Adaptability Evaluation of Polymer Flooding for Zahra Oil Field

Viscosity-concentration and temperature performance are the prerequisite constraint factor of the application of polymer flooding in the oilfield. The static and dynamic adsorption of the polymer in the core can affect the performance of polymer flooding. Based on the viscosity-concentration, temperature and the static, dynamic adsorption results of six kinds of polymers, DQ3500 is chosen as the most suitable polymer for Zahra oilfield. Its affects show that oil recovery is increased by 7% and water cut is reduced by 20%. Keyword: Enhanced oil recovery, Polymer viscosity-concentration, Static dynamic adsorption, Temperature performance, Water cut.


INTRODUCTION
Viscosity-concentration,temperature performance are the prerequisite constraint for polymer flooding in an oilfield [1] .Static adsorption and dynamic adsorption of the polymer not only can restrict the dissemination properties of polymers directly [2][3][4][5], but also can affect the economic effects of polymer flooding in a specific reservoir.In this paper, adaptability of polymer flooding of 6 kinds of polymers in Zahra oil field is researched.Based on the viscosityconcentration, temperature relationship and the static, dynamic adsorption results of the 6 kinds of polymers, the polymer DQ3500 is selected as the most suitable polymer for polymer flooding in Zahra oil field.Its effects on enhancing oil recovery are analyzed.

Formation Brine in Zahra field
The composition of formation brine in Zahra field is shown as in Table 1.In the artificial simulation, the water based on the composition of the formation brine in Zahra field is prepared and used for preparing chemical solution in this research.
There are 6 polymers have been selected in the evaluation of polymer flooding for Zahra crude oil.The parameters of M w , hydrolyzing degree, filtration factor, water insoluble matters, dissolving time and solid content of the 6 polymers are selected and listed in Table 2. From Table 2, it can be found that the content of all four polymers is higher than 86.5%.The dissolving time of all four polymers is less than 2 hours.The results show that all of the polymers selected can meet the polymer flooding standard.

Relationships between the Apparent Viscosity and Polymer Concentration
The relationship between the apparent viscosity and the polymer concentration of polymer solution for the six kinds of polymers with formation brine are examined.The shear rate is 7.34 s -1 and the results are as shown in Table 3. From Table 3, it can be seen that the viscosity of polymer solution is increased with the increasing of polymer concentration.The viscosity of polymer solution for polymer FP6040s, DQ3500 and KYPAM-2 are larger than the others.Under the shear rate of 7.34 s -1 , the apparent viscosity of 1500mg/L polymer solution for polymer FP6040s, DQ3500 and KYPAM-2 are 28.0,39.0 and 29.7 mPa•s at temperature of 72 o C. From the perspective of the relationship between the apparent viscosity and the polymer concentration of polymer solution, polymer FP6040s, DQ3500, and KYPAM-2 are suitable for polymer flooding in Zahra's oil field.

Relationship between the Apparent Viscosity and Temperature
The relationship between the apparent viscosity and the temperature of polymer solution for the six kinds of polymers with formation brine is examined.The concentration of polymer solution is 1500mg/L and the shear rate is 7.34 s -1 .The results are as shown in Table 4. From Table 4, it can be seen that the viscosity of polymer solution is decreased with the increasing of temperature.Under the shear rate of 7.34 s -1 , the apparent viscosity of 1500mg/L polymer solution for polymer FP6040s, DQ3500 and KYPAM-2 are 25.2, 33.0, 23.8mPa•s at temperature of 85 .The viscosity of polymer solution for polymer FP6040s, DQ 3500 and KYPAM-2 are larger than the others.From the perspective of the relationship between the apparent viscosity and the temperature, polymer FP6040s, DQ 3500, and KYPAM-2 are suitable for polymer flooding in Zahra's oil field.

Thermo-stability Characteristics of Polymer Solution
Fig.
( 1) is apparent viscosity changing of the polymers with thermal aging time, with the polymer concentration of 1500 mg/L in anaerobic condition at temperature of 72ºC.It can be seen from Fig. (1) that the apparent viscosity of polymer solution is basically remained unchanged.The polymers solution has long time thermo-stability.

Transmission Performance of Polymer in Cores
The static and dynamic adsorption of the polymer in core can affect the performance of the polymer flooding.Flow properties of the three kinds of polymer DQ3500, FP6040s and KYPAM-2 in the reservoir rocks are investigated through core flow test.

(1). Polymer of FP6040s
Fig. (2) is the pressure curve of four different measuring points in a core when 1500 mg/L FP6040s polymer solution flows through the core.The core permeability is 73.0 mD.From Fig. (2), it can be seen that the pressure at different position in the core is raised slowly when polymer solution is   3) that the pressure at different position in the core is raised slowly when polymer solution is injected into the core.And then water replaces the polymer solution when the polymer solution injection volume reaches 0.5 PV (pore volume).The pressure at different position in the core rose slowly along with the water injection, the pressure at different position in the core is declined slowly and then to be in stability state.The dramatic change of pressure is also not appeared.4) that the pressure at different position in the core is raised slowly when polymer solution is injected into the core.And then water replaces the polymer solution when the polymer solution injection volume reaches 0.5 PV (pore volume).The pressure at different position in the core is also risen slowly.But along with the water injection, the pressure at different position in the core is declined slowly and then to be in stability stat.The pressure does not present dramatic changes.

Static Adsorption of the Polymer on Natural Rock Particles
Static adsorption of the polymer FP6040s, DQ3500 and KYPAM-2 on the surface of the natural rock particles (100 mesh) is examined [5] at 72 ºC.The experimental results are shown in Table 8.It can be seen from Table 8 that static adsorption of the polymer FP6040s, DQ3500 and KYPAM -2 do not change a lot with the increasing of concentration of polymer solution.Static adsorption amount of polymer FP6040s, DQ3500, KYPAM -2 is approximately 1000 mg/g, especially the static adsorption amount of DQ3500 is the smallest.

Dynamic Adsorption of Polymer on Surface of Natural Core
Dynamic adsorption of polymer FP6040s, DQ3500, KYPAM-2 with a concentration of 1500 mg/L on the surface of the natural rock is measured by core flow test at 72 ºC.Parameters of cores are shown as in Table 9.The results are shown in Table 10.It can be seen from Table 10 that dynamic adsorption of polymer FP6040s, DQ3500, KYPAM-2 on the surface of the core are 196, 125 and 159 mg/L, respectively.The dynamic adsorption of the polymer DQ3500 is the smallest.
It can be seen from the static and dynamic adsorption results that DQ3500 adsorption is significantly smaller than the other two polymers.In practical application of polymer solution flooding the reducing of the polymer adsorption can reduce the amount of injection volume which is helpful to displacement efficient.So DQ3500 is chosen as the most suitable polymer for polymer solution flooding in Zahra oilfield.

Flooding Effect on Natural Core
Polymer solution flooding effect is evaluated by displacement experiment in natural core.Fig. (6)     2. The core is flooded with the dewatered Zahra crude oil until no more water could be displaced.The oil saturation is calculated by the ratio of saturation oil volume to the saturation water volume in step 1.
3. The core is water flooded at a rate of 0.1ml/min.When water cut reaches 98%, the core is flooded by of polymer solution of DQ3500.When the injected polymer solution volume reaches 0.5 PV, the core is flooded by water again until no more oil could be displaced.
The cores used in the displacement experiment are shown in Table 11 and the results of the displacement experiment are shown in Table 12.
It can be seen from Table 12 that enhanced oil recovery by polymer solution of DQ3500 in natural core is about 7% and the water cut is significantly decreased about 20%.

( 3 )
Fig. (4) is the pressure curve of four different measuring points in a core when 1500 mg/L DQ3500 polymer solution flows through the core.The core permeability is 59.3 mD.It can be seen from Fig. (4) that the pressure at different position in the core is raised slowly when polymer solution is injected into the core.And then water replaces the polymer solution when the polymer solution injection volume reaches 0.5 PV (pore volume).The pressure at different position in the core is also risen slowly.But along with the water injection, the pressure at different position in the core is declined slowly and then to be in stability stat.The pressure does not present dramatic changes.

Table 12 ,
[6,7]ctively.The oil used in displacement experiment is the dewatered Zahra crude oil.The displacement procedure was as follows[6,7]:1.The core is saturated with water in a vacuum state.So the pore volume is calculated by the water volume.