Abstract

Scaling phenomenon is a major problem that occurs when water is injected for oil displacement and pressure maintenance in oilfields. This phenomenon of precipitation and accumulation of oilfield scale due to incompatibility between formation and injected water is induced around the well bore after water breakthrough at reduced reservoir pressure. The effect results in formation damage which may negatively impact on reservoir performance well bore performance and the success of water flooding project that depends on mobility ratio.

This paper presents an analytical model based on existing thermodynamic models for predicting brine mobility, hydrocarbon mobility and mobility ratio of water flooded reservoir with possible incidence of scale precipitation and accumulation. The key operational and reservoir/brine parameters which influence the mobility ratio such as salt concentration in the brine, produced water rate, pressure drawdown, reservoir temperature were identified using this model.

Results of the study shows that the mobility ratio of a water flooded reservoir remains constant until water breakthrough and achieves an increasing local maximum at 10% pore volume injected water as the flow rate of produced water increases with a significant jump beyond the critical flow rate observed at mobility ratio of 1. Similar results corroborating above were obtained with variation in skin factor.

This model therefore can be used to diagnose, evaluate and simulate mobility ratio and skin factor in a water flood scheme enabling production engineers plan an economically efficient water flood scheme.

Introduction

The process of precipitation and deposition of mineral scale due to flow of fluid containing saline solution in porous media is a major problem during water flooding project {(Oddo et al 1991), (Atkinson et al 1991), (Fadairo 2004), (Fadairo & Omole 2004 & 2008), (Moghadasi et al 2005, 2006a & 2006b), (Kalantari et al (2006), (Faruk (2001)}. The process pronouncedly occurs around the well bore resulting in permeability damage after water breakthrough at reduced reservoir pressure (Fadairo & Omole 2007). The phenomenon may provoke loss in productivity, injectivity and efficiency of water flooding scheme that is generally dependent on mobility ratio.

Several authors {(Kalantari et al (2006), (Faruk (2001) (Frank et al 1991, 1996) and (Civan et al 1989)} have developed models on permeability damage due to migration of mineral scale particles in porous media and indicated that permeability damage is more likely to be severe near the well bore. Among other authors, (Rachon et al, 1990) presented a relationship between initial permeability and instantaneous permeability as a porosity exponential function. Civian et al (1989) developed a power law model that is valid for a solid mineral deposition inducing permeability reduction of up to about 80%. Chang & Civian (1996) presented a relationship between the initial permeability and instantaneous permeability as functions of altered porosity and initial porosity, by assuming power law of 3.0. Moghadasi et al (2005, 2006a & 2006b) modified Civian et al (2001) model by introducing variable parameters such as particle concentration in the fluid, solid particles density against the depth and time of invasion. Fadairo and Omole (1996) recently presented an improved model of Moghadasi et al (2005, 2006a & 2006b) and Civian et al (2001) formulation on formation permeability damage due to mass transfer of particles flowing through the porous media.

Effect of oilfield scale induced permeability on the success of water flooding project that depends on mobility ratio is rarely reported by precious authors. Tahmasebi et al (2007) recently formulated an empirical correlation for predicting the permeability and apparent mobility reduction due to calcium sulphate scale.

Mobility ratio is important in determining the success or failure of water flooding project (Kumar et al 2005); hence it is pertinent to determine the key operational and reservoir/ brine parameters that influence the mobility ratio.

For better understanding of mobility ratio behavior after water breakthrough in a water flooded reservoir, various essential parameters need to be taken into account that were not reported by previous authors. These parameters include salt concentration in the brine, viscosity of brine, formation volume factor of the brine, solid scale density, produced water rate, pressure drawdown, reservoir temperature, reservoir thickness, brine velocity against injection time and radial distance from the well bore vicinity.

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