An Investigation of Countercurrent Imbibition Recovery in Naturally Fractured Reservoirs With Experimental Analysis and Analytical Modeling

Naturally fractured reservoirs occur worldwide containing potentially economic and strategic fluids such as gas, oil and water. Modeling of naturally fractured reservoirs has advanced considerably because of the desire to increase the recovery from naturally fractured oil reservoirs and to exploit the vast storage capacity of naturally fractured formations for underground disposal of nuclear wastes. Countercurrent expulsion of oil from matrix blocks to the surrounding fractures by capillary imbibition of water is one of the more important fluid flow mechanisms in naturally fractured reservoirs. Transfer functions are essential for dual porosity simulators to characterize the countercurrent fluid flow between matrix blocks and surrounding fractures. The primary objectives of this study are:

1. to conduct experimental studies with single matrix blocks to better understand the general characteristics of countercurrent imbibition, and 2) to develop a comprehensive analytical matrix/fracture transfer function. New experimental methods for cleaning laboratory cores, establishing initial water saturation in odd shaped rocks and obtaining a transparent epoxy seal on core pieces to observe imbibition fronts have been developed to examine the general characteristics of countercurrent imbibition in single matrix blocks. A new analytical model (matrix/fracture transfer function) capable of modeling 1D, 2D and 3D countercurrent imbibition flow inside single matrix blocks has been derived. Imbibition characteristics are identified by analyzing the results of experimental data. The examined characteristics are: 1) types of imbibition and recovery trends, 2) flux and transition time, 3) imbibition front and average saturation, 4) effect of core size and shape, 5) effect of temperature, 6) effect of initial water saturation, 7) reproducibility, and 8) long term recovery. Based on the identified characteristics, solution proce~ures are developed to use with the new analytical model for recovery predictions. Results indicate that some imbibition parameters stay the same regardless of the geometry of the matrix block and imbibition type. The concept of an equivalent dimensionless distance is shown to reduce the number solution steps by combining the two periods of countercurrent imbibition under one set of equations. This greatly simplifies the analyses of countercurrent imbibition recovery with the new model. An apparent relative permeability concept shows that relative permeabilities are independent of temperature, and the change of imbibition with temperature is primarily due to the change of fluid viscosity and interfacial tension. The results in this study can easily be incorporated into a dual porosity simulator for multiphase fluid flow in naturally fractured reservoirs.