Publications of year 2001

Conference articles

A. R. Kovscek. Mechanistic Modeling of Solution Gas Drive in Viscous Oils. In , Porlarmar, Margarita Island, Venezuela, March 2001.
[pdf]
Keywords: Solution Gas Drive, Heavy Oil.

Abstract

Solution gas drive in reservoirs containing heavy and viscous oil is not well understood. This paper develops a mechanistic population balance model for describing the process of bubble nucleation and growth. The model is applied to both light and viscous oils. The primary modeling concept is a continuum bubble population balance. Appropriate rate equations are derived for two theories of bubble nucleation described in the literature—instantaneous nucleation (IN) and progressive nucleation (PN). The results of simulations for the IN and PN models are compared to experimental data reported elsewhere for light oil and to new data for viscous oils. Model parameters are all physically based. Within the IN model, the number density of bubbles must be specified while the PN model requires the cavity size distribution of the porous medium as input. The PN model matches the experiments somewhat better, but is more demanding computationally. Interestingly, the population balance description of either model does not require a critical supersaturation to be exceeded before the onset of bubble nucleation and growth. Supersaturation is the difference between the equilibrium and dynamic liquid pressure of a system. Liberation of gas from solution at the thermodynamic bubble point and the bubble growth equations presented here well describe the kinetics of the gas phase and pressure response of the systems examined.

BibTex Entry:

@CONFERENCE{SPE69717,
TITLE ={Mechanistic Modeling of Solution Gas Drive in Viscous Oils},
AUTHOR ={A. R. Kovscek},
JOURNAL={SPE International Thermal Operations and Heavy Oil Symposium},
year ={2001},
month =mar,
address ={Porlarmar, Margarita Island, Venezuela},
KEYWORDS ={Solution Gas Drive, Heavy Oil},
URL ={http://ekofisk.stanford.edu/supria/publications/public/tr118.pdf},
ABSTRACT ={Solution gas drive in reservoirs containing heavy and viscous oil is not well understood. This paper develops a mechanistic population balance model for describing the process of bubble nucleation and growth. The model is applied to both light and viscous oils. The primary modeling concept is a continuum bubble population balance. Appropriate rate equations are derived for two theories of bubble nucleation described in the literature—instantaneous nucleation (IN) and progressive nucleation (PN). The results of simulations for the IN and PN models are compared to experimental data reported elsewhere for light oil and to new data for viscous oils. Model parameters are all physically based. Within the IN model, the number density of bubbles must be specified while the PN model requires the cavity size distribution of the porous medium as input. The PN model matches the experiments somewhat better, but is more demanding computationally. Interestingly, the population balance description of either model does not require a critical supersaturation to be exceeded before the onset of bubble nucleation and growth. Supersaturation is the difference between the equilibrium and dynamic liquid pressure of a system. Liberation of gas from solution at the thermodynamic bubble point and the bubble growth equations presented here well describe the kinetics of the gas phase and pressure response of the systems examined.},

}

D. Zhou, L. Jia, J. Kamath, and A. R. Kovscek. An Investigation of Counter-Current Imbibition Processes in Diatomite.. In , Bakersfield,CA, USA, March 2001.
[pdf]
Keywords: Imbibition, Diatomites, Analytical Work.

Abstract

Oil recovery from low permeability reservoirs is strategically important because of the large resources locked in such formations. Imbibition is fundamental to oil recovery from such reservoirs under most secondary and improved recovery processes of practical interest. It is also characteristic of porous medium wettability. The rate and the extent of imbibition depend critically on the viscosity of the wetting and nonwetting phases. In this study, we present our recent work on imaging imbibition in low permeability porous media (diatomite) with X-ray computed tomography. The viscosity ratio between nonwetting and wetting fluids is varied over several orders of magnitude yielding different levels of imbibition performance. We also perform a mathematical analysis of counter-current imbibition processes and develop a modified scaling group incorporating the mobility ratio. This modified group is physically based and appears to improve scaling accuracy of countercurrent imbibition significantly.

BibTex Entry:

@CONFERENCE{SPE68837,
TITLE ={An Investigation of Counter-Current Imbibition Processes in Diatomite.},
AUTHOR ={D. Zhou and L. Jia and J. Kamath and A. R. Kovscek},
JOURNAL={SPE Western Regional Meeting},
year ={2001},
month =mar,
address ={Bakersfield,CA, USA},
KEYWORDS ={Imbibition, Diatomites, Analytical Work},
URL ={http://ekofisk.stanford.edu/supria/publications/public/tr118.pdf},
ABSTRACT ={Oil recovery from low permeability reservoirs is strategically important because of the large resources locked in such formations. Imbibition is fundamental to oil recovery from such reservoirs under most secondary and improved recovery processes of practical interest. It is also characteristic of porous medium wettability. The rate and the extent of imbibition depend critically on the viscosity of the wetting and nonwetting phases. In this study, we present our recent work on imaging imbibition in low permeability porous media (diatomite) with X-ray computed tomography. The viscosity ratio between nonwetting and wetting fluids is varied over several orders of magnitude yielding different levels of imbibition performance. We also perform a mathematical analysis of counter-current imbibition processes and develop a modified scaling group incorporating the mobility ratio. This modified group is physically based and appears to improve scaling accuracy of countercurrent imbibition significantly.},

}

Internal reports

I. Diabira, L. M. Castanier, and A. R. Kovscek. Porosity and Permeability Evolution Accompanying Hot Fluid Injection Into Diatomite.. Technical report, Stanford University, CA, USA, March 2001.
[pdf]
Keywords: Permeability, Diatomites, Porosity, Experimental work.

Abstract

An experimental study of silica dissolution was performed to probe the evolution of permeability and porosity in siliceous diatomite during hot fluid injection such as water or steam flooding. Two competing mechanisms were identified. Silica solubility in water at elevated temperature causes rock dissolution thereby increasing permeability; however, the rock is mechanically weak leading to compression of the solid matrix during injection. Permeability and porosity can decrease at the onset of fluid flow. A laboratory flow apparatus was designed and built to examine these processes in diatomite core samples. At the core level, we measured the pressure drop as a function of time for fixed injection rates to determine permeability variation and utilized an X-ray Computerized Tomography (CT) scanner to measure in-situ porosity. At the pore level, a scanning electron microscope (SEM) was used to observe changes in pore morphology. We found that porosity decreased initially due to compaction caused by the imposed pressure drop across the core. Later, porosity increased as silica dissolved. Dissolution of the rock matrix appeared to be relatively uniform; wormholes were not observed even after tens of pore volumes of fluid injection.

BibTex Entry:

@TECHREPORT{TR123,
TITLE ={Porosity and Permeability Evolution Accompanying Hot Fluid Injection Into Diatomite.},
AUTHOR ={I. Diabira and L. M. Castanier and A. R. Kovscek},
YEAR ={2001},
MONTH =mar,
INSTITUTION = {Stanford University, CA, USA},
KEYWORDS ={Permeability, Diatomites, Porosity, Experimental work},
URL ={http://ekofisk.stanford.edu/supria/publications/public/tr123.pdf},
ABSTRACT ={An experimental study of silica dissolution was performed to probe the evolution of permeability and porosity in siliceous diatomite during hot fluid injection such as water or steam flooding. Two competing mechanisms were identified. Silica solubility in water at elevated temperature causes rock dissolution thereby increasing permeability; however, the rock is mechanically weak leading to compression of the solid matrix during injection. Permeability and porosity can decrease at the onset of fluid flow. A laboratory flow apparatus was designed and built to examine these processes in diatomite core samples. At the core level, we measured the pressure drop as a function of time for fixed injection rates to determine permeability variation and utilized an X-ray Computerized Tomography (CT) scanner to measure in-situ porosity. At the pore level, a scanning electron microscope (SEM) was used to observe changes in pore morphology. We found that porosity decreased initially due to compaction caused by the imposed pressure drop across the core. Later, porosity increased as silica dissolved. Dissolution of the rock matrix appeared to be relatively uniform; wormholes were not observed even after tens of pore volumes of fluid injection.},

}

K. T. Elliot and A. R. Kovscek. A Numerical Analysis of the Single-Well Steam Assisted Gravity Drainage Process.. Technical report, Stanford University, CA, USA, June 2001.
[pdf]
Keywords: SAGD, Steam, Heavy Oil, Thermal Recovery.

Abstract

Steam assisted gravity drainage (SAGD) is an effective method to produce heavy oil and bitumen which are important energy resources. In a typical SAGD approach, steam is injected into a horizontal well located directly above a horizontal producer helping to displace heated oil. Single-well (SW) SAGD attempts to create a similar process using only one horizontal well. To improve early-time response of SW-SAGD, it is necessary to heat the near-wellbore area to reduce oil viscosity and allow gravity drainage to begin. Ideally heating should occur with minimal circulation or bypassing of steam. We have investigated early-time processes to improve reservoir heating. A numerical simulation study was performed to gauge combinations of cyclic steam injection and steam circulation prior to SAGD in an effort to better understand and improve early-time performance. Results from this study, include cumulative recoveries, temperature distributions, and production rates. It is found that cyclic steaming of the reservoir offers the most favorable option for heating the near-wellbore area to create conditions that improve initial SAGD response. More favorable reservoir conditions such as low viscosity, thick oil zones, and solution gas, improved reservoir response. Under unfavorable conditions, response was limited.

BibTex Entry:

@TECHREPORT{TR124,
TITLE ={A Numerical Analysis of the Single-Well Steam Assisted Gravity Drainage Process.},
AUTHOR ={K. T. Elliot And A. R. Kovscek},
YEAR ={2001},
MONTH =jun,
INSTITUTION = {Stanford University, CA, USA},
KEYWORDS ={SAGD, Steam, Heavy Oil, Thermal Recovery},
URL ={http://ekofisk.stanford.edu/supria/publications/public/tr124.pdf},
ABSTRACT ={Steam assisted gravity drainage (SAGD) is an effective method to produce heavy oil and bitumen which are important energy resources. In a typical SAGD approach, steam is injected into a horizontal well located directly above a horizontal producer helping to displace heated oil. Single-well (SW) SAGD attempts to create a similar process using only one horizontal well. To improve early-time response of SW-SAGD, it is necessary to heat the near-wellbore area to reduce oil viscosity and allow gravity drainage to begin. Ideally heating should occur with minimal circulation or bypassing of steam. We have investigated early-time processes to improve reservoir heating. A numerical simulation study was performed to gauge combinations of cyclic steam injection and steam circulation prior to SAGD in an effort to better understand and improve early-time performance. Results from this study, include cumulative recoveries, temperature distributions, and production rates. It is found that cyclic steaming of the reservoir offers the most favorable option for heating the near-wellbore area to create conditions that improve initial SAGD response. More favorable reservoir conditions such as low viscosity, thick oil zones, and solution gas, improved reservoir response. Under unfavorable conditions, response was limited.},

}

D. S. George and A. R. Kovscek. Visualization of Solution Gas Drive in Viscous Oil.. Technical report, Stanford University, CA, USA, July 2001.
[pdf]
Keywords: Solution Gas Drive, Heavy Oil.

Abstract

Several experimental studies of solution gas drive are available in the literature, but almost all of the studies have used light oil. Solution gas drive behavior, especially in heavy oil reservoirs, is poorly understood. Hence, experiments were performed in which pore-scale solution gas drive phenomena were viewed in water/carbon dioxide and viscous oil/carbon dioxide systems. A new pressure vessel was designed and constructed to house silicon-wafer micromodels that previously operated at low (< 3 atm) pressure. The new apparatus is used for the visual studies. Several interesting phenomena were viewed. The repeated nucleation of gas bubbles was observed at a gas-wet site occupied by dirt. Interestingly, the dissolution of a gas bubble into the liquid phase was previously recorded at the same nucleation site. Gas bubbles in both systems grew to span one or more pore bodies before mobilization. Liquid viscosity affected the ease with which gas bubbles coalesced. More viscous solutions result in slower rates of coalescence. The transport of solid particles on gas-liquid interfaces was also observed.

BibTex Entry:

@TECHREPORT{TR126,
TITLE ={Visualization of Solution Gas Drive in Viscous Oil.},
AUTHOR ={D. S. George and A. R. Kovscek},
YEAR ={2001},
MONTH =jul,
INSTITUTION ={Stanford University, CA, USA},
KEYWORDS ={Solution Gas Drive, Heavy Oil},
URL ={http://ekofisk.stanford.edu/supria/publications/public/tr126.pdf},
ABSTRACT ={Several experimental studies of solution gas drive are available in the literature, but almost all of the studies have used light oil. Solution gas drive behavior, especially in heavy oil reservoirs, is poorly understood. Hence, experiments were performed in which pore-scale solution gas drive phenomena were viewed in water/carbon dioxide and viscous oil/carbon dioxide systems. A new pressure vessel was designed and constructed to house silicon-wafer micromodels that previously operated at low (< 3 atm) pressure. The new apparatus is used for the visual studies. Several interesting phenomena were viewed. The repeated nucleation of gas bubbles was observed at a gas-wet site occupied by dirt. Interestingly, the dissolution of a gas bubble into the liquid phase was previously recorded at the same nucleation site. Gas bubbles in both systems grew to span one or more pore bodies before mobilization. Liquid viscosity affected the ease with which gas bubbles coalesced. More viscous solutions result in slower rates of coalescence. The transport of solid particles on gas-liquid interfaces was also observed.},

}

A. R. Kovscek, L. M. Castanier, and W. E. Brigham. Heavy and Thermal Oil Recovery Mechanisms Annual Report for 2000-2001.. Technical report, Stanford University, CA, USA, 2001.
[pdf]
Keywords: Multiphase Flow, Reservoir Definition, In-Situ Combustion, EOR, Heavy Oil.

Abstract

Supri-A yearly research report.

BibTex Entry:

@TECHREPORT{TR127,
TITLE ={Heavy and Thermal Oil Recovery Mechanisms Annual Report for 2000-2001.},
AUTHOR ={A. R. Kovscek and L. M. Castanier and W. E. Brigham},
YEAR ={2001},
INSTITUTION ={Stanford University, CA, USA},
KEYWORDS ={Multiphase Flow, Reservoir Definition, In-Situ Combustion,EOR, Heavy Oil},
URL ={http://ekofisk.stanford.edu/supria/publications/public/tr127.pdf},
ABSTRACT ={Supri-A yearly research report.},



}

E. R. Rangel-German and A. R. Kovscek. Experimental and Analytical Study of Multidimensional Imbibition in Fractured Porous Media.. Technical report, Stanford University, CA, USA, October 2001.
[pdf]
Keywords: Imbibition, Fractures, Analytical Model.

Abstract

Capillary imbibition is an important mechanism during water injection and aquifer influx in fractured porous media. Better understanding of matrix-fracture interaction and imbibition in general is needed to model effectively these processes. Using an X-ray computerized tomography (CT) scanner, and a novel, CT-compatible core holder, we performed a series of experiments to study air and oil expulsion from rock samples by capillary imbibition of water in a three-dimensional geometry. The air-water system is useful in that a relatively large number of experiments can be conducted to delineate physical processes. Different injection rates and fracture apertures were utilized. Two different fracture flow regimes were identified. The "filling-fracture" regime shows a plane source that grows in length due to relatively slow water flow through fractures. In the second, "instantly-filled fracture" regime, the time to fill the fracture is much less than the imbibition time. Here, imbibition performance scales as the square root of time. In the former regime, the mass of water imbibed scales linearly with time. A new analytical model is proposed for filling fractures incorporating implicit matrix/fracture coupling. Good agreement is found between experiments and calculation. This analytic coupling was obtained by means of solving the saturation diffusion equation with appropriate initial and boundary conditions. The solution provides the location of the wetting phase front in the fracture and the saturation distribution in the matrix. The solution is analogous to that obtained by Marx and Langenheim (1959) for the areal extent of an equivalent heated zone in thermal recovery methods. Analogous terms among flow and heat transfer in porous media were found and are also presented.

BibTex Entry:

@TECHREPORT{TR129,
TITLE ={Experimental and Analytical Study of Multidimensional Imbibition in Fractured Porous Media.},
AUTHOR ={E. R. Rangel-German and A. R. Kovscek},
YEAR ={2001},
MONTH =oct,
INSTITUTION ={Stanford University, CA, USA},
KEYWORDS ={Imbibition, Fractures, Analytical Model},
URL ={http://ekofisk.stanford.edu/supria/publications/public/tr129.pdf},
ABSTRACT ={Capillary imbibition is an important mechanism during water injection and aquifer influx in fractured porous media. Better understanding of matrix-fracture interaction and imbibition in general is needed to model effectively these processes. Using an X-ray computerized tomography (CT) scanner, and a novel, CT-compatible core holder, we performed a series of experiments to study air and oil expulsion from rock samples by capillary imbibition of water in a three-dimensional geometry. The air-water system is useful in that a relatively large number of experiments can be conducted to delineate physical processes. Different injection rates and fracture apertures were utilized. Two different fracture flow regimes were identified. The "filling-fracture" regime shows a plane source that grows in length due to relatively slow water flow through fractures. In the second, "instantly-filled fracture" regime, the time to fill the fracture is much less than the imbibition time. Here, imbibition performance scales as the square root of time. In the former regime, the mass of water imbibed scales linearly with time. A new analytical model is proposed for filling fractures incorporating implicit matrix/fracture coupling. Good agreement is found between experiments and calculation. This analytic coupling was obtained by means of solving the saturation diffusion equation with appropriate initial and boundary conditions. The solution provides the location of the wetting phase front in the fracture and the saturation distribution in the matrix. The solution is analogous to that obtained by Marx and Langenheim (1959) for the areal extent of an equivalent heated zone in thermal recovery methods. Analogous terms among flow and heat transfer in porous media were found and are also presented.},

}

Y. Wang and A. R. Kovscek. A Streamline Approach for History Matching Production Data.. Technical report, Stanford University, CA, USA, June 2001.
[pdf]
Keywords: History-Matching, Streamlines, Inverse Problems.

Abstract

This study proposes and develops a streamline approach for inferring field-scale effective permeability distributions based on dynamic production data including producer water-cut curve, well pressures, and rates. The streamline-based inverse approach simplifies the history-matching process significantly. The basic idea is to relate the water-cut curve at a producer to the water breakthrough of individual streamlines. By adjusting the effective permeability along streamlines, the breakthrough time of each streamline is found that reproduces the reference producer fractionalflow curve. Then the permeability modification along each streamline is mapped onto cells of the simulation grid. Modifying effective permeability at the streamline level greatly reduces the size of the inverse problem compared to modifications at the grid-block level. The approach outlined here is relatively direct and rapid. Limitations include that the forward flow problem must be solvable with streamlines, streamline locations do not evolve radically during displacement, no new wells are included, and relatively noise-free production data is available. It works well for reservoirs where heterogeneity determines flow patterns. Example cases illustrate computational efficiency, generality, and robustness of the proposed procedure. Advantages and limitations of this work, and the scope of future study, are also discussed.

BibTex Entry:

@TECHREPORT{TR125,
TITLE ={A Streamline Approach for History Matching Production Data.},
AUTHOR ={Y. Wang and A. R. Kovscek},
YEAR ={2001},
MONTH =jun,
INSTITUTION = {Stanford University, CA, USA},
KEYWORDS ={History-Matching, Streamlines, Inverse Problems},
URL ={http://ekofisk.stanford.edu/supria/publications/public/tr125.pdf},
ABSTRACT ={This study proposes and develops a streamline approach for inferring field-scale effective permeability distributions based on dynamic production data including producer water-cut curve, well pressures, and rates. The streamline-based inverse approach simplifies the history-matching process significantly. The basic idea is to relate the water-cut curve at a producer to the water breakthrough of individual streamlines. By adjusting the effective permeability along streamlines, the breakthrough time of each streamline is found that reproduces the reference producer fractionalflow curve. Then the permeability modification along each streamline is mapped onto cells of the simulation grid. Modifying effective permeability at the streamline level greatly reduces the size of the inverse problem compared to modifications at the grid-block level. The approach outlined here is relatively direct and rapid. Limitations include that the forward flow problem must be solvable with streamlines, streamline locations do not evolve radically during displacement, no new wells are included, and relatively noise-free production data is available. It works well for reservoirs where heterogeneity determines flow patterns. Example cases illustrate computational efficiency, generality, and robustness of the proposed procedure. Advantages and limitations of this work, and the scope of future study, are also discussed.},

}

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Last modification: Thu Dec 02 13:25:51 2004
Maintained by Herve Gross, SUPRI-A PhD Candidate.

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Publications of year 2001

Conference articles

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Internal reports

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