The formation damage caused by the injection of water containing suspended particles, which are stable and are not adsorbed spontaneously onto pore surfaces under Brownian motion, has recently been analyzed at a pore scale level. Formation damage is the result of four more or less overlapping successive steps:
deposition on a grain surface,
formation of mono- or multiparticle bridges with subsequent accumulation upstream from the bridges,
internal cake formation as soon as the nonpercolation threshold has been reached near the core entrance, and
external cake formation.
The surface deposition is not uniform over the grain surface and varies from the upstream stagnation point to the near pore throat zone according to a function depending on flow rate and surface forces. The bridging of pore throats is strongly dependent on the effective pore throat-to-particle size ratio, and the pore-throat size is often reduced by previous surface deposition.
A new model has been developed to predict formation damage while taking into account these different steps. The dominant mechanism in each step is governed by parameters that have a clear physical meaning. However, due to the complexity of natural systems, these parameters cannot be quantitatively predicted from theoretical considerations but can easily be determined by specifically designed lab experiments. The model predicts the retention by deposition, by bridging and by subsequent accumulation upstream from bridges, the concentration in flowing particles and the local permeability reduction as a function of the distance from the inlet, as well as the overall permeability reduction, and the beginning of external cake formation.
This new model appears to be an effective tool for analyzing the consistency of a set of laboratory data and for selecting the values of the parameters that must be introduced in a near-well bore field simulator for the proper prediction of formation damage in a given application.
The early models proposed in the oil literature aimed to fit the decrease in relative permeability observed when a particle suspension is injected into a permeable core. Thus these models are empirical or at best purely phenomenological in the sense that they simulate observed phenomena by using equations without any clear physical meaning. They are very attractive however for petroleum engineers since they are very simple and easily introduced in conventional field simulators without increasing computing time too much. A good review of these empirical models can be found in Ref. 1.