Effect of vapour–liquid phase behaviour of steam–light hydrocarbon systems on steam assisted gravity drainage process for bitumen recovery

doi:10.1016/j.fuel.2011.10.044

Fuel

Volume 95, May 2012, Pages 159-168

https://doi.org/10.1016/j.fuel.2011.10.044 Get rights and content

Abstract

The vapour–liquid phase behaviour of steam–solvent (light hydrocarbon) systems used in an oil sands recovery process called steam assisted gravity drainage (SAGD) is examined. Analysis shows that condensation occurs over a temperature range and that a concentration gradient exists between the liquid and vapour phases. In a large range of solvent concentrations, water condenses first from the vapour phase. Solvent only condenses first from the vapour phase at extremely high solvent volume fractions. Due to water condensing first at most concentrations, the ability of the solvent to directly contact the bitumen in the reservoir depends on the orientation of the vapour–liquid interface and the relative position in the vapour chamber. Addition of solvent into a mature SAGD operation can also cause a temporary suppression of the steam–oil ratio (SOR) due to the change in the temperature at the vapour–liquid interface. This effect must be taken into account in interpreting experimental or simulation results. The addition of solvent not only changes the temperature but also decreases the heat of condensation of the mixture. As the concentration of solvent changes, the SOR is also expected to change.

Graphical abstract

Highlights

► Steam–solvent phase behaviour during bitumen recovery is examined for the first time. ► Solvent only condenses first from a mixture at very high solvent concentrations. ► Water condensing first from the mixture reduces solvent contact with bitumen. ► Addition of solvent changes phase transition temperatures and enthalpies.

Introduction

Steam assisted gravity drainage (SAGD) is a thermal process currently being developed for widespread commercialization of bitumen recovery in Western Canada [1]. When it is applied to suitable reservoirs, SAGD is capable of high production rates and recovery factors exceeding 65% [2]. A major drawback of the process is its high energy intensity and associated CO₂ emissions.

A process called expanding solvent SAGD (ES-SAGD) or solvent co-injection with SAGD is currently under intensive research to help address the high energy costs of SAGD. In general, the new technique continuously injects vaporized hydrocarbons, such as propane, butane, up to C12, or in various mixtures, along with the steam [3], [4], [5]. The injected solvent is intended to assist bitumen recovery through dissolution into the oil phase to reduce bitumen viscosity in conjunction with the viscosity reduction through heating. Many studies and trials project that the addition of solvent will increase recovery rates and simultaneously reduce the steam usage or steam oil ratio (SOR) [6], [7], [8].

One aspect of solvent co-injection with steam that has not been studied is the vapour–liquid phase behaviour involved in solvent–steam co-injection process. It is not well understood how the co-injection can impact the gravity drainage process. In a constant pressure process like SAGD where only one component (water) is used, the dew point and bubble point temperatures are the same. When hydrocarbon solvents are mixed with water, partial pressures come into play such that the dew point and bubble point of the system separate. In the steam chamber, the concentrations of water and solvent in the liquid and vapour phases are transient between the dew point and bubble point. The appearance of a temperature range for condensation and a concentration gradient between liquid and vapour phases introduces additional considerations when applying solvent co-injection. In this paper, both the phase behaviour and the potential impact on SAGD are examined.

Section snippets

Water–hydrocarbon vapour–liquid phase behaviour

To illustrate the vapour–liquid phase behaviour of water–hydrocarbon systems, a phase diagram of water–hexane (C6) mixture is generated for a system in a typical pressure–volume–temperature (PVT) test cell. Similar to a SAGD vapour chamber, the pressure in PVT cell is maintained constant. In this case, the phase change is achieved by the change in cell temperature through heat transfer between the mixture inside the cell and the heating system on the exterior of the cylinder. When the

Temperature and condensation profiles

Many studies [5], [6], [7], [11] point to hexane or heptane (C7) as the ideal solvent of the co-injection in SAGD since their saturation temperatures are the most similar to that of water. A common assumption is that a similar saturation temperature will promote co-condensation. Fig. 8 shows the comparison of saturation pressure and temperature for C6, C7, and water over the typical range of pressures seen in heavy oil reservoirs. As shown in Fig. 8, at the same pressure between 0.01 and 2.7

Conclusions

Use of steam–solvent combinations to enhance recovery from horizontal well pairs presents a significantly number of additional variables to control from the operations perspective. The addition of solvents significantly changes the phase behaviour such that concentration gradients and temperature gradients will form in the vapour chamber where none previously exists for SAGD). This presents the problems of dealing with solvent contact with the bitumen and ensuring appropriate level of subcool

References (13)

R.M. Butler
Some recent developments in SAGD
J Can Pet Technol
(2001)
R.M. Butler
Thermal recovery of oil and bitumen
(1991)
O.R. Ayodele et al.
Laboratory experimental testing and development of an efficient low pressure ES-SAGD process
J Can Pet Technol
(2009)
Ivory J, Zheng R, Nasr TN, Deng X, Beaulieu G, Heck G. Investigation of low pressure ES-SAGD, SPE117759, paper present...
Li W, Mamora DD. Drainage mechanism of steam with solvent co-injection under steam assisted gravity drainage (SAGD)...
Mohebati MH, Maini BB, Harding TG. Optimization of hydrocarbon additives with steam in SAGD for three major Canadian...

There are more references available in the full text version of this article.

Cited by (49)

A systematic comparison of solvents and their concentrations in bitumen gravity drainage under controlled thermodynamic conditions
2024, Fuel
Solvent-assisted SAGD (SA-SAGD) has been studied as an alternative to improve the efficiency of SAGD. This paper reports a new set of experimental data for SA-SAGD and analyzes the characteristics of bitumen-solvent mixing in the experiments using history-matched numerical models. The dimensions of the sand pack were 3 in. in diameter and 15 in. in length, and the sand pack was contained in a 25-L cylindrical pressure vessel. An annular void space surrounded the sand pack with a gap thickness of 1 in., within which a steady state flow of the injected vapor phase maintained the controlled pressure, temperature, and composition. Therefore, the gravity drainage in the sand pack occurred at a set of specified thermodynamic conditions for the surrounding annular steam chamber, unlike transient conditions near the edge of a steam chamber in larger-scale steam injection processes. In addition to SAGD as the base case, five sets of SA-SAGD were performed: 20 mol% C₄, 40 mol% C₄, 10 mol% C₈, 20 mol% C₈, and 10 mol% condensate coinjected with steam at 3500 kPa.
The peak oil production rate was 9.84 cm³/min with SAGD, 14.61 cm³/min with 20 mol% C₄-SAGD, 16.34 cm³/min with 40 mol% C₄-SAGD, 31.33 cm³/min with 10 mol% C₈-SAGD, 12.36 cm³/min with 20 mol% C₈-SAGD, and 12.33 cm³/min with 10 mol% condensate-SAGD. SAGD was the least effective in bitumen gravity drainage, while the 10 mol% C₈-SAGD was the most effective. Increasing the C₄ molar concentration from 20 to 40 mol% slightly increased the peak rate; however, the peak rate in C₈-SAGD significantly was decreased by increasing the C₈ concentration from 10 to 20 mol%. This counter-intuitive result was reported in a few simulation studies with heavy solvents (e.g., C₇ and C₁₂) in the literature, but had not been experimentally confirmed before this research.
Although the condensate contained 93 mol% C₇-like pseudo component (C_{7, eq}), the peak rate of 10 mol% condensate-SAGD was much smaller than that of 10 mol% C₈-SAGD. These experimental observations highlight the practical importance of understanding the sensitivity of bitumen gravity drainage to heavy-solvent concentrations near the edge of a steam chamber in SA-SAGD.
Vapor-liquid equilibria (VLE) of the binary mixture of normal hexane and water at P = 2.5 MPa and T=(456.85 – 487.85 K)
2023, Fluid Phase Equilibria
We report the vapor-liquid equilibria (VLE) of the binary mixture of the n-C₆/H₂O system at P=2.5 MPa and T=(456.85 – 487.85 K). The result showed an azeotropic behavior with a co-condensation temperature of ∼456.85 K at an n-C₆ mole fraction of ∼0.6029. The measured experimental data are modeled with the Cubic Plus Association Equation of State (CPA-EoS) and activity coefficient-based Non-Random Two Liquid (NRTL) model. The infinite dilution activity coefficient of n-C₆ and H₂O are also estimated over the temperature range of interest. The overall root mean square deviation (RMSD) of the n-C₆ mole-fraction in vapor and liquid phases for the CPA EoS and NRTL models are 0.0370 and 0.0765, respectively.
Dimethyl ether-steam assisted gravity drainage: Physical 2D heavy oil simulation
2023, Fuel
Steam-assisted gravity drainage (SAGD) is a mature heavy oil thermal recovery technology but inherently limited to low recovery efficiency at mid-late stage due to the increasing heat loss in the steam chamber. Here, the dimethyl ether (DME)-assisted SAGD technology and its relevant transport and interfacial properties are specifically investigated through physical 2D heavy oil simulation. A total of four-set physical experiments, with different DME injection ratios, were conducted and compared with the traditional SAGD. The experimental results indicate DME is capable of reducing the heavy oil’s viscosity and interfacial tension at high temperatures. Three stages, rising, lateral expansion, and descending stages, are determined in the SAGD steam chamber’s development; the mid-late SAGD stage is defined when the steam chamber extends to the caprock boundary. With DME additions at the mid-late SAGD stage, higher oil recovery factor and sweep efficiency could be reached and the dew point temperature of steam chamber’s lead decreases by 0.7–1.8 °C. The optimum gas–water ratio is determined to be 3:1, with the fastest oil recovery growth and the highest final cumulative oil-vapor ratio of 0.163. This study provides reliable experimental bases and valuable analyses for the DME-SAGD technology and its application in heavy oil reservoirs.
The impact of permeability barriers on steam-solvent coinjection – A mechanistic study using a physical model
2023, Geoenergy Science and Engineering
Coinjection of steam and condensate has been studied for improving the energy efficiency of steam-assisted gravity drainage (SAGD) as solvent-assisted SAGD or SA-SAGD. However, it is not well understood how tortuous hydraulic paths under heterogeneous permeability can influence the compositional and thermal flow characteristics in SA-SAGD. We addressed this question by performing an experiment of the coinjection of steam and condensate for bitumen recovery. This paper discusses the experimental results and gives our mechanistic analysis of how the tortuous hydraulic paths affected the thermal and compositional flow and the properties of the produced bitumen in the experiment.
The physical model was a cylindrical pressure vessel with a diameter of 0.425 m and a length of 1.2 m, which was filled with unconsolidated sands. Two shale plates were horizontally installed above the well pair at different elevations so that they could represent permeability barriers to cause tortuous flow paths during the SA-SAGD experiment. The porosity and permeability of the sandpack was 0.34 and 5.6 D, respectively. Initially, the oil and water saturations were 95% and 5%, respectively.
After preheating for 1 day, a mixture of 98.2 mol% steam and 2.8 mol% condensate was injected at 3500 kPa for 4 days (35 cm³/min of steam, cold water equivalent). The injection and production histories along with the real-time temperature distribution were recorded during the experiment. Produced oil samples were frequently taken and analyzed for density and asphaltene content. After the experiment, the sandpack was excavated and sampled from different locations to analyze the saturation and asphaltene content of the remaining oil. The experimental results in this paper were compared with the previous SAGD and SA-SAGD experiments that used the same experimental set up with a homogeneous sandpack.
Results showed that, in comparison to the homogeneous SAGD case, SA-SAGD was able to lower the cumulative steam-oil ratio (SOR) by a factor of two to three even in the presence of shale plates. Analysis of the temperature profiles indicated that the steam chamber vertically expanded from the lower part to the upper part through tortuous paths at lower temperatures. An emerging steam chamber above the shale plates occurred by convective heat from the injection well through lower-temperature flow paths between shale plates, involving light to intermediate solvent components that enabled the steam chamber to expand away from the injection well. This highlights the important role of volatile solvent components in the growth of a steam chamber in SA-SAGD under heterogeneity. The produced bitumen density in this research was closer to the original bitumen than in the homogeneous SA-SAGD case because the bitumen dilution and the solvent retention increased by the tortuous flow regime resulted in efficient drainage of oil at lower temperature.
EOR potentials in the poststeam-injected heavy oil reservoirs
2023, Thermal Methods
Thermal methods, particularly the in situ steam-based practices, used for oil production from oil sands and heavy oil reservoirs are the leading technology for enhanced oil recovery (EOR) in such reservoirs. However, a majority of these techniques, such as steam-assisted gravity drainage, cyclic steam stimulation, and steam flooding, are on the verge of exhaustion. Yet, the enduring interactions between rock and steam, bitumen recovery, and improving heavy oil recovery are the main challenges in the poststeam injection era. Here, we reviewed the experimental and field applications of the EOR methods and the recovery mechanisms behind each method in the poststeam injection era, in heavy oil reservoirs with different lithologies. Furthermore, several other thermal techniques, their applications, and their attributed pros and cons are discussed in this chapter.
Rising-phase performance evaluation of Expanding-Solvent SAGD: Considering the horizontal-wellbore effect
2021, Journal of Petroleum Science and Engineering
Expanding-Solvent Steam-Assisted Gravity Drainage (ES-SAGD) is proposed to develop the oil sands more efficiently than conventional SAGD. Even in the rising phase when the steam hasn't reached the overburden, the heat-mass transfer in the wellbore can be one of the reasons for the non-uniform chamber growth of ES-SAGD along the horizontal well. Furthermore, the time and oil production during the rising phase greatly determine the final benefits of the ES-SAGD. In this study, a mathematical method is established to evaluate the performance of ES-SAGD in the rising phase. The calculated results show that: pentane can help increase the oil rate in the rising phase significantly, while the effect of heptane is not quite obvious. Although propane is also a widely used solvent, the oil rate with it is even lower than that of conventional SAGD. The differentials between the cases, which consider the wellbore or not, is different for different solvent-steam co-injections. Specifically, the oil rate differential between the situations considering the wellbore effect or not for propane-SAGD is the highest and is almost twice that of conventional SAGD which has the least differential. Pentane-SAGD has the second-largest differential which is followed by heptane-SAGD. Therefore, for the sake of a more accurate evaluation of ES-SAGD performance in the chamber rising phase, the effects of solvent diffusion into the bitumen and the heat-mass transfer in the wellbore should be comprehensively considered.

View all citing articles on Scopus

View full text