System for measuring the pore volume and permeability of very tight core plugs and method therefor

Abstract

A system and method for measuring Klinkenberg permeability and pore volume of a tight core plug sample is provided wherein a pressure transducer is positioned between the sample cell and the manifold of the system.

Description

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a system and method for measuring permeability and pore volume of core samples and, in particular, to an automated system and method therefore for measuring Klinkenberg permeability and pore volume of very tight oil core samples (i.e., permeability of less than one-hundred microdarcies).

2. of or P_FILL for a period of time from several seconds to one or two minutes. During this time, the core plug 70 will also become partially pressurized. Then, valves 130 and 110 are closed. The pressure transducer 50 is monitored until the pressure reaches equilibrium. This pressure P_E1 PE1 is then recorded.

During the period when the pressure in the core plug is approaching PE1 P_E1 equilibrium, valve 100 is opened venting the manifold 40 to atmosphere. In a second or more, the manifold quickly reaches zero psig, then valve 100 is closed. After pressure equilibrium, PE1 P_E1, has been achieved valve 100 is reopened so that the pressurized helium in the sample holder 60 and core plug 70 is allowed to expand back into manifold 40. Eventually, after a period of time, the helium gas in the manifold 40, the sample holder 60, and the core plug 70, will reach a second equilibrium pressure, P_E2.

This is illustrated in the graph of FIG. 3 wherein P_FILL is the initial pressure to which the manifold 40 and pressure transducer 50 are charged. The core plug is also subjected to this pressure, but because of its very low permeability the core plug does not become fully pressurized throughout its entire length. At time T₁, valves 110 and 130 are closed, after which time the volume of gas contained in line 114 down to the core face 400 (i.e., the dead volume) and the gas in the core plug 70 and in the volume below it, to valve 120, gradually reaches pressure equilibrium, P_E1 at time T₂, after which there is no further change in pressure. Before time T₂, the manifold 40 is vented to atmospheric pressure by opening valve 100 for several seconds and then closing it. At time T₂, pressure P_E1 sensed by transducer 50 is recorded and then valve 110 is openend. Gas from lines 114 and 52, and from transducer 50 above core face 400 quickly expands into manifold 40, causing a sudden decrease in pressure to pressure P t. Gas from the core plug and the volume below it flows more slowly into the manifold 40 and into pressure transducer 50, causing a gradual pressure build-up until equilibrium pressure, P_E2 is achieved at time T₃, after which time no further pressure change occurs.

Hence, under the teachings of the present invention, both P_E1 and P_E2 are pressures that are recorded and delivered over lines 162 to a suitable device 160 such as a computer for ascertaining the pore volume. It is to be expressly understood that device 160 is further capable of controlling the operation of valves 80, 90, 100, 110, 120, and 130 over suitable connections not shown in FIG. 4. Based upon the ratios of the two pressures, the pore volume of the core plug 70 can be ascertained. The curve 300 of FIG. 3 represents the solid curve for a very tight core plug whereas the dashed line curve 300a represents a curve for a core plug of higher permeability, but having the same pore volume as the plug represented by curve 300. In typical embodiments, pressure P_E1 is 180 to 240 psig and pressure P_E2 is between 90 to 120 psig. It is desirable to keep the equilibrium pressures in this range as will be explained more fully in the following.

The time to reach equilibrium is approximately proportional to the compressibility of the fluid used. For example, and in way of illustration, suppose the manifold 40 were filled with water at 250 psig, and the core plug were filled with water at zero psig. Because water has such a low compressibility, only a fraction of a drop flowing from the manifold into the core plug would equalize the pressures in both. If the fluid were instead a gas at low pressure, considerable volumetric flow from manifold 40 must take place into the core plug 70 in order to obtain pressure equilibrium.

Less time is required to reach the initial pressure equilibrium, P_E1, when the teachings of the present invention are followed, compared to the methods of the prior art as shown in FIGS. 1 and 2. The initial equilibrium pressure for a pore volume measurement, according to the prior art, is zero psig, whereas it is preferably 180 to 240 psig according to the present invention. The higher pressures result in a thirteen-fold to seventeen-fold reduction in the gas compressibility over that performed in prior art and a corresponding reduction in time to reach equilibrium.

In the present invention, the second equilibrium pressure, P_E2, according to FIG. 3, is somewhat lower than the corresponding equilibrium pressure, P_E, of the prior art (see FIG. 2). This causes a higher gas compressibility and would require a longer equilibrium time were it not for the smaller manifold volume of the present invention. The time required to achieve this second equilibrium should be about half that required in the prior art. Thus, the time saved for both equilibria involved in a pore volume measurement under the teachings of the present invention is considerable.

There is another important benefit found in the configuration of FIG. 4. An absolutely leak-free system is required to obtain accurate pore volume measurements. Leaks are difficult to ascertain in the prior art configuration of FIG. 1. The approach to equilibrium (see FIG. 2) is accompanied by a declining pressure. One cannot easily tell whether the decline is due to helium diffusing into the core plug or due to a leak in the system. Both would cause the pressure to decline. On the other hand, the approach to the final equilibrium, P_E2, as shown in FIG. 3 for the configuration of the present invention, is achieved with an increasing pressure as helium diffuses from the plug, which is at a higher pressure than the manifold. If a leak is present, then the pressure rise will be followed by a decline; i.e., the pressure will pass through a maximum with time.

The configuration of FIG. 4 offers yet another benefit. The time required for measuring the permeability of very tight core plugs is reduced. To accomplish a permeability measurement, poppet valve 120 is closed as is valve 100. The manifold 40 and the core plug 70 are pressurized to about 240 psig with helium by opening valve 130. After a few seconds, valve 130 is closed and valve 120 is opened, allowing helium that exits from the core plug to be vented to atmospheric pressure. Permeability can be calculated from the instantaneous time rate of pressure decay based upon a number of measurements under methods that are well known; e.g., Jones, S. C., "A Rapid Accurate Unsteady-State Klinkenberg Permeameter", Soc. of Petroleum Engineers, J., (Oct. October 1972), 383-397.

This rate of pressure decay is directly proportional to the volume of the manifold 40, lines 112 and 114 and pressure transducer 50 up to the upper core face 400. Because this volume is reduced, in the configuration of FIG. 4, by a factor of 2 to 2.5 from that taught in the prior art, so, too, is the time for a permeability measurement reduced by a factor of 2.0 to 2.5.

A further time reduction can be made in measuring the permeability of an extremely tight plug, i.e., less than about 0.1 md. To accomplish this, the core is charged with helium to pressure P_FILL as before. But now, before poppet valve 120 is opened to start the pressure decay, valve 110 is closed, leaving only the volume below valve 110 as the helium reservoir to be delivered through opened valve 110. This results in approximately a eight-fold time decrease from the prior art configuration. This is made possible because the pressure transducer is now connected below valve 110, and can monitor the pressure decay when valve 110 is closed.

In measuring high permeability plugs (i.e., greater than 50 or 75 md.), one or both helium tanks 20, 30 are charged with helium as taught in the prior art by selectively opening valves 80 and 90 and this feature is also implemented in the present invention.

In making pore volume measurements, an error analysis shows that the maximum accuracy is obtained when: (1) pressure P_E1 is as close as practicable to the full-scale reading of the pressure transducer, which, in the preferred embodiment, is 250 psig, (2) pressure P_E2 is about one-half the value of pressure P_E1, and (3) the dead volume, V_d, i.e., the volume in sample holder 60 above and below core plug 70 and below valve 110 and above valve 120 is as small as possible. The minimum practical dead volume under the teachings of the present invention, without severely restricting the flow paths to and from the core, is about 3 to 4 cubic centimeters. The second requirement under the teachings of the present invention sets the most desirable manifold volume, V_o. It should be equal to the sum of the core's pore volume V_p and the dead volume V_d. If the dead volume, V_d, is 3 to 4 cubic centimeters and typical pore volumes, V_p, range from 2 to 8 cubic centimeters, then the manifold volume, V_o, should fall in the range of 5 to 10 cubic centimeters. The lower end of this range is favored because the maximum accuracy is desired for plugs having the smallest pore volume. Here, a given error in the absolute measurement causes the largest percentage error. The smaller manifold volume is also desired because it results in a higher P_E2 than would a larger volume. Hence, the time to reach equilibrium is reduced.

In FIG. 5, a core sample holder 60 is set forth which corresponds to the type of core sample holder disclosed in co-pending patent application Ser. No. 06/651,558 U.S. Pat. No. 4,573,342 which issued March 4, 1986. Sample holder 60 has a main body 500 with a top cap 510 firmly holding a connector portion 520. The conduit 114 connects to the valve 110 so that a fluid path 540 is provided from the valve 110 to the sample holder 60. The fluid path 112 is established from the valve 110 to the manifold 40 and a fluid path 52 is established to the pressure transducer 50. The valve 110 works in a conventional fashion through actuation of a plunger 570 to open and close a fluid path between fluid paths 940 and 112. Such a valve may be modified from a conventional valve of the type manufactured by Nupro Company, 4800 E. 345th Street, Wilboughby, Ohio 44094 as Model SS-4BK-V51-1C. The modification consists of counterboring and tapping the bottom of the valve to accommodate the fitting that leads to transducer 50, and of providing an O-ring seal therefore. Fluid path 540 accesses a perforated plate 580 and then the upper surface 400 of sample core 70.

FIG. 5 is intended to be only an example of one way to achieve the flow configuration shown in FIG. 4. Several variations are possible under the below described formulas; for example, a compact arrangement can be configured by incorporating valves 80, 90, 130, 100, and 110 into the manifold body itself by drilling appropriate holes in the manifold to accommodate the air-operated valve stems. Several seals are also eliminated when these valves as well as helium tanks 20 and 30 are integrated into the manifold block.

Hence, upon inspection of FIG. 5, the volume, Vb V_b , equals the volume of the transducer along path 52 plus the volume of the line 114 plus the volume of the perforated plate 580 plus the volume of the cavity 111 beneath the valve plunger when it is in the closed position. And, the lower volume, Vd V_d , equals volume, Vb V_b , plus the volume at the bottom of the plug 70 and valve 120. Hence, the following formulas:

V_b=V(TRANSDUCER)+V(LINE)+V(PLATE)+V(CAVITY) (Formula 2)

V_d =V_b Vb+V(Bottom) (Formula 3)

In the preferred embodiment, the total volume V_d is designed to be typically 3 to 4 cubic centimeters.

The upper volume V_o is equal to the volume of the manifold 40, the volume of the line 112 and the volume to the valves 130, 100, and their associated lines. Hence, the upper volume can be expressed as:

V_o =V(MANIFOLD)+V(IN VALVES 80, 90, 100, and 130)+V(LINES 84, 94, 104, and 134) (Formula 4)

In an ideal situation, under the teachings of the present invention, the upper volume V_o should equal the lower volume V_d plus the pore volume of the core plug VP V_p as set forth in Formula 3:

V_o =V_d +V_p (Formula 5)

In the construction of the permeability/porosity apparatus of the present invention, valves 100, 130, 110 and the associated lines, the manifold 40, the pressure transducer 50 and the sample holder 60 are all designed to be within the ranges provided by Formulas 2 through 4. It is to be understood that when the plunger 570 opens, a displacement of about 0.1 to 0.3 cubic centimeters occurs, and that this plunger displacement can be accounted for. The embodiment of FIG. 4 is desirable over that shown in FIG. 1 since monitoring of the pressure by pressure transducer 50 is located near and is in fluid communication with the core plug 70. Furthermore, since equilibrium time is inversely proportional to compressibility of the helium gas, the embodiment shown in FIG. 4 is several times faster than conventional pore volume measurements. Finally, in the prior art embodiment of FIG. 1, the smallest available reservoir volume for the measurement of permeability was the combination of both the upper and lower volumes, V_o plus V_b or typically 25 cubic centimeters. Under the teachings of the present invention, the smallest is Vb V_b or 3 to 4 cubic centimeters. This relationship provides a factor of at least six times in speed up of measurement for permeability. For example, under the conventional approach shown in FIG. 1, typically if 45 minutes passed for a permeability measurement, now under the embodiment shown in FIG. 6, only six or seven minutes would pass. Hence, for very tight core plugs a definite speed up occurs.

Although the system and method of the present invention has been specifically set forth in the above disclosure, it is to be understood that modifications and variations can be made thereto which will still fall within the scope and coverage of the following claims.

Claims

1. A method for determining the pore volume of a very tight core samples sample with an apparatus having a manifold in selective fluid communication with a sample holder holding said core sample, said sample holder having an outlet, said method comprising the steps of:

(a) sealing the outlet of the sample holder while maintaining the sample holder in fluid communication with the manifold,

(b) pressurizing the manifold volume and the sample holder with a gas to a predetermined pressure, P_FILL,

(c) sealing the core sample holder and a pressure transducer from the manifold while maintaining the pressure transducer in fluid communication with the sample holder,

(d) waiting for the gas pressure in the core sample holder to reach a first equilibrium state pressure, P_E1,

(e) measuring the value of said pressure P_E1 with the pressure transducer at a location between the manifold and the sample holder,

(f) venting the manifold to the atmosphere so that the manifold pressure obtains zero psig is at atmospheric pressure,

(g) sealing the manifold in response to said venting so that the sealed manifold contains zero psig remains at atmospheric pressure,

(h) delivering the pressurized gas at said first equilibrium pressure, P_E1, from the sample holder into the manifold such that the sample holder, manifold and pressure transducer are in fluid communication wiht one another,

(i) waiting for the delivered pressurized gas to reach a second equilibrium state pressure, P_E2,

(j) measuring the value of said said pressure PE2 PE₂ with the pressure transducer at the aforesaid location, and

(k) determining the pore volume, Vp V_p , of the core plug sample in the sample holder based upon the measured values of said first and second equilibrium pressures, P_E1 and P_E2.

2. A method for determining the pore volume of a very tight core samples sample with an apparatus having a manifold in selective fluid communication with a sample holder holding said core sample, said sample holder having an outlet, said method comprising the steps of:

(a) providing a manifold volume, V_o, that is substantially equal to the pore volume, V_p, plus the dead volume, V_d, of the line and the sample holder and a line between the manifold and the sample holder, V_d , (FORMULA 5),

(b) sealing the outlet of the sample holder while maintaining the sample holder in fluid communication with the manifold,

(c) pressurizing the manifold volume and the sample holdersholder with a gas to a predetermined pressure, P_FILL,

(d) sealing the core sample holder and a pressure transducer from the manifold while maintaining the pressure transducer in fluid communication with the sample holder,

(e) waiting for the gas pressure in the core sample holder to reach a first equilibrium statepressure, P_E1,

(f) measuring the value of said pressure P_E1 with the pressure transducer,

(g) venting the manifold to the atmosphere so that the manifold pressure obtains zero psig is at atmospheric pressure,

(h) sealing the manifold in response to said venting so that the sealed manifold contains zero psigremains at atmospheric pressure,

(i) delivering the pressurized gas at pressure, P_E1, from the sample holder into the manifold such that the sample holder, manifold and pressure transducer are in fluid communication with one another,

(j) waiting for the delivered pressurized gas to reach a second equilibrium state pressure, P_E2,

(k) measuring the value of said pressure, P_E2, with the pressure transducer, and

(l) determining the pore volume, V_p , of the core plug sample in the sample holder based upon the measured values of said first and second equilibrium pressures, P_E1 and P_E2.

3. A method for determining the pore volume of a very tight core samples sample with an apparatus having a manifold in selective fluid communication with a sample holder holding said core sample, said sample holder having an outlet, said method comprising the steps of:

(a) providing a manifold volume, V_o, that is substantially equal to the pore volume, V_p, plus the dead volume, V_d, of the line and the sample holder and a line between the manifold and the sample holder, V_d, (FORMULA 5),

(b) sealing the outlet of the sample holder while maintaining the sample holder in fluid communication with the manifold,

(c) pressurizing the manifold volume and the sample holders holder with a gas to a predetermined pressure, P_FILL,

(d) sealing the coresample ep holder and a pressure transducer from the manifold while maintaining the pressure transducer in fluid communication with the sample holder,

(e) measuring the value of the a first equilibrium pressure, P_E1, with the pressure transducer at a location above on said line between the core sample holder and below the manifold,

(f) venting the manifold to the atmosphere so that the manifold pressure obtains zero psigis at atmospheric pressure,

(g) sealing the manifold in response to said venting so that the sealed manifold contains zero psigremains at atmospheric pressure,

(h) delivering the pressurized gas at said first equilibrium pressure, P_E1, from the sample holder into the manifold such that the sample holder, manifold and pressure transducer are in fluid communication with one another,

(i) measuring the value of a second equilibrium pressure, P_E2, with the pressure transducer at the aforesaid location, and

(j) determining the pore volume, V_p, of the core plug sample in the sample holder based upon the measured values of the first and second equilibrium pressures, P_E1 and P_E2.

4. An improved apparatus for determining the pore volume of a very tight core samples sample, said apparatus having a manifold (40) in selective fluid communication through a line lines (112, 114) with a sample holder (60) holding said core sample (70), said sample holder (60) further having an outlet valve (120) being capable of being opened to atmospheric pressure, said improved apparatus comprising:

a valve (110) connected to said linelines (112, 114) for selectively opening and closing said line lines (112, 114),

a transducer (50) connected to said line (112, 114) at a location below between said valve (110) and said sample holder (60),

said manifold (40) being formed to have a volume, V_o, substantially equal to the pore volume, Vp V_p , of said core sample plus the a dead volume, V_d, of said line (114) below said valve (110, said sample holder (60), and a line (52) to said transducer (50), V_d ,

means (130, 132, 134) for delivering gas into said manifold (40) and sample holder (60) at a fill pressure, P_FILL, when said valve (120) is closed, said transducer (50) being capable of ascertaining a first equilibrium pressure, P_E1, when said valve (110) is closed,

means (100, 102, and 104) for venting gas into the atmosphere from said manifold (40) when said valve (110) is closed in response to said ascertainment of said first equilibrium pressure, said venting means becoming closed when said manifold (40) obtains zero psig,

said transducer (50) being capable of ascertaining a second equilibrium pressure, P_E2, when said valve (110) is opened, and

means (160) connected to said transducer (50) for determining the pore volume of the core sample from said first and second equilibrium pressures, P_E1 and P_E2.

5. A method for determining the permeability of an extremely tight core samples sample with an apparatus having a manifold in selective fluid communication with a sample holder holding a said core plug sample, said sample holder having an outlet, said method comprising the steps of:

(a) closing the outlet of the sample holder,

(b) pressurizing the manifold volume and core the sample holder with a gas to a predetermined pressure, P_FILL,

(c) selectively sealing the manifold from the core sample holder,

(d) opening the outlet of the sample holder in order to obtain atmospheric pressure to the atmosphere,

(e) allowing the pressure P_FILL in the sample holder to decay to atmospheric pressure as the gas flows from the a volume at the seal above between the core sample holder and the manifold through the core plug sample, and through the outlet, thereby venting the gas to atmospheric pressure,

(f) measuring a predetermined number of values, in time, of the instantaneous pressure values over time near said seal during the period of pressure decay, and

(g) determining the permeability of the core plug sample from said instantaneous time and pressure measurements values over time. 6. The method of claim 2 wherein the sample holder and pressure transducer are sealed from

the manifold by sealing the line therebetween. 7. The method of claim 3 wherein the sample holder and pressure transducer are sealed from the manifold by sealing the line therebetween.