8.2. Sar adsorption

Adsorption is a process of separation of SAR from the water solution and its settling-out on the surface of pore channels under the forces of the inter-molecular interactions.

Adhesion is a sticking of molecules of two different bodies, caused by mutual attraction. It occurs between liquid and solid bodies.

Film-type oil covers the hydrophobic part of the pore surface of the formation in the form of a thin layer or in the form of sticky drops held by the forces of adhesion. The forces work of adhesion W_а, required to remove the film-type oil from the pore surface unit to the water phase, filling the pores is determined by Dupre or Young-Dupre equations [1]. Rock well-wetting waters or solutions wash the oil off better.

An addition of SAR to the water causes the change in the ratio of the values of free surface energy due to adsorption processes of SAR on the inter-phase boundaries. Under the forces of molecular attraction SAR separate from water solution and deposit on the surface of pore channels (they are sorbed). This process is largely determined by the specific reservoir surface and adsorption activity of the porous medium surface.

Surface active reagents adsorption on the hydrophobic parts of the pores’ surface that can exist in the result of sorption of some oil components, leads to the change of surface tension in accordance with the rule of molecules orientation. These circumstances facilitate the separation of oil from the surface of pore channels.

On the hydrophilic parts of the pores’ surface SAR sorption leads to the unproductive losses of reagents, and promotes oil drops adhesion to these parts.

The quantity of A, sorbing on the surface of grains of hard rock of SAR, is determined by Langmuir sorption isotherms. [7].

(8.1)

And Henry

(8.2)

Where с - is the specific concentration of SAR in water, kg/m³, a and b are the coefficients determined experimentally. As SAR is added to the injection water then to determine the quantity of the sorbed SAR on the surface of grains of hard rock is necessary to use water filtration equation with SAR and oil, the equations for determination of water saturation and calculation of concentrations that are based on Henry or Langmuir isotherms.

There are two physical models that are used to describe the processes happening in the formation: reciprocating and nonreciprocating oil displacement by water and in the second case, the models of Backley-Leverett, Rappoport-Liss and others are used. The solution of the formulated problems made up either by approximate or numerical methods. For straight-parallel filtration with reciprocating oil displacement by water the exact solution is possible [7]. In Figure 8.1 there is shown a diagram of the reciprocating oil displacement by water solution of SAR. The increase of oil in area 2, in comparison with the area 1 (oil bank), is associated with additional oil displaced from the region 1 by SAR adsorption in area 1.

Fig.8.1. The diagram of reciprocating oil displacement by SAR solution.

1 - 0<=х<х_сор – an area where SAR is adsorbed; х_сор – sorption front;

2 - х_сор<х<х^* an area, occupied by oil, displaced due to SAR stimulation; x* - oil displacement front by SAR water solution;

3 – an area of oil displacement by pure water х^*<х<х_ф, где х_ф –oil displacement front by water coordinate.

4 – the stratum area, uncovered by the development.