exacerbating the conflict between the limited space and carrying capacity of the offshore platform and the long polymer dissolution time [38,39,40]. Therefore, the problem of hydrophobic polymer formulation on offshore platforms has become a technical bottleneck that restricts the large-scale application of polymer drives in offshore oil fields [41,42], and there is an urgent need for a method that can quickly and effectively accelerate polymer dissolution while reducing the footprint of polymer injection systems [43,44,45,46].

This study introduces super gravity technology into the polymer fast dissolution process. By studying and analyzing the mechanism of super mass transfer fast dissolution, the super mass transfer fast dissolution device is designed and combined with the existing forced stretching device to form a polymer solution dispensing device suitable for offshore platforms. The device can significantly shorten the polymer dissolution time and thus reduce the number of dissolution tanks on offshore platforms; it also covers a small area and is light in weight, which is a good solution to the problem of large dispensing systems due to large polymer dispensing volume and long dissolution time, and is of great significance to the large-scale application of polymer drive in offshore oil fields.

Super-gravity technology refers to applied technology created by using scientific principles developed by studying the physical and chemical change processes in a super-gravity environment [47,48,49,50,51], usually in gas-liquid and liquid-liquid contact and reaction processes [52,53,54]. Super-gravity technology began to be more widely used in the chemical industry in the 1980s and is a qualitative leap forward in enhancing mass transfer processes [55]. The strength of the super-gravity field is usually characterized by the super-gravity factor [56], which is defined as the ratio of the centrifugal acceleration to the gravitational acceleration at any place (or at any point) under the super-gravity field, and is expressed as [47]:

$$\beta =\frac{{\omega }^{2}r}{g}$$

(1)

2. Polymer Dissolution Processes

The structure of the polymer AP-P4 is complex, and the molecular chains are interpenetrated and entangled with each other, which can be regarded as an aggregation system containing certain remaining space and many irregular clusters interpenetrated with each other. After the polymer dry particles enter into the solvent, the solvent molecules gradually enter into the interior of the dry particles and the polymer molecular chains start to become loose, but the polymer molecular chains are affected by their gravity and also need to overcome the interactions between molecular chains, so the diffusion is very slow, resulting in a long dissolution time of the polymer.

From an overall perspective, the polymer dissolution process is divided into the swelling phase, the dissolution phase and the curing phase (Figure 1).

Polymer particles in the dissolution process will appear in the infiltration layer, solid swollen layer, gel layer, and liquid layer (Figure 2).

➀ After polymer dry powder particles are mixed with solvent, solvent molecules move into the free volume of polymer particles under the action of osmotic pressure, forming a thin permeable layer on the surface of the polymer particles; these will become a solid swelling layer of certain thickness with the invasion of solvent molecules.

➁ With the gradual expansion of the polymer particles’ volume, as there is not enough space for the polymer to adjust the molecular chains, the polymer chains become untangled and start to gradually leave the swollen particles into the solvent. After the polymer chains enter the solvent, the large difference in concentration gradient makes the solvent continuously enter the swollen particles and the chains outside the swollen clusters form a gel layer.

➂ With the increase of dissolution time, the polymer chain segments are surrounded by the free-flowing solvent, and most of the chain segments of polymer molecules gradually lose their restraint, but are not completely free-flowing but suspended in the solvent without fixed shape and volume; they finally slowly diffuse into the solvent, and the polymer molecules mix with the solvent molecules until they are completely dissolved.

3. Super-Gravity Rapid Dissolution Method

From the polymer dissolution process, it is known that reducing the length of polymer swelling and dissolution phases is the nucleus of solving polymer fast dissolution. The principle of introducing super-gravity technology into polymer fast dissolution and designing a super-gravity fast dissolution device is to use the super-gravity field to enhance interphase mass transfer and rapidly renew the interphase interface, which greatly improves the interphase transfer rate coefficient. By driving the packing at high speed through a mass transfer ring, the polymer swollen particles are trapped by the packing layer when passing through the mass transfer ring, and the polymer swollen particles are dispersed and broken by the packing under the action of super-gravity, which in turn increases the interphase interface between the dispensing water and the polymer swollen particles, thus accelerating the polymer dissolution.

3.1. Super-Gravity Rapid Dissolution Mechanism

(1) Accelerated water-polymer bi-directional diffusion rate

In the high speed centrifugation, the packing filament on the film receives liquid droplets in two forms, in addition to a small amount of liquid filament. Due to the high shear force caused by rotation, the liquid in the packing film is very thin, only a few dozen microns or less, that is, the Reynolds number is very low, so the liquid flow on the packing can be treated as laminar flow. The liquid flow on the packing can be seen as a composite of axial and circumferential flow along the packing in the form of a film. Based on the above analysis, the mathematical model of liquid flow in the packing may be established.

➀ Boundary Conditions

The physical model of fluid flow is assumed as follows:

The packing layer is composed of coaxial i layers of mesh packing;

The liquid is ejected from the spout. When the liquid meets the first layer of packing, it can be divided into two parts: part of the liquid passes between the gaps in the packing, not affected by the packing, and continues to maintain the original direction; another part of the liquid is captured by the packing, attached to the silk, and the packing but with the same circumferential velocity, and due to centrifugal force continues to flow outward. The liquid reaching the second layer of packing can be divided into two parts, part of the original form of liquid jet; the other part is the liquid flowing from the first layer of packing. Some liquid is captured by the second layer of packing, while the other part maintains the original state. The situation at the third layer of packing on the situation is roughly similar, and the process continues until the liquid reaches the outer edge of the packing layer.

➁ Mathematical model

The microstructure is shown in Figure 3. Assuming that the fluid exists in the form of liquid microelements inside the packing, the computational model of fluid flow on the packing wire is shown in Figure 4.

From the Navier–Stokes equation [57,58], the velocity equations for the axial descending film and circumferential winding flow of the liquid can be obtained as follows [59,60,61], respectively:

$$u_z=\frac{1}{v}{\omega }^2{R}_{\mathrm{i}}\left[\frac{1}{4}\left(r_0^2-r^2\right)+\frac{1}{2}{\left(r_0+\delta \right)}^2\ln \left(\frac{r}{r_0}\right)\right]$$

(2)

$$u_{\theta }=\frac{1}{3v}{R}_i{\omega }^2\sin \theta \left(\frac{r_0^3+2r_1^3}{r_0^2+r_1^2}r+r_0^2{r}_1^2\frac{r_0-2r_1}{r_0^2+r_1^2}·\frac{1}{r}-r^2\right)$$

(3)

In the study of the mechanism of super-gravity accelerated polymer dissolution, it is first assumed that the packing layer is one layer, and the mixture formed by the polymer swelling particles and the dispensing water passes through the packing layer [62], the small size swelling particles pass through the packing layer directly [63,64,65], while the larger size swelling particles are trapped by the rotating packing layer and obtain the same circumferential velocity as the packing layer, and the instantaneous axial velocity of the polymer swelling particles is zero at this moment [66,67], that is:

$$u_{zp}=0$$

(4)

$$u_{\theta p}=\omega R$$

(5)

At the same time, part of the dispensing water passes directly through the packing layer under the action of centrifugal force, and the other part is trapped by the packing to pass through the gap between the polymer-swollen particles and the packing filaments, obtaining circumferential and axial velocities.

The liquid film formed by the liquid on the packing wire is very thin and can be neglected, i.e., $\delta =0$, the radius of the liquid film $r=0$, at this time $r_1=r_0+\delta =r_0$. Bringing in Equation (1), Equation (2) can be simplified to obtain the circumferential velocity and axial velocity of the water in the packing wire calculation formula:

$$u_{zw}=\frac{1}{4v}{r}_0^{2}R{\omega }^2$$

(6)

$$u_{\theta w}=\frac{1}{6v}{r}_0^{3}R{\omega }^2\sin \theta$$

(7)

The resulting difference in velocity between the polymer-swollen particles and the dispensing water on the packed filament is:

$$\Delta u_z=\frac{1}{4v}{r}_0^{2}R{\omega }^2$$

(8)

$$\Delta u_{\theta }=\omega R(\frac{1}{6v}{r}_0^3\omega \sin \theta -1)$$

(9)

In turn, the difference in velocity between the i-th layer of polymer-swollen particles and the dispensing water on the packed filament can be obtained as:

$$\Delta u_z=\frac{\pi n^2}{v}{R}_i{r}_0^2$$

(10)

$$\Delta u_z=2{(\pi n)}^{2}R(\frac{1}{3v}{r}_0^3\sin \theta -\frac{1}{\pi n})$$

(11)

Under this velocity difference condition, the dispensing water can forcibly penetrate into the interior of the polymer swollen particles, causing the undissolved portion inside to come into contact with water to form a new swollen layer. At the same time, after the initial swollen layer of polymer particles comes into contact with water, the molecular chains start to become looser and more easily detach from the particle surface, thus accelerating dissolution.

(2) Reducing the particle size of dissolved particles

The main action stage of super-gravity accelerated polymer dissolution is the polymer swelling stage, which accelerates the conversion to polymer solution by the action of the packing layer. The dry polymer powder particles are mixed with solvent in the dissolution tank to form the swelling particles, which are transferred to the super-gravity speed dissolution device by the transfer pump, and the swelling particles and solvent are trapped by the packing layer after entering the super-gravity speed dissolution device. As the size of polymer swollen particles is much larger than the pores of the filler, when the polymer swollen particles pass through the filler, under the acceleration of super-gravity, the swollen particles are squeezed and deformed by the force, resulting in the swollen layer on the surface of the particles being gradually peeled off, and the un-swollen part inside is exposed to form a new swollen layer in contact with the solvent molecules, and so forth to accelerate the dissolution of polymer (Figure 5).

(3) Enlarge interphase interface

In the process of high-speed rotation, the mass transfer ring distributes the polymer swollen particles and the dispensing water evenly to the mass transfer ring, which makes the two mixed more evenly and increases the contact area between the dispensing water and the polymer swollen particles. At the same time, the polymer swollen particles become smaller and the contact area between water and polymer swollen particles increases per unit of time after repeated stripping by the mass transfer ring.

3.2. Super-Gravity Rapid Dissolution Realization Method

The mass transfer ring is the core structure to realize the super-gravity rapid dissolution. With a support network on both sides and copper foam inside, the polymer swollen particles and the dispensing water are solubilized in the aperture of the copper foam to realize the super-gravity rapid dissolution. The size of the super-gravity factor can be changed by changing the size of the mass transfer ring and the motor speed.

The polymer solubilized particles dispersed uniformly by the liquid distributor get very high kinetic energy after being captured by the high-speed rotating mass transfer ring, which is a prerequisite for super-gravity rapid dissolution.

The pore size of the filler plays a crucial role in accelerating the contact between the polymer swollen particles and the dispensing water. Too large a pore size will not play the role of super-gravity rapid dissolution, while too small sized pores will be blocked, bringing inconvenience to the operation. Given the particle size range of hydrophobic polymer dry powder (

Table 1. Particle size distribution of hydrophobic associative polymers.

Particle Size (mm)	Quality (g)	Percent Content (%)
>0.850	0.35	3.52
0.425~0.850	4.57	45.74
0.250~0.425	3.29	32.93
0.178~0.250	1.12	11.25
0.150~0.178	0.66	6.60

) used in the offshore oil field, the pore sizes of copper foam metal used here are 200 μm, 300 μm, and 450 μm (Figure 6).

According to the mechanism of quick dissolution, a super-gravity quick dissolution device (Figure 7) was designed to meet the requirements. The device mainly consists of a mass transfer unit, liquid collection chamber, and housing.

4. Experimental Section

In this section, we present detailed information regarding the fluids, experimental equipment, and experimental procedures used in this work.

4.1. Materials

In this work, hydrophobic associative polymer AP-PA with solid content = 90%, insoluble matter content in water = 0.154%, degree of hydrolysis = 18.1%, the molar fraction of hydrophobic groups of 2–5% and molecular weight = 13 × 10⁶ was purchased from Sichuan Guangya Polymer Chemical Co., Ltd. (Chengdu City, Sichuan Province, China). Unless otherwise stated, AP-P4 solutions were prepared using simulated water from the Bohai oilfield formation in all experiments. Its ionic composition is shown in

Table 2. Ion components of simulated formation water in Bohai oil field.

Ions	Na+ + K+	Ca2+	Mg2+	CO32−	HCO3−	SO42−	Cl−	TDS
Concentration (mg/L)	3091.96	276.17	158.68	14.21	311.48	85.29	5436.34	9374.13

.

4.2. Methodology

4.2.1. Micro-Dimensional Measurement

The micro-dimensional of AP-P4 was measured with a Zeiss microscope. A certain concentration of polymer AP-P4 solution was prepared, and the polymer dry powder particles were evenly spread into a beaker with the preparation water; the dry powder particles were mixed with the preparation water under the action of a stirrer to swell, and after stirring for about 3 min, the swollen particles were taken in the observation dish as the comparison group, and the size was observed and marked using Zeiss microscope. Some of the solution was taken, and under the action of certain super-gravity, the swollen particles were passed through the pore size of 200 μm, 300 μm, 450 μm packing. The swollen particles were taken into the observation dish as the control group after passing the packing solution.

4.2.2. Dissolution Time Measurement

This study investigated the effect of a super-gravity instant dissolution device on the dissolution time of the polymer AP-P4. The experimental apparatus is shown in Figure 8. The polymer concentration was 5000 mg/L and the preparation temperature was 65 °C. The process was as follows. Firstly, prepare AP-P4 solution in a 2 L beaker using a conventional indoor stirrer and measure the dissolution time as a comparison group. Secondly, adjust the process parameters of the super-gravity rapid dissolution device, connect the process according to the experimental flow chart, carry out the test run of the device, and check for leaks and other conditions. Take 15 L of prepared simulated formation water and add it to the dissolution tank; weigh 75 g of dry polymer AP-P4 powder with an electronic balance; turn on the stirrer of the dissolution tank, and set the stirrer speed at 200 r/min. When the polymer AP-P4 dry powder and simulated formation water in the dissolution tank are premixed for 180 s, turn on the transfer pump and the stirrer speed in turn. After the dry polymer AP-P4 powder and the simulated formation water are premixed for 180 s, the transfer pump and the super-gravity rapid dissolution device are turned on in turn. After the polymer solution concentration at the outlet of the super-gravity rapid dissolution device is stabilized (running for 2 min), samples are taken in the beaker at the outlet of the device for testing. Dissolution is finished when the viscosity of the solution remains almost constant.

4.2.3. Solution Viscosity Measurement

The apparent viscosity of AP-P4 solutions was measured by a Brookfield viscometer (RVDV-Ⅲ; Brookfield, Middleboro, MA, USA) at 7.34 s⁻¹ at 65 °C. The viscosity of the solution was measured at the same time as the polymer dissolution time, and samples were taken every 5 min until the viscosity of the solution remained basically constant.

5. Results and Discussion

5.1. Effects of Micro-Dimensional Polymer Size

The effect of packing on the micro-dimensional size of polymer AP-P4 is shown in Figure 9. The average particle size of polymer swollen particles was 2617.57 μm after 3 min of stirring and swelling, and the average particle sizes were 1939.70 μm, 1563.25 μm, and 1326.56 μm after packing with pore sizes of 450 μm, 300 μm, and 200 μm. The particle sizes of polymer swollen particles decreased significantly after packing, and the internal color of the particles became lighter as the pore size of the packing decreased. The smaller the size of the polymer swollen particles, the lighter the color. This means that after the polymer swollen particles pass through the filler, the gel layer on the surface of the particles is peeled off by the filler, the particle size becomes smaller, the internal un-swollen part is exposed, and more solvent enters the polymer particles to form a new gel layer, thus accelerating the dissolution of the polymer.

5.2. Effects of Dissolution Time

The effect of the super-gravity rapid dissolution device on the dissolution time of polymer AP-P4 is shown in Figure 10. It is clear that under the same packing pore size condition, the dissolution time of AP-P4 gradually shortens with the increase of super-gravity factor, and the basic dissolution time shortens from 80 min to 36 min with the increase of super-gravity factor from 0 to 1298, and the dissolution time shortening rate can reach up to 56.3%. When the super-gravity factor increased to 1031, the dissolution time of AP-P4 basically ceased to change. Under the same super-gravity factor, the dissolution time of AP-P4 was gradually reduced with the decrease of packing pore size, and the degree of polymer dissolution time reduction was most obvious when the packing pore size was 200 μm. This indicates that the larger the super-gravity factor and the smaller the pore size of the packing, the better the effect of the super-gravity quick dissolution device to accelerate the dissolution of AP-P4.

With the increase of the super-gravity factor, the dissolution time of AP-P4 was gradually shortened and the basic dissolution viscosity was gradually reduced after the super-gravity quick dissolution device. This is because with the increase of super-gravity factor, the interphase interface between water and AP-P4 dissolved particles was renewed rapidly during the process of aggregation and dispersion in the packing, which strengthened the mass transfer between water and polymer dissolved particles; in addition, due to the increase of the super-gravity factor, the centrifugal force on water and AP-P4 dissolved particles increased, which accelerated the penetration of water into the interior of AP-P4 dissolved particles.

5.3. Effects of Solution Viscosity

The effect of the solution viscosity device on the dissolution time of polymer AP-P4 in Figure 11. It is evident that under the same packing pore size condition, the solution viscosity at the basic dissolution of AP-P4 gradually decreased with the increase of super-gravity factor, and the lowest solution viscosity retention rate at basic dissolution was 51.8%. Under the same super-gravity factor, the solution viscosity at the basic dissolution of AP-P4 gradually decreased with the decrease of packing pore size. This means that the larger the super-gravity factor is, and the smaller the pore size of the filler, the lower will be the retention rate of solution viscosity at the basic dissolution of AP-P4 by the super-gravity quick dissolution device.

From the results of the super-gravity rapid dissolution device on the dissolution time and solution viscosity, it is clear that with the increase of the super-gravity factor, the shortening rate of AP-P4 dissolution time showed an increasing trend, which was most obvious when the packing pore size was 450 μm; 200 μm, and 300 μm were more stable and had the same trend. The retention rate of solution viscosity at basic dissolution showed a decreasing trend, which was most obvious when the packing pore size was 450 μm, followed by 200 μm; 300 μm was the most stable. The results of the effect of packing pore size on polymer dissolution time and solution viscosity at basic dissolution were integrated, and the range of super-gravity factor was 1031~1298. In subsequent formulations of polymer solution using the super-gravity rapid dissolution device, the packing pore size can be selected according to the actual situation.

5.4. Magnification Experiments

In this section, in order to better achieve the effect of the super-gravity rapid dissolution device, the existing forced stretching rapid dissolution device [68] was combined with the super-gravity rapid dissolution device to form a new rapid dissolution device (Figure 12). The prepared polymer solution is first passed through the forced stretch rapid dissolution device and then through the super-gravity rapid dissolution device. This was used to conduct field scaling experiments, and the experimental flow chart is shown in Figure 13.

The core structure of the forced stretching rapid dissolving device adopts a two-stage grinding structure, which consists of a dynamic tooth disc and two fixed tooth discs (upper fixed tooth disc and lower fixed tooth disc), and the fixed tooth disc and dynamic tooth disc mesh with each other to form the rapid dissolving ring. The gap size of the first-stage instant structure ring is 450 μm to ensure that the particle size of dissolved particles at the outlet is ≤450 μm; the gap size of the second-stage instant structure ring is 350 μm to ensure that the particle size of dissolved particles at the outlet is ≤350 μm. In this experiment, the packing pore size of the super-gravity rapid dissolution device is chosen to be 300 μm.

The polymer AP-P4 solution viscosity was measured at 5 min intervals using beakers sampled at the outlet of the forced stretch fast dissolution device and the outlet of the supergravity fast dissolution device, respectively, and the test results are shown in Figure 14.

The experimental results showed that the basic dissolution time of hydrophobic associative polymer AP-P4 was shortened from 75 min to 15 min by the compulsory stretching + super-gravity rapid dissolution device, and the viscosity retention rate was above 53.4%.

6. Conclusions

The super-gravity rapid dissolution device accelerates the two-way diffusion of polymer swollen particles and solvent under the dual action of super-gravity and filler. At the same time, it strips off the particles’ surface swollen layers and increases the interphase interface between the remainder of the particles and the solvent, thus accelerating the dissolution. The basic dissolution time of the polymer AP-P4 was shortened by 40 min compared with the conventional formulation method after using the super-gravity rapid dissolution device. The optimal super-gravity factor range of 1031~1298 was obtained when the polymer solution was passed through the super-gravity rapid dissolution device. The dissolution time was shortened to 15 min when combined with the forced stretch rapid dissolution technology.