Two-Wellhead SAGD Scheme Increases Efficiency of Heavy-Oil Development
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Variants of steam-assisted gravity drainage (SAGD) for heavy-oil recovery include crosswell SAGD, single-well SAGD, and horizontal alternating steam drive. The main goal of these modifications is to increase the efficiency of the SAGD process and control steam-chamber development. This paper shares experience gained in the Ashalchinskoye heavy-oil field with a two-wellhead SAGD modification. As a result of a pilot for this technology in Russia, the accumulated production of three pairs of these wells is greater than 200,000 tons.
Heavy-Oil Development in Tatarstan, Russia
The depletion of the traditional oil reserves of the Republic of Tatarstan makes the development of hard-to-recover hydrocarbon reserves an urgent matter. These reserves include natural bitumen and heavy oil, the reserves of which in Tatarstan’s Permian deposits, according to various estimates, amount to 7 billion tons.
These deposits are at a shallow depth. The low mobility of such oil and bitumen is the result of their high viscosity (greater than 10,000 cp). Therefore, thermal methods of heavy-oil recovery are used to ensure an inflow to producing wells. Among such methods are steam injection, huff ’n’ puff, and in‑situ combustion. In the operator’s zone of activity, 149 heavy-oil deposits have been identified.
The first heavy-oil development projects in Tatarstan began in the 1970s with the implementation of pilots in two deposits using vertical wells. The methods of in-situ combustion, steam injection, and combined steam and gas injection were tested. All pilots in this area were not considered to be successful, because the efficiency of the tested technologies for those particular reservoir conditions was low and the costs of heavy-oil production exceeded the cost of heavy-oil mining. By the end of 2005, approximately 206,000 tons of heavy oil were produced, with an average daily production rate of 0.4 t/D.
For the first time in Russia, a pair of horizontal wells for the implementation of SAGD technology was drilled in 1998 at the Mordovo-Karmalskoye field. Because of technological limitations, the length of the horizontal section of the wells could not be increased beyond 150 m. In addition, it was not possible to keep the equal distance between production and injection wells. These problems affected the oil production of the well pair (production did not exceed 4–5 t/D). As a result, achieving profitability was impossible for this first SAGD implementation in Russia.
The next trial for SAGD in Tatarstan, with some modifications, was introduced in the Ashalchinskoye heavy-oil field.
Ashalchinskoye Heavy-Oil Field
The Ashalchinskoye reservoir is shallow, with a formation depth of 80–100 m. The maximum formation thickness is 32 m, with an average permeability of 2.6 darcies. Oil is heavy and highly viscous, with a density of 960 kg/m3. The average oil viscosity at reservoir conditions is 27,000 mPa·s. The reservoir temperature is 8°C, and the reservoir pressure is 4.4 bar, which is less than the hydrostatic pressure.
The reservoir is characterized by a high zonal heterogeneity. Oil/water contact is characterized by uneven surfaces caused by deterioration of reservoir properties at the bottom of the deposit. The reservoir is complicated by the presence of clay interlayers within the deposit. The reservoir rock is composed of unconsolidated sandstone cemented by bitumen.
Under these conditions, the pilot project using two-wellhead SAGD wells was launched (Fig. 1). First, pilot wells were drilled with core sampling, laboratory work was performed to simulate the technology on formation models, and thermal hydrodynamic modeling was performed to determine the optimal parameters of the technology evaluation. Other critical tasks included determination of basic drilling technology at shallow depths, the principles of heavy-oil refinement, and consideration of the environmental aspects of development.
Two-Wellhead SAGD Wells
For the pilot project, three SAGD well pairs were drilled and completed in 2006. For instance, for the pair consisting of Wells 232 and 233, production Well 232 has two wellheads with a horizontal section length of 200 m. Part of the horizontal well section is drilled through the water-bearing formation and completed by blank pipes. The rest of the horizontal section is completed by wire-wrap sand screens. Approximately 37% of the total length of the screen zone is located in the low-productivity zone.
At the preheating phase, steam was circulated in both wells to establish the hydrodynamic connectivity between the well pair. The SAGD process was launched by injecting steam into the upper horizontal well, and the lower horizontal well was put into production.
The process of switching to the production phase was initiated by use of the swabbing method. Swabbing was necessary to clean the bottom of the production well from clay-mud residue, remove sand particles during the start of the production phase, and create a gravel filter around the production well.
A special construction swab was applied for these purposes. The swabbing unit consists of two slickline units and wellhead equipment, which includes all necessary elements installed at both wellheads. The swab is moved in the wellbore by means of slickline connected to the swab at both ends.
The wellbore fluid is carried out by movement of the swab at a speed of 1 m/s. Then the swab is moved to the other side of the wellbore, which makes it possible to extract all of the incoming fluids at every stroke from the well. This allows fluid to be produced, but the sand entering the well also is removed from the wellbore completely. This type of artificial lift for SAGD operations has a number of advantages, including cost-effectiveness, ease of operation and maintenance, and tolerance to sand production. This approach is not, however, appropriate for highly productive wells. Other methods of wellbore cleanup, including gas lift and nitrogen injection through coiled tubing, were used in the Ashalchinskoye heavy-oil field. For the continuous SAGD production phase, high-temperature electrical submersible pumps (ESPs) are installed for production wells. These ESPs usually are equipped with pressure and temperature gauges for well control and monitoring. In addition, distributed-temperature-sensing cables delivered by coiled tubing are installed for every well for real-time data gathering and steam-trap control.
The two-wellhead SAGD well approach allows the use of different operating modes with changes in the proportion of fluid production and steam injection from each side of the wellbore. In total, there are nine modes of operation, allowing the achievement of uniform warming of the interwell zone under the conditions of geological heterogeneity within the formation and the intersection of the wellbore with zones featuring low oil saturation. During the operation, all nine modes were tested.
As a result of fluid production from different points (toe and heel), as well as the reduction of heat losses and improvement of steam quality because of shorter tubing suspensions from both wellheads, well production increased from 5 to 30 t/D on average. The maximum oil production from one of the SAGD well pairs exceeded 60 t/D.
SAGD production-phase optimization is made possible by the mode of operation that uses production of liquid from both sides of the production well and the injection of steam into both sides of the steam-injection well in different proportions. It is possible to achieve equalization of the heating of the interwell zone and, accordingly, an increase in the oil-production rate.
For a limited time, the complete paper SPE 189743 is free to SPE members.
Two-Wellhead SAGD Scheme Increases Efficiency of Heavy-Oil Development
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