Enhanced Imbibition Through Wettability Alteration During Shut-In Treatment After Hydraulic Fracturing

ABSTRACT Tight oil reservoirs are characteristic of low porosity and permeability, complex pore-throat network, and typically oil-wet, which results in a low oil recovery rate of less than 10%. Surfactant can alter the rock wettability while maintaining an ultralow adsorption rate on rock surface, and thus can be a promising method to enhance the oil recovery. This paper uses the numerical simulation method to understand how shut-in pressure affects the imbibition of fracturing fluid when rock wettability changes. Results show that wettability has a great influence on water imbibition depth of fracturing fluid and the change of water saturation at the invasion front. There is a positive correlation between the imbibition rate and the water-oil interfacial tension in a water-wet reservoir, but a negative correlation in an oil-wet reservoir. In water-wet reservoirs, when the shut-in pressure keeps the same, the water imbibition rate is linear to sqrt (K/ϕ), and increases with the interfacial tension. When the oil-wet reservoir is altered to water-wet, although the water imbibition rate is reduced by 37%, water saturation in the invasion front significantly increases from 0%-1.75% to 8%-15%, resulting in an enhanced water imbibition efficiency of 6.25% to 15%. INTRODUCTION Abundant oil and gas resources are present in tight reservoir formations, which are widely distributed all over the world. Tight reservoirs have small pore radius, few effective pores, and complicated pore throats with abundant microfractures[1]. In this kind of reservoir, the traditional water flooding method is difficult to achieve the effective use of crude oil. Hydraulic fracture is needed to improve the conductivity of the matrix[2]. Research and practice indicate imbibition flooding can significantly improve recovery in fractured tight reservoirs[3]. In general, imbibition refers to the spontaneous entry of the wetting-phase fluid into the pore throat of the rock under the action of capillary forces, displacing the non-wetting phase fluid in the pores[4]. To study the imbibition characteristics and imbibition displacement mechanism of tight reservoir can provide an effective method for improving the recovery efficiency of tight reservoir.