Development and Performance Evaluation of a High-Temperature Profile Control System.

In the late stage of the steam flooding development of heavy oil reservoirs, the contradiction between layers is prominent, which makes the development of the reservoir difficult. To obtain a profile control agent suitable for steam flooding in heavy oil reservoirs, the modification technology was used to improve the temperature resistance of the traditional NH-1 main agent, as the conjugation effect may enhance the stability of the molecular structure. Thus, we obtained the modified NH-1 main agent, which was used in combination with graphite particles. To explore the oil displacement effect of the profile control system, the single-variable method was used to optimize the profile control system and evaluate its performance. Moreover, the multimedium steam flooding experiment was carried out to further verify the profile control capability of the profile control system. The results show that the formula of the graphite particle-gel compounding system is 0.03% coagulant BK-A05 + 2.2% cross-linking agent I + 1.8% cross-linking agent II + 6% modified NH-1 main agent (prepared by reacting NH-1 main agent with 65% concentrated nitric acid at a solid-liquid ratio of 1:6) + 0.7% graphite particles (10 000 meshes) + 0.3% suspending agent CMC. The gel viscosity of the profile control system can reach 2 × 106 mPa·s, the gel temperature range is wide (160-280 °C), and the temperature resistance is 300 °C. The profile control system has good injection performance, plugging effect, washing resistance, and thermal stability and is significantly better than the gel system alone. The experiment results also show that the profile control system has a strong profile control capability and expands the swept volume. The crude oil recovery increases by 8.89%, and it can be effectively applied to the profile control operation of heavy oil multimedium steam flooding.

Introduction

Low-permeability and ultralow-permeability reservoirs are widely distributed in oilfields throughout the world. Low permeability, low porosity, and poor reservoir properties lead to water injection difficulties and low oil recovery in these oilfields.[1] At present, the water content in many oilfields is up to 95%. In heterogeneous reservoirs, water channeling results in only oil displacement in high-permeability zones, but unswept oil needs to be recovered to improve the recovery efficiency.[2−5] Based on the profile control technology of water injection reservoirs, the high-temperature profile control technology has gradually developed and high-temperature profile control technologies based on gels, particles, foams, and other media have been developed.[6−9] At present, preformed particle gel has been successfully synthesized and applied to control excess water production in most of the mature, waterflooded oilfields in China.[10,11] Zhao et al. successfully prepared phenolic resin dispersed particle gel (PDPG) by a mechanical shearing method and systematically analyzed the effects of bulk gel strength, shearing time, shearing rate, and bulk gel–water ratio on the PDPG particles. Zhao et al. believed that the shearing rate is one of the most critical factors in the preparation. In addition, the bulk gel–water ratio has a slight impact on the prepared PDPG particles. The experiment indicated that PDPG particles can easily enter the deep formation and effectively block high-permeability channels.[12] Ge et al. developed a partially hydrolyzed polyacrylonitrile gel profile control system. This system is suitable for sandstone formations, with a temperature resistance of 150 °C and a pH value of 4–8.[13] Al-Muntasheri et al. mixed polyvinylamide with aldehyde and phenolic cross-linking agents, with low initial viscosity and easy injection, and its gel temperature is around 205 °C; it can be used for profile control operations in steam flooding reservoirs.[14] Micron-size polyacrylamide elastic microsphere (MPEM) is a newly developed profile control and oil displacement agent for enhanced oil recovery in heterogeneous reservoirs.[15,16] Yang et al. synthesized three kinds of fluorescent polymer microspheres P(AM-BA-AMCO), P(AM-BA-Ac-Flu), and P(AM-BA-RhB) and researched effects of the fluorescent monomer on the performance of polymer microspheres. The study showed that the addition of fluorescent monomer could destroy the structure of hydrophilic polymer microsphere bulk gel to a certain extent and affect the creep performance of polymer microsphere bulk gel.[17] For high-salinity reservoirs, Yang et al. researched a betaine-type binary amphiphilic polymer PAMA-n with different hydrophobic group contents. Yang et al. found that a higher hydrophobic group content could enhance the ability to reduce surface tension and the association function between hydrophobic groups of PAMA.[18] Tang et al. synthesized polymer/nano SiO2 composite microspheres (PNSCMs) via inverse suspension polymerization with 3-methacryloxypropyltrimethoxysilane (MPS)-modified SiO2 as the reinforcing filler.[19] Many enhanced oil recovery (EOR) techniques have been applied like polymer flooding to decrease the mobility of water.[20,21] Similarly, another widespread example of EOR is polymer gel[22,23] that can be used as a plugging agent in high-permeability zones to transfer the displacing fluid toward the low-permeability zone, thus displacing the existing reservoir and increasing the oil production.[24−26]

With the increasing profile control requirements of different formation conditions and interlayer characteristics,[27−29] the compound high-temperature profile control technology[30] is increasingly favored for its advantages of strong plugging ability and deep profile control. The compound profile control agent is generally compounded by the principle of bridging between the gel and the particles. The traditional gel profile control agent has good selectivity and deformability, while the main agent plays a vital role in the temperature resistance of the gel system.

The traditional NH-1 main agent is a nontoxic high-molecular natural organic substance; its molecules contain oxygen-containing functional groups such as phenolic hydroxyl groups and quinone groups, and its temperature resistance is only 180 °C. Aiming at the harsh profile control environment caused by steam channeling in high-temperature reservoirs with steam flooding, as the conjugation effect may enhance the stability of the molecular structure, the modification technology was used to improve the temperature resistance of the traditional NH-1 main agent in which nitro was added. The modified NH-1 main agent can be copolymerized with coagulant (HPAM, BK-A05), aldehyde, and phenolic cross-linking agents to produce a gel with temperature resistance of 300 °C. The gel was used in combination with high-temperature graphite particles to further improve the scouring resistance of the profile control system and extend the use period of the profile control agent. Through the laboratory evaluation experiment, the applicable limit of the profile control agent was obtained from the perspective of formation condition adaptability. Through the high-temperature dynamic performance evaluation, the plugging rate, scouring resistance, and thermal stability of the profile control system were investigated. The purpose of this work is to develop a high-temperature profile control agent suitable for steam flooding, to improve the contradiction between layers in the later stage of steam flooding, and to increase the economic benefit.

Results and Discussion

Optimization of the Modified High-Temperature Gel System

Content of Coagulant BK-A05

The mass fraction of the modified NH-1 main agent was 6%, that of the cross-linking agent I was 2.2%, and that of the cross-linking agent II was 5%. The effect of the mass fraction of coagulant BK-A05 (0.015, 0.02, 0.025, 0.03, and 0.035 wt %) on the gelling performance of the gel solution is shown in Table . With the increase in the content of coagulant BK-A05, the gel viscosity of the gel solution increased, and after the content of coagulant BK-A05 was higher than 0.015%, the gel time was shortened slightly.

Table 1

Change of the Gel Viscosity of Gel Solution with Different Mass Fractions of BK-A05

	BK-A05 mass fraction (%)
time (h)	0.015	0.02	0.025	0.03	0.035
	Viscosity (mPa·s)
0	2.31	3.01	3.92	4.33	5.21
1	48 587	39 382	43 831	50 511	69 015
2	71 183	88 324	88 124	87 654	91 134
3	102 000	133 000	137 000	151 000	173 000
4	272 000	261 000	303 000	397 000	392 000
5	379 000	485 000	515 000	685 000	868 000
6	490 700	745 000	778 000	998 000	1 140 000
7	878 000	912 000	962 000	1 090 000	1 110 000
8	889 000	933 000	982 000	1 020 000	1 150 000
9	880 000	908 000	974 000	1 050 000	1 160 000
10	882 000	918 000	968 000	1 050 000	1 160 000

The content of BK-A05 has a greater influence on the initial viscosity of the solution. As the content of BK-A05 increases, the initial viscosity of the solution increases. As can be seen from Figure , as the content of BK-A05 increased, the gel viscosity increased and the gel time decreased. Therefore, considering the gelling performance and time, the mass fraction of coagulant BK-A05 was selected as 0.03% (Figure ).

Figure 1

Effect of the mass fraction of coagulant BK-A05 on gelling performance.

Figure 2

Effect of the mass fraction of NH-1 main agent on gelling performance.

Content of the Modified NH-1 Main Agent

The mass fraction of coagulant BK-A05 was 0.03%, that of cross-linking agent I was 2.2%, and that of cross-linking agent II was 1.5%. The gel viscosity of the gel solution at different modified NH-1 main agent contents (4, 6, 8, 10, and 12%) is shown in Table . When the content of the modified NH-1 main agent increased from 4 to 12%, the gel viscosity increased positively. The phenolic hydroxyl group in the molecular structure of the modified NH-1 main agent and the aldehyde cross-linking agent formed a phenolic pre-condensate. However, with low reaction, a long-molecular-chain coagulant was needed to provide the amide group. Furthermore, it should be copolymerized with the active functional groups in other cross-linking components, so that the gel net structure becomes denser and stronger. As the content of the modified NH-1 main agent increased, the probability of copolymerization and cross-linking reaction among the four molecules increased, resulting in a decrease in gel time.

Table 2

Variation of the Gel Viscosity of the Gel Solution with Different Mass Fractions of the Modified NH-1 Main Agent

	NH-1 main agent mass fraction (%)
time (h)	4	6	8	10	12
	Viscosity (mPa·s)
0	3.31	3.89	4.01	4.33	4.74
1	43 452	52 312	38 494	58515	78 895
2	77 274	78 324	88 285	88 315	135 000
3	141 000	149 000	173 000	171 000	1 080 000
4	387 000	391 000	523 000	538 000	2 000 000
5	595 000	662 000	758 000	778 000	1 890 000
6	773 000	823 000	1 340 000	1 410 000	2 000 000
7	788 000	1 100 000	1 340 000	1 430 000	1 910 000
8	810 000	1 090 000	1 330 000	1 490 000	1 950 000
9	798 000	1 110 000	1 350 000	1 410 000	2 000 000
10	790 000	1 100 000	1 330 000	1 410 000	2 000 000

As the mass fraction of the main agent increased, the gel viscosity increased. When the mass fraction of the main agent was 6%, the gel time was up to 7 h. When the mass fraction of the main agent was 12%, the generated gel had the highest viscosity, with short gel time and fast reaction. When the mass fraction of the main agent further increased, the main agent reached saturation in the solution and was difficult to continue to dissolve. Based on the gel time and gel viscosity, the mass fraction of the main agent was selected as 6%.

Content of Cross-Linking Agent I

The mass fraction of the modified NH-1 main agent was 6%, that of coagulant BK-A05 was 0.03%, and that of cross-linking agent II was 1.5%. The variation of the gel viscosity of the gel solution with different mass fractions of cross-linking agent I (1.65, 2.2, 2.75, 3.3, and 3.85%) is shown in Table .

Table 3

Variation of the Gel Viscosity of the Gel Solution with Different Mass Fractions of Cross-linking Agent I

	cross-linking agent I mass fraction (%)
time (h)	1.65	2.2	2.75	3.3	3.85
	Viscosity (mPa·s)
0	3.25	3.27	2.98	3.01	3.33
1	44 315	45 461	57 125	65 362	87 874
2	61 365	85 154	98 878	102 000	117 000
3	63 351	151 000	141 000	203 000	215 000
4	93 258	385 000	441 000	558 000	601 000
5	132 000	877 000	838 000	725 000	585 000
6	353 000	1 110 000	912 000	718 000	587 000
7	625 000	1 160 000	914 000	731 000	592 000
8	830 000	1 200 000	908 000	727 000	596 000
9	821 000	1 110 000	899 000	720 000	578 000
10	827 000	1 200 000	932 000	717 000	588 000

As the mass fraction of the cross-linking agent I increased, the gel time of the system decreased. When the mass fraction of cross-linking agent I was 2.2%, the gel had the maximum viscosity, which was 1.2 × 106 mPa·s. According to Table and Figure , the gel viscosity increased first and then decreased as the mass fraction of the cross-linking agent I increased, that is, when the mass fraction ranged from 1.65 to 3.85%. Therefore, considering the gel time and gel viscosity, the mass fraction of the cross-linking agent I was selected as 2.2%.

Figure 3

Effect of the mass fraction of cross-linking agent I on gelling performance.

Content of Cross-Linking Agent II

The mass fraction of the modified NH-1 main agent was 6%, that of coagulant BK-A05 was 0.03%, and that of cross-linking agent I was 2.2%. The variation of gel viscosity of the gel solution with different mass fractions of cross-linking agent II (1.2–2.4%) is shown in Table .

Table 4

Variation of the Gel Viscosity of the Gel Solution with Different Mass Fractions of Cross-linking Agent II

	cross-linking agent II (%)
time (h)	1.2	1.5	1.8	2.1	2.4
	Viscosity (mPa·s)
0	2.84	3.27	3.15	3.34	3.15
1	2.69	45 461	86 781	101 000	98 905
2	1620	85 154	153 000	188 000	1 010 000
3	2932	101 000	1 030 000	1 560 000	1 470 000
4	2696	175 000	1 920 000	2 000 000	2 000 000
5	3099	577 000	2 000 000	2 000 000	1 800 000
6	6764	910 000	1 980 000	1 960 000	2 000 000
7	8938	986 000	2 000 000	1 890 000	1 950 000
8	37 592	912 000	2 000 000	2 000 000	2 000 000
9	36 890	998 000	2 000 000	2 000 000	2 000 000
10	37 487	942 000	1 880 000	2 000 000	2 000 000

As shown in Table and Figure , with the increase of the content of cross-linking agent II, the system’s gel viscosity increased and the gel time decreased. The hydrolysate of cross-linking agent II was able to react with the other three agents. When the content of cross-linking agent II exceeded 1.8%, the final gel was semisolid. At this time, the gel was strong but the gel time was short.

Figure 4

Effect of the mass fraction of cross-linking agent II on gelling performance.

The formula of the modified high-temperature gel system finally obtained through the ratio optimization experiment was as follows: 0.03% coagulant BK-A05 + 2.2% cross-linking agent I + 1.8% cross-linking agent II + 6% modified NH-1 main agent.

Optimization of the Graphite Particle–Gel Compounding System

Optimization of CMC Content of the Suspending Agent

As shown in Figure , when the mass fraction of CMC was 0.3%, the suspension rate of graphite particles reached more than 98%, and there was almost no sedimentation at the bottom after 48 h. Graphite particles have good fluidity and dispersibility, without water precipitation. Therefore, the CMC mass fraction of the suspending agent was determined to be 0.3%.

Figure 5

Suspension rate of particles with different meshes under different CMC contents.

Optimization of Graphite Particle Content

The content of graphite particles was optimized based on the plugging rate of the compounding system to the sand-filled pipe and the plugging rate after 30 PV steam flushing. As shown in Figure , with the increase of graphite particle content, the plugging rate of the compounding system to the sand-filled pipe increased, reaching more than 98.4%. When the graphite particle content was greater than 2%, the injection became difficult. As the graphite particle content increased, the plugging rate of the sand-filled pipe after 30 PV steam flushing increased until it became stable. The inflection point appeared when the content was 0.7%. Therefore, the content of graphite particles (10 000 meshes) in the compounding system was determined to be 0.7%.

Figure 6

Blocking rate before and after the flushing of the compounding system with different particle contents.

Therefore, the formula of the graphite particle–gel compounding system was determined as follows: 0.03% coagulant BK-A05 + 2.2% cross-linking agent I + 1.8% cross-linking agent II + 6% modified NH-1 main agent + 0.3% CMC + 0.7% graphite particles (10 000 meshes).

Evaluation of Adaptability of the Modified High-Temperature Gel System

Temperature

The actual injection wells needed to be shut down for 1 week for artificial water injection to reduce the temperature before manual profile control to simulate the temperature near the well zone (160, 180, 200, 240, and 280 °C). The influence of temperature on the gelling performance of the modified high-temperature gel system (0.03% coagulant BK-A05 + 2.2% cross-linking agent I + 1.8% cross-linking agent II + 6% modified NH-1 main agent, the same below) is shown in Figure .

Figure 7

Gelling type of the profile control agent at different temperatures.

In the above temperature range, the profile control agent is a semisolid gel with a dark reddish-brown appearance. The gel has a certain elasticity, and the gel surface is relatively smooth. Except for 160 °C, no liquid appeared around the gel at other temperatures. This shows that the gel reaction of this system is fully carried out and the gelling effect is good.

As shown in Table , in the simulated formation temperature range, the modified high-temperature gel system has a gel viscosity of 4.3 × 105 mPa·s or more, which can meet the plugging needs, and has the advantages of wide gelling temperature range and high utilization rate.

Table 5

Effect of Temperature on the Gelling Performance of the Gel System

temperature (°C)	160	180	200	240	280
gel viscosity (106 mPa·s)	0.43	1.25	2.00	1.35	2.00

pH Value

In the experiment, the reaction temperature was set at 200 °C and the pH value of the experimental water used to prepare the modified high-temperature gel system was adjusted by NaOH or HNO3. The influence of pH value (6–10) on the gelling performance of the gel system is shown in Table . The modified high-temperature gel system was suitable for neutral formations with a pH between 6 and 8. In the over-alkaline environment with pH = 9, the reaction rate of the profile control system slowed down and the gel viscosity decreased greatly. At pH = 10, the gel system was ungelled.

Table 6

Effect of pH on the Gelling Performance of the Gel System

pH	gel time (h)	gel viscosity (mPa·s)
6	18	2 × 106
7	16.5	2 × 106
8	17	1.57 × 106
9	24	1.15 × 104
10		nonadhesive

Salinity

The reaction temperature was set at 200 °C, and the water used for preparation was changed to aqueous NaHCO3 solution, with a salinity range of 1600–4800 mg/L.

As shown in Figures and 9, the experimental results show that when the salinity was between 1600 and 3200 mg/L, the viscosity of the plugging agent changed a little and the gel time increased. When salinity was more than 4800 mg/L, the gel time became significantly shorter.

Figure 8

Effect of salinity on the gelling performance of the profile control agent.

Figure 9

Gelling type of the profile control agent under different salinity conditions.

High-Temperature Dynamic Performance Evaluation of the Profile Control System

Plugging Ratio and Residual Resistance Factor

The plugging ratio and the residual resistance factor of the gel system and the compounding system are compared at different temperatures.

As shown in Figures and 11, the compounding system has good plugging ability, and the plugging effect is better than that of modified high-temperature gel or graphite particle alone. The blocking rate at high temperature is more than 99%, and the blocking effect changes little with the reaction temperature. This shows that the profile control agent can still react and block channeling in the formation environment. Therefore, the profile control agent has a perfect plugging performance.

Figure 10

Blocking rate curve of a sand-filled pipe with different systems.

Figure 11

Variation curve of the plugging rate and the residual resistance factor of the graphite particle–gel compounding system at different reaction temperatures.

Scouring Resistance

We evaluated the scouring resistance of the modified high-temperature gel system, the graphite particle, and the compounding system. Table shows the permeability and plugging rate of the sand-filled pipe after steam flushing with different PV number. In the compounding system, after injection with 30 PV high-speed and high-temperature steam flushing, the system showed excellent scouring resistance and the plugging rate only dropped by 0.35%. The sand-filled pipe using the modified high-temperature gel system or graphite particles alone had poor scouring resistance. The compounding system can improve the retention ability of the plugging agent in the formation and extend the validity period.

Table 7

Scouring Resistance of Different Profile Control Systems for Plugging Sand-Filled Pipes

	water permeability measurement (10–3 μm2)
system	before blocking	after blocking	15 PV	30 PV	30 PV blocking rate %
gel system	680	14	71	336	50.59
graphite particles	703	14	34	101	85.63
compounding system	912	8	9	11	98.78

Thermal Stability

At 300 °C, the thermal stability of the profile control agent was evaluated by the change in the plugging rate of the sand-filled pipe with profile control by the compounding system over time. The thermal stability test results of the three systems are shown in Figure . High-temperature aging had basically no effect on the stability of graphite particles. The blocking rate dropped to 85.9% on the 50th day. At the 50th day of aging, the blocking rate of the sand-filled pipe by the compounding system was 97.63%, only down 1.63%. The compounding system had good thermal stability; the graphite particles and the gel system supported each other. When the gel degraded, the graphite particles can still block the dominant pores with high strength.

Figure 12

Comparison curves of the thermal stability of different systems.

Through the above performance evaluation, the graphite–gel compounding system has excellent scouring resistance and good thermal stability. The blocking rate can reach more than 99% at high temperature.

Evaluation of the Effect of Profile Control System

In the experiment of profile control agent evaluation, the relationship between the cumulative volume of injected pore volume and the cumulative recovery factor of heavy oil is assessed and is shown in Figure . It can be found that before the injection of the profile control agent, the cumulative recovery rate of the multimedium steam flooding for heavy oil was 53.03%. At this time, the water content was as high as 98%, and there was no longer any heavy oil production (Figures and 15).

Figure 13

Pore volume multiple and cumulative recovery factor.

Figure 14

Pore volume multiple and injection pressure.

Figure 15

Reflux condensation reaction device.

After injection of the profile control agent, aging was performed according to the optimal aging time 5 days, and then the multimedium steam flooding was resumed. It can be seen from Figure that the recovery factor and injection pressure of heavy oil increased gradually with the increase of pore volume multiple. It shows that the profile control agent forms a blockage on the original steam channeling channel, so that the injected hot fluid starts to advance to the unswept region and displaces the heavy oil not produced previously. When the water content reached 98%, the cumulative recovery of heavy oil was 61.92%.

After calculating the increase in the rate of the recovery factor, the results show that after injection of the profile control system, the recovery factor of heavy oil increases by 8.89%, with significantly enhanced oil recovery. It shows that the compounding system can have a good profile control and plugging effect in the process of heavy oil multimedium steam flooding. It can improve the oil displacement efficiency, expand the swept volume, and improve the crude oil recovery. This system can be used in profile control operations of heavy oil multimedium steam flooding.

Conclusions

The formula of the graphite particle–gel compounding system is 0.03% coagulant BK-A05 + 2.2% cross-linking agent I (phenols) + 1.8% cross-linking agent II (aldehydes) + 6% modified NH-1 main agent + 0.3% CMC suspending agent + 0.7% graphite particles (10 000 meshes). The gel temperature range is wide (160–280 °C), and the temperature resistance is 300 °C. The system is suitable for formation conditions with a pH range of 6–8 and alkaline water salinity below 8000 mg/L.

The compounding system has good injection performance and a high blocking rate, greater than 99%. After 30 PV high-temperature steam flushing, the blocking rate can still reach 96.78%. After aging at 300 °C for 50 days, the blocking rate is 97.63%. The scouring resistance and thermal stability of the compounding system are significantly better than those of the gel system alone.

The compounding system contains a small amount of graphite particles. The gel can degrade with steam flushing. Due to the lubricity of particles, the system has no damage to the pump during injection and production and has broad development prospects.

The results of multimedium steam flooding experiments show that the profile control system has a strong profile control capability and expands the swept volume. The crude oil recovery increases by 8.89%. Therefore, this system can be effectively applied to the profile control operation of heavy oil multimedium steam flooding.

Experimental Materials and Methods

Development of the High-Temperature Profile Control System

Experimental Agents

The experimental agents used were as follows: NH-1 main agent, chemically pure, Tianjin Fine Chemical Research Institute; cross-linking agent I (phenols), cross-linking agent II (aldehydes), and coagulant BK-A05 (HPAM relative molecular mass: 12 million), Henan Baike Company; graphite particles (2000–10 000 meshes), Qingdao Jinrilai Graphite Co., Ltd.; suspending agent CMC (content ≥ 95%), Renqiu City Yanxing Chemical Co., Ltd.; NaOH, NaCl, and NaHCO3, analytically pure and commercially available; HNO3 (65–70%) commercially available.

Experimental water was distilled water. The sand-filled pipe was formed by pressing high-temperature-resistant hydraulic powder and quartz sand. The permeability of water measurement ranges from 600 × 103 to 1000 × 103 μm2, with a diameter of 2.5 cm and a length of 30 cm.

Experimental Instruments

The following instruments were used: A YZHR-type high-temperature and high-pressure hydrothermal synthesis reactor (50 mL), Beijing Yanzheng Biotechnology Co., Ltd.; a 60227iec53 pressure monitoring device, Tianzhou Electric Group Co., Ltd.; a Brookfield rotational viscometer, American Brookfield Corporation; a SIN-PH100 pH test pen, Hangzhou Sinomeasure Automation Technology Co., Ltd.; a PL4002-IC electronic balance, Shanghai Youyi Instrument Co., Ltd.; a constant-temperature magnetic stirrer, Shanghai Sile Instrument Co., Ltd.; and a beaker, test tube, glass rod, condenser tube, three-necked flask, and thermometer.

Development of the Modified NH-I Main Agent

The quantitative NH-1 main agent and concentrated nitric acid were added into a three-necked flask for reflux condensation in a constant-temperature water bath at 90 °C. After 3 h of reaction, the product was taken out and placed in a flask. Then, the product was evaporated to dryness using a constant-temperature magnetic stirrer to obtain the modified NH-1 main agent.

The substituent nitro group was added to the benzene ring structure of the NH-1 main molecule. Based on the conjugation effect, the strong electronegativity generated by the nitro group changed the molecular electron cloud density and improved the molecular stability. The mass ratio of the NH-1 main agent to nitric acid (solid–liquid ratio) was 1:6. The modified NH-1 main agent was synthesized with different nitric acid mass fractions (10, 40, and 65%). The gel solution was prepared with 0.015% coagulant (HPAM, BK-A05) + 2% cross-linking agent I + 1.6% cross-linking agent II + 6% modified NH-1 main agent. As shown in Table , when the mass fraction of nitric acid was 65%, the modified main agent formed a semisolid gel.

Table 8

Effect of the Nitric Acid Mass Fraction on the Gelling Performance of the Gel Solution (Solid–Liquid Ratio, 1:6)

mass fraction of nitric acid (%)	gel viscosity (mPa·s)	gelling
0	20.1	ungelled
10	32.5	ungelled
40	32 558	gel with medium mobility, low strength
65	2 000 000	rigid gel with compact gel structure

The modified NH-1 main agent was synthesized with different solid–liquid ratios (1:2, 1:3, 1:4, 1:5, 1:6, 1:7). The gel solution was prepared with 0.015% coagulant BK-A05 + 2% cross-linking agent I + 1.6% cross-linking agent II + 6% modified NH-1 main agent. The gel solution was gelled at 300 °C and placed for 24 h. The comparison shows that when the solid–liquid ratio was 1:6, the dehydration amount of the gel tended to be stable, no scorch phenomenon was observed on the gel surface, and the structure was still compact.

Therefore, when synthesizing the modified NH-1 main agent, the mass fraction of nitric acid was 65% and the solid–liquid ratio was 1:6. The aqueous solution of the modified NH-1 main agent was dark red, and it was saturated when the mass fraction was 14%. At this time, the viscosity reached 2112 mPa·s. Considering that the initial viscosity of the field system should not be too large, the content of the modified NH-1 main agent in the following optimization experiment should be controlled within 12%.

Preparation of the Modified High-Temperature Gel System

An appropriate amount of coagulant (HPAM, BK-A05) was weighed and dissolved in distilled water until it was fully swollen, and quantitative cross-linking agent I and cross-linking agent II were added in sequence. Under sealed conditions, it was stirred at a speed of less than 100 r/min for 10 min until it mixed well, and then, a certain amount of modified NH-1 main agent dissolved in distilled water was added to obtain the gel solution after mixing well.

Preparation of the Gel–Graphite Particle Compounding System

Graphite particle solution with CMC as the suspending agent and the modified high-temperature gel system solution were prepared. The dosage of each agent was twice the target ratio content. The gel–graphite particle compounding system was obtained by mixing two solutions at a mass ratio of 1:1.

Optimization of the High-Temperature Profile Control System

Optimization of the Modified High-Temperature Gel System

The experiment was aimed to optimize the content of each composition, and we used the gelling properties (gel time and gel viscosity) as the screening criteria. The temperature of the simulated formation was 200 °C, and the pH of the plugging agent solution was 7.4.2.1.1

Experimental Agents

The experimental agents used were as follows: coagulant, modified main agent, cross-linking agent I, cross-linking agent II, pH adjusting agent, and formation water.

Experimental Instruments

The experimental instruments used were as follows: a hydrothermal synthesis reactor, an electric balance, a Brookfield rotary viscometer, an incubator, an electric blender, a pH meter, a measuring cylinder, a beaker, and so on.

Experimental Method

An appropriate modified NH-1 main agent, cross-linking agent I, and coagulant were weighed and sufficiently dissolved in water, respectively, and the pH value was adjusted to 7. The three kinds of solutions were mixed and added with the cross-linking agent II. The mixed solution was divided into 10 portions approximately, each of which was put into the hydrothermal synthesis reactor, sealed, and placed in the reaction environment (200 °C). One reactor was taken out every 1 h and cooled to room temperature in the air. Then, the reactor was opened (the reactor cannot be opened at a high temperature); the routine method was used to measure the viscosity of gel in the reactor with a viscometer. The appropriate rotor was selected for rotation. When the number fluctuated in a range with a small amplitude, the average should be recorded. The ratio of each composition in the solution using the above procedure should be changed.

Optimization of the Graphite Particle–Gel Compounding System

Optimization of CMC Content of the Suspending Agent

Taking the suspension rate as the criterion for evaluating suspension performance, the suspension rate is calculated as followswhere Q is the suspension rate in %, h1 is the volume of suspension solution in mL, and h2 is the volume of water precipitated from the suspension solution at 48 h in mL.

Furthermore, weigh a certain amount of CMC solution and dissolve it in water using an electric mixer to stir at 1000 r/min until it is completely dissolved. Weigh the graphite particles with a mass fraction of 2% and a mesh range of 2000–10 000, disperse them in a beaker, and stir at high speed for 30 min. Pour suspension solutions with different meshes into the corresponding 100 mL measuring cylinders and allow them to stand for 48 h. Record the amount of precipitated water, and calculate the suspension rate.

Optimization of Graphite Particle Content

A certain amount of 10 000 mesh graphite particles was added to the CMC solution with a mass fraction of 0.3% and then mixed with the gel system at a mass ratio of 1:1 to obtain the graphite particle–gel compounding system. The content of graphite particles was optimized based on the plugging rate of the compounding system to the sand-filled pipe and the plugging rate after 30 PV steam flushing.

Evaluation of Adaptability of the Modified High-Temperature Gel System

The gelling performance of the gel system was measured by simulating different formation conditions in the mine (changing the reaction conditions of the modified high-temperature gel system).

Preparation of the sand-filled pipe: Turn the sand-filled pipe into vacuum with the vacuum pump and saturate it with water after half an hour. Then, measure the pore volume and permeability before plugging.

Seal Test

Connect the experiment devices according to Figure . Turn on the valves c and d, turn off the valves a and b. Inject water through the pump to find the place that is not sealed. Change or tight the connection until it is well sealed.

Figure 16

Flow chart for determination of the residual resistance factor.

Plugging Experiment

Fill the profile agent solution in the piston container. Turn on the valves a, b, and d, and turn off the valve c. Displace 3 PV profile control agent into the sand-filled pipe at a constant speed of 1 mL/min, keep it sealed, and put it into the thermostat at 200 °C for 12 h until it is gelled.

After the profile agent gels, measure the water phase permeability in the air. Inject water at speeds of 1, 3, 5, 7, and 9 mL/min; calculate the water phase permeability, respectively; and then get the average. The plugging ratio and residual resistance factor are calculated by formulas and 3.

Plugging ratio

Residual resistance factorwhere K1 is the water phase permeability before plugging, K2 is the water phase permeability after plugging.

Evaluation of the Effect of Profile Control System

To evaluate the effect of the graphite–gel compounding system on the oil displacement in the process of heavy oil multimedium steam flooding, we simulated the multimedium steam flooding process by injecting air and steam mixed in a certain proportion. The experimental process is shown in Figure . The heavy oil used in the experiment was taken from Block Jin 91 in the Liaohe Oilfield, and the viscosity at 80 °C was 598.4 mPa·s.

Figure 17

Flow chart of multimedium steam flooding.

The multimedium steam flooding process was simulated by mixed injection of air and steam at a mass ratio of 98:2. After flooding to 98% of the limit water content, the graphite–gel compounding system was injected and aged for a period of time before observing the change in recovery factor. The description of specific experimental steps is as follows.

Fill two sand-filled pipes with a permeability of 2000 × 103 μm2 with 50–70 mesh quartz sand, and conduct gas permeability measurements first and then water permeability measurements.

Saturate the sand-filled pipe with formation water by the vacuum method and record the pore volume.

Saturate the sand-filled pipe with heavy oil at 80 °C and record the saturated oil volume.

Connect the experimental instruments according to Figure . Check the air tightness of the instrument. Set the thermostat temperature to 80 °C. Pressurize the back pressure valve by a hand-operated metering pump, which is always 2 MPa higher than the injection pressure.

Set the internal temperature of the steam generator to 200 °C. Set the steam injection rate to 2 mL/min using the ISCO pump. Open the piston container with compressed air, adjust the injection pressure through the pressure regulating valve, and read the air injection rate by the wet gas meter. The air injection rate was controlled at 34.91 mL/min, so that the injection mass ratio of steam to air was 98:2.

At the steam–air mass ratio of 98:2, carry out air-steam flooding on the sand-filled pipe with saturated heavy oil. Record the pressure, the volume of the liquid, and the volume of produced water in the measuring cylinder every 5 min.

When the water content reached 98%, the profile control system was injected into the sand-filled pipe and aged at 80 °C for 5 days.

After the aging was over, continue the air-steam flooding for the sand-filled pipe in the same manner. Record the pressure and the volume of oil and water in the measuring cylinder. Stop the experiment when the water content reached 98%.

Calculate the cumulative recovery of heavy oil at different times. Observe the cumulative recovery of heavy oil before and after injection of the profile control system. Calculate the increase in the rate of recovery factor. Evaluate the effect of this profile control system on the oil displacement during the multimedium steam flooding process.