Qualification of Biopolymer in an Offshore Single-Well Test
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A single-well polymer-injection and back-production test has been performed in an oil and gas field offshore Norway. The objective of the test was to verify at field conditions the properties measured in the laboratory for the biopolymer schizophyllan. The data confirm that the biopolymer is not biodegraded during the shut-in period because of efficient application of biocide injected simultaneously with the biopolymer. Rate and pressure data also support that the injectivity is maintained during the test compared with water injectivity.
Polymer injection has been considered as a method to increase oil recovery from the Heidrun Field. The field is located at Haltenbanken in the Norwegian Sea, producing from a large concrete floating tension-leg platform with access to 56 well slots in addition to tie-in of five subsea templates and two production and three injection templates. The recovery factor for the different formations ranges from 60% for the homogeneous high-permeability zones to 10% for the most-heterogeneous low-permeability zones.
Schizophyllan has a linear structure without charged functional groups, which results in virtually no loss of viscosity in high-salinity brine, especially in the presence of di- or multivalent ions, and comparably low adsorption on rock surfaces. Furthermore, schizophyllan shows short-term stability at temperatures up to 135°C and has shown half-life values of several years, exceeding 10 years at lower temperatures. Finally, schizophyllan provides high viscosity efficiency, with strong shear-thinning behavior but no shear-thickening behavior, at high shear rates.
A standard laboratory program was performed to qualify the biopolymer for Heidrun Field conditions. In addition, a special laboratory study was performed to determine long-term thermal and biological stability.
The standard laboratory program included static bulk tests and coreflooding tests. The special laboratory program included long-term thermal-stability tests, long-term biological-stability tests using actual injection water, and microbial-degradation tests.
The polymer had good rheological properties, was stable at high shear rates, demonstrated low adsorption and good filterability and injectivity, and was thermally stable for at least 10 months at 85°C. As expected, schizophyllan will biodegrade in the injection waters available at Heidrun unless protected by biocide. Glutaraldehyde and formaldehyde are both compatible with the biopolymer.
Baseline Information of the Test Well. Challenging bacterial conditions were present in an injection zone in which the reservoir had been cooled to between 30 and 40°C and in which the bacterial level was assumed to be high. An injector at Heidrun was selected that met these criteria. It was a water injector with three downhole instrumentation and control system (DIACS) valves for zonal isolation. The well was gravel packed with sand screens and completed with three separate zones isolated with swell packers. Downhole gauges for pressure and temperature were installed in both tubing and annulus at each DIACS zone.
The well had been injecting sulfate-reduced seawater since 2012, and the formation around the injector was cooled to 37°C. The test zone was 36 m high with an average permeability of 270 md. Because of poor cleanup during completion, the injectivity was only 6.5 m3/d/bar.
A baseline production test was performed to obtain production data and collect samples of formation water for baseline analysis of the microbial community, formation water composition, and content of hydrogen sulfide (H2S) and oxygen. The production data showed much higher productivity (80 m3/d/bar) than injectivity and a rapid reservoir pressure decline. Gas lift was planned to ensure enough lift for the back production after the 39-day shut-in period.
To homogenize and dilute the polymer mother solution to the required injection concentration of approximately 220 ppm, a high-shear rotor/stator mixing device was used (Fig. 1a). Before shipping the test equipment offshore, an onshore yard test was performed to check functionality and train the personnel.
Offshore Rig Up and Logistics During Injection and Production. Drilling operations are conducted continuously in parallel with well interventions at the Heidrun platform. Approximately 20 tanks of schizophyllan, ranging from 4 to 8 m3, were stored at the top deck along with high-quality nitrogen for blanketing during unloading of the tanks. The low-pressure part of the rig up contained the mixers (Fig. 1a), feeding pumps, a biocide pump, and a filter pod. This part was placed close to the high-pressure rig pump, three levels below the polymer-tank-storage area. Balconies between the levels were used for placement of the unloading polymer tanks and biocide tanks, providing gravity feed to the pumps.
The high-pressure part of the rig up consisted of an injection line and a back-production line. The injection line was rigged up between the high-pressure rig pump and the swab valve on the injector well and consisted of 5,000-psi lines, a tracer injection pump, and a high-pressure sampling point (Fig. 1b). The back-production line was rigged up between the kill wing valves on the injector well and a plugged and abandoned producer well (the latter to enable flow from the test well to the separators).
During pumping, the polymer was mixed with sulfate-reduced seawater. Five sample stations were included in the low-pressure lines and were used to ensure correct quality of the polymer solution.
For the final back production, two 16-bottle racks with nitrogen, six tanks with liquid nitrogen, and dedicated pumps were rigged up to the annulus wing valve (AWV) for gas lift. The nitrogen was pumped through the AWV, down the production riser, and into the well through the gas-lift valve (GLV) to lighten the fluid column and provide sufficient drive to produce back the remaining polymer after 39 days of shut-in.
Offshore Injection and Back Production
Sampling and Analysis Program. A detailed sampling and analysis program was prepared for the test, including the sampling point location, frequency, volume, type (sterile, anaerobic, or aerobic), and preservation. Multiple sampling points were established for representative sampling and troubleshooting if necessary.
An offshore arrival check with respect to viscosity, pressure in the container, H2S, and filter ratio was conducted for all received tanks at the platform. Samples were collected anaerobically during the injection and back-production period at a high frequency for onsite analysis and stored for later shipment to onshore laboratories for additional analysis. Some of the samples were collected in sterile bottles anaerobically for bacterial analysis, while some of the samples were stabilized with biocide to avoid any further degradation during transport and storage and to capture a profile of the samples from the reservoir.
Injecting Biopolymer, Biocide, and Water Tracer. Biocide at a concentration of 1,100 ppm was injected together with schizophyllan at a concentration of approximately 220 ppm. A water tracer was injected simultaneously to distinguish between injected water and crossflow water during back production. A second water tracer was added during injection of the last 90 m3 (tubing volume) to determine if the produced water came from the wellbore or from the reservoir. Both water tracers gave trends consistent with the viscosity measurements taken.
The test started with a preflush of biocide for 1 day at an injection rate of 580 m3/d. The injection rate for the biopolymer solution was 500 m3/d. This gave a very slow, small pressure increase throughout the test, and, consequently, the injectivity calculated from rate and pressure data was reduced slightly.
First Back Production (Baseline). After injection of 3500 m3 of water solution with biopolymer, biocide, and water tracer, the injector was changed to a producer and 195 m3 of water solution was produced from the well. Samples were taken according to the program. Some samples were analyzed immediately to confirm the properties of produced water. The pressure declined rapidly, and the decision was made that the gas lift valve should be installed to secure production in the second back-production step.
Shut In. The well was shut in for 39 days to allow microbes in the injection water and in the reservoir to degrade the biopolymer.
Second Back Production. The well was started by nitrogen gas lift, and 365 m3 water solution was back produced.
- This offshore single-well biopolymer test was an operational success. No health, safety, or environmental issues were reported, and delivery of an extensive data set was achieved.
- The analysis of data confirms that the biopolymer is not biodegraded during the shut-in period because of efficient application of biocide.
- The recorded injection data confirm that the injectivity is maintained during the test compared with water injectivity.
- Extensive laboratory and well analysis, reservoir surveillance, and high-quality sampling was necessary for verifying laboratory data in the offshore field test.
- The successful execution of the test was possible through extensive collaboration between the operator, biopolymer suppliers, and service providers.
Qualification of Biopolymer in an Offshore Single-Well Test
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12 June 2018