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Steam-assisted gravity drainage (SAGD) is a successful commercial recovery method to produce heavy oil and bitumen. In this method, a pair of horizontal wells is used as injection well and production well, by using the gravity as the driving force, it not only ensures the stability of the steam front flooding and is also very economical. Currently, there are several ways to improve the performance of SAGD, but the research of changing the structure of the wellbore to improve the performance of SAGD has not yet appeared. By means of the central and northern Alberta's Athabasca and Cold Lake reservoir testing program, researchers verify the impact of SAGD performance by changing the well structure.
SAGD operation process
The researchers compared the gas injection’s SAGD performance of vertical wells and horizontal wells, finding that the maximum recovery is obtained through horizontal wells, and the bitumen recovery is higher near the top of net pay on account of steam injection (that is, through vertical steam injection well). SAGD operations are divided into three phases: pre-heating, steam injection / production and failure. The target of the first phase is to warm and move crude oil between steam injection well and production well. At the end of the first phase, it will establish fluid communication between the steam injection well and production well. The researchers used discrete wellbore of thermal reservoir simulator to carry through thermal sensitivity analysis and optimization of the first phase of SAGD, finding that well spacing have an impact on development effectiveness, that is in the SAGD process long spacing could enhance oil recovery and expansion of the steam chamber. In order to simulate SAGD cycle preheating stage, both two wellbores in base model are made discrete dispose, which aims at considering the fluid flow and energy equations, so that wellbore at each lattice for each group are used in the same way. In the cycle stage, by injecting steam into the tube and case production, thus every well is composed of steam injection pipeline and production casing pipe. In the late stage of the cycle, steam injection pipeline and production casing pipe will be closed. After the reservoir between the wells becomes sufficiently hot, the operation transitions to the SAGD’s second stage, that is the top of the well injects steam and the lower recovery oil. The researchers developed a strategy to optimize the connection between the injection and production well, which is in the case of the minimum cumulative gasoline ratio to maximize oil recovery. The researchers also introduced the X-SAGD structure used for low-pressure reservoir, and they found that by increasing the vapor pressure could make thermal efficiency of this structure worse than the standard SAGD structure. So they proposed a new method of well structure. Thermal efficiency of this method is even higher than the standard SAGD structure. Production well located in bedrock above 2.5m. Injection well is above the production well, only five meters away from it, which is the best vertical spacing of such reservoirs between two horizontal wells.
Changing well structure affects SAGD performance
Researchers explored the applicability of Athabasca and Cold Lake reservoir in central and northern Alberta, Canada. The researchers use the fully implicit thermal reservoir simulator (CMG's STARS 2007) and fully coupled borehole to combine, so that we could calculate the frictional pressure drop and heat loss along the wellbore, build a three-dimensional numerical simulation model, and analyze sensitivity of steam injection pressure. After optimizing the steam injection pressure, we used these models to analyze the new wellbore structure. The results show that by changing the wellbore structure, SAGD process performance of Athabasca and Cold Lake reservoir can be significantly improved. Researchers list the operating conditions of production stage and the preheating stage steam cycle of foundation program: that is 120 days was chosen as thermal connection setup time (the temperature between the two wells is near 100 ℃) between two horizontal wells; SAGD production and recycling stages of steam injection and production wells define two conditions, that is when the steam dryness of steam injection wells is 0.9, the maximum bottom hole pressure is set, minimum bottom hole pressure and steam chamber control of production wells are set, produced fluid is maintained at a temperature which is 10 ℃lower than the saturated steam temperature.
Researchers define the basic program model of Athabasca and Cold Lake reservoir respectively. The vertical distance between steam injection well and production well is 5m, and when the horizontal direction is the same plane, they conduct the sensitivity analysis of both reservoirs at an average steam injection pressure.
The results showed that reservoir cumulative oil production is increasing with steam injection pressure increased. In Athabasca, when the steam injection pressure is 2500KPa, the cumulative oil production has essentially the same result with the pressure higher than 2500KPa. Therefore, 2,500kPa was selected as the optimal steam injection pressure, and the optimal steam injection pressure of Cold lake reservoirs is 5,500kPa.
SAGD of tilt steam injection well has more advantages
The difficulty of drilling a deviated well and a horizontal well is quite equal, it requires the corner is approximately 89 °instead of 90 °, and the accuracy of orientation forecast is the same with parallel SAGD well. Before SAGD production, thermal communication between the two wells toe loop is established by circulation. Compared with conventional SAGD, connection is established in the smaller vertical distance, so the cycle is shorter and it is less energy intensive. After establishing the connection between the toe-end well, steam is continuously injected by tilt steam injection well, and produced by bottom horizontal well. With the continued injection of steam, like SAGD program, steam chamber not only extends towards the direction of vertical wells, but also towards the above toe to the end of the well. This expansion of the steam chamber makes the steam chamber cover the entire length of the horizontal section of the well. In the Athabasca and Cold Lake reservoir, the researchers research many plans of different well structures (for example, vertical steam injection well is above horizontal production well, the combination of horizontal and vertical steam injection well and branch SAGD), which shows that, in all the structures, laying up tilt steam injection well above production well could improve the most potential of SAGD performance. Because slant wells increase the distance between the two wells with the end, compared with conventional SAGD, it is more easily to control SAGD steam chamber and incline steam injection well. This ensures all horizontal sections in SAGD production stage could be utilized. Compared with ordinary SAGD, steam chamber of tilt steam injection wells extended more uniform; tilt steam injection wells intersect with most of these geological heterogeneities, which helps improve the expansion of the steam chamber.
In order to demonstrate the advantage of this new well structure, researchers compared this structure of Athabasca and Cold Lake reservoirs with SAGD performance of the conventional structure.
The foundation program of Athabasca tilt steam injection well’s SAGD has the same operating conditions with normal SAGD. Steam injection pressure is 2,500kPa. By comparing the cumulative oil production and cumulative gasoline ratio after producing 1,750 days, and considering the steam chamber began to expand from tilt steam injection well, we could block the end of the tilt steam injection well, centralize heating steam injection well with the end in order to improve the efficiency of the heating process. At the end of steam injection wells 200m be blocked for 150 days, and the steam injection pressure should drop to 2,000kPa. By closing the injection of steam at the end of the steam injection well, thermal efficiency is increased by 5%. In Cold Lake Reservoir, in order to compare effect between this new well structure and the normal SAGD, the range of steam injection pressure is 3,500 ~ 5,500kPa. The production cycle of these two programs ends when the gasoline ratio is approximately equal to 4. The cycle of tilt steam injection well SAGD is even shorter, failure time is earlier about 2 months, but recovery ratio is the same. Test proved that oil and gas reservoirs use tilt injection wells mechanism body, SAGD will have more advantages.
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