An Examination of Countercurrent Capillary Imbibition Recovery from Single Matrix Blocks and Recovery Predictions by Analytical Matrix/Fracture Transfer Functions

Abstract

Abstract Countercurrent capillary imbibition is an important oil recovery mechanism in naturally fractured reservoirs. This paper presents a comprehensive study which examines 8 important characteristics of countercurrent imbibition as observed from experiments conducted with single matrix blocks. These characteristics are (1) types of imbibition and recovery trends, (2) flux and transition time, (3) imbibition front and average saturation, (4) effect of block sizes and shapes, (5) effect of temperature, (6) effect of initial water saturation, (7) reproducibility, and (8) long-term recovery. Existence of two flow periods during countercurrent imbibition were verified. The transition time between the periods was identified as an excellent factor for scaling core size and temperature effects on countercurrent imbibition recovery. The advancement rate of the imbibition front was confirmed to be proportional to the square root of time for 1 D water imbibition into gas-saturated matrix blocks. Experiments showed that initial water saturations ranging from zero to near 20% had no significant effect on recovery behavior. Long term recovery from gas-saturated matrix blocks reached 100%. Analytical matrix/fracture transfer functions are often used to predict imbibition recovery. Over 23 such functions have been identified from the literature. In this study, these functions are categorized as:diffusivity functions,scaling relations,empirical functions, andmaterial balance functions. One critical finding has been that analytical functions can provide alternative solutions when modeling oil recovery from naturally fractured reservoirs. We have shown that better functions can easily be developed by simply following the examples of previous functions. In this regard, three new functions and their variations are presented and verified. The findings in this study should provide a powerful toolbox for reservoir engineers in modeling oil and gas recovery from dual porosity/permeability structures which include naturally fractured reservoirs. P. 237