The objective of this study is to investigate the effect of Wettability, matrix blocks' shape factor and interfacial tension on the rate of spontaneous imbibition and the level of oil recovery from naturally fractured reservoirs. This study followed the Amott experimental procedure yielding data that could be used to determine the spontaneous imbibitions oil recovery vs. time as well as predict the rock wettability using Amott Wettability Index.

Three different brine solutions (NaCl, KCl, CaCl2), Kerosene and Berea cylindrical core plugs with four boundary conditions were utilized in this paper. The paper derived the shape factors as well as the characteristic lengths of the cylindrical cores of All Sides Open (ASO), Two Sides Open (TSO) and two types of Half Side Open (HSO-1, and HSO-2).

Experimental data showed that cores with highest shape factor would enjoy highest imbibtion rate as well as oil recovery for the same brine type, core permeability and wettability. Due to its higher permeability, some TSO cores could achieved higher imbibition rate, despite its low shape factors. The high capillary forces associated with higher interfacial tensions instigated NaCl-Brine and KCl-Brines to consistently achieve highest spontaneous oil recovery regardless of the imbibition rate which may be influenced by cores' shape factor, permeability and wettability. Finally, core samples of this work were found to be neutrally wet with some water-wetness tendency resulting in minor effect of wettability on the performance of the different core samples of this work.

This study will help to estimate the possible level of oil recovery from naturally fractured reservoirs, which would lead to have a proactive plan for possible secondary or tertiary recovery mechanism if needed. This study will also help operators to optimize the salinity of the injected water for better recovery as well as compatibility with formation water.

1. Introduction

Spontaneous imbibition can be described as the absorption of a fluid into a porous rock by capillary attraction. Imbibition is the displacement of a nonwetting fluid with a wetting fluid in a rock and is vital for oil recovery in porous media, especially when considering fluid flow behavior in fractured reservoirs1. Spontaneous imbibition process is controlled by the properties of the porous medium and fluids, such as porosity, permeability, pore structure, matrix size and shape, boundary condition, fluid viscosity, initial water saturation, wettability, interfacial tension, relative permeability and gravity2.

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