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Abstract Fractured reservoirs contain a large fraction of the world supply of oil. For viscous crudes, steam is the most successful technique and field tests indicate that steam has the best potential to recover significant amounts of oil from fractured reservoirs. Unfortunately, there has been little laboratory work done on steam injection in such systems. The experimental system discussed here was designed to understand the mechanisms involved in the transfer of fluids between the matrix rock and the fracture as a result of steam injection. Both continuous and cyclic steam injection experiments were performed on a fractured laboratory system. Saturations were measured in-situ both in the fracture and the consolidated matrix by a CT scanner. The results indicated that there was no steam saturation in the matrix, and that conduction was the dominant heat transfer mechanism. Numerical simulations were used to model both continuous and cyclic steam injection experiments. To model heat losses, heat loss models in the simulator had to be adjusted based on the analysis of the heat losses from the laboratory system with analytical models. After this adjustment, the results from the simulations agreed well with the experiments. When pressure cycling was applied in the simulations with no external heat losses, a considerable amount of steam saturation was observed in the matrix. While the experiments were done with water and steam, simulation runs were also performed for the laboratory system with oil present. Again, steam only flowed in the fracture. Oil recovery was found to be mainly caused by water imbibition into the matrix and heat conduction. Results of this work should be useful in modeling matrix/fracture transfer in dual porosity thermal models. Introduction Fractured reservoirs are estimated to contain 25-30 % of the world supply of oil. Steam injection is required for most of the reservoirs containing heavy oils and tars. There have been quite a few field studies on steam injection for fractured systems (Sahuquet and Ferrier (1982), Britton et al. (1982), Stang and Soni (1987), Closmann and Smith (1983), Duerksen et al. (1984), Couderc et al. (1990) and Hartemink et al. (1995)). Most of these applications were successful or promising. Thomas (1964), Lesser et al. (1966), Abdus Satter (1967) and Satman (1988) presented theoretical models for conduction heating of formations by injecting a fluid through a high permeability streak or fracture. Van Wunnik and Wit (1992) developed a detailed analytical model to study the improvement of gravity drainage by steam injection in a fractured reservoir containing heavy oil. Pooladi-Darvish et al. (1994) developed analytical solutions for heat flow and nonisothermal gravity drainage from a block surrounded by fractures filled with steam. In addition to these analytical models, there have been some numerical simulation studies for steam injection in fractured systems (Pruess and Narasimhan (1985), Lee and Tan (1987), Chen et al. (1987), Pruess and Wu (1989), Nolan et al. (1980), Abad and Hensley (1984), Lin (1988), Briggs (1989), Jensen et al. (1992) and Oballa et al. (1993)). P. 591
- North America > United States > California (0.68)
- North America > United States > Texas (0.46)
SPE Members Abstract To simulate steam injection in fractured reservoirs either double porosity or double permeability models are used. Both incorporate a matrix-fracture transfer term in the mass and energy balances. Due to the difficulty in modeling the physical processes taking place in the matrix-fracture transfer and the lack of experimental data, the matrix-fracture transfer term is not fully understood. This is especially true for nonisothermal processes. Fine grid simulation results are used to design a 3-D laboratory matrix-fracture model to study the matrix-fracture transfer function for steam injection. A CT-Scanner will be used to measure the three-phase insitu saturations in the fractured model. The flow parameters were determined by using the results of several simulation runs. Among these are the steam injection rate, maximum expected pressure in the system and the number and the locations of the injection and production wells needed to clean and saturate the model for each run. Analytical heat transfer models were used to determine the heat losses from the model. Fine grid simulations were used to investigate the sensitivities of the flow process to matrix-fracture properties such as capillary pressure and relative permeability. The simulations showed that matrix water-oil capillary pressure increased the oil recovery by increasing the water imbibition rate. On the other hand, fracture water-oil capillary pressure decreased the water imbibition rate and recovery. Matrix gas-oil capillary pressure had no effect on the oil recovery but fracture gas-oil capillary pressure had a positive effect on recovery by allowing flow of steam into the matrix thus increasing oil mobility. The effect of matrix relative permeability was found to be less important. The experimental design was modified based on the numerical results. We give details of the experimental apparatus and show some results from preliminary experiments. Introduction Several flow models have been developed in the literature for isothermal processes in fractured media. The models used in thermal simulators are generally extensions of models for isothermal processes. They are classified into two groups, dual porosity and dual permeability models. All dual porosity models assume that fractures constitute the main path for fluid flow, and matrix blocks act as sources or sinks to the fracture network. In the basic dual porosity model, matrix and fracture communicate through a single exchange term in the flow equations. Kazemi et al. (1976) developed a three-dimensional 2-phase model to simulate such a fractured system. The simulator equations are two-phase extensions of the single-phase equations derived by Warren and Root (1963) Thomas et al. (1983) extended the model to 3-phase flow in fractured systems. In the multiple interacting continua model which is a different type of dual porosity model, the matrix is divided into nested volume elements which communicate with each other. In this model, flow of fluid and heat between matrix and fracture is transient. The model was first used in geothermal reservoir simulation by Pruess and Narasimhan (1985). Oilman (1986) applied this model to hydrocarbon reservoirs. Pruess and Wu (1989) developed a semianalytical model to simulate matrix-fracture transient flow. Dutra and Aziz, (1992) developed an analytical transfer function, which results in a model having the same form as single-porosity models. P. 49^
Interpretation of Simulated Falloff Tests
Onyekonwu, M.O. (Standford U.) | Ramey, H.J. (Standford U.) | Brigham, W.E. (Standford U.) | Jenkins, R.M. (Consultant)
Abstract A study was made of the method for determination of swept volume and the proper average temperature to use for interpretation of combustion falloff data using the "pseudo-steady" state concept. Two thermal simulators were used for this study to include non-uniform reservoir temperature and variable saturation effects. Skin and storage effects were not included. Interpretation of the data was based on the finding that, because of the very large contrast between the conductivity of gas in the swept volume and that in the unswept sand ahead, transient effects caused by the swept volume would be characteristic of a section of very high transmissivity (kh/w). This implied that the transition period characteristics of the falloff data will form a straight Cartesian line whose slope will be related Lo the swept volume. This follows from the concept of "pseudo-steady" state. Results obtained from the analysis of simulated data showed good agreement between calculated swept volume and actual swept volume. However, the swept volume was found to include both the burned volume and also the high gas saturation zone ahead of the combustion front. Thus a volume correction is necessary to relate the swept volume to the burned volume. In addition, average temperatures within the swept volume were calculated so that the appropriate physical properties can be included in the interpretation. properties can be included in the interpretation. Graphs which can he used to make these corrections are presented for use in interpreting similar field falloff data. Although a one-dimensional radial model was used for this study, the concept should apply in multi-dimensional cases where gravity override is common. Introduction Many authors have applied different methods to estimate the swept or burned volume from pressure falloff analysis. These methods include pressure falloff analysis. These methods include the radius of investigation method, the semilog intersection method, the deviation time method and material balance methods. Some of these methods yield swept volume that is larger than the actual swept volume. Recently, there has been interest in using the "pseudo-steady" state concept, a material balance method, in the analysis of thermal falloff data. Due to the large contrast between the mobility of the gas in the swept volume and the fluid in the unswept sand ahead of the front, the swept sand tends to behave like a large tank closed except at the well. This phenomenon has been observed by Mangold et al in their study of geothermal reservoirs. They observed that the presence of zones of different temperatures in presence of zones of different temperatures in non-isothermal reservoirs, resulted in a fluid mobility contrast that may resemble permeability barriers during well testing. The use of this concept has been investigated with different models. The results show that a typical pressure falloff curve will normally consist of an initial semilog straight line followed by a transition period and finally by a second semilog straight line. The transition period may contain a straight Cartesian line whose slope is related to the swept volume. In the models used in these investigations, several assumptions were made. These include; that the reservoir temperature is uniform, no saturation gradient exists at the front, the gas in the swept volume behaves as fluid of slight but constant compressibility, and the front interface is an isopotential surface. This study used finite difference thermal simulators to relax some of the assumptions, and to evaluate the appropriate temperature to be used in the analysis of combustion falloff data. Also the swept volume as determined from pressure analysis was compared with the actual simulated volume. THEORY The slopes from the initial semilog straight line and the Cartesian straight lines are related to the reservoir and fluid properties by the following equations: (1) P. 205
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Pressure transient analysis (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Drillstem/well testing (1.00)