Abstract

Despite the numerous experimental studies, there is a lack of fundamental understanding about how the local and global heterogeneity control the efficiency of polymer flooding. In this work a series of water and polymer injection processes are performed on five-spot glass micromodels which are initially saturated with the crude oil at varying conditions of flow rate, water salinity, polymer type and concentration. Three different pore structures in combine with different layer orientations are considered for designing of five different micromodel patterns. It has been observed that the oil recovery of water flooding is increasing with the salinity concentration, for the ranges studied here. While, it shows there is an optimum value of concentration in which maximize the oil recovery in polymer flooding. The results confirmed that the highest oil recovery is obtained when the layers are perpendicular to the mean flow direction for both water and polymer flooding. Also, the oil recovery in polymer flooding increases with the increase of layer inclination angle, however it does not increase for waterflooding. In addition, the oil recovery is strongly affected by the local heterogeneity which is near injection zone. This study demonstrates the applicability of micromodel for studying of enhanced oil recovery techniques in locally and globally heterogeneous five-spot models.

Introduction

Water flooding is being widely used in the petroleum industry and has been considered as a simple inexpensive secondary recovery method. Craig (1980) summarized the reasons of waterflooding popularity among other fluid injection methods as

  1. availability of water,

  2. easy way of water injection,

  3. ability of water to spread through pay zone, and

  4. water efficiency to displace oil through pay zone.

On the other hand, one of the most popular drawbacks of waterflooding was recognized to be its poor sweep efficiency.

Polymer flooding can yield significant increases in percentage recovery accompanied with a complex mechanism. However, analyzing the performance of a polymer flooding in a field project requires a vast knowledge not only of the polymer solution's rheological behavior, but also of the local and global heterogeneities which affect areal sweep efficiency of the process.

Adding suitable polymer solution to the injected water would result in reduction of water mobility, and so the oil recovery will increase. According to Needham (1987), polymer solutions may lead to an increase in oil recovery over that from conventional waterflooding by three potential ways:

  1. through the effects of polymers on fractional flow,

  2. by decreasing the water/oil mobility ratio, and

  3. by diverting the injected water from zones that have been swept.

The above three effects can make the polymer flooding process more efficient.

Sweep efficiency is defined as the ratio of volumes of oil contacted by displacing agent to volume of oil originally in place, and is negatively affected by many factors such as the unfavorable mobility ratio (greater than one), complicated pore structure and characteristics of oil-wet on the rock surfaces (which impede oil transport by capillary force), and the reservoir rock heterogeneity (Craig 1980; Han et al. 1999; Lake 1989). However, nearly all oil reservoirs are heterogeneous because of the wide variations in porosity, permeability, depositional environments, and existence of naturally fractured systems. According to van Poollen (1980) in terms of enhanced oil recovery, the divergence of reservoir permeability is a significant factor. The permeability variation can have a profound effect on the flow of fluids in a reservoir and thereby influence oil recovery.

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