US2679159A

US2679159A - Determination of irreducible water and other properties of core samples

Info

Publication number: US2679159A
Application number: US139839A
Authority: US
Inventors: Elmer S Messer
Original assignee: Continental Oil Co
Current assignee: ConocoPhillips Co
Priority date: 1950-01-21
Filing date: 1950-01-21
Publication date: 1954-05-25
Anticipated expiration: 1971-05-25

Description

May 25, 1954 5, MESSER 2,679,159

DETERMINATION OF IRREDUCIBLE WATER AND OTHER PROPERTIES OF CORE SAMPLES Filed Jan. 21, 1950 2 Sheets-Sheet 1 INTOR. ZLMEI? 5. #7555512.

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E. S. DETERMINATION OF IRREDUCIBLE WATER AND y 5, 1954 MESSER OTHER PROPERTIES OF CORE SAMPLES 2 Sheets-Sheet 2 Filed Jan. 21,. 1950 figtvmtti 8 an Oh Id O M OM mN ON INVENTOR.

ELMZA 5. H5565 BY GTTC/l VEYS- Patented May 25, 1954 .DETERMINATION OF IRREDUCIBLE WATER AND OTHER PROPERTIES OF CORE SAM- PLES Elmer :S.;Messer, Ponoa City, Okla., 'assignor to Continental .Oil Company, Ponca :City, 0kla., a

corporation of Delaware Application January 21, 1950,'Serial No. "139,839

.8Claims.

, 1 T'I'he present inventionrelates amethod for adetermining :the :irreducible interstitial water content, capillary pressure, and related char- :acteristics of cores or :samples of .zsub-surface 'formations.

in sorder to obtain :a more'rcomplete analysis of .coresamples :to determine the petroleum reserves, :it :is necessary and .essential to determine the irreducible interstitial watercontent of the formation. This quantity is generally referred "to :as a ::percentage *of the available 51),01'8 volume "of the nore thatisloccupied by water.

The term interstitial water is defined .as that rwaterth'at iCOeXiStS in the pore space with the nil :prior to exploitation. The irreducible inter- 'StitidlfWflliBI' content of the formation is that "which islheld in *a state -of equilibrium with the nil by capillary pressure in all of that .zone of the forma'tiomwhich sufliciently tar above the water table to be :abovethe transition zone. .The .water content ofthe transition zone is also .a

asynonymously with these terms; .however, this ageclogicall-y, connate water is that which was entrapped in the pore spaces of the rock at the time -of :deposition. :Forethe sake :of brevity throughout the specification and claims, ir-

-reducib1e interstitial water content will be referred to as the irreducible water.

:Since the irreducible water in the .-formation above the transition "zone is dependent -on the mock structure, .it is a :function of the capillary pressure; and several techniques have 'been developed to measure this pressure and its degree of Water saturation.

'Themethods'used to determine the capillary ,pressure and the irreducible water may be divide'dinto three classifications. Themost prominent-of these :is generally referred .to as the restored state method. The techniqueis described --in detail in the literature and essentially consists of saturating a core sample with the reser- 'voirsfluid'and placing it in contact'withaporous mem'brane saturated with the samefluid. Oil,

gas or another liquid under pressure :is then 'applied to the exposed area :of the :core and :one

side of the membrane driving the saturant-irom The interstitial water content and 2 thelpores of thesample. The .membrane is such that it will pass the saturating fluid but is .impervious to the driving liquid or gas. At each applied pressure, an equilibrium saturation is obtained the core is weigl'ied to determine the fluid content. This'pressure-saturation .rep-

*resents one point :of a curve generally known .as a capillary pressure curve. With increased pressures there is adecrease .in saturation .until the irreducible water is attained "when there is no longer an appreciable change in the liquid content. The amount of liquid remaining in the core represents the irreducible liquid content and corresponds to-arelated capillary pressure.

This method has :certain limitations since the core sample must :have a flat 'face .for complete contact with the membrane; the .amount of ap- :plied pressure :is limited by -the capacity of the membrane.

Since at every pressure there is a time required to attain an equilibrium, ;a :period of several days is usually required "to obtain the :data for thelcapill'ary pressure curve and the irreducible vvater ;value.

The second classification includes the method of :determining the capillary :pressure by :cenitrifuga'l'iorce. v'Ihismethool of determination :is essentially similar to the restored .state .method, :exceptxthat :the :force driving'the liquid from the :pore spaces is :a centrifugal force. .In the determination, the speed of rotation of the sample is increased in steps until "a'speedrreached where any increase does not appreciably lower theawatercontent nf'the core. The Water driven rout-is measured-in atpiipette; :and in the final vdetermination, the loss of weight in the :core is compared with the water collected -in this pi- .rpette. This method is much faster than the :restored state method; however, it is limited in that the speed of rotation must be constant and any wariation will nullify the test. In'thistmeth- :Ud, the sample must have a'fiat face for :contact with the semi-pervious imernbrance. :Calculation of the capillary force :from the rotating speed is long and involved.

The third method ofmeasuring capillary pressure is based on the force "necessary to drive "mercury into "the pore space of the core "sample. This method utilizes the variation "in forces necessary to change the saturation of aporous medium with mercury. The main disadvantages of this method are the core samples used 'arenot suitable for'further testing; mechanical failures and corrections for the apparatus are numerous.

:All of the foregoing :procedures, as well as my own new method to be described hereinafter, usually require that the core sample be first cleaned to remove the crude oil and related organic substances which may be present in the sample. A convenient way of cleaning, commonly used, is to extract the organic material with carbon tetrachloride in a Soxhlet extractor.

The principal object of my invention is to provide a method for determing the irreducible water with speed and accuracy.

Another object of my invention is to provide a method having calculations that are simple and direct.

Still another object is to provide a method which does not depend upon a uniformly sized and shaped sample of material and that does not alter the structure or characteristics of such sample for further testing.

Other objects and advantages will become apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends, said invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various Ways in which the principle of the invention may be employed.

Broadly stated then, this invention comprises a method for determinin the irreducible water of a core sample of the sub-surface strata comprising the steps of saturating said sample with a vaporizable liquid, and while evaporating the liquid from said sample, periodically noting the decreasing weight of said sample during evaporation and subsequently determining the irreducible water of said sample from a graphical interpretation of the evaporation time-weight change relationship.

More specifically stated, this invention comprises a method for determining the irreducible water of a core sample taken from the sub-surface strata comprising the steps of saturating said core sample with a liquid evaporating the liquid from said saturated sample, determining from the inflection of the rate of evaporation curve the point at which only irreducible water remains, and calculating subsequently the actual percentage of the pore space of said sample occupied by the irreducible water.

In the further explanation of my improved method, it becomes convenient to illustrate apparatus by which such method may be carried on; two forms of such apparatus are illustrated in the drawings in which:

Fig. 1 is a diagrammatic illustration of one iform of apparatus that may be used in my inven- Fig. 2 is a diagrammatic illustration of another form of apparatus that may be used in the practice of my invention; and

Fig. 3 illustrates a typical curve showing liquid in sample versus time.

Fig. 4 illustrates capillary pressure vs. interstitial water content.

Referring now more particularly to the drawings, Fig. 1 illustrates one form of apparatus, a commercially available automatic balance, which may be used for practicing the present invention. Core sample It is first saturated preferably with a liquid that does not leave a residue when evaporated or cause any chemical action with the sample itself.

Examples of some of the liquids used and found satisfactory are Water, toluene, tetrachloroethane, benzene, and normal propyl alcohol. It is possible also to use liquids which have a vapor pressure greater than atmospheric at room.temperatures. Examples of such liquids are a wide variety of petroiemn fractions and their derivatives. Liquified propane is an example of such a liquid which will be found satisfactory for use. It will be understood, of course, that the time necessary to reach the irreducible liquid content is a function of the evaporation rate of the particular liquid used.

The reservoir fluid would, of course, be the most appropriate to use; but, since it contains salts in solution, compensation must be made for the amount of salts deposited in the pores after evaporation. This requires extra care and calculation. Distilled water, resembling most nearly the properties of the reservoir fluid, may be used on certain rock samples; however, on some structures, principally those of shaly or clay nature,

' it causes a swelling of the sample and a change of its characteristics.

Any of the liquids named above can be used, however, there should be a correction applied to the volume retained depending on the nature of the liquid, other than water used. This correction is a ratio of the absorption factor of the crystals to the liquid used as compared to that of water. When the core sample consists principally of CaCOx crystals, the correction factors for the various liquids are: 1.24 for toulene, 1.24 for benzene, 1.16 for tetrachloroethane, and 1.75 for normal propyl alcohol. If the principal core component is siliceous material then the correction factors are: 1.11 for toluene, 1.13 for benzene, and 1.11 for tetrachloroethane. A simple test to determine the principal constituents of the sample is the placing of a drop of dilute hydrochlorid acid on the rock. A vigorous evolution of carbon dioxide means an abundance of CaCOs, while no reaction means the principal coonstitucut is that of siliceous material. Since most core samples are made up of a combination of the two types, an average correction factor can be used for routine purposes. A value of the irreducible water as determined by using the various fluids will be shown later.

Figure 1 shows one form of balance I which may be used in carrying out my invention. It consists of a beam 2, a fulcrum 3, a pan 4, a weight pan 5, weights 8, and a mirror 1 onto which is projected a beam of light from a light source 8 and which beam is then reflected onto a scale 9.

The balance of the type illustrated here is in equilibrium with its full set of weights in place on platform 5 before it is put to use.

The liquid treated core sample It is placed in a tube H usually of glass, and tube H is placed on platform 4 of balance i. A sufiicient number of suitable weights are then removed from platform a which will correspond to the weight of the prepared sample and tube placed upon pan 4; the balance is thereby brought to equilibrium after which the periodic measurement of evaporation weight loss is begun. A stream of gas, ordinarily air from tube 12 is now passed over and around core sample it with only enough velocity to effect a rapid evaporation but not enough to agitate the balance so that a reading might be hard to obtain. Gases other than air may also be used with favorable results. Examples of these gases are nitrogen, carbon dioxide, neon, and others that do not react with the sample.

.The stream of airpr gaS'lPQSSiIIgRDVGI the evaporated from the sample. This continuous v decrease inweight is noted-on the scale 9. There :willbe a markeddecrease in the rate of evaporation at a point where the forces "causing 'the 1 weight loss by evaporation begin to be overcome .by the -.capillary or cohesive forces which exert the greatest effect when residual amounts of liquid remain. The water .or other liquid used.

7 remaining in the core, represents the irreducible ,liqu-id present vin the formation.

Weight readings are taken at suitably spaced intervals of time, and these transferred to volumetric readings, are plotted on cartesian co-ordinates as a function of time which results in a curve such as is shown in Fig. 3.

The data for the curves, Figs. '3 and 4, were obtained. by saturating 'test'plug No. 8 (shown in Table I) with tetrachloroethan'e, wa-terfand toluene; the points A, Band C of Fig. 3 indicate the inflection point corresponding to'the irreducible liquid content for the various fluids respectively. The points A, B, and C referred to :are each the measure of the maximum amount of fluid remaining as an adsorbed 'film upon the internal surfaces of the sample after [free evaporation of the bulk liquid from the interstitial spaces has occurred. Such'remaining surface film of liquid is not free-flowing :nor is it subject to free evaporation but rather to a much slower desorptive vaporization. This desorption region of the curve is substantially linear with time. Therefore, to graphically determine' the point of irreducible liquid content requires that a sufiicient number of readings be taken .in the desorption region to plot a straight line. This straight line such as a-a, bb', or 0-0, in Fig. 3 is then extended to the left as shown. The tangent or inflection points A, B, and C thus Y :maximum :quantity-of unflowable =.adsorbediiuuid will remain substantially constant and reproduciwble within the normal variations :of .room "item-- formed show graphically where free liquid has disappeared and where desorption of the remaining internal surface-adsorbed liquid begins. The inflection point values are tabulated below together with the saturant absorption factors and the pore volume of the particular sample under study in Fig. 3.

The d column expressing the desired irreducible water value isderived by applying the data for a given saturant from the first three columns according to the formula,

Reasonably constant temperature conditions "such as ordinarily prevail under room conditions are satisfactory for the accuracy'of the method.

The reason for this is that the desorptive vaporization of the adsorbed liquid film is relatively slow and little affected by temperature change in comparison'with'theinitialevaporation of tree 'Iliquid. Thustheinfiectionpoint marking the perature.

The core may .be considered as being made up of two prinicipal parts: .(:1) the sand grains :held together by a cementing material and :(2) the pore volume or free space between the :grains. The pore volume .may be considered :as :being :composed of capillaries capable of transmitting soil, gas, .or water, and :a certain volume occupied by the irreducible water retained .by "the smaller capillaries. This latter water, which is to "be determined, will .not flow "from the core sample even when flushed with an oil, natural gas, or other reservoir fluid.

It is to be noted that the treated core sample It may be placed either in a glass tube II as illustrated, or simply in front of a funnel through which air is passed. The saturated sample l0 mayrbe suspendedin air at room temperature and pressure with no air blowing around it.

Another apparatus which may be used in the practice of my improved method is that illustrated in Fig. 2 in which the numeral 53 represents a container used as an enclosure for the apparatus. Tube 1.1, provided with Valve 19 admits air, or other suitable gas to the container 3. The container 53 is provided with valved exhaust tubes is which may be connected to a vacuum pump or merely left open to the atmosphere. A pressure gauge 2?: is provided to indicate the pres sure within the container H5. The core sample 2| is treated in the same manner with a liquid suchas wateror other vaporizable liquid as before described, and is held suspended from a coiled fiat spring it by means of holder or clamp 14. The coiled flat spring it; is rigidly attached to the top of container i3 and has a small mirror [6 fastened thereto .50 that any movement of the spring .15 is directly transmitted to the mirror I6. A light'beam is directed on the mirror 66. As the sample changes weight, the mirror is moved by consequent change in position of the spring and by passing .thebeam of light reflected by the mirror 16 through a window in .theiside of the container l3 and. onto a photographic film that is moving'w'ith constant velocity at right angles to the rotation .of the mirror, the evaporation rate of the liquid from the sample may be recorded. Such photographic equipment hasnot been'illustrated as .it is felt that it is well known to those skilled in the art and further mention is not necessary. The determination of the irreducible water is then accomplished in the same manner 'asbefore'd'escribed.

till

At this point it may be noted that when the liquid used has a vapor pressure less than atm'ospheric'at room temperature, the valved outlet tubes i8 may be'left open'to the atmosphere tior connected toa vacuum pump if the evaporation rate is to "be accelerated by having the pressure within the container at less than atmospheric. if the fluid used hasa'vapor pressure greaterithan atmospheric at room temperature,.thenthe valves '13 and 19 may be so adjusted that the pressure withinthe container is greater than atmospheric. Regardless of the 'specificliquid used, the control of the pressure within the container |.3 may be utilized to control the evaporation rate of the liquid .from the sample. By usinga liquid ,havfine a vapor pressure greater than atmospheric at room temperatures .or by suitablapressure control over the pressure in .the containertheliguid may be caused to evaporate atthe desired .r.a'te

without the necessity of blowing a gas onto the sample.

Table I given below lists the results of tests made with various liquids and types of core samples. The agreement in the values of irreducible water shows that any of the liquids mentioned may be used in this evaporation method.

Furthermore, the evaporation method is equally applicable to core samples over a wide range of permeability to air as will be noted in Table 1. Samples having very low air permeability, e. g., Nos. 7 and 8 of Table I, are subject to erratic results when tested by other methods.

up of fluid passing through the small capillaries and spreading over the surface of crystals too greatly separated to be classified as microcapillaries. The energy causing this flow has been referred to as immersional wetting and expresses the free energy change when a solid surface in equilibrium with the vapors of the liquid is covered by the liquid.

To calculate the capillary pressure, consider this energy as causing a flow of liquid to be hydrostatically balanced by an applied capillary pressure at any saturation. The following equation can be derived from the calculation of the capillary pressure and a typical curve, as ob- Table I IRREDUCIBLE WATER OF CORES Weight Air Perme- Irreduciblo l Pore Porosity of ability Water (per- Plug No. I mmation Sample Vtglglle (351;. (mum saturating Liquid cent pore (grams) darcys) volume) letrachlorethane 6. 5 1 Dolomite 10.573 2. 13 39. 4 5, 123 Toluene 6. 7 Benzene 7. 4 Tetraclilorethane.. S. 2 2 Sandy Dolomite. 10. 451 1.36 25. 7 2, 648 8. 3 8. 2 10.8 3 Sandstone 12. 829 1. 13 1o. 0 539 Toluene 11.6 Benzene 10.9 Tetraehlorethane. 7. 4 4 .do 12. 054 1. O0 19. 1 295 Toluene 7. 9 N-Propyl Alcohol. 7. 5 when... 17.0 5 do 14. 416 1. 37 20. 6 115 Tetraehlorcthane.... 17. 7 Toluene l7. 3 'letrachlorethanan. l6. 8 6 Limestone 20. 657 1.53 17. 9 l9 Toluene l7. 1 Benzene. 17. 0 7 do 13 461 1 1 1s 2 3m 27. 2 8 -d0 13. 380 1.03 16. 9 28. 4 27. 8

Table II IRREDUCIBLE WATER USING NORMAL SIZE PLUGS AND CORE CHIPS Air Perm- Irreducible Weight of Pore Core eabihty Water (per- Sample Formation vrgicnine (ml-1m saturating Liquid cent Pore darcys) volume) 19. 407 2. 79 674 Tetrachlorcthane 16. 9 9 1. 966 .29 674 d 16.7 22. 622 2. 75 825 Toluene 14. 0 10 do 2.415 .33 825 -do 13.6 2. 415 S3 825 Tetraehlorethane. 13. 7 13.360 1. 75 16 .d0 16.8 11 do .r 2.252 .33 16 .do 17.4 2. 252 33 16 Toluene l7. 2 12 do 21. 853 2. 03 10 Tetraehlorethane.. 23. 6 2.908 .27 10 do 24.0

As before mentioned, my method of determining irreducible water can be made on either standard plugs with flat faces, or on pieces which may be smaller with irregular surfaces which are generally known as core chips. Table II given above shows the agreement of the evaporation method, testing the same sample either as a normal test plug or as a chip of approximately 2 grams.

By my method of determining irreducible water, capillary pressures, and other related characteristics of the core sample, these properties are derived from the observed data, calculated by a hydrodynamic treatment of the data. From a standpoint of surface relationship, the test sample undergoing a saturation change by evaporation is also experiencing the flow of fluid from the inner section to the exposed surface. This flow is made tained by this method, is shown in Figure 4, which was derived for core sample 8 shown in Table I.

aSAA, n Tr.

Where The change in internal surface area of the rock can be calculated from the liquid flow deter mined from the curves represented by Figure 3. This method is, therefore, also a means of obtaining data for the calculation of the internal 9 surface area of the rock exposed to fluids available to flow.

An observed data curve such as shown in Figure 3 gives the liquid volume content as a function of time and may be represented by a decay equation of the form:

V =Vo where V=volume of liquid in core sample at any time (t) Vo=volume of liquid in core sample when i=0,

i. e., at start of evaporation test a=an exponential constant of the equation depending on the characteristics of the core sample.

From a hydrostatic consideration it can be mathematically shown that the constant (a) of the equation is the resistance of the core sample to fluid flow and therefore an inverse function of its permeability. By applying the correct units to the constants associated with the exponent (a), the permeability can be calculated in millidarcys. My method, therefore, enables the obtaining of data sufficient for the calculations of the permeability of the core sample.

Since the core surface is exposed in all directions, the permeability found by this method is more representative of the ability of the rock structure to produce oil and/or gas than the permeability determined by methods that are unidirectional determinations.

From the foregoing description it can be seen that the present invention comprises steps of obtaining a sample, saturating it with a vaporizable liquid, and evaporating the liquid from the core sample while taking accurate weight determinations at suitably spaced intervals of time. For the purposes of this invention it has been found that an overall evaporation time of approximately one hour affords the desired results. However, it is to be noted that this time limit is not critical and may be of any duration that is practical, while still affording a rapid determination. It is also possible to determine, with the data obtained by this method, (1) the irreducible water of a porous rock sample, (2) the permeability of a porous medium, and (3) internal rock surface area and (4) the capillary pressure. This invention may be valued by the fact that the data required for calculations is obtained within one hour while these values by methods heretofore used require several days.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims, or the equivalent of such, be employed.

I therefore particularly point out and distinct- 1y claim as my invention:

1. A method for determining the irreducible water of a subsurface stratum comprising the steps of saturating a core sample taken from said stratum with a vaporizable liquid of known properties, evaporating the liquid from said sample, periodically weighing the sample during the evaporation, plotting a rate of evaporation curve from the findings, determining the inflection point on the curve, and determining the irreducible water of said sample therefrom.

2. A method for the determination of the irreducible water of a sample taken from the subsurface strata comprising the steps of treatin the sample to remove approximately all of its petroleum content, saturating said sample with a volatile liquid of known properties, evaporating the liquid from said sample, and determining the irreducible water of said sample from the inflection point on a plotted rate of evaporation curve.

3. A method for the determination of the irreducible water of a sample taken from the subsurface strata comprising the steps of treating the sample to remove approximately all of its petroleum content, saturating said sample with tetrachloroethane, evaporating said tetrachlorethane from said sample, and determining the irreducible water of said sample from the inflection point on a plotted rate of evaporation curve.

4. A method for the determination of the irreducible water of a sample taken from the subsurface strata comprising the steps of treating the sample to remove approximately all of its petroleum content, saturating said sample with toluene, evaporating said toluene from said sample, and determining the irreducible water of said sample from the inflection point on a plotted rate of evaporation curve.

5. A method for the determination of the irreducible water of a sample taken from the subsurface strata comprising the steps of treating the sample to remove approximately all of its petroleum content, saturating said sample with benzene, evaporating said benzene from said sample, and determining the irreducible water of said sample from the inflection point on a plotted rate of evaporation curve.

6. A method for the determination of the irreducible water of a sample taken from the subsurface strata comprising the steps of treating the sample'to remove approximately all of its petroleum content, saturating said sample with n-propyl alcohol, evaporating said n-propyl alcohol from said sample, and determining the irreducible water of said sample from the inflection point on a plotted rate of evaporation curve.

7. A method for the determination of the irreducible water of a sample taken from the subsurface strata comprising the steps of treating the sample to remove approximately all of its petroleum content, saturating said sample with water, evaporating said added water from said sample, and determining the irreducible water of said sample from the inflection point on a. plotted rate of evaporation curve.

8. A method for the determination of the irreducible water and other physical properties of a sample taken from the sub-surface strata comprising the steps of treating the sample to remove approximately all of its petroleum content, saturating said sample with an inert, residue free volatile liquid of known properties, evaporating the liquid by the exposure of said saturated sample to a stream of inert vapor, determining from the inflection of the rate of the plotted evaporation curve the point at which irreducible water remains, and calculating from the curve other physical properties of the subsurface strata represented by said sample thereof.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 648,868 Hartshome May 1, 1900 694,782 Prinz Mar. 4, 1902 2,359,278 Allen et a1 Oct. 3, 1944