US2330721A

US2330721A - Method of determining connate water content of cores

Info

Publication number: US2330721A
Application number: US443544A
Authority: US
Inventors: Miles C Leverett
Original assignee: Standard Oil Development Co
Current assignee: Standard Oil Development Co
Priority date: 1942-05-18
Filing date: 1942-05-18
Publication date: 1943-09-28
Anticipated expiration: 1960-09-28

Description

Sept. 28, 1943. c, L E 2,330,721

METHOD OF DETERMINING CONNATE WATER CONTENT OF CORES Filed May 18, 1942 30 Cm. of Mercury ATTORNEY 5/, l00% ,Per cent suturofion'woier BY 7724124 INVENTOR.

Patented Sept. 28, 1943 WATER CONTENT OF CORES Miles 0. Leverett, Houston, Tex., assignor to Standard Oil Development Company, a corporation of Delaware Application May 18, 1942, Serial No. 443,544

6 Claims.

The present invention is directed to a method of determining the connate water content of cores or samples of subsurface formations.

It is essential to determine the connate water content of producing formations in order to determine the petroleum reserves therein. By connate water is meant that water which is found associated with the rock in a petroleum reservoir 'at the time of first tapping by the drill.

The present methods of determination of connate water content do not adequately take into account the fact that the cores frequently are flushed by water from the drilling fluid. Further, cores taken by any method may lose part of their liquid contents by evaporation prior to laboratory examination. Both these features of conventional core analysis methods are overcome by employing the method of the presvent invention.

Other objects and advantages of the present invention may be seen from a reading of the following description taken in conjunction. with the drawing in which- Figs. 1 and 2 are diagrammatic illustrations of apparatus suitable for use when carrying out the determination of connate water in accordance with the present invention; and

Fig. 3 is a curve indicating the relationship between the capillary pressure and per cent of saturation of a core from a typical producing formation.

It is postulated that in a petroleum containing reservoir superimposed over a water stratum, the connate water of the petroliferous parts of the reservoir is in equilibrium with the water in the underlying water stratum. It can be shown that, in order for this to be true, the connate water within the petroliferous parts of the reservoir must exist under a pressure which is less than that of the water in the water table; that is, the water immediately underlying the petroleum bearing sections. It now has been found that the amount of water retained by a core sample at equilibrium depends on this pressure deficiency or capillary pressure.

It is therefore proposed that any core sample which has not in some way been irreversibly altered since removal from the reservoir, can be returned to the water content which it bore in the reservoir by bringing it again into equilibrium with the water-at a capillary pressure equivalent to that under which itexisted in the reservoir. This capillary pressure can be evaluated from the fact that it is equal to the product:

G-A -H where G is gravitational constant, delta rho is the difference in density between the water and the non-aqueous fluid in the reservoir, and H is the difierence in elevation of the core in its originalposition in the reservoir and that of the water table.

One form of apparatus which has been found suitable for practicing the present invention is that shown in Fig. 1. It will be understood that this is merelya diagrammatic illustration, and

5 that the actual apparatus employed in the laboratory will have conventional refinements. In the drawing, numeral II is a wide-mouthed thistle tube, to the lower end of which has been attached a flexible tube l2 and leveling bulb I3. Mercury i placed in the apparatus so that itwill fill a portion of the leveling bulb, the amount of mercury employed being such that a difference in pressure of approximately 30 centimeters may be obtained in the enlarged portion of th thistle tube, as will be explained hereafter.

In the enlarged portion of the thistle tube there is a porous membrane of a fine granular solid, such as barium sulfate, saturated with a solution having approximately the same salinity as the water in the well from which the core was taken.

This membrane is preferably formed by arranging a fritted glass plate [4 in the thistle-tube, placing saline solution therein, and then depositing a layer [5 of the granular barium sulfate on the fritted glass plate.

To test a core, the level of the saline solution is pulled to the level of the top of the porous membrane I5 by lowering the leveling bulb, care being taken that the suction applied does not exceed the amount which the porous membrane i5 is capable of sustaining. The sample i6 is then placed in the thistle tube with its lower surface resting on the upper surface of the membrane l5. Either before or after the sample has been placed, the leveling bulb I3 is positioned so that the diiference in pressure above and below the porous membrane is equivalent to the capillary pressure under which the core existed in the reservoir. Preferably, the water used should have the same surface tension as the interfacial ten sion of the water and petroleum in the reservoir, but this I have found to be not absolutely essential in many cases. When the leveling bulb has been so arranged, a loose cover I! is placed on top of the thistle tube to prevent the undue loss of water by evaporation, and the apparatus is allowed to stand in this position for a suitable period, such as several days, in order to allow the moisture content of the sample to com to equilibrium with the saline solution. The sample is then removed from the apparatus, weighed, dried,'and reweighed. The loss in weight is the water content of the core. The volume occupied by this water within the core can be computed from the known salinity of the water.

Another apparatus suitable for practicing the present invention is that illustrated in Fig.2.

and porous membrane of fine granular solid l5 corresponding with like elements described in Fig. 1. In setting up the apparatus, a saline so-'- lution is placed in thistle tube 18 and the membrane I is laid down in this solution. Excess pressure is then employed to force the solution down to the upper surface of membrane IS. A suitable means of applying the excess pressure is by the use of a stopper l9, clamp 20 and tube 2| connected to a reducing valve 22, through which compressed air is. supplied. The amount of pressure used may be determined from mercury filled U-tube 24. The excess pressure is then relieved, as by closing a valve 23 and removing the stopper l9, and sample It is placed in position on bed IS. The stopper i9 is again put in position and the pressure above membrane l 5 adjusted to the desired value by using regulation valve 22. The sample is allowed to remain in this apparatus until the moisture content of the sample has reached an equilibrium and the sample is then removed and treated as described in connection with Fig. 1.

As stated above, it is preferable to adjust the difference in pressure to correspond with the difference in pressure between the sample in position in the earth and the height of the water table. It has been found, however, that when this difference is equivalent to or exceeds approximately 30 centimeters of mercury, the employment of a pressure differential of 30 centimeters of mercury in analyzing the sample gives substantially the same results as the pressure difference actually present in the undisturbed reservoir. This is illustrated by Fig. 3. Fig. 3

is a curve showing the manner in which the the density of the reservoir fluids. The water content, after thecore has been allowed to come to equilibrium in this situation, is substantially connate water content petroleum reservoir, comprising the steps of.

physically contacting the sample with water, inducing a difference in pressure between the .sample and the water comparable to the pressure difference between the water table and the connate water in the sample in its original terrestrial position, allowing the moisture content of the sample to come to equilibrium, and removing and determining the water content of the sample.

2. A method of determining connate water content of a sample taken from an underground petroleum reservoir comprising the steps of saturating a membrane with water and retaining it thereafter in at least capillary contact with a body of water, placing the sample in physical contact with said membrane, exerting a difierential pressure between said sample-and said body of water through said membrane comparable to the pressure differential between the connate water of the sample and the water table in the undisturbed reservoir, and maintaining this pressure differential until the moisture content of the sample becomes substantially constant, and subsequently determining this moisture content.

3. A method in accordance with claim 2 in which the pressure diiferential between the connate water and the water table in the undisturbed reservoir is determined as less than 30 cm. of mercury, and the pressure differential between the sample and the body of water is lent pressure differential is more than 30 cm.

mercury, these cores can be run by the present technique without precise knowledge of the elevation of the water table in the reservoir. This fact increases the utility of the present method.

From the facts set forth in the foregoing paragraph, coupled with conventional thermodynamic reasoning, it can be shown that for cores taken from well above the water table it is also unnecessary that the surface tension of the water equal the interfacial tension of the water and petroleum in the reservoir. This fact further increasesthe utility of the method. The phrase well above the water table usually implies a distance of more than 15 to 40 feet vertically. The exact distanceis variable and depends on the texture of the reservoir rock.

From the foregoing description, it can be seen that the present invention comprises essentially bringing the core sample into communication with water having a surface tension approximately equal to the interfacial tension of the water and petroleum in the reservoir, which water in the test exists under a pressure less than that of the surroundings of the core in the test by an amount which is computable from the position of the core within the reservoir and maintained at substantially this determined value.

4. A method in accordance with claim 2 in which the pressure differential between the connate water and the water table in the undisturbed reservoir is greater than 30 cm. of mercury, and the pressure differential between the sample and the body of water is maintained at 30 cm. of mercury.

5. A method of determining connate water content of a sample taken from'an underground petroleum reservoir comprising the steps of saturating a membrane with a saline solution of the same salinity of the water of the well from which the sample was taken, maintaining capillary contact between a body of the saline solution and said membrane, placing said sample in contact with the membrane on'the opposite side of said membrane from the body of solution, adJusting the pressure across the membrane to a constant value so that when equilibrium is established between the sample and the saline solution the moisture content of the sample will correspond to that of the sample when the sample was in the undisturbed reservoir, and determining said moisture content.

6. The method in accordance with claim 2 in which the surface tension of the water comprising said body is substantially equal to the interfacial tension between the water and petroleum within the reservoir.

MILES C. LEVEREI'I'.