SAGD (Steam Assisted Gravity Drainage) is commercially adopted as a main development methods for heavy oil reservoirs and oil sands. Improving recovery rate and heat efficiency of SAGD process is the main pursuit of all researchers. This paper aims to utilize a new additive with carbamide to reduce the steam consumption and lower down the residual oil saturation, hence to improve the recovery efficiency of this method. Study on improving recovery efficiency of SAGD with carbamide in super heavy oil reservoir was carried out. 1-D flooding experiment were carried out at 50°C and 150°C to compare the sweep efficiency between different additives including carbamide, CO2 and alkali. Then simulation were run to evaluate the influence of carbamide to SAGD process. Different additives were compared and influences of different products from carbamide were analyzed in detail. Study reveals that SAGD process can be greatly influenced by carbamide. Carbamide can gradually decompose into ammonia and CO2 in steam chamber. Great solubility of ammonia in water can effectively increase PH value of reservoir liquid and decrease residual oil saturation. The core flooding test results show that the oil displacement efficiency can be improved by 17.7% when 25% mass percent of carbamide was added at 150°C. And at 50°C, NH4OH showed 9% improve of sweep efficiency comparing to water. Solution of CO2 into oil can improve oil water mobility ratio. And in the simulation case the oil saturation in the core part of steam was reduced to 0.12 by ammonia (this value is 0.2 for steam). Distribution of CO2 and ammonia shows that oil drainage maybe start with viscosity reduction with CO2 solution and ends with ammonia flooding in steam chamber. With the result of this paper, the recovery factor of SAGD process can be improved by 15.4% and SOR can be improved by 20% when carbamide was co-injected with steam.
Guantao formation in Du84 block, Liaohe oilfield is a typical massive block extra-heavy oil pool with top-water. The oil viscosity is very high and the formation thickness is great. Bitumen shell with 3~4m thickness exist between the top-water layer and oil column. If the bitumen is dissolved when heating in the steam injection recovery process, the top-water will flow down. Moreover, the field production showed that conventional steam stimulation recovery with vertical well is poor. New methods and technologies have to be researched and tested to improve the oil recovery, decrease operation cost and enhances economic efficiency. Numerical simulation results showed that the reservoir can be developed efficiently by implementing Steam Assisted Gravity Drainage (SAGD) process using existing vertical well and horizontal wells to be drilled following steam stimulation. To avoiding top water going down, the new well completion methods have to apply, and the upper portion of the formation in the steam injector shouldn't be perforated. If the viscosity of the bitumen shell is greater than 500×104mPa.s, the 20m thickness of upper portion shouldn't be perforated to inhibit the top-water effectively. If the viscosity of the bitumen shell is 50~200×104mPa.s, the thickness of upper portion not perforated should be more than 30m to inhibit the top-water effectively. At the same time, the results also showed that the heat communication between the wells is an important factor to control the top-water. The SAGD process can be implemented only when heat communication is established by steam stimulation in order to inhibit the top-water. Introduction China has abundant super-heavy oil resource, Which is mainly distributed, in Liaohe Oilfield, Karamay Oilfield of Xinjiang and Shengli Oilfield. Up to now, about 200million tons of super-heavy oil reserves has been gone into production. But for the super-heavy oil reservoirs with top water, it is first time to be developed with steam injection. For Super-heavy oil, its main property is very high viscosity, so its mobility is very poor or there is no mobility at all in formation. Therefore, the technical challenge encountered in Super-heavy oil recovery is how to reduce oil viscosity and increase its mobility. As we known, steam injection recovery has been the most effective technology for heavy oil, but for Super-heavy oil reservoir, the good recovery performance can't be achieved only through conventional steam injection method. The field production showed that the cycle period was short, with only two month generally, oil production and oil/steam ratio per cycle were low, and the oil rate decline dramatically, so the final economic profit was poor. Therefore, the effective recovery technologies have to be researched and tested to further improve the steam injection recovery(1). In the meantime, how to control the top-water is also a new technology. There is no experience about developing the Super-heavy oil reservoir with the top-water in the world. Therefore, controlling the top-water is one of the technical challenges to develop the reservoir. Using the exiting vertical wells, Steam Assisted Gravity Drainage (SAGD) process(2) through combination of vertical and horizontal well(3) can enhance the oil recovery further. The new well completion methods that the upper portion of the formation in the steam injector shouldn't be perforated have to apply, in order to avoid top water going down, and the heat communication between the wells is an important factor to control the top-water. The SAGD process can be implemented only when heat communication is established by steam stimulation in order to inhibit the top-water.
Guantao formation in Du 84 block, Liaohe oilfield is a typical massive extra-heavy oil pool with top-water. The oil viscosity is 23.2×104mPa.s (@50') and the average formation thickness is 106m. Bitumen shell of 3–4m thick exists between the top-water layer and oil column. If bitumen is softened when heating in the steam injection recovery process, the top-water will flow down. Moreover, the field production showed that conventional steam stimulation recovery with vertical well is poor. Experiment and simulation results have shown that the injection of N2 as an additive can be effective in inhibiting the breakup of bitumen shell hence to prolong the life of SAGD. Also the recovery factor can be improved. After series of simulation, the injection of N2 and steam is optimized as follows: steam injection rate for single well: 100t/d, steam quality: higher than 70%, volume ratio of N2/Steam (Cold water Equivalent) at downhole conditions: 0.5, effective thickness of N2 layer: 10–15m. Under such operating conditions, the life of SAGD can be prolonged by 3–4 years and the recovery factor can be improved by 11.4%. Introduction China has abundant extra-heavy oil resource, which is mainly distributed in Liaohe Oilfield, Karamay Oilfield of Xinjiang and Shengli Oilfield. Up to now, about 200×106 tons of super-heavy oil reserves have been put into production. But for the extra-heavy oil reservoirs with top water, it is first time to be developed with steam injection. Steam Assisted Gravity Drainage (SAGD) has been pilot tested in Du-84 Block, Liaohe Oil field. But the main technical challenges existing in the test are how to avoid flow down of top water and prolong life of SAGD hence to improve production performance. This paper has proposed a new method based on the research results and reservoir characteristics, that is injecting non-condensable gas (N2) into SAGD steam chamber. N2 accumulates at the top of steam chamber and forms a heat-insulating layer, inhibiting the heat conduction of steam chamber to cap rocks. It can delay the down flow of top water during the life of SAGD. Reservoir Description The study case is Guantao Formation in Du 84 Block, Liaohe Oilfield, which is a extra-heavy oil reservoir. Guantao Formation is a monocline structure which dip 2o~3o to the southeastern direction. The Formation is a massive block reservoir with top and edge water aquifer, with the depth of between -530~-649m. The formation thickness is between 150 and 210m. The initial reservoir temperature is 28~32?, and the initial pressure is 6.14MPa. Guantao Formation is a shallow, unconsolidated reservoir. The average porosity is 36.2%, and the average permeability is 5539´10–3mm2, so it is a high porosity and high permeability formation. There are no continuous mud barriers in the formation. The reservoir directly contacts with top and bottom water. Especially, the top-water exists upon all the reservoir and the bottom-water exist partly. Thus, these unfavorable conditions have caused great difficulty for the development of the reservoir. The properties of Guantao oil are as follows: ?high oil density: 1.007g/cm3 at 20', ?high oil viscosity: dead oil viscosity is 16´104mPa.s. at 50', ?high gel and asphaltene content: up to 52.9%. ? high pour point: 27'; the wax content is 2.44%. According to the screening criterion of the heavy oil, it is extra-heavy oil.
The SAGD pilot test in Guantao oil reservoir, Du 84 block Liaohe oil field has achieved good results. But the existence of the top water is harmful to SAGD process. Non-Condensable Gas injection with steam (NCG-SAGD) is a beneficial technique for this kind of reservoir. Gas selection is an important step for NCG-SAGD. Based on the high temperature and high pressure phase-equilibrium experimental devices, the solubility of NCG like CH4, CO2 and N2 in super-heavy oil has been measured. Also the thermal property of heavy oil saturated with these NCG were tested separately. A numerical simulator is used to forecast the performance of different NCG in this reservoir, and optimize the operating parameters to maximize the effect of NCG additive. After series of experiments, the main results are as follows: The solubility of the three kind of gases is increasing along with the reduction of the temperature and the increase of the pressure. With the rise of the temperature and pressure, the viscosity of the crude saturated with the three kind of gases will reduce, and the density of the crude saturated with the three kind of gases will also decrease. The dissolving capacity in the Guantao oil and the viscosity reduction effect for the three kind of gases are listed as follows from high to low: CH4>CO2>N2. Simulation results shows that the production process for N2 is the longest because of its good heat insulation property. N2 is determined as the assistant gas in the SAGD technology according to the demands for top-water reservoir and mechanism of NCG-SAGD technology.
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