7.5 The reasons of residual oil condition.

The main reason of impossibility of full oil displacement achievement by water from the reservoir during the water flooding is oil and water immiscibility; as a result the surface boundary between the liquids occurs and the holding of oil in a porous medium by capillary forces happens. Incomplete displacement in the formation water flooded areas is determined by the hydrophobization of the reservoir-rocks due to adsorption of heavy crude oil components on the surface of the rocks’ grains; by the difference of viscosities of displacing and displaced fluids, resulting hydrodynamic instability of oil-water contact; there are the oil drops or globules, oil accumulation behind the displacement front [7].

O il remains in a porous medium in the form of films on the rock grains and globules, located in the dead end pores or in the porous medium that is not water flooded.

Fig. 7.1. 1- hard rock grains; 2 – residual oil in the dead end pores; 3 – film-type oil; 4 – water.

If oil is displaced from the reservoir by miscible liquid, then the result of molecular diffusion liquid-solvent would penetrate to the oil, but oil hydrocarbons would penetrate to the solvent and over the time the oil would have been completely washed out the reservoir. Solvents, having washed out the oil, will remain in the reservoir, therefore, they should be cheaper than oil.

As the solvents, displacing oil from the reservoir, can be applied alcohols, ethers, carbon hydrogen sulphide, but they are rather expensive. That’s why natural gas, water, carbon dioxide, air are used for oil displacement.

7.6. The conditions of effective enhanced oil recovery methods use.

An important condition for the effective enhanced oil recovery methods use is the right choice of an object for a method or, on the contrary, a method for an object.

Criteria for the methods use define the range of favorable physical properties of fluids and reservoir for the effective method application or the best technical and economic development indicators realization. These criteria are defined on the basis of experience generalization of its application in different geological and physical conditions, and broad theoretical and laboratory investigations use, analysis of technical and economic indicators of the method application.

Usually, there are defined three categories of methods’ criteria application:

Geological - physical : the properties of reservoir fluids, depth and bedding conditions of oil-saturated reservoir, pool-reservoir properties and features of oil-bearing formation, saturation of porous space by formation fluids;

Technological: the size of the flooding bank, the concentration of the agents in the solution, well pattern, injection pressure, the choice of the drive of the producing wells.

Material-technical: availability of equipment, chemical reagents, their properties.

The criteria of the first category are decisive, the most significant and independent. Technological criteria depend on geological-physical and are selected in accordance with them. Material-technical conditions are also independent, in major part, and define the possibility of technological criteria implementation.