Abstract:
A system for non-destructively measuring the strength of a cement slurry sample includes an elongate sample container for receiving a cement slurry sample. The elongate sample container has a mass mounted at its first end. A transducer mounted at a second end of the elongate sample container vibrates the elongate sample container and mass. The elongate sample container, mass and transducer have a known resonance. The system calculates the strength of a tested cement slurry within the elongate sample container as a function of variation in resonance of the elongate sample container, mass and transducer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to methods and apparatus for testing material samples which are initially fluid but which may change to a solid during testing. More specifically, the present invention relates to method and apparatus for determining the gel strength and compressive strength of a cement slurry sample. 
     2. Description of the Prior Art 
     Cement is utilized at different times during the drilling, completion, and repair of oil and gas wells to bond the well casing to the well bore. This technique is utilized to seal off any flow of fluids or gas along the length of the casing. The cement utilized is formulated specifically for the temperature and pressure conditions found in the well being cemented. Because of the wide range of conditions present in typical oil wells there are a wide range of diverse cement formulations utilized in this cementing procedure. In order to ensure proper cementing within the well the cement utilized is typically tested in a laboratory utilizing the specific conditions believed to be present in a particular well. Multiple measurement systems are described, for example, in U.S. Pat. No. 4,259,868 issued to Rao, et al.; U.S. Pat. No. 4,567,765 to Rao, et al.; U.S. Pat. No. 5,412,990 to D&#39;Angelo, et al.; and U.S. Pat. No. 5,992,223 to Sabins, et al. 
     A primary requirement in such operations is that the cement slurry remain fluid for a period long enough to permit the slurry to be pumped to desired locations within the well. A high temperature-high pressure consistometer is often utilized to measure the thickness or consistency of the cement during the pumping operation. Once in place the cement must develop adequate gel strength to prevent fluid or gas movement before the cement develops compressive strength. Eventually, it is necessary to know that adequate compressive strength has developed before beginning certain other operations within an oil well. An ultrasonic cement analyzer may be utilized to measure the gel strength and the compressive strength of the cement as it cures. Previous versions of devices utilized to obtain these measurements typically utilized one or two acoustic transducers to measure the sound velocity within the cement to obtain the compressive strength or the amplitude of the signal through the cement in order to obtain the gel strength. 
     These prior art devices typically utilized relatively large samples of cement contained within a pressure vessel. After the cement has set and the test is complete the pressure vessel would then be disconnected from the high pressure fluid source and electronics to allow its removal from the housing which contains the electronics, pressure source and heat source. These pressure vessels, typically weighing 20 lbs. or more, would then be taken to a large bench vice to allow removal of both the top and bottom plugs from the pressure vessel. The cement sample may then be driven from the pressure vessel. Once the cement has been removed grease is typically applied to all interior surfaces of the pressure vessel before filling the pressure vessel with a subsequent cement sample. This test equipment does not lend itself to affordability or portability in view of the massive pressure vessels required. Additionally, this test equipment is quite expensive. 
     SUMMARY OF THE INVENTION 
     The present invention describes an alternate method and apparatus for obtaining the gel strength and compressive strength of a cement slurry. This technique provides a comparable accuracy with a much smaller and less expensive test instrument, making the measurement much more widely available within the industry. Increasing the availability of testing will greatly improve the quality of the typical cementing job within an oil well by increasing access to the testing of the cement slurry prior to placing the cement within the well. 
     A cement sample, in accordance with the present invention, is placed within a small tube within pressure vessel. The tube includes a steel plug at a first end and the opposite end thereof is connected to a plug threaded into a pressure vessel. The tube is then attached to the plug through a diaphragm, which allows a piezoelectric ceramic device outside of the pressure vessel to vibrate the tube across a range of frequencies. This continuous vibration is in direct contrast to the prior art wherein short acoustic pulses were typically utilized to characterize the cement slurry and wherein the cement slurry typically filled a large pressure vessel. A steel weight, either integral with the tube or attached to one end of the tube opposite piezoelectric ceramic element, can be utilized to trap the acoustic energy generated by the piezoelectric ceramic element within the tube. The electrical admittance of the piezoelectric ceramic element thereafter produces a strong indication of the mechanical characteristics of the tube and the cement material within the tube. The electrical admittance may then be utilized to determine both the gel strength and compressive strength of the cement slurry contained within the tube under appropriate signal analysis. Upon completion of the test the cement sample is simply removed by disconnecting the plug from the pressure vessel so that the tube and cement sample may be removed. The pressure vessel typically contains water, a temperature sensor and a fluid inlet to provide the appropriate pressure. A cement sample contained within the tube may be thereafter simply removed from the tube or the tube may be replaced for subsequent tests. No specialized heavy equipment is required to open the pressure vessel or remove the cement sample, making the present test much more amenable to field testing than previously known devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. However, the invention, as well as a preferred mode of use, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a sectional view of the test system of the present invention depicting the sample tube within a pressure vessel; 
         FIG. 2  is a sectional view depicting the sample tube and sensor system configured for filling with cement slurry; 
         FIGS. 3A and 3B  are high level block diagrams of the electrical and hydraulic components of the test system of the present invention; 
         FIG. 4  is a graphically depiction of variations in the admittance of the piezoelectric ceramic device at various frequencies; 
         FIGS. 5A-5D  depict measured data, computed gel strength and computed compressive strength obtained utilizing the test system of the present invention; and 
         FIG. 6  depicts the processed output of the test system of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the figures and in particular with reference to  FIG. 1  there is depicted a sectional view of the test system of the present invention. As illustrated, the test system includes a pressure vessel  1 , which, in the depicted embodiment, typically contains water. An end plug  2  may be removed to permit placement of cement slurry sample for testing or for removal of the cured cement subsequent to testing. 
     Depicted at the bottom of pressure vessel  1  are ports  3  and  4 , which permit placement of a temperature sensor and allow pressurized fluid to be added or removed from pressure vessel  1  in order to obtain the appropriate temperature and pressure required to match conditions within a particular well. As depicted, pressure vessel  1  is generally disposed in a vertical position so that when end plug  2  has been removed the pressure vessel may be filled with water or other appropriate fluid. Once end plug  2  has been replaced, pressure vessel  1  may, as those having ordinary skill in the art will appreciate upon reference to the foregoing, be placed either in a horizontal or vertical position for testing. In the depicted embodiment of the present invention horizontal placement is preferred to eliminate any effects of free water formation within the cement slurry. 
     As illustrated, a flange  7  is mounted to the upper surface of end plug  2  utilizing bolts of sufficient strength, which also may be utilized to mount electrical connector  6 . Flange  7 , as those having ordinary skill in the art will appreciate, provides a suitable grip to permit hand removal of end plug  7  and also allows end plug  7  to stand vertically when placing a cement slurry sample into testing tube  21 , when testing tube  21  is inverted, as will be described herein. 
     As depicted, a port  24  is provided at the lower end of testing tube  21 . An end mass  23  is also attached to testing tube  21  having a tapered fit  22  within testing tube  21 . Of course, those having ordinary skilled merit will appreciate that mass  23  may be integrally formed with testing tube  21  in an alternate embodiment. In the depicted embodiment of the present invention a small amount of vacuum grease is typically placed on mass  23  to lubricate and seal the mating surfaces of mass  23  and testing tube  21 . Thus, as testing tube  21  is forced into a mating relationship with mass  23 , a self-holding contact can be achieved. 
     An identical procedure may be utilized on the upper end of testing tube  21 , as indicated at reference numeral  20 . At this point testing tube  21  is attached to an acoustic driving fitting  16 . Acoustic driver fitting  16  is preferably sealed to end plug  2  utilizing an O-ring  17 , a torque isolation ring  18  and a threaded retaining ring  19 . In alternate embodiments the acoustic driver fitting may simply be welded to the end plug so as to eliminate any possibility of seal failure. 
     A piezoelectric ceramic element  15  is then placed against the opposite side of the driver fitting. A steel mass  13  is then placed against the opposite side of piezoelectric ceramic element  15 . This steel mass  13  then acts as a positive electrode for the piezoelectric ceramic element  15  while the driver fitting  16  acts as the ground electrode. Steel mass  13  is then connected to the center electrode of an electrical connector  6  utilizing a wire attached at reference numeral  14 , utilizing a small machine screw. 
     A ground connection is then made through the metal body of end plug  2  and a ceramic washer  12  may then be placed against the opposite face of steel mass  13 . A steel washer  11  and a nut  10  are then placed on threaded rod  9 , which passes through driver fitting  16 . An insulating sleeve  49  then covers threaded rod  9 . Tightening nut  10  then clamps all components tightly together as a single mechanical entity. 
     The relatively high mass of end mass  23  and steel mass  13  cause a major resonance at the half wave length as determined by the speed of sound within testing tube  21 , based upon its length. This resonance may be simply measured by detecting the electrical admittance of piezoelectric ceramic element  15 . Thus, as the cement slurry cures, this and other resonances will change and develop within the described test structure. 
     Referring now to  FIG. 2  the manner in which testing tube  21  is filled is depicted within the sectional view thereof. As illustrated, end plug  2  is placed on a bench with flange  7  resting on the bench. A small vessel  28  is then placed on end mass  23 . An O-ring seal  26  may be utilized to seal the vessel to end mass  23 . A plastic pipette may then be utilized to place a quantity of cement slurry through port  27  to fill approximately one-third of the volume of vessel  28 . In the depicted embodiment of the present invention side walls  25  of vessel  28  are preferably transparent, in order to make the fill amount relatively easy to determine. 
     Next, a hand vacuum pump (not shown) is connected to port  27  and a vacuum is pulled, removing, the air within testing tube  21  and vessel  28 . This vacuum also removes most of the air trapped within the cement slurry. Releasing the vacuum thereafter allows atmospheric pressure to push the cement slurry through port  24  into the volume of testing tube  21 . Vacuum may be applied several times in order to be certain that there is no air left in testing tube  21 . Once testing tube  21  has been filled, vessel  28  can be removed. Port  24  is of sufficiently small diameter that cement will generally not flow from testing tube  21  even when testing tube  21  has been inverted. However, a small semi-permeable cap may also be place on end mass  23  to eliminate any loss of cement into the pressure vessel but to allow fluid within the pressure vessel to pressurize the cement sample and supply any water absorbed into the cement as it cures. 
     A high level block diagram of the electronic and hydraulic components of the test system of the present invention are depicted within  FIGS. 3A and 3B . As illustrated in  FIG. 3A , electronic components  31  include a computer  30  for providing timing and control logic in order to produce a proper heat cycle and monitor the pressure within the pressure vessel. Computer  30  also processes the admittance of piezoelectric ceramic element  15  (See  FIG. 1 ) as a function of frequency. The computer  30  then applies processing functions to calculate the gel strength and compressive strength of the cement slurry as it cures. These functions are derived from experimental data. A heater control  35  is included within electric components  31  and is utilized to modulate the line voltage, which provides an appropriate current to an electronic heater element (not shown) surrounding pressure vessel  1 . (See  FIG. 1 ) Temperature sensor  34  is preferably a thermocouple within the cell, the output voltage of which is converted to a digital representation of the temperature. Pressure sensor electronics  33  are provided in order to convert the output of a strain gauge pressure sensor to a digital value. An auxiliary heater  36  and controller may be utilized to heat a separate a reservoir to compensate for fluid take-up as the cement cures. This fluid take-up may also be compensated by a slight increase in the temperature within pressure vessel  1 . 
     Network analyzer  32  is also depicted. The network analyzer  32  may be a suitable device such an AD5934 made by Analog Devices, which contains a complete network analyzer or separate signal generator and envelope detector in order to measure the input current of the piezoelectric ceramic element. In either case, the electrical admittance of piezoelectric ceramic element  15  at a range of frequencies can then be utilized to evaluate the gel strength and compressive strength of the cement slurry. 
     Depicted within  FIG. 3B  are the hydraulic components  37  of the test system of the present invention. As illustrated, a pressure pump  38  is provided. In the depicted embodiment of the present invention pressure pump  38  is preferably a manual screw piston pump. A valve  39  is provided in order to isolate pump  38  from pressure vessel  1  once the desired pressure has been achieved. A pressure relief valve  40  is provided to allow excess fluid to exit the pressure vessel as the pressure vessel is heated to a proper temperature. Pressure sensor  41  may be utilized to monitor the pressure within pressure vessel  1 . A small auxiliary pressure vessel  43 , preferably having a small fraction of the volume of pressure vessel  1  may also be added to produce the fluid volume change which occurs as the cement cures. By heating this small volume the pressure may be controlled in the system automatically without an otherwise required slight increase in the sample temperature. No additional pumping devices are required to control the pressure during a test. 
     In operation, end plug  2  is placed in pressure vessel  1  and the fluid therein is pressurized. Computer  30  then controls the heating of the cement slurry, at a proper rate, to the desired temperature. Computer  30  also controls the acquisition of the electrical admittance of piezoelectric ceramic element  15 . The admittance of the piezoelectric ceramic element  15  indicates the capacitance of the piezoelectric ceramic element and the mechanical resonances of testing tube  21  and the associated components, including the cement slurry contained therein. 
     Referring now to  FIG. 4  there is depicted a typical signal generated by the test system of the present invention. As illustrated, when the admittance of piezoelectric ceramic element  15  is graphed against the frequency, the upward slope indicates the capacitance of piezoelectric ceramic element  15 . The inflection in admittance at reference numeral  48  indicates that a resonance of testing tube  21  and the associated end masses has occurred. As the cement begins to develop gel strength it begins to reduce the ability of the walls of testing tube  21  to move, causing the depicted inflection at reference numerals  48  to decrease in magnitude. 
     With reference now to  FIG. 5A  it can be seen at trace  44  that the magnitude of the inflection decreases as the gel strength increases. The change in the mechanical character of testing tube  21 , which is filled with cement, as indicated by the admittance of piezoelectric ceramic element  15 , may be utilized to directly indicate the gel strength of the cement sample therein. 
     Referring to  FIG. 5B  trace  45  shows gel strength development in another cement sample at a slightly lower temperature after application of the appropriate function to the admittance data. This function can be developed utilizing regression analysis on data from physical measurement of the slurry gel strength and admittance data. As the cement slurry hydrates further, another inflection with a changing frequency indicates the development of compressive strength as is shown within trace  46  with  FIG. 5C . This inflection is indicative of the compressive strength of the cement slurry. Again, a regression analysis is utilized to develop the function utilized to translate the frequency of the inflection in admittance to compressive strength, as depicted within trace  47  of  FIG. 5D . 
     Finally, referring to  FIG. 6 , the pressure, temperature, gel strength, and compressive strength of a single cement slurry sample are depicted within a single graph. Temperature  50  increases at a specified rate to a set point temperature. Once the desired temperature has been achieved the temperature is modified slightly to compensate for any volume changes of the slurry within the test system once the pressure  52 , decreases below a set value, as determined by the pressure relief valve. The pressure within the test system then remains constant while the temperature is allowed to change a small amount while the slurry sets. Gel strength  54  is then indicated as is compressive strength  56  as the cement slurry sets within the test system of the present invention.