Free Fall Gravity Drainage Research Articles

New empirical scaling equations for oil recovery by free fall gravity drainage in naturally fractured reservoirs

Gas-oil gravity drainage is a recognized major contributor to production in fractured reservoirs. While various empirical and analytical methods have been proposed to model this process, many of them contain assumptions that are questionable or require parameters that are not accessible at the field level. The aim of this work is to provide new, easy-to-use scaling equations for estimating the recoverable oil through gravity drainage in naturally fractured reservoirs, considering the effects of resistance capillary pressure. To accomplish this, data from four oilfields undergoing gravity drainage, including rock properties (eight sets), block height (three sets), and fluid properties (four sets), were used to generate a wide range of recovery curves using a single porosity numerical simulation model. Aronofsky's and Lambert's functions were then utilized to match the generated recovery curves. Statistical analysis revealed that the Aronofsky's function is more accurate in replicating the recovery patterns, while the Lambert's function tends to overestimate the early-time oil recovery and underestimate the oil recovery at a later stage in the majority of cases. A sensitivity analysis was subsequently performed, revealing that parameters such as absolute permeability, viscosity of oil, height of block, gas and oil density, characteristics of relative permeability and capillary pressure curves and interfacial tension (IFT) influence the amount of time taken to achieve the final recovery. Of these parameters, absolute permeability has the most significant effect on the amount of time needed to attain the final recovery, while the effect of difference between oil and gas densities is the lowest. Consequently, two different expressions were developed using nonlinear multiple regression analysis of simulated gravity drainage data which can be combined with the Aronofsky model to substitute the rate convergence constant. The new scaling equations include the effects of capillary pressure and other relevant factors in gravity drainage simulations. Both forms show satisfactory accuracy, as evidenced by the statistical parameters obtained (R2 = 0.99 and MSE = 0.0019 for both established correlations). The new correlations were verified using a wide range of oilfield data and are expected to provide a better understanding of the recovery process in naturally fractured reservoirs.

Experimental evaluation of Carbon Dioxide-Assisted Gravity Drainage process (CO2-AGD) to improve oil recovery in reservoirs with strong water drive

The Gas-Assisted Gravity Drainage (GAGD) process has been developed to improve and enhance oil production in both secondary and tertiary recovery stages. In the GAGD process, oil is produced through horizontal wells above the oil-water contact. Therefore, high levels of water cresting are probably happening, especially in reservoirs with active water aquifers. However, all the previous GAGD feasibility experimental studies have never been addressing the existence of active water aquifers. Consequently, the current research was conducted to test the immiscible GAGD process feasibility to improve oil recovery and minimize water cut in brown reservoirs with strong water coning tendencies.A Hele-Shaw model consists of two parallel glass plates packed with silica sand to visually discern the process performance. Specifically, the CO2-AGD process feasibility versus the Free Fall Gravity Drainage (FFGD) mechanism was investigated on the physical model with large, moderate, and without bottom water drive. The experimental results indicated that the GAGD process performance is higher performance than the FFGD in all runs because of its fast high oil recovery, especially when implemented on the reservoir without a bottom water drive. The immiscible CO2-AGD process is not only important in terms of increasing oil recovery, but it also significantly delays the gas breakthrough. The strength of the aquifer inversely impacts the gas breakthrough time as increasing the aquifer strength results in fast oil production and early gas breakthrough. Additionally, the immiscible GAGD experiments illustrated that oil recoveries in the FFGD and GAGD processes improved to 35% and 47% of OOIP for the model without bottom water drive, respectively. However, the oil recovery increased in the same FFGD and GAGD processes to 61% and 63% of OOIP for the model with a large bottom water drive, respectively. Specifically, the ultimate oil recovery during the GAGD process with a large bottom water drive was higher than the FFGD run with only 2%; while there is increasing in ultimate oil recovery for the model without the bottom water drive of about 12%. Consequently, the GAGD process is more efficient to improve oil recovery in reservoirs with limited or without bottom water drive.

New insights into forced and free fall gravity drainage performance in a fractured physical model

Gravity is one of the major driving forces for fluid flow in naturally fractured reservoirs. Gravity drainage appears to be a vital recovery mechanism over the oil production process in a majority of fractured reserves. In this paper, free-fall gravity drainage (FFGD) and forced gravity drainage (FGD) production approaches are investigated through employing a new fractured sand pack with a rectangular geometry. Distilled water and condensate are used as the wetting phase. Nitrogen, carbon dioxide, and air are also utilized as the non-wetting phase. A systematic sensitivity analysis is conducted to find the effects of the key factors such as fracture blockage, injection rate, dip angle, and type of wetting phase and injection fluid on the performance of FFGD and FGD processes. Also, a numerical simulation is performed to simulate the experiments. Results reveal that the FGD has a better performance under controlled process conditions, compared to the FFGD so that gas injection with a controlled rate improves the recovery. Moreover, it was found that increasing the gas injection rate improves the recovery factor (RF) of wetting-phase when the up and bottom sides of the fractures are blocked; the RF is decreased in the case with open fracture sides. The recovery of the wetting phase decreases in both FFGD and FGD modes when the porous system has a deviation from the vertical orientation. According to the study outcomes, the immiscible gas-assisted gravity drainage offers a higher condensate recovery, compared to water recovery, due to lower viscosity and interfacial tension (IFT) of the condensate phase. The comparison of the experimental data and simulation results implies that the simulation approach can satisfactorily predict the performance of the FGD process. Based on the error analysis, there is a good match between the simulation results and the experimental measurements; the fluid pair of CO2-condensate exhibits the highest accuracy with an R-squared of 0.9961 and an average relative error (ARE) of 0.0054.

Experimental and Modelling Study of Gravity Drainage in a Three-Block System

Gravity drainage is known as the controlling mechanism of oil recovery in naturally fractured reservoirs. The efficiency of this mechanism is controlled by block-to-block interactions through capillary continuity and/or reinfiltration processes. In this study, at first, several free-fall gravity drainage experiments were conducted on a well-designed three-block apparatus and the role of tilt angle, spacers’ permeability, wettability and effective contact area (representing a different status of the block-to-block interactions between matrix blocks) on the recovery efficiency were investigated. Then, an experimental-based numerical model of free-fall gravity drainage process was developed, validated and used for monitoring the saturation profiles along with the matrix blocks. Results showed that gas wetting condition of horizontal fracture weakens the capillary continuity and in consequence decreases the recovery factor in comparison with the original liquid wetting condition. Moreover, higher spacers’ permeability increases oil recovery at early times, while it decreases the ultimate recovery factor. Tilt angle from the vertical axis decreases recovery factor, due to greater connectivity of matrix blocks to vertical fracture and consequent channelling. Decreasing horizontal fracture aperture decreases recovery at early times but increases the ultimate recovery due to a greater extent of capillary continuity between the adjacent blocks. Well match observed between the numerical model results and the experimental data of oil recovery makes the COMSOL multiphysics model attractive for application in multi-blocks fractured systems considering block-to-block interactions. The findings of this research improve our understanding of the role of different fracture properties on the block-to-block interactions and how they change the ultimate recovery of a multi-block system.

Application of a Novel Ultrasonic Technology to Improve Oil Recovery with an Environmental Viewpoint

It is proven by recent studies that sonication has a positive influence over the oil flow within the porous media. Accordingly, the researchers in this paper evaluated the influence of sonication over the oil recovery by means of free fall gravity drainage. Furthermore, the influence of sonication on the oil permeability was assessed in three samples that had different bead size in average. By use of the Hagroot backward method and Matlab simulation, the optimal petrophysical situation for sonication was determined. The authors concluded that sonication positively affects the oil recovery for the non-asphaltenic samples, while it has a reverse effect on the asphaltenic samples because of increasing the viscosity in long-term. Furthermore, it was witnessed that gravity drainage was heightened by increase of beads’ size in the non-asphaltenic sample. Accordingly, this mechanism can be useful in oil recovery by means of gravity drainage, specifically in fracture reservoirs.

Residual liquids saturation development during two and three phase flow under gravity in square capillaries at different temperatures

An experimental study on heavy oil with air (two phase flow) and water and air (three phase flow) at different temperatures was carried out in square capillaries under gravity drainage conditions. Fluid retention characteristics (in the corners of capillaries) were determined and evaluated using the trapping number (NT). In air–heavy oil systems, when NT<2.7E−2, the residual oil saturation (Sor) was constant and equal at 55 and 85°C. The Sor was controlled by capillary forces regardless of viscous and gravity forces, including free fall gravity drainage (FFGD). For higher NT, the Sor was a function of competition between gravity, viscous and capillary forces. The Sor was always higher at 85°C compared to 55°C for the same gas injection rate and the difference increased as the NT augmented. FFGD experiments demonstrated that heavy oil retention depended on the Bond number and increased linearly as the Bond number increased.In the three phase systems (air–heavy oil–water) the oil retention did not diminish with the presence of water, which was also constant for the entire interval of NT at 55 and 85°C. High viscous forces originated from heavy oil were responsible for no change in the Sor. However, due to the water-wet nature of the capillary tubes, water was not completely swept. More Sor and residual water saturation were observed in air–water–heavy oil configuration, especially at 85°C, due to the unfavorable viscosity ratio oil/water and the negative spreading coefficient of water in oil in the presence of air. Finally, the change of wettability from water wet to oil wet did not modify the Sor but the water saturation decreased slightly for air–heavy oil–water systems.

Experimental Study of the Fracture and Matrix Effects on Free-Fall Gravity Drainage With Micromodels

Summary Free-fall gravity drainage (FFGD) is the main production mechanism in the gas-invaded zone of fractured reservoirs. The gravity and capillary forces are two major forces that control the production performance of a fractured system under an FFGD mechanism. Gravity force acts as a driving force to remove oil from the matrix block whereas the resistive capillary force tends to keep oil inside the matrix. In this study, a series of experiments was performed to study the effects of the geometrical characteristics of the fracture and matrix on the oil-production rate under an FFGD mechanism by use of a glass micromodel. The oil-recovery factor (RF) was also obtained for a single matrix block by use of different patterns. Results from the experiments show that different flow regimes occur during the production life of a single matrix block under a FFGD mechanism. The fluid flow is controlled by the capillary-dominated regime at the early stage and late time of production life, whereas it shows a stabilized bulk flow under a gravity-dominated regime is exhibited at other times. Experimental results revealed that for a narrow fracture opening, fracture capillary pressure has a form similar to that of the matrix block. Also, it was observed that the oil-production rate and RF of the matrix block decreased as the permeability ratio between two media (matrix block and fracture) increased. Lower production rate is achieved in larger-fracture-spacing micromodels. In addition, wider vertical fractures lead to an early breakthrough of gas in bottom horizontal fracture that makes up the main portion of oil traps in the matrix block, and this reduces the RF. Results from this study show that in a heterogeneous layered matrix block, both the drainage rate and RF decrease in comparison with a homogeneous matrix block. Finally, a multiple linear-regression analysis was performed to understand the dimensionless groups affecting the RF of the FFGD process. It was found that the Bond number cannot truly describe the process and other parameters such as the fracture-/matrix-permeability ratio; fracture spacing and fracture opening should also be considered.

An Estimation of Wave Attenuation Factor in Ultrasonic Assisted Gravity Drainage Process

It has been proved that ultrasonic energy can considerably increase the amount of oil recovery in an immiscible displacement process. Although many studies have been performed on investigating the roles of ultrasonic waves, based on the best of our knowledge, little attention has been paid to evaluate wave attenuation parameter, which is an important parameter in the determination of the energy delivered to the porous medium. In this study, free fall gravity drainage process is investigated in a glass bead porous medium. Kerosene and Dorud crude oil are used as the wetting phases and air is used as the non-wetting phase. A piston-like displacement model with considering constant capillary pressure and applying Corey type approximation for relative permeabilities of both wetting and nonwetting phases is applied. A pressure term is considered to describe the presence of ultrasonic waves and the attenuation factor of ultrasonic waves is calculated by evaluating the value of external pressure applied to enhance the flow using the history matching of the data in the presence and absence of ultrasonic waves. The results introduce the attenuation factor as an important parameter in the process of ultrasonic assisted gravity drainage. The results show that only a low percentage of the ultrasonic energy (5.8% for Dorud crude oil and 3.3% for kerosene) is delivered to the flow of the fluid; however, a high increase in oil recovery enhancement (15% for Dorud crude oil and 12% for Kerosene) is observed in the experiments. This proves that the ultrasonic waves, even when the contribution is not substantial, can be a significantly efficient method for flow enhancement.

Analytical Modeling of Oil Production from a Matrix Block by Free Fall Gravity Drainage Mechanism

Free fall gravity drainage is the most effective mechanism in gas invaded zone of fractured reservoirs. Although several analytical models have been proposed to characterize this mechanism, most of them suffer from inadequate reality, such as neglecting capillary pressure. In this study, a new analytical model was proposed to predict the oil recovery versus time for a homogeneous matrix block under a free fall gravity drainage mechanism. Considering the effect of viscous, gravity as well as capillary forces, the model was developed. This model is applicable to different conditions of gravity and capillary force, as well as when both forces are active. Along with core scale experimental data available in the literature, a series of micro model experiments was also conducted and used to check the model's validity. In addition, a synthetic computer model was constructed and used for further model validation. Results showed that the suggested model predicts the physics of the gravity drainage mechanism very well. The results of this work can be helpful to develop a new transfer function for gravity drainage process modeling in fractured reservoir.

Effect of Temperature on VAPEX Performance

Summary Incorporating some heat injection along with solvent injection appears to bethe most viable option for improving the oil-drainage rate of vapour-assistedpetroleum extraction (VAPEX) in extraheavy-oil formations. This study wasintended to quantify the maximum possible increase in VAPEX drainage rate thatcan be obtained by heating the formation to a target temperature. Theexperimental phase of this study involved conducting VAPEX experiments in alarge high-pressure physical model, packed with 250-darcy sand, using propaneas the solvent. The physical model was preheated to 40, 50 and 60°C, andpropane was injected at the same test temperature but different injectionpressures to observe how injection pressure affects oil-drainage rate atelevated temperatures. In the experiments at elevated temperatures, but withoutincreasing the injection pressure, higher rate of oil production was achievedin the early stages of the process. However, a stabilized rate of oilproduction did not show pronounced improvement caused by a lower solubility ofpropane in the oil at higher temperatures. Increasing injection pressure alongwith increasing the test temperatures was successful in accelerating the oilproduction. The oil used in these experiments was found to become mobile withthe increase in temperature even without solvent dissolution. As a result, thetotal rate of oil production appeared to be controlled by two mechanisms: (1)by solvent dissolution and oil mobilization at the boundaries of the vapourchamber and (2) by pure free-fall gravity drainage beyond the vapour chamberwherever gravity head was sufficient to push the mobile oil toward theproduction well. The results of this these tests define the upper limit of oilrates achievable with heated solvent injection. They can also be used to assessthe applicability of VAPEX to warm reservoirs naturally (e.g., in Venezuela)and reservoirs with mobile oil in place.

The Proper Simulation of Free Fall Gravity Drainage in the Commercial Simulator Environment

Free fall gravity drainage is one of the most efficient mechanisms identified in the gas-invaded zone of naturally fractured reservoirs. It is believed that gravity drainage could result in very high displacement efficiency. In spite of this importunity, there is a lack of theoretical, experimental, and simulation works on investigation of this recovery process. In this study, a new approach is proposed for simulation of free fall gravity drainage called tank model approach. The commercial numerical simulator could be used as a platform to be able to perform this new approach properly. A simulator is selected as an appropriate environment since its fine grid simulation will lead to obtain the most proper results. The fine grid simulation as a numerical approach includes fewer simplifications comparing with available mathematical modeling and, consequently, the results are more accurate by considering numerical models. As a result, the tank model approach is properly performed for simulating the free fall gravity drainage process. Eventually, having the proper simulation of free fall gravity drainage could lead to obtaining a powerful tool for verifying the mathematical modeling introduced for description of this process. In addition, it could be used for statistical analysis in order to achieve better understanding of the block to block interaction, such as re-infiltration and capillary continuity phenomenon.

Experimental investigation of secondary and tertiary oil recovery from fractured porous media

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.

An Experimental Investigation of the Effect of Fracture Dip Angle on Oil Recovery and Drainage Rate in Free Fall Gravity Drainage in Fractured Reservoirs Using a Glass Micromodel (A Pore Level Investigation)

Gravity drainage is the main production mechanism in the gas invaded zone in naturally fractured reservoirs. However, there are large ambiguities and complexities, resulting from the dynamic of oil depletion from matrix blocks toward the fracture network. Visualization of drained oil at pore scale using glass micromodels provides the opportunity to better understand the effects of different parameters which might affect oil recovery from fractured reservoirs. In this work a micromodel apparatus generated by laser etching is used to perform some gravity drainage tests on the network patterns. The experiments were performed on double block systems using crude oil. The block to block interactions and the formation and changes of capillary bridges between matrix blocks were visually investigated. A suitable empirical model of gravity drainage is used for the prediction and evaluation of experimental data. The empirical model will be used for sensitivity analysis of fracture dip angle which might affect oil recovery efficiency and oil drainage rate, and as seen in experiments this parameter has a direct relationship to and positive effect on oil recovery. We also found the important rule of the block to block effect on the recovery of blocks and the higher recovery of upper matrix blocks than lower ones shows this reality; also it was claimed that the threshold height of fractured reservoir blocks doesn't show the true matrix capillary pressure of matrices, because of the block to block effect that helps the gravity forces in the matrix draining procedure. By increasing the block tilting angle up to α = 90 – tan – 1(y/x) as y and x are the 2-dimensional block sides lengths, the recovery of the block will increase but its initial drainage rate will decrease. It's a result of an increase in effective block height and increasing the capillary bridges stability which increases recovery, and the decrease of rate is due to capillary bridges frequency decreasing, which decreases the area open to flow.

An Experimental Investigation of Fracture Tilt Angle Effects on Frequency and Stability of Liquid Bridges in Fractured Porous Media

Liquid bridges are believed to play an important role in improving the recovery of fractured reservoirs. However, little is known about the stability of liquid bridges in fractured media at the pore scale. In this work, a glass micromodel representing a stack of two blocks was used at different tilt angles to monitor the frequency and stability of liquid bridges formed during free-fall gravity drainage as a function of tilt angle. It was observed that by increasing the tilt angle, the liquid bridge frequency decreased but its stability increased. This resulted in higher ultimate recovery. In addition, it was found that during the first half of the experiments, the number of bridges was higher but their stability was lower than during the second half of the tests. Moreover, no more than one stable liquid bridge was observed at tilt angles above 20°, and the bridge cross-sectional area was gradually decreased as the stability was maintained. A sequence of bridges that were formed and broken one after the other results in a higher drainage rate than a single bridge with stability equal to the overall stability of the sequence.

EXPERIMENTAL INVESTIGATION OF WETTABILITY EFFECT AND DRAINAGE RATE ON TERTIARY OIL RECOVERY FROM FRACTURED MEDIA

Vertical displacement of oil by gas is one of the most efficient methods for oil recovery from naturally fractured reservoirs. Unlike the homogeneous media, the ultimate oil recovery by gravity drainage in fractured media is more dependent on the production rate. Hence finding the optimum production rate for more oil recovery with respect to the properties of media seems to be essential. In this work, unconsolidated packed models of cylindrical geometry surrounded by fractures were utilized to perform a series of flow visualization experiments during which the contribution of different parameters such as the extent of matrix wettability and the withdrawal rate were studied. In addition, mutual effects of wettability and production rate on tertiary oil recovery efficiency through controlled and free fall gravity drainage processes were also investigated. Experimental results obtained from tertiary gravity drainage experiments demonstrated that just before gas breakthrough, lower withdrawal rates facilitate the tertiary oil recovery under the film flow mechanism, which leads to a higher ultimate recovery factor. However, after gas breakthrough, monitoring oil recovery by gravity drainage showed that higher production rates recovered more oil. Furthermore, under tertiary recovery processes in low-production cases, oil-wet systems achieved higher recovery factors, while at high withdrawal rates, more oil was recovered for 50% oil-wet media.

Effect of Wettability in Free-Fall and Controlled Gravity Drainage in Fractionally Wet Porous Media with Fractures

Wettability was found to significantly influence the oil recovery behavior in heterogeneous porous media in gravity drainage processes. Despite the considerable efforts conducted on this issue, there are still challenging aspects remaining. The objective of this study is to investigate the wettability effects in fractured and homogeneous porous media during free-fall gravity drainage (FFGD) and controlled gravity drainage (CGD) conditions. Different test fluids (water and Varsol oil) were employed for completely water-wet, completely oil-wet, and fractionally wet porous media in this research. This experimental work enabled us to capture important characteristics of gravity drainage processes such as the production performance, matrix–fracture flow communication, and liquid production rate by the film-flow mechanism. It was concluded that the physical properties of the test fluids and the fractional wettability composition of the porous medium govern the fluid recovery mechanism during FFGD and CGD processes. The oil production rate by film flow was found to be appreciably affected by the packing composition when water-wet and oil-wet beads were randomly mixed. A percolation threshold of oil-wet beads was obtained at about 63%, after which no further change was experienced in the production rate magnitude when the film flow was a dominant recovery mechanism. Moreover, the results revealed that the wetting properties of the test fluid considerably affected the matrix-to-fracture transfer and the fluid saturation. The water-wet beads condition was favored in oil recovery for both FFGD and CGD processes. The oil-wet conditions provided greater film-flow rates and matrix-to-fracture transfer rates.

Laboratory Investigation of Free Fall Gravity Drainage in Fractured Porous Systems Using Unconsolidated Macromodels

Naturally fractured reservoirs (NFRs) contain a significant amount of the world's oil reserves. Oil recovery by gravity drainage in a NFR strongly depends on the capillary height of the porous medium. Capillarity and gravity forces are usually the major driving forces in NFRs. To address this issue, a series of flow visualization experiments were performed using unconsolidated packed models of rectangular geometry with two fractures on the side. Parametric sensitivity analyses were performed considering effects of different system parameters such as fracture aperture, matrix height and permeability, and fluid viscosity on liquid drainage rate. These experiments enabled us to capture some aspects of the flow communication between matrix block and fracture during gravity drainage. Results from this study showed that the rate of liquid flowing from matrix to fracture is proportional to the difference of liquid levels in the matrix and in the fracture (H f ― H m ). In addition, the characteristic rate and the maximum liquid drainage rate from these fractured models are determined for stable gravity-dominated processes. Also, it was concluded that the characteristic rate depends only on the dimensions of the fracture and properties of the test fluid, and not on the properties of the matrix. For a given fracture―matrix system with different initial liquid saturation conditions, it is observed that the production history can be correlated by plotting the fraction of recoverable liquid as a function of time. Furthermore, ultimate recovery factor and capillary threshold height can be correlated using dimensionless numbers such as the Bond number and the porosity ratio.

Modeling of oil production by gravity drainage

Gravity drainage is one of the most effective mechanisms of developing an oil field because of high oil recovery yielded. There have been several models to characterize and model the process of gravity drainage. However the existing models do not work well in many cases. In this study, an analytical model was developed to determine the ultimate oil recovery by free-fall gravity drainage. This model demonstrates clearly the effect of capillary pressure on ultimate oil recovery. The ultimate oil recovery increases with the decrease in entry capillary pressure and the increase in pore size distribution index. The model was modified based on the ultimate recovery model to match and predict oil production. The model was tested against experimental, numerical, and field data of oil production by free-fall gravity drainage. The results demonstrated that the modified oil recovery model could work satisfactorily in the oil–gas cases studied. Initial oil production rate, entry capillary pressure, and average residual oil saturation can be estimated using the oil recovery model. An approach was also developed to infer capillary pressure curves from the oil production data by free-fall gravity drainage.

The Prediction of Oil Recovery by Gravity Drainage

An approximate theory of free-fall gravity-drainage recovery is expanded to account for residual oil saturation. The recovery equation is derived and seven comparisons are made of observed gravity-drainage recovery with calculated recovery. Examples show how the theory can be applied. Introduction The gravity-drainage mechanism is one of the most efficient ways of producing an oil field. Unfortunately, most oil fields cannot be produced economically under free-fall gravity alone because the effective oil permeability is too low, the oil viscosity is too high, or the dip of the formation is too small. Nevertheless, many reservoirs take advantage of the gravity component, especially under pressure maintenance in areas where gas is available, such as overseas, or where inert gas can be used. The most ideal system would be one where oil is produced only under free-fall gravity drainage, giving the highest possible recovery at the most efficient rate. However, because a producing mechanism must be justified economically as opposed to another producing mechanism must be justified economically as opposed to another producing mechanism, an engineer must calculate gravity-drainage recovery producing mechanism, an engineer must calculate gravity-drainage recovery as a function of time.An approximate theory of strict gravity drainage has been presented by Cardwell and Parsons. They ignored capillary pressure terms and show how gravity-drainage equations can be solved. They give only one comparison of recovery calculated from theory with recovery obtained experimentally from a draining column.In addition to that study, many other papers have been published describing how to calculate reservoir performance where gravity effects influence the production mechanism. An early investigation describing the potentially high recovery with gravity drainage is Katz's discussion of potentially high recovery with gravity drainage is Katz's discussion of Oklahoma City Wilcox sand. Katz also presented data on the low (6 to 8 percent) residual oil saturations that could be obtained from a completely percent) residual oil saturations that could be obtained from a completely drained portion of the Wilcox sand. Lewis enlarged upon the gravity- drainage mechanism for oil recovery and gave four examples of gravity- drainage fields. Beginning in 1949, more studies discussed methods of calculating the effect of gravity component on oil recovery during depletion or pressure maintenance, and case histories of fields where gravity was an important factor in production. All the papers describing a calculating procedure used the Buckley-Leverett method. These studies also used historical rates of production or a pre-selected rate of production to get the advance of gas-oil front and to calculate recovery. More recently, high-speed computers were used to solve mathematical equations of various reservoir simulators. These reservoir simulators still required that rates or drawdown be pre-selected as a boundary condition to the solutions of mathematical equations. Thus, these simulators could not be used to determine the free-fall, gravity-drainage rate of production.One study attempted to determine production rate based on the location of the gas-oil front below a stated level. Essley et al. determined an approximate producing rate by arbitrarily assuming a maximum capacity for each well remaining on production below a given subsea level and determining the total production rate from that. They did not state how the arbitrary assumption was made, nor how this rate was related to a true gravity-drainage rate. JPT p. 818