Ion Exchange and Osmotic Mechanisms of Low Salinity Water Flooding and Modeling of Oil Displacement in Clay Reservoirs

Most reservoir sands contain clay minerals. It is the well known fact that fresh water injected into the clay reservoirs causes swelling of clays. The swelling clays partly block the capillary openings in the sand and therefore reduce the rate of flow to the well bore. In addition clay minerals are susceptible to destruction of its molecular structure by exposure to waters [1]. Clay particles emerged from swelling process can block capillary openings as well. However, a great number of laboratory tests [ 1 ] showed that enhanced oil recovery can be obtained when performing a low salinity waterflooding (LSW). Despite increasing interest in LSW, none of the proposed mechanisms have so far been accepted as the “true”, none of the mathematical models of LSW have been created.

Mathematical modeling is based on analysis of electrokinetic and physicochemical effects at micro-level.

This process includes description of electro-osmotic flow in a capillary, ion-exchange process in diffusion layer of a capillary and also osmotic swelling of clays. Porous medium is generally modeled by the parallel conducting chains [ 2 ] bound up with interconnecting capillaries so that current could flow into the other chain. The main characteristic of this capillary system is described by the probability density function f(r). After the all micro- processes having been described, we go on with modeling at macro-level by measuring reservoir and two-phase flow characteristics (porosity, permeability, relative permeabilities, capillary pressure curves) depending on clay factor and mineralization of injected water.

Rapoport-Leas model has been chosen to estimate efficiency of oil displacement. This model allows us to take into account capillary pressure taking place during low salinity waterflooding. Salt transport in porous medium is described by convective diffusion equation which includes ion-exchange reaction rate, diffusivity and hydrodynamic dispersion.

The results of the calculations show the growth of oil production rate, water cut decrease and as a consequence an increase in recovery factor when performing LSW. The results fit well with experimental data.

1. Tang, G., Morrow, N. R. Influence of brine composition and fines migration on crude oil/brine/rock interactions and oil recovery. Journal of Petroleum Science and Engineering, 1999, 99–111.
2. VI. Selyakov and VV Kadet, Percolation Models for Transport in Porous Media With Applications to Reservoir Engineering. Kluwer Academic Publishers. Dordrecht/Boston/London, 1996, 241 p.