Recovery of Heavy Matrix Oil by Capillary Imbibition in Naturally Fractured Reservoirs

Abstract

Abstract If matrix is water wet and enough amount of water is supplied in fractures, capillary imbibition becomes the governing recovery mechanism in naturally fractured reservoirs. When the matrix contains heavy oil, however, this process becomes highly ineffective due to slow recovery rate and high residual oil saturation. Additional methods, other than waterflooding, such as injection of surfactant solution, polymer solution or hot water should be considered to increase the effectiveness of the capillary imbibition recovery of heavy matrix oil. This paper includes the results and analysis of the laboratory scale experimentation of these three methods. Experiments are conducted on Berea Sandstones (water-wet sample) using heavy oil samples with the viscosities ranging between 185 and 630 cP. Carbonate cores (oil-wet limestone samples) were also used with light oil samples for comparison purpose. Different boundary conditions are created using epoxy coating. Based on boundary conditions, co- or counter-current type capillary imbibition takes place. Performances of the three methods are compared in terms of ultimate recovery and recovery rate. It is observed that all three methods yield higher and faster capillary imbibition recovery compared to waterflooding. The contribution of chemical additives and high temperature of water becomes more pronounced as the viscosity of oil increases. This contribution is more prominent on ultimate recovery than the recovery rate, especially for chemical solution cases. Introduction Capillary imbibition can be an effective recovery mechanism if matrix is water wet and enough amount of water exists in fractures of naturally fractured reservoirs (NFR). However, recovering matrix oil by capillary imbibition becomes challenging as the viscosity of oil increases. Beside the oil viscosity, other unfavorable conditions such as oil wet matrix, matrix boundary conditions limiting the dynamics of oil displacement, large matrix sizes, low matrix permeability, and high IFT add more restrictions towards the recovery rate and ultimate recovery. As such, additional effort to enhance the capillary imbibition recovery becomes indispensable. The most economic and therefore, the most common way to recover matrix oil by capillary imbibition are to inject water. The properties of water can be changed to enhance the capillary imbibition when the unfavorable conditions outlined above exist1. One of the methods is to inject steam or hot water2–7 to reservoir resulting in a reduction of oil viscosity and IFT. Surfactant2,8–16 and polymer solutions1,17,18 have been also tested in laboratory conditions previously and observed that they might have significant contribution to the capillary imbibition recovery. Field applications of chemical injection into NFR are not widespread even though enough evidence was found that surfactant injection yields remarkable increase in ultimate recovery19–21. Limited number of field applications of polymer22 and steam injection into NFR was reported23,24. The main factor restricting the implementation of field applications is the cost25 and controllability26 of the process. The high permeability fracture streaks may yield an undesired movement of injected fluid without exploiting the matrix oil properly. Therefore, cost efficiency of the process is a critical issue27. Despite of the fact that the processes become costlier and create technical difficulties, additives to water phase or additional heat effect is necessary for heavy-oil recovery in NFRs. In this study, chemical (polymer and surfactant) and hot water injection to recover heavy-matrix oil by capillary imbibition were tested for different matrix and oil properties. The conditions to enhance the capillary imbibition were clarified.