Factors Governing Accumulation of Oil and Gas in Stratigraphic Traps. Experimental Studies on the Characteristics of the Electrochemical Potentials Encountered in Drill Holes. Studies on Infiltration of Porous Formations by Drilling Fluids in Relation to the Quantitative Analysis of Electrologs in Drill Holes

Citation

deWitte, Leendert (1950) Factors Governing Accumulation of Oil and Gas in Stratigraphic Traps. Experimental Studies on the Characteristics of the Electrochemical Potentials Encountered in Drill Holes. Studies on Infiltration of Porous Formations by Drilling Fluids in Relation to the Quantitative Analysis of Electrologs in Drill Holes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/trf9-g451. https://resolver.caltech.edu/CaltechTHESIS:02032021-150144083

Abstract

Factors Governing Accumulation of Oil and Gas in Stratigraphic Traps

This thesis is a study of those characteristics of sedimentary rocks, that determine their capability to function as cap rocks for petroleum accumulations.

The function of the cap rock is to prevent the oil from upward migration. The active upward forces on the oil (or gas) are due to the buoyancy of the oil (or gas) on water. The forces opposing upward migration of the oil are due to the capillary retention of the water in the cap rock. For any oil trap it is necessary that the capillary forces in the cap rock are larger than the buoyancy of the accumulating oil on the subsurface waters.

For a given combination of oil or gas and subsurface water, the magnitude of the capillary-retention forces on the water in the cap rock is given by the displacement pressure of that rock.

An outline is given of the theories, relating the displacement pressure to the basic characteristics of the rock; such as permeability, porosity; interstitial surface area and the Kozeny constant.

The displacement pressure for a given rock is directly proportional to the interracial tension between the water and the non-wetting phase. The buoyancy of the oil or gas on the water depends upon the density difference between the non-wetting phase and the water. The variations of densities and interfacial tensions with temperature and pressure are discussed and graphical relations are given between these quantities and the depth of the accumulation in question.

Displacement pressures for various types of sandstones were determined from experimental studies or the capillary pressure versus saturation relationships. From the experimentally determined displacement pressures, permeabilities and porosities, the interstitial surface area and the Kozeny constants for the various rocks were computed.

It was found that the interstitial surface areas could be correlated with the electrochemical formation coefficients of the sediments in question, which were measured in the course of another research project by this author. This correlation indicates the possibility of a subsurface method for the determination of displacement pressures, using the S. P. curves of electrologs.

Different types of stratigraphic traps are described and the relative positions or the cap rooks are considered for the various cases.

A schematic example is given or the location or a "permeability pinch-out" type of stratigraphic trap by computation of the displacement pressures from electrologs of drill holes in the vicinity.

Experimental Studies on the Characteristics of the Electrochemical Potentials Encountered in Drill Holes

This thesis describes the results of experimental studies on the characteristics of the electrochemical potentials encountered in drill holes and registered by electrologging apparatus.

The experiments were carried out by bringing samples of sediments into contact with two solutions of different salinities on opposite sides and measuring the difference in potentials between the solutions.

The following conclusions were drawn from the measurements thus obtained:

The potentials are independent of the amount of material involved. They are proportional to the logarithm of the concentration ratio of the two solutions. The potentials decrease slowly with time.

There is no sharp demarcation between the electrochemical behavior of sandstones and shales, but rather there exists a complete continuity in the potentials exhibited across sediments of the sandstone shale series.

This continuity is explained using the concept of "apparent ion mobilities."

To classify sediments according to their electrochemical Behavior, the electrochemical formation coefficient, C, is introduced. Dense argillaceous shales have a formation coefficient of 1. Inert clean sands have C = O. It is shown however that in practice sands may very seldom be considered inert.

A correlation exists between the formation coefficient and the ratio of the amount of conductive solids or argillaceous materials over porosity (see references 8 and 13). The influence of temperature on self potentials is computed theoretically and confirmed experimentally.

It is pointed out that dense formations may give considerable self potential kicks on electrologs. This conclusion is confirmed by measurements of the potentials across quartzites.

The general conclusions concerning the character of the electrochemical potentials are extended to the self potentials across calcareous formations.

The application of the analysis of the electrochemical potentials to electrolog interpretation is outlined . Also the possibility of electrochemical testing of sediments as a new tool in surface geology is indicated.

Studies on Infiltration of Porous Formations by Drilling Fluids in Relation to the Quantitative Analysis of Electrologs in Drill Holes

This thesis presents a discussion of a method, by which certain characteristics of the infiltrated zones around bore holes can be to good use in determining the fluid content and the formation factor, of the horizons in which we are interested.

To obtain the necessary field data a new combination of electrode spacings is proposed, namely a short lateral spacing, with two normal spacings. It is shown that apparent resistivities, obtained with these spacings, enable us to obtain the true resistivity, the resistivity of the invaded zone and the diameter of the latter.

To be able to use the invaded zone resistivity in the determination of the formation factor and fluid contents a group of laboratory experiments were carried out, using consolidated sandstone samples and drilling muds or liquids having the properties of the mud filtrate. The sandstones were invaded by the muds or their equivalent filtrates for different conditions of initial fluid contents. Then the resistivity changes were measured by a four electrode arrangement coupled to an amplifier-rectifier circuit and a sensitive test meter.

For the case of oil and gas sands the displacement of the non-wetting phase by water is treated analytically using the concept of relative permeabilities of the reservoir rocks and the results are compared with the experimental data.

From measurements on coresamples too actual resistivity profiles in the infiltrated zones of formations were computed. These showed that in all cases the resistivity of the invaded zone may be approximated by a constant.

The relations between resistivities of rocks and their fluid contents are considered in the light of experimental results and several new formulae are introduced, governing these relations.

Finally the water saturations of the invaded zones of oil and gas horizons are correlated with the water saturations of the undisturbed formations.

A brief outline is given of the computation of fluid contents and formation factors of porous rocks from the measured values of infiltrated zone- and true resistivities, using the new resistivity formulae and data obtained from analysis of the self potential curve of electrologs.

Thesis Files