US2345535A - Method of determining porosity - Google Patents

Description

March 28, 1944. w HORNER 2,345,535

7 METHOD or" DETERMINING POROSITY Original Filed Jan. 5, 1938 MW-H1 INVENTOR William L. Home] ATTOR Patent eii Mar. 28, 1944 METHOD OF DETERMINING POROSITY William L. Horner, Houston, Tex, assignor to Core Laboratories, lnc.,-Dallas, Tex., a corporation of Delaware Original application January 3, 1938, Serial No. 183,018. Divided and this application July 8, 1942, Serial No. 449,969

4 Claims.

This invention relates to the art of determining the porosity of solid, non-compressible, permeable bodies, and more particularly to the determination of the porosity of rock samples taken during the drilling of oilwells.

Such rock' samples are commonly referred to as cores, and-the rock strata from which the cores are removed are called sands. Such sands, and hence their cores, may contain liquids such as water and hydrocarbons, and possibly gas. By the time the core samples are received at the surface of the well, part of the liquid has usually been pushed out of th pores and replaced by gases which, in this specification, will be referred to as free gas. A single core section may be as long as four or five feet and have a diameter of from one and a halt to three inches.

For many years such cores have been analyzed for difierent properties, and in recent years the importance such analyses has gained increasing recognition. One of the properties that is becoming increasingly important to determine is that known as the porosity of the sand, that is, the amount of pore space or the sand (the space which may be filled with water, extractable liquid, hydrocarbons, or gas, as distinguished from the mineral matter comprising the body of the sand) It is an object of the present invention to provide an improved method for determining this porosity value, which method is more rapidly carried out and more accurate in its results than those methods previously used. The rapidity with which the porosity determination is made becomes important when the cores are analyzed as they are taken from the well, and the results of the analyses are used to tell at what sand stratum the well is to be completed, that is, the sand horizon from which the well is to produce to the exclusion of other sand horizons through which the well passes. Under such circumstances, long delays, occurring as a result of time taken to make the analyses of the cores, become expensive and unless rapid analysis is possible the results 01 core analysis cannot be used in deciding upon the best horizon at which to complete the well. 5

The matter of accuracy is important because, on the basis of the porosity determination, estimates are made as to the amount of hydrocarbons that a sand of given area and depth contains, and since the area may be a matter of square miles and the depth a matter 01' feet, a small error in the porosity determination may make very considerable error in the total estimate obtained using the porosity determination. This application is a division of my copendlng application Serial No. 183,018, filed January 3,

A core, as it is received at the top 01 the well, usually contains in its pores a mixture of fluids consisting primarily or tree gas, hydrocarbons usually in liquid form, andwater. In the present embodiment, the total volume of these fluids is measured in order to' obtain the amount 01 the pore space and the pore space is then figured in terms of, for example, cubic centimeters of pore space per unit volume of core sample. The amount of pore space occupied by free gas is first determined on one part of a divided core sample taken from a section of a core length. On the other part of the same core sample the amount of extractable water and hydrocarbons present is determined. It is assumed that the pore space is filled with gas, water and fluid, or extractable hydrocarbons. The results of these determinations are then added to obtain the total volume of extractable fluids present in the core sample. From this volume is obtained the porosity of the section of the core from which the sample was taken.

Referring to the drawings,

In Figure 1 apparatus is shown for making rapid determinations oi (1) the volume oi. a core sample, which measurement is utilized in determining the density o! the sample, and (2) for determining the free gas content of a core sample.

The apparatus shown in Figure 2 is a distillation apparatus for distilling oil. and collecting the liquid content of the core sample. The novel subject matter of the apparatus shown in Figure 1 is described and claimed in my copending application Serial Number 351,891, flled August 8, 1940, which is a division of my copending application Serial Number 183,018, filed Jan. 3, 1938, Patent No. 2,296,852. The apparatus shown in Figure 2 is described and claimed in my Patent No. 2,282,654, which issued from application Serial Number 351,892, filed August 8, 1940, which is a division of my said 00- pending application Serial Number 183,018. Therefore, this application is a division of my said copending application Serial Number 183,- 018, Patent No. 2,296,852.

In practicing the known methods of and in the apparatus for determining the porosity of such core samples it is necessary first to extract from the core sample all extractable material. This is accomplished by means of flowing a solvent through the core sample and in instances where the permeability of the core sample is rel atively low, that is, wherein it has a relatively high resistance to passage of the liquid through the sand, this extracting step may involve a con= siderable period of time. even as much as twenty hours or so, for a single small sample. Further, these known methods require theme of samples of small sizes, that is, from 5 to 20 cubic centimeters, so the possibility of error in a determination is increased. Moreover, because the determinations are made on such small samples of the core taken from a well, it frequently turns out that the small samples selected are not representative of the core itself and that the porosity determinations are inaccurate to the extent that the total liquid content as measured on. one

portion of a core rock exceeds the total porosity as determined on a small core sample taken from an adjacent section of the core.

The practice of the present invention not onlyeiiminatesthe extracting step but permits the use of large samples for the determination. Thus the determination of the porosity in accordance with the present invention not only materially reduces the time required for the determination but also gives far more accurate results.

As pointed out, practicing the present invention to determine the porosity of a core taken from a well includes first separating into two part a sample of the core, one weighing, for example, approximately 40 grams, and the other weighing approximately 180 grams. The first part, which may be called part A, is used for determining the amount of free gas present in the pores of the sample comprising part A; and the second, or part B, of the sample is used for determing the amount of extractable liquid present in the pores of part B. By the term extractable is meant those liquids present in and removable from the pores.

The apparatus shown in Figure 1 may be used on the part A of the sample to determine its free gas content. to determine the density of the core sample so that the volume of the part B sample can be calculated from its weight. In thi apparatus the core sample is first immersed in a liquid which does not enter the pores of the sample immersed in it. As will be described, the amount of liquid displaced by the immersed sample may be This apparatus is also usedmeasured and the volume of the sample calculated from such measurement. Then the liquid in which the sample is immersed is subjected to a high pressure to force the liquid into the pores of the core sample to compress and substantially occupy the space previously filled by free gas. The amount of the liquid that thus enters the pores of the core sample is measured and this measurement becomes the total amount of pore space of the core sample occupied by free gas. Now, by dividing the amount of space thus occupied by free gas by the total volume of the sample, the amount of pore space occupied by free gas per unit volume of core sample is obtained.

In using this method of analyzing the free gas content of a porous body it is desirable to use a liquid which does not chemically react with the materials comprising a sample and which has a surface tension such that the liquid does not penetrate the pores without being forced into them.. One liquid that I have found which fulfills these requirements is mercury.

Other liquids, however, may be used which chamber indicated at it of a mercury pump.

The chamber in is closed at its top by a suitable cover plate it which may be bolted in place by bolts i8 after the sample is placed in the chamber. The joint between cover and chamber is made fluid-tight by asuitable gasket 20. The inside of the cover is conically shaped, as shown, and provided with a small vent 22 at the apex of the cone. The small outlet may be closed off by a needle valve 24 which is suitably threaded down through a support 26 to set in the opening 22. As shown in the drawings, the sample chamber is so constructed that it has a suitable air vent at its top and an inlet at its bottom, and is so shaped that as the non-compressible mercury is flowed into the chamber through the bottom all the gas is driven out of the chamber ahead of the liquid as the liquid reaches the vent. Furthermore, the core sample used is preferably suitably haped and trimmed to eliminate pits and cavities onits surface that mightretain air bubbles.

With this construction the sample may be placed in the chamber while th chamber is empty by simply removing the cover I6 and then bolting the cover back on. It is possible to tell when the chamber is full of mercury with no occluded air by filling the chamber with mercury until the mercury appears in the small vent 22.

The inlet E2 of the chamber Iii communicates with the plunger chamber in which a plunger 28 is reciprocally mounted and passes out of the chamber through suitable bearing and stufling box generally indicated at 32. The outer end of the plunger carries a slider 33 sliding along suitable guide rods 34. Further, the plunger is hollow and threaded to receive a plunger-operating screw 35, one end of which is suitably rotatably mounted in thrust bearings, generally indicated at 36, mounted in a journal 31 secured with respect to the plunger chamber I 4 by means of a heavy base 39 and the guide rods. Keyed to the operating screw 35 is a hand-wheel 38 used for operating the screw.

The top guide rod 34 is suitably marked oil in units of measurement such, for example, as cubic centimeters, and the hub of the hand-wheel is also suitably marked in cubic centimeters, but in thousandths of cubic centimeters, and in accordance with the pitch or the threads on the operating screw so that the position of the plung er may be read in terms 01 cubic centimeters and hundredths of cubic centimeters from the scale In making a determination oi. a sample the cover is first bolted on, with no sample in the chamber, and the needle valve 24 is left open 22. The position ot-the plunger at this'moment is noted from the scale. The plunger is then retracted to lower the mercury level in the chamber, the lid is taken of! and the part Act the sample, suitably prepared and-accurately weighed, is put into the chamber and the lid bolted back on. With the needle valve still open the plunger is again moved inwardly by the hand-wheel until the mercury again appears at the outlet 22, at which time the position of the plunger is again noted. From the difference of the two readings the volume of the sample is obtained. The needle valve is now closed so that no mercury can escape and the hand-wheel is operated to move the plungerinwardly to maintain a pressure on the gauge equivalent to 50 atmospheres. th hand-wheel being moved as fast and for such length 01 time as is required to maintain this ring I28. held together by three tie rods I28. The ends of the resistance wire come out through the top I24 and are secured to suitable insulated terminals I from which extends a cord adapted to connect the resistance wire with a supply or electricity. As shown in the drawings, the heating mechanism does not extend to the lower part of the steel shell H4 and consequently the lower portion 0! the shell and the bottom I34 are not heated to the temperatures to which the upper parts of the apparatus are heated.v

The annular supporting flange H8 is provided with downwardly extending bolts I32 for clamping a base plate I34 to the supporting flange. The inside of the base plate is conicallydished at its center, as shown, and at the apex of the conical pressure. After it is no longer necessary to move the hand-wheel to keep the pressure at 50 atmospheres, the readings on the two scales are again noted and the diflerencebetween this reading and the last succeeding reading gives correctly the volume of mercury necessary to compress the gas in the sample and to fill the pore space occupied by the gas. This latter reading, as above pointed out,-is increased by two percent in order to take care of the volume of space occupied by the gas now under 50 atmospheres of pressure. By dividing the gas space volume thus obtained by the volume of the sample the cubic centime-- ters of pore space occupied by free gas per unit volume is obtained.

Further, by dividing the weight or this part A by its total volume the density of the sample is obtained.

The second part of the determination to determine the pore space occupied by liquid may be conveniently carried out in a still such as shown in Figure 2, in which still the weighed part B of the sample is heated to cause the liquid contents therein to distill oil, which contents are condensed and collected in a measuring tube. By dividing the weight of this part B by the previously determined density the total volume of the part B is obtained, and then by dividing the volume of the liquid condensed from the part B by the total volume of the sample the amount 0! liquid present in the pores per unit volume of the sample is obtained.

The still shown is adapted to distill oil and water from core samples S and to collect the products of distillation in a graduated tube generally indicated at I02 supported below the still by a connecting tube generally indicated at I04.

The still comprises an insulated heater having an inner chamber -I 08 adapted to receive a sample container I08. Around chamber I06 is a ceramic shell, such as porcelain, and indicated at H0. The shell is spirally grooved on its outside, as shown, and is open at its bottom and is closed at its top with a porcelain top I I2. Closely fitted inside the porcelain shell is located a polished steel shell II4 having sides and a top and sup tormation is provided'an outlet I38 into which usbrazed a short bronze tube I38. The outlet tube is connected by suitable rubber connections to a relatively long copper tube I40 serving as a condenser and terminating in a rubber stopper I42 in the top of the graduated glass tube I02. A suitable vent I44 also extends upwardly from the stopper.

The sample container I08 is preferably made of some corrosion-resisting material, such as Monel metal, or is plated with some corrosion-resisting plating. It is open at its top and has a' periorated bottom. A spacing rod I48 extends downwardly from the container. 'a

In operation the sample container is filled with pieces of core samples that have previously been weighed. While the base cover I34 is removed the container is inserted in the heating chamber I08 and is pushed to the top of the chamber by the spacing rod I48 as the base I34 is moved into its closing position. After the container and sample have been placed in the heating chamber I06 and the base plate I34 bolted in place, the heat is turned on and the sample is eventually heated to a dull red temperature which insures all oil, gas and water being driven oil. The oil, which is driven off in the form of vapors, is always caused to move to cooler regions, and as it passes downthrough the tube I40 is sufflciently ported at its open bottom by a flange ring 6.

cooled so that it condenses and drips into the graduated tube I02.

The above method of distilling the water and oil from the sample has the advantage of being rapid and accurate. The heavy vapors of the distillate fiow by gravity to the colder regions and the condensing tube. A further advantage of this method is that the heating of the sample is carried out uniformly so that the sample of water obtained is free from wateror crystallization (which is not extractable) and the sample of oil obtained represents practically 98% recovery.

The distillation operation takes approximately forty-five minutes and may be started before the free gas determination and so may be running during the time the free gas determination is being made in the apparatus shown in Figure 1.

For convenience, a chart of the above operations is shown as comprising eleven steps. Whereas other apparatus than that shown in Figures 1 and 2 may be used for carrying out the method of the invention, the apparatus shown has the particular advantages above pointed out.

, Chart of operation 2. Weigh part A =grams Measure total volume of part A by immersing in mercury i 4. Measure total free gas oi! part A by subjecting it while immersed in mercury to elevated pressure to comp! -ss free gas and iill its por Divide cubic centimeters of free gas (4) b total volume (8) to obtain cubic centmeters of pore space occupied by free gas per unit volume of core sample =cc./unit volume 6. Calculate density oi! sample by dividing its weight (2) by its volume (3) ..=cc.,/gram 7. Weigh part B =grams 8. Distill oil! extractable liquids from part B and collect and measure to obtain total volume of liquid in pore space =cc.

9. Calculate volume of part B by dividing its weight (7) by its density (6) =cc,

l0. Divide cubic centimeters of liquid (8) by total volume (9) to obtain cubic centimeters of pore space occupied by liquid per unit volume of core sample =cc./unit volume 11. Add cubic centimeters of pore space occupied by free gas per unit volume of core sample to cubic centimeters of pore space occupied by-liquid per-Emit volume of core sample (10) to obtain cubic centimeters of pore space per unit volume of core sample =cc. pore space/unit volume As various embodiments might be made of this invention, and as various changes might be made in the construction herein described, all without departing from the scope of the invention, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a the sample into two parts, weighing each of the parts, immersing one of the parts in a liquid to determine its volume, subjecting the immersing liquid to pressure to cause the liquid to fiovwinto the pores and compress the gas therein to such a small volume that thepore space is substantially filled with the liquid and measuring the liquid that thus enters the pores, combining the volume measurement and the measurement of the amount 01 liquid that enters the pores to obtain the amount of pore space per unit volume of core sample occupied by gas, distilling oii, collecting and measuring the extractable liquid from the pores of the other part, obtaining the den sity of the sample from the volume and weight of the first part and the volume of the second part from its weight and density, combining the volume of the collected liquid and the vol- A ume of the second part to obtain the amount of pore space per unit volume of core sample occupied by liquid .and adding the latter determination to the free gas obtained to obtain the pore space per unit volume of core sample.

3. The method of determining the porosity of a core sample comprising the steps of dividing the sample into two parts, weighing each of the parts, measuring the free gas content of the pores of one of said parts without extracting the liquid content therefrom, measuring the volume of said part to obtain the pore space occupied by free gas per unit volume of core sample, distilling ofi the extractable liquid from said second part, condensing and measuring the same, determining the volume of said second part by dividing its weight by its density obtained from the volume and weight measurementson the first part to obtain the pore space occupied by liquid per unit volume of core sample, and adding the free gas and extractable liquid measurements to obtain the pore space per unit volume of core sample.

4. The method as in claim 3 wherein the weight of the first part is made approximately 40 grams and the weight of the second part is made approximately grams.

WILLIAM L. HORNER.

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