Improvement in polymer water flooding efficiency using a low shear choke valve
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Primary and secondary oil depletion only recovers 20 – 50 % original oil in place (OOIP), so large amount of oil is still trapped in the reservoir after conventional processes. To recover more oil, tertiary methods are used to increase the amount of oil that can be extracted from an oil field. Enhanced oil recovery (EOR) is a tertiary method used to maximize oil recovery from the reservoirs. There are many different EOR methods, however, polymer injection and polymer flooding is the most important chemical EOR method used in sandstone reservoirs. It has been found that polymer flooding can recover from 2 – 5% OOIP over traditional water flooding. Polymers are long chain molecules, often, with high molecular weight. These molecules increase the viscosity of the injection water and improve the mobility ratio, which is the mobility of the displacing phase divided by the mobility of the displaced phase. A mobility ratio of 1 between water and oil will increase the sweep efficiency. However, one of the main problems with polymer flooding is that polymers are very shear sensitive. During polymer injection, shear is especially to be found in process equipment, like fluid flow devices (valves, pumps), but also in the reservoir. When polymers are subjected to shear, they are readily broken down and degraded, which results in irreversible loss of viscosity and inefficient oil recovery. There are however low shear technologies and equipment which purpose is to reduce mechanical degradation of polymers during injection. Some of the state of the art low shear techniques rely on pipes and coils, while other relies on cyclonic principles. It is thought, that a reduction in pressure drop over a longer length (spiral) or in a bigger volume (cyclonic) will reduce shear forces, hence mechanical degradation of polymers. This thesis consists of and is part of two industry projects, a pre-project and a main project between Typhonix, Total and the International Research institute of Stavanger (IRIS) and the Research Council of Norway. The aim of the pre-project and the ongoing main project is to study mechanical degradation of polymers by low shear valves and fluid flow devices, especially low shear Typhoon technologies. Typhonix AS, which is a company with knowledge within low shear process equipment, have patented a cyclonic low shear valve, the Typhoon Valve, and a low shear spiral fluid flow control device, the Shark. Both devices have proven to have a positive effect on oil in water emulsions, due to less shear forces and turbulence. In the Typhoon Valve, the pressure drop is reduced in a bigger volume than in a conventional valve, while in the Shark, the pressure drop is reduced over a longer length, resulting in less shear forces and turbulence. From previous study, it is to believe that a new low shear process for EOR polymer flooding can increase the recovery rate from 3 to 7 % . The Typhoon Valve was tested at Total’s division for polymer injection in Lacq, France (PERL). The Shark was tested at Typhonix’s own laboratory at Varhaug, together with a small scale Typhoon Valve and a standard valve. In both test programs, a high concentration polymer solution was pumped through a test-rig where the Typhoon Valve (France) or the Shark, small scale Typhoon Valve or standard valve were installed (Varhaug), at different flow rates and pressure drops. Different configurations of the Shark were tested, to see if different lengths and diameters had influence on polymer degradation. Samples were taken downstream the test-section, and the viscosity of the samples was measured with a rheometer. % degradation was then calculated, and the Typhoon Valve, the Shark, and the Typhoon configuration were compared to the results with the Standard valve. The testing in France with the 2” Typhoon Valve did not give the positive results that were expected. It was observed that the degradation with the Typhoon Valve was in the same range as with the Standard Valve. It was also seen that the degradation was a function of both pressure drop and flow rate, as they increased, the degradation also increased. Especially with high pressure drop was % degradation in both valves was high (> 60% ). It was found that the internals in the Typhoon Valve was not optimized for the test conditions, so this may be the reason for the disappointing results. However, the testing at Varhaug with the different Shark configurations, gave very promising results. It was not observed any degradation of the solution, with any of the different cones and spirals, even at the highest pressure drop, and % degradation remained low. It was seen that the shear rates in the cones and spiral were less than the critical shear rate of the polymer. The viscosity did differ a little at very low shear rates when viscosity measurements were conducted, but this can be explained by high uncertainties and sound at low shear rates. Also, with the Typhoon configuration, the results were promising. It was observed that the valve gave less degradation of the polymer solution compared to a standard valve, especially at high pressure drops, and % degradation was lower than with the standard valve. The experiments conduced, both in France and at Varhaug, gave knowledge about shear rates and shear degradation of polymer solutions in mechanical facilities. From the results further suggestions and further tests are planed to be conducted, for both the Typhoon Valve and the Shark. The Typhoon Valve are planed to be tested with new internals, such that optimized capacity is achieved. This testing is going to take place at Total’s facilties in Lacq in autumn 2013. Further tests with the Shark are also suggested. Tests with smaller diameter coils, and higher flow rates, to initiate shear should be conducted. The results should be compared to the shear degradation with a Standard valve. The shark can be constructed to have shear rates which is under the critical shear rate of the polymer, which means that polymer degradation is avoided. This is of outermost importance, to increase oil recovery, and reduce the amount of added polymer.
Master's thesis in Environmental technology