Summary

We conducted a pilot test of polymer flooding in the Shuanghe reservoir located in the southeast Henan oil field, China. The target reservoir has a net thickness of 15.56 m (50 ft), an average permeability of 420 md, and a temperature of 75°C (167°F). The polymers used are two types of modified partially hydrolyzed polyacrylamides, named S525 and S625, which have molecular weights of 16,700,000 and 19,670,000 daltons, respectively. The objective of this pilot test is to investigate the feasibility of polymer flooding for improving oil recovery in an elevated-temperature reservoir.

The polymer flooding started in February 1994. Through December 1995, a total of 246 tons (about 0.5×106 lbm) of dry polymer had been used with an injection concentration of 900 to 1,100 ppm. The pore volume (PV) injected reached 0.2164. As a result, oil production increased by 22,000 tons (184,000 bbl) and water production decreased by 153,000 tons (962,000 bbl), which accounts for the incremental oil recovery of 3.8% and water-cut reduction of 5.6% in the test block. We estimate that, by the end of this project, the ultimate increase in oil production will exceed 63,000 tons (528,000 bbl) with the enhanced oil recovery going up to 9.8%. The yield is 0.2 tons more oil produced per kilogram of polymer injected or 0.7 barrel of oil produced per pound of polymer.

We attribute the success of the pilot test to the techniques used during the implementation of the flooding, including prevention of polymer-thermal degradation, good reservoir description, and the profile modification carried out before and after the polymer injection. This pilot test illustrates a case where polymers with extra-high molecular weight are successfully injected in an elevated-temperature reservoir to control the mobility ratio and modify the permeability profile.

Introduction

In recent years, polymer flooding along with other enhanced oil recovery (EOR) projects has been downsized, even suspended, in many research organizations and oil and gas companies because of low oil prices and high operation costs. However, polymer-flooding technology in China has gained prominence for mobility-ratio control and permeability-profile modification. There, laboratory experiments, computer simulation, treatment design, and performance prediction concerning polymer flooding are active research subjects. The need to use polymer flooding in China is caused by the severe heterogeneity of reservoirs, especially in vertical profiles, and the high oil/water viscosity ratio often present. These reservoirs generally are continental formations with complex geological structures, a large variation in reservoir types, and great differences in the fluid properties. To stabilize oil and gas production and to improve oil recovery in the developed fields, China Natl. Petroleum Corp. (CNPC) has adopted policies to encourage and support various EOR projects, including chemical, miscible, thermal, and microbial technology.1 In the past 5 years, several large polymer-flooding projects have been successfully conducted in the Daqing, Dagang, Shengli, Liaohe, and Jilin oil fields.2

Most of the current polymer projects are intended for low-temperature reservoirs. There are very few studies reported on polymer applications in the elevated-temperature reservoirs because polymers generally tend to lose their effectiveness and become unstable under high-temperature conditions. Unfortunately, high-temperature, high-heterogeneity, and high oil/water viscosity ratio are very common in many reservoirs of east China, which is estimated to have a reserve of more than 1.7 billion tons of oil. The previous study showed the promising potential of polymer flooding to improve oil recovery from these reservoirs, if the polymers injected have sufficient thermal stability.

To test the feasibility of polymer flooding in elevated-temperature reservoirs, we conducted a pilot test in the Shuanghe reservoir, which has a temperature of 75°C. This paper reports the laboratory studies, reservoir simulation, flooding monitoring, and results of the pilot test.

Reservoir Description and Production History

Shuanghe Reservoir.

Discovered in 1976, the Shuanghe reservoir is located in central China in the southeast Henan oil field, a sizeable field containing 4.557 million tons original oil in place (OOIP). The major producing formation, found at a depth of 1480 m in Tertiary, is a complex sand body made of stacked deltaic cycles (multilayer reservoir). The reservoir is connected with an active aquifer in the east, surrounded by a sealed fault in the south, and smeared out in the northwest. The pilot-test area is located in the southwest area of the reservoir. Fig. 1 shows the well pattern for the pilot test.

The lithology of the reservoir formation is described as a sandstone, including 51.8% quartz, 24.8% feldspar, and 23.4% detritus containing 7.5% clay. The clay mineral consists of 44.5% kaolinite, 32.4% illite, 16.8% montmorillonite, and 6.3% chlorite. The reservoir is composed of four layers, known as, and, with permeabilities of 350, 510, 500, and 18 md, respectively. The reservoir formation has an average porosity of 0.216 and average permeability of 420 md, with a variation factor ranging from 0.73 to 0.79. The net thickness of the oil-bearing formation is 15.56 m, and the temperature is 75°C. The crude oil has a viscosity of 7.8 cp at reservoir conditions. Formation water contains 5,060 ppm total dissolved solids (TDS), including 23 ppm of Ca++ and Mg++ combined. The TDS in injection water is 12,000 ppm.

Production Histories.

The Shuanghe reservoir was put into production in early 1977 and began to be waterflooded in late 1978. To improve the productivity, reduce the water cut, and improve the ultimate oil recovery, infill drilling was done between 1988 and 1992. Before polymer flooding, 23 production wells and 18 injection wells were drilled in the reservoir with a well density of 13.7 wells per km2. The pilot test area is located in the southwest area of the reservoir and contains 19 wells, including seven injection wells and 12 production wells. By February 1994, 33% of oil in place (OIP) had been recovered. The cumulative oil production had reached 0.85 million tons with a water cut more than 92.6%. It is believed that the high water cut can be attributed to the unfavorable water/oil viscosity ratio (1/8) and the severe heterogeneity in vertical and areal directions in the reservoir (permeability variation factor is 0.73 to 0.79).

In early 1990, the screening work for EOR methods showed that the use of polymer to thicken water to be injected and to reduce water-phase permeability would improve the mobility ratio dramatically and increase the oil recovery significantly. From 1990 to 1994, extensive laboratory investigations and numerical simulations studied the feasibility of polymer flooding in Shuanghe reservoir.

Shuanghe Reservoir.

Discovered in 1976, the Shuanghe reservoir is located in central China in the southeast Henan oil field, a sizeable field containing 4.557 million tons original oil in place (OOIP). The major producing formation, found at a depth of 1480 m in Tertiary, is a complex sand body made of stacked deltaic cycles (multilayer reservoir). The reservoir is connected with an active aquifer in the east, surrounded by a sealed fault in the south, and smeared out in the northwest. The pilot-test area is located in the southwest area of the reservoir. Fig. 1 shows the well pattern for the pilot test.

The lithology of the reservoir formation is described as a sandstone, including 51.8% quartz, 24.8% feldspar, and 23.4% detritus containing 7.5% clay. The clay mineral consists of 44.5% kaolinite, 32.4% illite, 16.8% montmorillonite, and 6.3% chlorite. The reservoir is composed of four layers, known as, and, with permeabilities of 350, 510, 500, and 18 md, respectively. The reservoir formation has an average porosity of 0.216 and average permeability of 420 md, with a variation factor ranging from 0.73 to 0.79. The net thickness of the oil-bearing formation is 15.56 m, and the temperature is 75°C. The crude oil has a viscosity of 7.8 cp at reservoir conditions. Formation water contains 5,060 ppm total dissolved solids (TDS), including 23 ppm of Ca++ and Mg++ combined. The TDS in injection water is 12,000 ppm.

Production Histories.

The Shuanghe reservoir was put into production in early 1977 and began to be waterflooded in late 1978. To improve the productivity, reduce the water cut, and improve the ultimate oil recovery, infill drilling was done between 1988 and 1992. Before polymer flooding, 23 production wells and 18 injection wells were drilled in the reservoir with a well density of 13.7 wells per km2. The pilot test area is located in the southwest area of the reservoir and contains 19 wells, including seven injection wells and 12 production wells. By February 1994, 33% of oil in place (OIP) had been recovered. The cumulative oil production had reached 0.85 million tons with a water cut more than 92.6%. It is believed that the high water cut can be attributed to the unfavorable water/oil viscosity ratio (1/8) and the severe heterogeneity in vertical and areal directions in the reservoir (permeability variation factor is 0.73 to 0.79).

In early 1990, the screening work for EOR methods showed that the use of polymer to thicken water to be injected and to reduce water-phase permeability would improve the mobility ratio dramatically and increase the oil recovery significantly. From 1990 to 1994, extensive laboratory investigations and numerical simulations studied the feasibility of polymer flooding in Shuanghe reservoir.

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