An experimental study of spontaneous imbibition in fractured sandstone with contrasting sedimentary layers

Fluid transport in reservoir formations at low flow rates is strongly dominated by capillary forces. These forces are the macroscopic manifestation of complex molecular interactions between fluids and solids inside the intricate pore structure of rocks. In naturally fractured reservoirs, spontaneous capillary imbibition is an important recovery mechanism since waterflooding of the disjointed matrix cannot be accomplished by forced displacement. The goal of the present experimental work is to study the mechanisms of flow in heterogeneous fractured formations under conditions favoring capillary displacement. This paper presents experimental results of spontaneous capillary imbibition in a layered Berea sandstone sample with a single longitudinal fracture. The sample was artificially fractured using a modified Brazilian test that resulted in an extensional longitudinal fracture oriented perpendicularly to the natural bedding planes of the rock. Non-destructive high-resolution X-ray CT imaging allowed identification of localized cocurrent and countercurrent flow during spontaneous capillary imbibition. Three distinctive flow intervals were also identified: early, intermediate, and late time imbibition. The presence of bedding planes in the rock's structure affects the shape of the imbibing front during early-time imbibition. Cross-layer fluid exchange tends to level the imbibing front during the intermediate imbibition stage. Countercurrent flow controls fluid transport during the early and intermediate intervals, while both cocurrent and countercurrent flow mechanisms coexist at late time. Results from this experimental work present strong evidence of hysteretic behavior. Drainage, imbibition, and transition zones characteristic of countercurrent flow are clearly identified as a function of time.