**3.1 Development and application of novel salt-resistant polymers**

Bohai oil field has the characteristics of high crude oil viscosity, large well spacing, high water salinity, and limited platform space. Thus, the suitable polymer agent should have high-viscosity property, good salt tolerance, aging stability, and fast-dissolution ability. In order to expand polymer injection scale, two kinds of salt-resistant polymer were investigated and applied in Bohai Bay.

*Development and Application of Chemical EOR Technologies in China Offshore Oil Fields DOI: http://dx.doi.org/10.5772/intechopen.88942*

**Figure 1.** *Schematic of hydrophobic associated polymer.*

*Enhanced Oil Recovery Processes - New Technologies*

offshore chemical EOR projects:

characteristic.

reinjection.

and high-viscosity crude oil.

(350–600 m) and anti-nine-spot well pattern.

level, and the incremental oil was obviously enhanced.

**3.1 Development and application of novel salt-resistant polymers**

salt-resistant polymer were investigated and applied in Bohai Bay.

requires a large amount of freshwater supply, large injection allocation equipment, good reservoir conditions, and loose environmental requirements. It is difficult to meet these requirements in offshore oil fields. To realize the field application of polymer flooding technology, there are several challenges to overcome to conduct

1.Lack of fresh water on offshore platforms. It is almost impossible to use fresh water to prepare polymer for offshore polymer flooding. Only seawater (salin-

2.Narrow space of offshore platform. The equipment for polymer solution preparing and injection is required to be compact, flexible, and more efficient. At the same time, the polymer is required to have excellent and efficient viscosity properties, including temperature and salt resistance and instant solubility

3.The reservoir conditions are complex. For example, most Bohai oil fields have many high-permeability heterogeneous layers, high-salinity formation water,

4.Large well space. Most offshore oil fields were developed with large well space

5.Environmental requirements for offshore oil field development are high. It is difficult for produced fluid from polymer flooding to meet standards in the last treatment on the platform, which will have a great impact on water

6.There is a big difference between offshore oil field and onshore ones in development model and polymer injection timing, so it is necessary to establish evaluation methods and standard for offshore polymer flooding performance [5–9].

**3. Research and development of offshore chemical EOR techniques**

It could be seen that the research and application of enhanced oil recovery technology in offshore heavy oil reservoirs are of great significance for stale production and economic and efficient development of Bohai oil field. Based on about 20 years of research offshore chemical EOR techniques, a series of resolutions and techniques were developed including high-efficiency driving agents, platform polymer injection distribution technology, produced fluid treatment technology, and performance evaluation method for early-stage polymer flooding. These techniques had been carried out in S, L, and JW oil fields. The water cut was controlled at a low

Bohai oil field has the characteristics of high crude oil viscosity, large well spacing, high water salinity, and limited platform space. Thus, the suitable polymer agent should have high-viscosity property, good salt tolerance, aging stability, and fast-dissolution ability. In order to expand polymer injection scale, two kinds of

ity 32,000–35,000 mg/L) or formation water could be considered.

**114**

### *3.1.1 Hydrophobic associated polymer*

With reference to the research of hydrophobic acrylamide polymers and the literature on the synthesis of ultrahigh-molecular-weight hydrolyzed polyacrylamide, a new kind of polymer was designed with branched structure and hydrophobic associative units with micro-blocks (**Figure 1**). Meanwhile, a high-molecular-weight hydrophobic associative polymer industrial synthesis technology and polymer fast-dissolution technology was developed which could meet the requirements of large-scale application of high-efficiency offshore EOR technology [10, 11]. Under above conditions, the viscosity of hydrophobic associated polymer is 3–5 times that of ultrahigh-molecularweight HPAM, the hard water resistance is increased by three times, the shear and aging retention rate are increased by 1.6 times, the residual resistance coefficient are increased by 2–3 times, and the dissolution time is shortened to 40 min.

Hydrophobic associated polymer has been applied in Bohai oil field since September 2003 and has achieved significant oil and precipitation effects [7–10].

### *3.1.2 Structural composite polymer*

The performance of polymer depends on the molecular structure. Structural composite polymer is a kind of polymer with high sterically hindered side group on its skeleton, which exhibits relative rigidity in the solution, and the polymer molecule is strengthened under relatively harsh conditions by increasing the kinetic radius. Also a functional monomer which adjusts the amphiphilic nature of the polymer molecule is introduced into the molecular structure of the polymer to improve the solubility of the polymer. Structurally, polymer molecules contain diversified and multifunctional monomers, hence the name structural composite polymers (**Figure 2**). By adjusting the type and proportion of functional monomers, the salt and temperature resistance ability of polymers was improved. The polymer at lower concentrations could significantly reduce the viscosity of the heavy oil and significantly improves the fluidity of the heavy oil [12–14]. Above polymer had been injected in several pilot wells in Bohai oil field since December 2018.

### **3.2 Chemical flooding injection system on offshore platform**

In order to promote the application of polymer flooding technology in offshore oil fields, it was necessary to combine the special conditions of offshore platforms to break through the bottleneck limitation of the huge polymer dispensing system

### **Figure 2.** *Schematic of structural composite polymer.*

### **Figure 3.**

*The process of offshore platform injection system.*

and achieve miniaturization, high efficiency, and modularization of the polymer injection system [15, 16]. The offshore polymer dispensing equipment included four systems: the dispersing and dissolving system, the curing system, the high-pressure injection system, and the control system (**Figure 3**). Each system was independent and convenient to install according to the site conditions.

**Dispersing and dissolving system** was the core of the rapid dissolving device. According to their viscoelastic characteristics, swelling polymer micelles were stretched during going through the gaps of the stator and rotor by using moderate stretching method. More water molecules permeate into the inside of the polymer micelle. At the same time, polymer molecules at the micelle's surface are stretching down into the water. Curing system achieved the full dissolution and ripening of the polymer solution. The size was dependent on the injection volume and curing

### *Development and Application of Chemical EOR Technologies in China Offshore Oil Fields DOI: http://dx.doi.org/10.5772/intechopen.88942*

time of the polymer solution. In order to ensure fully dissolution, the interior of the curing tank was optimized. The curing tank was separated into the inlet chamber, ripening chamber, and outlet chamber through a choke plate [15].

**High-pressure injection system** was applied to achieve the flow distribution and pressurization of the polymer mother liquor. The polymer mother liquor is distributed by a flow distributor for each well and then pressurized by a high-pressure injection pump.

**Control system** was applied to achieve automatic control of the entire polymer dispensing system, automatic data acquisition, and variable frequency control equipment for power equipment.

The online polymer dissolving technology was combined with technologies of polymer powder transmission by positive pressure gas, multistage polymer dispersion and stretching, graded maturation, and high efficient mixing, as the polymer dissolving time shortened to 40 min with high-viscosity retention rate (91%). Under same injection amount, area was saved by 21%, and weight was reduced by 37%. The improved facility has operated steadily in Bohai oil field for 2 and a half years.

## **3.3 Reservoir dynamic analysis method for offshore chemical flooding fields**

The reservoir dynamic analysis method for polymer flooding in onshore oil fields is mature where the polymer injection is started when water cut is over 80%. However, in some offshore field, polymer injection was conducted at very low water cut (even <20% in some case) for the limited life of platform [6–8, 16]. The traditional polymer flooding evaluation method is not applicable in offshore early-stage polymer flooding projects for their short water flooding period, and the effectiveness of injection and the calculation method of incremental oil are not applicable. Based on the power law fluid's stable flow in pore media, considering non-Newtonian and multilayer heterogeneity of polymer solution, a Hall curve evaluation method of early polymer flooding is established. The principle is to use pressure and injection data to draw improved Hall curve of polymer flooding in linear form.

From **Figure 4**, it could be seen that, unlike the traditional Hall curve of polymer flooding, the slopes and intercepts of the improved Hall curve could be used to directly calculate the fluidity and resistance coefficients during polymer flooding, thus solving the reservoir dynamic analysis problem in early-stage polymer injection projects.

**Figure 4.** *Schematic diagram of calculation for resistance coefficient and residual resistance coefficient.*

*Enhanced Oil Recovery Processes - New Technologies*

and achieve miniaturization, high efficiency, and modularization of the polymer injection system [15, 16]. The offshore polymer dispensing equipment included four systems: the dispersing and dissolving system, the curing system, the high-pressure injection system, and the control system (**Figure 3**). Each system was independent

**Dispersing and dissolving system** was the core of the rapid dissolving device.

According to their viscoelastic characteristics, swelling polymer micelles were stretched during going through the gaps of the stator and rotor by using moderate stretching method. More water molecules permeate into the inside of the polymer micelle. At the same time, polymer molecules at the micelle's surface are stretching down into the water. Curing system achieved the full dissolution and ripening of the polymer solution. The size was dependent on the injection volume and curing

and convenient to install according to the site conditions.

*The process of offshore platform injection system.*

**116**

**Figure 3.**

**Figure 2.**

*Schematic of structural composite polymer.*
