Invasion And Impairment Of Formations By Particulates
Authors: Davidson Donald H. 
Published: December 2007

Abstract
Water injection operations associated with oil-field, geothermal, and disposal processes may be subject to formation damage by particulates. Over long periods of time it is unlikely that sufficient water treatment can be obtained to prevent injectivity declines. Selecting water sources and alternatives to water treatment, such as well stimulation or replacement, requires an assessment of the rate and location of formation damage. AS part of this evaluation, core tests and analysis of solids movement in porous media were used to study the mechanism of particle invasion of formations.

Introduction
The flow of particulates in a porous media is a complex process, which in an oil reservoir is further complicated by the presence of multiple fluid phases (oil, water, and/or gas). The final disposition of injected solids is related to both geometric and fluid dynamic factors. This paper is concerned with assessing the effects of pore velocity, particle size, pore size, and the method of core particle size, pore size, and the method of core testing (constant rate or pressure drop) on solids deposition in pores.
Doscher and Barkman and Davidson discussed the use of membrane filtration tests to evaluate formation damage. Although Barkman and Davidson proposed a method of quantifying rates of impairment, the analysis did not project how far particles might be expected to penetrate into the formation.
Examples of particulate invasion have been reported for both laboratory and field tests. Champlin demonstrated that the ionic composition of the liquid alters the electrostatic particle-sand matrix interaction, which in turn affects the penetrating distance of submicron particles into a penetrating distance of submicron particles into a porous media. Abrams used a radial sand model to porous media. Abrams used a radial sand model to simulate a 4-to-1 reduction in wellbore velocity and showed that at injection rates approximating 7 B/D/ft of interval, particles ranging in size from 1/2 to 12 microns (median 3 mircrons) could penetrate 40-micron pores up to 3 in. from the wellbore and reduce the pores up to 3 in. from the wellbore and reduce the local permeability by a factor of 10-to-1. Subsequent backwashing did not raise permeability significantly. Ogletree discussed economic alternatives associated with injection of waters of different quality and also presented field data that indicated that particulate invasion had led to formation damage at particulate invasion had led to formation damage at significant distances from the wellbore. Mitchell, evaluating water injection into the Forties reservoir, noted core and millipore filter tests correlated. McCune discussed the use of mobile field core-testing facility to evaluate all aspects of water quality on permeability damage.
Additional work is required to expand upon the data obtained from membrane filtration and core tests to quantify depths of invasion and rates of formation impairment by particulates. Testing principles and procedures must account for the principles and procedures must account for the large flow variations that exist between cores and the field, and this requires the impact of pore velocity on impairment. Core tests can be designed to simulate the dynamics of impairment at specific locations in the formation, by changing flow rates, but scale-up procedure must be developed to project overall formation damage.

DISCUSSION OF INVASION MECHANISM
Particles that are smaller than rock pore sizes will penetrate into the interior of the rock. The rate of impairment and depth of penetration are determined by the forces acting on the particle as it moves through the rock. Surface and gravitational forces cause particles to be separated from the fluid and deposit in pores, thus increasing the resistance to flow. The deposition process can be retarded by a force generated by the pore velocity. Two possibilities exist with respect to the final change in pore dimensions (see Figure 1).An equilibrium state is reached with no net deposition.