On The Necessity Of Respecting Reservoir Conditions In Laboratory Displacement Studies

Abstract

Abstract Prediction of the field behavior during primary or secondary recovery requires representative laboratory measurements. Insuring representativity of the rock surface properties and of the fluid distribution is not properties and of the fluid distribution is not possible, even for preserved samples. possible, even for preserved samples. A procedure for restoring original rock surface properties is presented, and results showing the properties is presented, and results showing the influence on oil recovery of both the method used for establishing initial water saturation and the saturation level attained are discussed. Introduction Choosing the recovery methods best suited for production from a reservoir goes via laboratory tests production from a reservoir goes via laboratory tests using reservoir-rock samples. Generally, the state of core samples brought to the surface is not representative of their state in situ. Even in taking precautions immediately upon arrival at the surface so as to prevent any effect of the oxygen in the air or of evaporation, the core has undergone transformations with regard to the distribution and nature of the fluids it contains. This results from pollution by drilling mud and variations in temperature and pressure during the time it is being brought up. Physics-chemical changes may be made in the solid-fluid interfaces by adsorption-desorption or even a deposit of the heavy constituents of the oil, e.g. asphaltenes, or salt contained in the water. Generally speaking, the wettability of the rock will be changed by a reduction in its affinity for water or by an increase in its affinity for oil. The sample then has to be cleaned so as both to restore the original state of wettability and to put fluids back in place with their original distribution. Yet the composition of fluids and thermodynamic conditions are known to govern its wettability. Therefore, operations must be done with fluids and under conditions found in the reservoir. This paper will successively describe a method for restoring the original state of wettability and then the influence of the initial fluid saturation process and of the saturation level on recovery by process and of the saturation level on recovery by different mechanisms. PROCEDURE FOR RESTORING ORIGINAL SURFACE STATE OF PROCEDURE FOR RESTORING ORIGINAL SURFACE STATE OF ROCK SAMPLES The importance of the surface properties of a rock on its behavior during various laboratory experiments has been well known for a long time. So-called "preserved" samples are often used. They are obtained by taking various precautions concerning the choice of mud, the use of special coring methods, the quick protection of the core sample as soon as it arrives on the surface, etc. Nonetheless, while it is being brought up to the surface, a decrease in temperature cannot be avoided, and pressure maintenance is not standard practice. Therefore, it is, a priori difficult to assert that the surface state of the so-called "preserved" rock has not been altered. In order to solve this important problem a method capable of clearing up this uncertainty has been developed for restoring the original surface state. Description of the method It consists in comparing the wettability of the rock upon reception and after restoration so as to justify either the direct use of the samples received or the application of a treatment for restoring the original surface state. A schematic diagram of the method is given in Figure 1. The restoration procedure first consists in cleaning the samples so as to make them as water wet as possible by flooding carefully chosen solvents.