Overview of Treatment Area
6 injection wells at the test area block and its interconnected 24 wells were selected as a test block. The test well group has an oil bearing area of 1.26 km2. The average effective thickness was 11.5 m, effective porosity was 15.0%, gas permeability measured as 4.2 X 10 -3 μm2, original oil saturation was 54.0%, reservoir temperature was 64.5 °C, and the saturation pressure was 5.34 MPa.
The average water injection pressure of 6 injection wells before test was 15.5 MPa. Daily single well water injection was 13 m3/d, accumulated single well water injection was 2.75 X 104m3. Of the 24 interconnected wells, there were 16 comparable wells. The daily oil production was 27.9 t/d, overall water content was 11.1%. The cumulative oil production of single well was 0.35 × 104 ton, cumulative water production was 0.06 X 104 m3.
Characteristics of Bio-enzyme:
Eliminate the influence of boundary layer of crude oil
Actively changing the wettability
Reducing the interfacial tension
Some formations have high power of surface adhesion, hence the oil droplets or oil films adsorbed on the rock surface is difficult to be flooded down, and bio-enzymes has the effect of reducing the adhesion power of rock surface, causing the oil droplets or oil films adsorbed on the rock surface can easily be peeled off, hence improving the efficiency of oil wash.
Analysis of Treatment Results
Field injection of bio-enzyme was started on December 26, 2007. All injections were completed on April 20, 2008. The cumulative injected bio-enzyme dope was 270 ton, and total injected volume of bio-enzyme solution was 11,422 m3.
Injection pressure of well dropped, water injectivity index rose
Bio-enzyme has been injected to test wells for 22 months. Within 1 a of the injected bio-enzyme, the maximum drop in injection pressure was 1.4 MPa, a rate of decline of 8.7%. After the test, all water flooded wells were able to complete the injection allocation, with an average daily single well water injection increase of 6 m3/d, cumulative increase of injection 2.304 X 104 m3. The water injectivity index increased from 0.84 m3/(d·MPa) before the measure to the current 1.25 m3/(d·MPa), a raise of 0.41 m3/(d·MPa). When compared with other well groups not being tested, the injection pressure increased from 14.4 MPa to 14.7 MPa, a rise of 0.3 MPa, while the water injectivity index dropped from 1.25 m3/(d·MPa) to 1.23 m3/(d·MPa).
Effect on the recovery pressure in wells.
Before and after the test, pressure recovery test were carried out to 2 interconnected wells respectively. Well logging data showed that, the formation pressure rose continuously after the test. Wherein the Fan 105-63-2 Well have had a formation pressure rise of 0.65 MPa after the test, within the 7 months period from April to November. The speed of pressure recovery was significantly increased.
Interlayer water injectivity improved
From the comparison of profile data on water injectivity before and after water flooding of bio-enzyme, the interlayer water injectivity difference became smaller. Comparable annual percentage of absorbent layers had increased by 12.5% and 25.0% respectively, the percentage of absorbent sandstone thickness increased by 11.0% and 23.0% respectively, effective thickness of absorption percentage increased by 12.7% and 26.0%. Affected volume of injected water had increased, residual oil has been effectively displaced.
Oil production increased, overall water content remained stable
There are a total of 16 wells in test area, including a long time shut-in well and 15 comparison wells. 10 months after the test, well comparisons found that the daily oil production rose from 21.5 t/d to 26.6 ton/d, the average monthly decreasing margin of present daily oil production was 1.1%. Up to now, the test area had finally achieved the accumulated oil increase of 5 860 ton, the final input-output ratio was 1:1.94, and the recovery rate by stage was increased by 0.35%. When compared to the continuous decline in oil production by water-flood wells in the same block, production decline control in bio-enzyme test area has achieved good results. The purpose of production decline control and improving the degree of recovery by stage was achieved.
Conclusions
Bio-enzyme is capable of changing the wettability of rock surface, reduce interfacial tension to 10-1 mN/m, oil wash rate more than 72%, able to reduce the formation pressure by 44.1%, with an average rate of recovery up to 9.4% or more.
In the case of block test wells completed with injection allocation, there was a drop of 10.8% in injection pressure when compared with wells of comparison block group. After the bio-enzyme injection, well water-flooding was effective for 18 months, indicating that bio-enzyme flooding agent has good pressure reduction and injection effect in low permeability reservoirs.
Liu, H., & Zhang, Z. (2011). Biology Enzyme EOR for Low Permeability Reservoirs. Society of Petroleum Engineers. doi:10.2118/144281-MS
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