Vapor extraction (Vapex) is an emerging technology to produce heavy oil and bitumen from subsurface formations. Single well (SW) Vapex technique uses the same concept of Vapex process but only with one horizontal well. In this process solvent is injected from the toe of the horizontal well with oil production at the heel section. The main advantage of SW-Vapex process lies in the economic saving and applicability in problematic reservoirs, where drilling of two horizontal wells is impractical. The performance of SW-Vapex seems to be comparable with dual horizontal Vapex process using proper optimization schemes. This study is grouped into two sections: (i) a screening study of early time operating performance of SW-Vapex and (ii) a sensitivity analysis of the effect of the reservoir and well completion parameters. Simulation results show that solvent injection rate can be optimized to improve oil production rate. Higher injection rates may not necessarily lead to increase in production. This study confirms that SW-Vapex process is very ineffective in reservoirs with high oil viscosity (more than 1,500 cp) and thin formations (less than 10 m).
Heavy oil and bitumen reserves represent a considerable portion of worldwide energy resources. It is estimated that these reservoirs contain six trillion barrels of STOOIP which is much higher than the total conventional oil reservoirs. Significant consumption of light oil reserves and therefore their dramatic decline encourage more interest in the exploitation of highly viscous oil and bitumen for future energy demands. For the optimum conditions, the primary recovery of these reservoirs would not exceed 10 percent of the STOOIP. Most of these heavy oil and bitumen deposits are located in Canada, Venezuela, China, and Indonesia. Production from these reservoirs is challenging and technically difficult. Recently, vapor extraction process (Vapex) is introduced as an attractive method for such viscous oil reservoirs [
In a reservoir where oil is very viscous and cannot flow in reservoir condition, adding of vaporized hydrocarbon solvents (HCS) can drastically reduce oil viscosity. This concept is the fundamental of Vapex. In a conventional Vapex approach, the solvent is injected into a horizontal well located directly above a horizontal producer. As the solvent diffuses and dissolves in the bulk of the reservoir, a chamber of solvent vapor grows around the injection well and helps the gravity drainage of diluted oil toward the production well, from where it is pumped out to the surface.
Depending on reservoir condition, several other well configurations are also possible. Butler and Jiang [
In single well (SW) Vapex a horizontal well is completed such that it plays the role of both injector and producer. The similar well configuration has been studied for SAGD process [
Vapex process has been extensively studied using laboratory experiments. The main limitation is the low oil production compared to other thermal recovery processes [
One of the alternative methods is to heat up the injected solvent to take advantage of both thermal and mass diffusion for oil viscosity reduction (N-Solv) [
Recently, solvent-assisted SAGD process showed promising potential for efficient heavy oil recovery. In this process, steam is used as the main heat carrier and the coinjected solvent is the axillary factor in the further reduction of oil viscosity. Nasr et al. [
Application of one single well in the Vapex process has not been addressed in the literature. However, Elliot and Kovscek [
Although the Vapex process is accomplished by mass transfer and the dominant mechanism in SAGD is heat transfer, the central idea of both processes is the growth of an egg-like vapor (steam or hydrocarbon solvent) chamber within the reservoir. Therefore, in this work, the same methodology was used to move toward the simulation of SW-Vapex. In the present study, a simulation model was developed to investigate the feasibility of single well Vapex. The approach includes, first, optimizing the early time performance of the SW-Vapex and, second, performing a sensitivity analysis to study the effect of the reservoir and well completion parameters on the process.
The reservoir model selected for simulation of SW-Vapex process is a homogeneous, anisotropic, three-dimensional system consisting of 5,568 grid blocks. The grid system is Cartesian with local grid refinement as detailed in Figures
Properties of the simulation model.
Property | Value |
---|---|
Grid type | Cartesian |
Length in | 1400 m |
Length in | 80 m |
Length in | 19.6 m |
Number of grid cells | 5,568 |
Average porosity | 0.33 |
Horizontal permeability | 1500 md |
Vertical permeability | 750 md |
Number of components | 4 |
Temperature | 20°C |
Connate water saturation | 0.15 |
Initial pressure | 1.1 MPa |
Initial oil viscosity @ 20°C | 8770 mPa·s |
Solvent composition | 0.7 C3 + 0.3 C1 |
Well length | 800 m |
Total hydrocarbon pore volume | 615,754 m3 |
The single horizontal well is simulated using discretized wellbore model [
Oil viscosity versus temperature.
Oil-water relative permeability curve.
Gas-oil relative permeability curve.
The criterion that should be considered in selecting the solvent composition is that it should be at or near to its saturation point or in the case of solvent mixture near its dew point. This decisive factor should be met to avoid any liquefaction of the solvent before it dissolves into heavy oil since this may cause higher solvent consumption, which results in higher oil production cost [
Phase diagram of injected solvent system with respect to reservoir temperature and pressure.
Vapor extraction is a slow process which is governed by mass diffusion, and therefore optimization schemes should be implemented to facilitate the process at the early time. Increasing of the gas-oil contact area at the early period of production should be considered to enhance the whole process by accelerating vapor chamber development. Thermal recovery methods especially steam injection can be used as an efficient alternative for this goal. For uniform heating of the near wellbore area and reducing the oil viscosity, the total length of the horizontal well can be used for steaming. In this way, an initial cavity is formed in the reservoir which increases the oil-solvent mass transfer area and therefore enhances oil dilution. To investigate the importance of initial thermal treatment and choose the appropriate heating method, different scenarios were considered with operation conditions summarized in Table
Operation condition of early time scenarios.
Case | Injection rate (CWE m3/day) | Description |
---|---|---|
Vapex | 2000 (m3/day) | Vapex from the beginning |
SAGD | 300 | SAGD for 180 days, followed by Vapex |
Cyclic 1x | 300 | 20 days of injection, 70 days of production, followed by Vapex |
Cyclic 2x | 300 | 20 days of injection, 70 days of production for 2 times, followed by Vapex |
The first scenario assumes thorough Vapex process from the beginning which can provide a reference case for comparison of the performance of other preheating strategies. In second case steam was injected at the toe of the horizontal well and simultaneously diluted oil is produced at the heel. The steam injection schedule which took 180 days was then followed by continuous solvent injection. The third and fourth cases include cyclic steam injection with different soak and production periods. In case 2, steam is continuously injected from the toe of the horizontal well. Low injectivity was one of the main issues for this preheating method. To improve the injectivity problem, steam was injected in a cyclic way. In case 3, the process starts with 20 days of steam injection and a soaking period of 40 days, then followed by 70 days of production (no steam injection). In case 4, steaming, soaking, and production sequence is repeated twice then followed by Vapex process. The oil production profiles are illustrated in Figures
Cumulative oil production for different early time preheating scenarios during the first 180 days. Cyclic steam injection with two periods of injection/production leads to highest oil production (voidage in the reservoir).
Oil production rate for different early time preheating scenarios during the first 180 days.
It should be mentioned that in case 1, the solvent is injected with operating pressure which is slightly higher than the average reservoir pressure. The gas breakthrough will happen after a while by immediate establishment of a short circuit between injection and production part of the horizontal well. The breakthrough time depends on oil mobility and the distance between well sections. Since the vaporized solvent remains in contact with a small portion of the reservoir, the efficiency of the process drastically reduces, and vapor chamber development is quite slow. Increasing the separation between the well sections does not improve the recovery, but it just delays the gas breakthrough time. In the following cases, it is attempted to maximize the initial contact area between the solvent and fresh heavy oil by creating a proper voidage in the reservoir.
Figure
Effect of early time scenarios on the performance of SW-Vapex, after 15 years.
Temperature profile after 180 days, cyclic steam injection 2x.
Vertical view of viscosity profile after 2,500 days; layer number in
Vertical view of saturation profile after 2500 days; layer number in
Here, a series of sensitivity studies has been carried out to investigate the effect of operating conditions and reservoir parameters on the performance of SW-Vapex. Solvent injection rate, the presence of a small gas cap, reservoir thickness, and permeability anisotropy were considered for sensitivity study. All cases are started after 180 days of initial cyclic steam injection. The results are compared with the reference case whose properties are listed in Table
Effect of initial gas cap and gas injection rate (
Effect of permeability anisotropy (
Effect of reservoir thicknesses (10, 19.6, 25, and 35 m) on ultimate oil recovery after 15 years.
Early time study indicated that applying vapor extraction process alone with single well configuration loses its viability due to inherent slow nature of the solvent. In this point, implementation of an optimization scenario can play a crucial role in the improvement of the oil recovery. By comparing the results of different scenarios, it was found that steam cycling was the most efficient method. The objective of early time heat treatment is to heat the reservoir, as much as possible, in a uniform manner to provide an initial cavity in the formation. It is worth mentioning that selection of early time strategy is fully dependent on reservoir property and operating conditions of the Vapex process and may differ from case to case.
Heavy oil viscosity has a pronounced effect on the success of SW-Vapex. Due to related difficulties in the mobilization of the crude oil with a viscosity greater than 15,000 cp, these highly viscous reservoirs may need to use other production methods. The same conclusion can be drawn for the height of the reservoir. A minimum height should exist to allow proper development of the vapor chamber in the vertical direction. For a reservoir with height that is less than 4 meters, the ultimate recovery would not exceed 5% of the STOOIP. Since the practical hydrocarbon solvents used in the operation are almost expensive, the solvent requirement and consumption are the key economic factor for the Vapex recovery. An optimum value should be specified to obtain minimum solvent to oil ratio. As expected the vertical permeability value plays the major role in the rate of vertical propagation of the vapor chamber. Since the diluted oil should be drained toward the producer and the solvent substitutes that, the vertical permeability reduction could reduce the oil recovery. Therefore, the presence of the shale layers in the reservoir is expected to make the process more challenging.
This study aimed at screening of applicability of single well Vapex process through numerical modeling. A sound analogy for simulation study of single well Vapex process has been developed to simulate both wellbore and reservoir sections. Preheating treatment is a must for creating initial proper voidage in the reservoir to enhance vapor propagation. Preheating has a significant effect on the total oil recovery in the long run. Cyclic steaming with two periods of soak and production in 180 days was selected as an efficient preheating scenario to heat the near wellbore area uniformly. Solvent injection rate should be adjusted to achieve maximum oil production rate by consuming minimum solvent. Higher injection rates just shorten the breakthrough time and leave the oil unaffected. Application of SW-Vapex in reservoirs with a reservoir thickness approximately less than 10 m and initial oil viscosity more than 1,500 cp seems to be not feasible. The presence of a relatively small gas cap aids the solvent diffusion by increasing the contact surface area.
Cold water equivalent
Expanding solvent steam assisted gravity drainage
Hydrocarbon solvent
Horizontal permeability
Vertical permeability
Millidarcy
Megapascal
Steam assisted gravity drainage
Steam oil ratio
Stock tank original oil in place
Single well
Vapor extraction
Centipoise
Viscosity.
The authors declare that they have no competing interests.