Fluid Flow in Fractures Visualized by MRI During Waterfloods at Various Wettability Conditions – Emphasis on Fracture Width and Flow Rate

Abstract

Abstract The mechanism of wetting phase flow into and across fractures was determined for stacked core plugs using highly water-wet and less water-wet chalk and several fracture widths and flow rates. Magnetic resonance imaging (MRI) was used to measure 2D saturation distributions in the matrix along the flow axis and 2D spatial distributions of the wetting and nonwetting phases in the fracture. For the strongly water-wet system, even at high flow rates, the inlet plug reached its spontaneous imbibition endpoint water saturation before the water entered the fracture. When water entered the fracture, gravity segregation resulted in the displacement of oil from the bottom of the fracture and upward. The rate of displacement was determined by the water injection rate. At less-water-wet conditions, the water produced a dispersed front that allowed water; at both high and low flow rates, to flow across the fracture and into the outlet plug as if there was no fracture. MRI images showed that water droplets formed on the outlet face of the inlet plug, formed bridges across the fracture and provided a path for water movement into the outlet plug while the oil phase was still being produced from the inlet plug. With time the bridges grew in size, coalesced and dropped to the bottom of the fracture eventually filling the fracture with water. At wider fracture widths the coalescence occurred earlier and the fracture was filled sooner. The capillary continuity provided by the bridges suggested a viscous component contributing to the total oil recovery in the fractured system for less than highly water-wet conditions.