Mechanisms of Formation Damage by Retention of Particles Suspended in Injection Water

Abstract

Mechanisms of Formation Damage by Retention of Particles Suspended in Injection Water. SPE Members Abstract This paper reports the main results of an experimental study of the process of permeability impairment by particles suspended in injection water. The experiments were carried out under well defined conditions both realistic and sufficiently well-known to yield clear and useful conclusions concerning actual retention mechanisms and their consequences for formation damage. Injection of waters containing particles of different sizes into sandstones of different permeabilities resulted in drastic permeability reductions, even for particles smaller than pore throats, with a very high flow rate sensitivity. The different possible steps in formation damage process have been characterized and related to phenomena at pore scale level. The results are analysed according to a new theory capable of predicting retention by two mechanisms for particles smaller than pore throats that do not absorb on pore walls without flow. This theory takes into account the competition between surface forces and hydrodynamic forces which can either induce or prevent retention depending on force balance at the location of capture. All experimental observations agree with this theory which, therefore, provides a reliable basis for quantitative predictions of formation damage. Introduction The retention of particles inside the reservoir rocks, which may cause substantial reduction in their permeability, is frequently at the origin of drastic declines in oil wells injectivity and productivity. In spite of its recognized importance, formation damage is still "a concept based on personal opinion and experience". The aim of this paper is to make clear what mechanisms are effectively responsible for retention, where retention takes place according to the type of retention involved and what are the consequences of this retention on permeability. Such a clarification gives the keys for a proper interpretation of experimental data and for a better prediction of formation damage characteristics as a function of the main parameters. The current approach of formation damage considers only two types of retention. The first one is surface deposition occurring spontaneously as soon as a particle arrives in close contact with a grain (or pore wall). This surface deposition requires surface attractive forces (Van der Waals electrostatic forces) or external forces like gravity for the layer particles. This type of retention has been studied extensively for particles much smaller than pores and reliable data and models are available, particularly for the kinetic of deposition onto isolated collectors and granular packs taking into account the respective effects of Brownian diffusion and convection. The effects of already deposited particles have also been taken into account. However, investigations concerning the case of particles having a size non negligible compared to grain size are quite recent. P. 329