Experimental Study of the Fracture and Matrix Effects on Free-Fall Gravity Drainage With Micromodels (original) (raw)
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Gas-oil gravity drainage in fractured porous media
Discovery Publication
"Gas-oil gravity drainage is considered to be one of the most dependable recovery mechanisms in naturally fractured reservoirs. The Production mechanism is as a result of the density difference between the phases and capillary contrast between the matrixes and the fractures. This mechanism is contingent upon certain factors such as capillary threshold height and capillary discontinuity, among others. To assess the efficiency and contributions of these factors, a simulation study was carried out on a modeled fractured porous system using ECLIPSE-100 simulator. The results obtained show that oil recovery from a single matrix block (RUN1) was higher than matrix blocks with two, three and five stacks with capillary continuity (RUNS 2, 3 and 4 respectively). Additionally, with capillary discontinuity (RUNS 5, 6 and 7), the results depicted an increase in oil recovery compared to the cases of capillary continuity. However, varying the degree of capillary discontinuity with the respective matrix block stacks in the fractured model yielded no significant increase in oil recovery. Thus, the results show that while both capillary threshold height and capillary discontinuity remain significant factors in gas-oil gravity drainage, capillary continuity and varying the degree of discontinuity between the matrixes degree has little or no effect on this recovery mechanism in fractured porous media. Keywords: Gas-oil gravity drainage, Oil recovery, Capillary threshold height, Capillary discontinuity, Fractured porous media."
Journal of Petroleum Science and Engineering, 2012
Spontaneous water imbibition into the matrix blocks is known as the main mechanism for increased oil recovery from naturally fractured oil reservoirs. The rate of oil recovery and its ultimate value is mostly affected by wettability of the rocks and their pore structure. Oil viscosity also greatly influences the rate of oil recovery. A novel experimental model was utilized to study the imbibition mechanism under different wettability conditions. Matrix blocks made from different grain types and size distributions of glass beads were saturated with two different types of synthetic oil, to mimic the oil-saturated matrixes. The wetting characteristic of the models used in this study were altered by a standard chemical treatment process. Wettability, grain type and size distribution, as well as oleic phase properties, were changed to find the effect of these parameters on oil recovery efficiency.
Pore-Level Observation of Free Gravity Drainage of Oil in Fractured Porous Media
Transport in Porous Media, 2011
This work presents results from two sets of experiments conducted to study, in pore level, the role of fracture aperture and tilt angle on the stability of liquid bridges and the shape of a front during free gravity drainage process. Glass micromodels of two different aperture sizes were used to monitor the mechanism of gravity drainage of air-crude oil system, rotating around a bottom corner to create different tilting angles. Oil content within the matrix blocks was determined as a function of time using a series of images obtained during the experiments, from which net drainage rate from the upper and lower matrix blocks is calculated. Liquid bridges are more frequent but less stable at early time of drainage. The liquid bridges, which have widths as thin as 50 µm, can resist instability to maintain continuity. Liquid bridges formed in stacks with higher tilt angles are more stable, enhancing oil drainage from the upper matrix block and causing higher recoveries. Quantitative analysis of the results shows that a wider fracture aperture increases the oil production rate, but reduces the ultimate recovery. Furthermore, stacks with higher tilt angles present larger ultimate recoveries and smaller production rates. The front geometry in the lower block deviates from linearity due to formation of liquid bridges in the middle fracture. The results of this work can be helpful to better understand the interaction between fractures and matrix blocks.
Journal of Canadian Petroleum Technology, 2010
Summary Miscible injection of carbon dioxide has seen a significant increase in interest for the purpose of enhanced oil recovery (EOR) in conventional oil reservoirs. However, naturally fractured reservoirs, which are among the largest oil reserves in the world, are considered poor candidates for this process because of presumed low-performance efficiency. This paper presents the results of an experimental study that explains the effect of connate water saturation, matrix permeability and oil viscosity on the performance of gravity drainage from the matrix (into fracture) when it is surrounded by a CO2-filled fracture. Experiments were performed in an experimental model under different operating pressures to cover both immiscible and miscible conditions. Experiments were conducted using synthetic oil (nC10) and light crude oil in two Berea cores having large differences in permeability. In addition, the effect of connate water saturation was studied by performing experiments in an ...
Experimental Investigation of Tertiary Oil Gravity Drainage in Fractured Porous Media
Special Topics & Reviews in Porous Media - An International Journal, 2010
The amount of residual oil trapped in the matrix of a fractured reservoir after water drive, either natural water drive or water injection, depends on the wettability of the matrix rocks. Gas oil gravity drainage (GOGD) has been proposed as the tertiary oil recovery process for this type of oil reservoir. The current work focuses on experimental investigation of tertiary GOGD in fractured porous media under different types of matrix wettability. Results of a set of experiments performed in artificial porous media composed of sand packs and glass beads of different wettability have been used to check the GOGD rate and the ultimate oil recovery for previously waterflooded models. A novel experimental setup to study flow behavior in fractured artificial porous media was designed to perform the GOGD process. Results show that tertiary gravity drainage increases oil recovery efficiency from a fractured matrix block, which also depends on the post-waterflood residual oil saturation and the wettability of the medium. Observation of the gas front location and oil recovery profile with time during the tertiary recovery stage reveals that the oil recovery of post-waterflood residual oil in a fractured matrix block starts before stabilization of the gas-liquid front. Oil recovery mechanisms similar to those presented in tertiary GOGD recovery in a conventional reservoir are proposed to explain the gradual drainage of matrix blocks in a naturally fractured stratum.
Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2014
During miscible displacements in fractured porous media, one of the most important factors that plays a significant role in oil production is the matrix-fracture interaction. In this work, a series of hydrocarbon injection experiments have been performed on a fractured glass micromodel that was designed specifically to study matrix-fracture interaction. A high quality image analysis method was used to determine the fluid flow behavior, solvent front movement, and viscous fingering associated with solvent movement in matrix and fractures. Observations showed that in the case of unit viscosity ratio, the injection rate increased the slope of recovery curve and consequently improved the final oil recovery. However, when using a viscosity ratio of 65, the injection rate increased the oil recovery at earlier times due to the breakthrough and fracture drainage. At later times, diffusion and dispersion became dominant and oil recovery decreased. Studying the effect of molecular diffusion revealed that by using an optimum solvent the oil recovery at both early and late times increased. Fingering phenomena appeared in the matrix as fingers and in the matrix as channeling, which drained fracture in early times of the process. In higher viscosity ratios, dispersion dominantly takes place in an upward direction, however, in the lower viscosity ratio the effect of fracture in flooding decreased and dispersion takes place in a left-to-right direction. It can be concluded that during miscible displacement in fractured porous media dispersion in fracture is directly proportional to viscosity ratio while dispersion in matrix is inversely proportional to it. The results of this work are helpful in understanding the matrix-fracture interaction, the effect of injection rate, molecular diffusion, and viscous fingering behavior, which are crucial for accurate prediction of oil recovery in fractured reservoirs during miscible displacements.
Empirical Modeling of Gravity Drainage in Fractured Porous Media
Energy & Fuels, 2011
Gravity drainage is considered to be the main mechanism in primary oil production from naturally fractured reservoirs, but mathematical models to adequately predict the oil recovery and flux rate between the matrix and fracture network under gravity drainage are rarely described in the literature. To address this lacuna, gas-oil contact movement and oil recovery rates in a thin glass-bead-packed simulator were measured, allowing for the capture of information about the matrix-fracture fluidtransfer process. A two-dimensional mathematical model was developed to numerically simulate the process under the same conditions as the experiments, and then empirical models were proposed for oil production in such fractured systems because the final liquid recovery was found to be correlated to dimensionless groups, such as the Bond number. The empirical model approach was then extended to predict the matrix-fracture liquid-transfer rate during the free-fall gravity drainage process. On the basis of experimental data and empirical correlations, the matrix-fracture liquid flux rate appears to be proportional to the liquid level difference in the matrix and fracture. These correlations were tested against numerical simulation results and actual field data of oil production by free-fall gravity drainage. The empirical models have been judged to perform acceptably in the prediction of the oil production and fluid-transfer rate in the oil-gas gravity drainage cases studied.
Experimental investigation of secondary and tertiary oil recovery from fractured porous media
Journal of Petroleum Exploration and Production Technology, 2013
Naturally fractured reservoirs (NFRs) contribute in large extent to oil and gas production to the ever increasing market demand of fossil energy. It is believed that the vertical displacement of oil during gas injection assisted by gravity drainage (GAGD) is one of the most efficient methods for oil recovery in these reservoirs. Hence, in this work, unconsolidated packed models of cylindrical geometry surrounded by fracture were utilized in order to perform a series of flow visualization experiments during which the contribution of different parameters such as the extent of matrix permeability, physical properties of oil (viscosity, density, and surface tension) and the withdrawal rate was studied. Furthermore, mutual effects of permeability, oil properties, and production rate on oil recovery efficiency through controlled and free fall gravity drainage processes were also investigated. Experimental results obtained from secondary and tertiary recovery experiments demonstrated that decreasing model permeability and increasing oil viscosity during secondary recovery process reduced the recovery efficiency for all production rates, while under tertiary recovery process these phenomena lead to more oil production for all production rates.
Journal of Petroleum Exploration and Production Technology
Oil extraction often accompanied with relatively large amounts of water production causes several environmental and mechanical problems, particularly in oil-fractured reservoirs. One way to alleviate this problem is to use gels as a blockage agent. In this study, several micromodels with different geometries of fractures including a simple fracture, step fracture, fracture with variable mouth and fracture with a tiny crack were prepared. The amount of oil recovery was measured from these micromodels in the presence or absence of gel. In situ and preformed particle gels (PPGs) were injected into the micromodels to compare their ability in blocking fractures and increasing oil recovery. The BaCl 2 •2H 2 O salt solution was used to swell PPGs. The amount of salt concentration was optimized so that PPGs had maximum ability to block fractures. Several PPG concentrations, 1000, 2000, and 3000 ppm, were examined to optimize the amount of PPGs, which have to be injected into the micromodels. Injection flow rate was optimized so that salt water could efficiently sweep the matrix structure. Results showed that both PPG and in situ gels have a considerable effect on decreasing water production and increasing oil extraction. However, PPGs were superior in comparison to in situ gels to increase oil recovery. It was found that increasing salt concentration decreases the swelling percentage of PPGs. The optimum amount of injection flow rate was found to be 0.1 mL/hr for all the prepared micromodels. The optimum concentration of PPGs was different for each micromodel.
Gravity-Enhanced Transfer between Fracture and Matrix in Solvent-Based Enhanced Oil Recovery
Solvent injection has been considered as an efficient method for enhancing oil recovery from fractured reservoirs. The success of this method therefore depends on the degree of enhancement of the mass exchange rate between the solvent residing in the fracture and the oil residing in the matrix. If the mass transfer would be solely based on diffusion, oil recovery would be unacceptably slow. A series of soak experiments have been conducted to investigate the mass transfer rate between the fracture and the matrix. In a soak experiment, a porous medium containing oil is immersed in an open space containing the solvent to simulate the matrix and the fracture, respectively. We use a CT scanner to visualize the process. The experimental data are compared with a simulation model that takes diffusive and gravitational forces into account. We find that the initial stage of all experiments can be described by a diffusion-based model with an enhanced "effective diffusion coefficient". In the second stage enhancement of the transfer rate occurs due to the natural convection of solvent in the fracture. The experiments are quantitatively modeled by numerical simulations. We find that transfer rates depend on the properties of the rock permeability, the viscosity and the density of solvent and oil. The gravity enhanced transfer is quantified by comparison of experimental and simulated results.