Abstract: As a favorable two-phase dispersive soft mater, foam is being extensively used for conformance control in heterogeneous reservoirs, gas channeling prevention in fractured reservoirs, and fracturing acidification in unconventional reservoirs. However, during transporting in matrix-fracture systems, foam usually leads to local capture, retention and accumulation, which might affect the permeability of the whole system especially matrix. In this study, we propose a method that assess the impact of foam on the permeability of matrix-fracture dual systems by using a surfactant foam and a cellulose nanofibrils (NCF) strengthened foam. The flow behaviors and distribution of foam systems in matrix-fracture systems are revealed. In total of four permeability-grade cores were used (0.13~239.60 mD), and the correlations between differential drop, invading depth, residual resistance factor and permeability are established. The results show that the foam could successfully flow through the cores with the permeability larger than 23.00 mD. The invasion of foam into low permeability(less than 3.13 mD) and tight-matrix leads to the reduction of permeability because of adsorption and/or mechanical retention. The influence degree of NCF foam is found to be analogous to that of surfactant foam. In addition, the foaming agent in the presence of NCF mainly retains on the fracture surface with few mass loss especially for the matrix with permeability lower than 8.50 mD. The permeability of the matrix is rapidly retrieved once the deposition is rinsed off the surface. The results of this study guide the investigation of foam transport and distribution in matrix-fracture systems especially for low permeability and tight matrix, and also provide method to assess the impact of foam on the permeability of matrix-fracture systems.

Key words: matrix-fracture system, low permeability/tight matrix, permeability, foam system, evaluation method