We present an analytical solution to estimate the minimum polymer slug size needed to ensure that viscous fingering of chase water does not cause its breakdown during secondary oil recovery. Polymer flooding is typically used to improve oil recovery from more viscous oil reservoirs. The polymer is injected as a slug followed by chase water to reduce costs; however, the water is less viscous than the oil. This can result in miscible viscous fingering of the water into the polymer, breaking down the slug and reducing recovery. The solution assumes that the average effect of fingering can be represented by the empirical Todd and Longstaff model. The analytical calculation of minimum slug size is compared against numerical solutions using the Todd and Longstaff model as well as high resolution first contact miscible simulation of the fingering. The ability to rapidly determine the minimum polymer slug size is potentially very useful during enhanced oil recovery (EOR) screening studies.
We study the life-cycle of miscible fingering, from the early fingering initiation, through their growth and nonlinear interactions to their decay to a single finger at late times. Dimensionless analysis is used to relate the number of fingers, the nature of their nonlinear interactions (spreading, coalescence, tip splitting), and their eventual decay to the viscosity ratio, transverse Peclet number, and anisotropic dispersion. We show that the initial number of fingers that grow is approximately half that predicted by analytical solutions that neglect the impact of longitudinal diffusion smearing the interface between the injected solvent and the displaced fluid. The growth rates of these fingers are also approximately one quarter that predicted by these analyses. Nonetheless, we find that the dynamics of finger interactions over time can be scaled using the most dangerous wavenumber and associated growth rate determined from linear stability analysis. This subsequently allows us to provide a relationship that can be used to estimate when predict when the late time, single finger regime will occur.
The job results from an operation using a wireline-operated lateral access tool (LAT) with a production logging tool (PLT) on E-coil are presented. The objective was to successfully identify, enter and acquire production data in each of the openhole laterals as well as the main borehole in order to quantify production and identify any cross-flow. This operation is enabled by the use of a wireline-operated LAT. The tool can identify where the lateral window is located in the well and provide orientation data. With the LAT, the bottom sub can be indexed to enable entry into the lateral, while sensors package would provide positive confirmation and identification of a particular, targeted lateral. The system is compatible with a number of mono-cable logging tools and can be deployed using both E-line as well as Coiled Tubing. This paper describes the operation in detail and discusses the output and evaluates the results, which demonstrate the efficiency and accuracy of finding and entering the laterals. The operation was conducted on a well in Saudi Arabia which was drilled using underbalanced coiled tubing drilling (UBCTD) technique in 2013 and included three, slim openhole laterals. In early 2014, the well was put on production with unknown contribution from each of the lateral sections, but interlateral cross-flow was suspected, leading to the need for intervention. A number of approaches were considered with special consideration given to a new technology that had been developed locally and run with success on some other oil and water wells in the region. This technology had not been utilized previously in slim openhole wells with predominantly gas production. Challenges were anticipated regarding how some of the sensors would perform over two critical areas: identification of the lateral windows and confirmation that the lateral had in fact been entered successfully. The results of this operation demonstrate that the sensors can operate successfully in this environment. The operator was able to acquire critical reservoir information about each of the openhole laterals as well as the main bore, enabling further understanding of well production and reservoir depletion efficiency. This case study demonstrates the innovative application of LAT to enable the production logging (PLT) and evaluation of slim openhole laterals in a gas well drilled with UBCTD compared to previous cases which were predominantly oil producers and water injectors.
This paper investigates the impact of aspect ratio on the growth rate of viscous fingers using high resolution numerical simulation in reservoirs with aspect ratios of up to 30:1. The behaviour of fingers in porous media with such high aspect ratios has been overlooked previously in many previous simulation studies due to limited computational power. Viscous fingering is likely to adversely affect the sweep obtained from any miscible gas injection project. It can also occur during polymer flooding when using chase water following the injection of a polymer slug. It depends upon the viscosity ratio, physical diffusion and dispersion, the geometry of the system and the permeability heterogeneity. It occurs because the interface between a lower viscosity displacing fluid and a higher viscosity displaced fluid is intrinsically unstable. This means that any small perturbation to the interface will cause fingers to grow. It is therefore almost impossible to predict the exact fingering pattern in any given displacement although many previous researchers have shown that it is possible predict average behaviour (such as gas breakthrough time and oil recovery) provided a very refined grid is used such that physical diffusion dominates over numerical diffusion. It is impossible to use such fine grids in field scale simulations. Instead engineers will tend to use standard empirical models such as the Todd and Longstaff or Koval models, calibrated to detailed simulations, to estimate field scale performance. At late times in high aspect ratio systems, we find that one finger dominates the displacement and that this finger grows with the square root of time, rather than linearly. We also observe that this single finger tends to split, during which time the solvent oil interface length grows linearly with time before one finger again dominates and grows with the square root of time. This cycle can repeat several times. We also find that industry standard empirical models cannot properly capture the average behavior of the fingering in these cases because they assume linear growth as a function of time. We show that a modified Peclet number can be used to estimate when these empirical models are no longer valid.
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