Abstract:
An apparatus  300  for simulating a pulsed pressure induced cavitation technique (PPCT) from a pressurized working fluid (F) provides laboratory research and development for enhanced geothermal systems (EGS), oil, and gas wells. A pump  304  is configured to deliver a pressurized working fluid (F) to a control valve  306 , which produces a pulsed pressure wave in a test chamber  308 . The pulsed pressure wave parameters are defined by the pump  304  pressure and control valve  306  cycle rate. When a working fluid (F) and a rock specimen  312  are included in the apparatus, the pulsed pressure wave causes cavitation to occur at the surface of the specimen  312 , thus initiating an extensive network of fracturing surfaces and micro fissures, which are examined by researchers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is related to U.S. application Ser. No. ______, filed on DD-MM-YYYY and entitled, “A CAVITATION-BASED HYDRO-FRACTURING TECHNIQUE FOR GEOTHERMAL RESERVOIR STIMULATION”, and U.S. patent application Ser. No. 12/945,252 filed on 12 Nov. 2010 and entitled, “REPETITIVE PRESSURE-PULSE APPARATUS AND METHOD FOR CAVITATION DAMAGE RESEARCH” the entire contents of which are included herein by reference as if included at length. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT. 
       [0002]    This invention was made with government support under Contract No. DE-AC05-000822725 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    None. 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    The present disclosure relates to enhanced geothermal system (EGS) production and particularly to apparatuses and methods for simulating a cavitation-based hydro-fracturing technique. 
         [0006]    2. Description of the Related Art 
         [0007]    Geothermal energy is an important part of the nation&#39;s renewable energy initiative.  FIG. 1  illustrates a simplified schematic of a geothermal plant that generates electricity for the electrical grid. A working fluid (F) such as water is transferred with a pump  100  down into the hot rock formations through an injection well  102 , where it absorbs heat energy from the fractured rock formation. The heated working fluid (F) is then pumped to an energy conversion plant  104  through a production well  106 . Depending on the fluid&#39;s (F) temperature, it may directly be used to power a turbine or may be used to heat a secondary working fluid, which, in turn, is used to power a turbine. The turbine is coupled to a generator through a common shaft (not shown), to generate electricity for the electrical grid  108 . The cooled working fluid (F) is then injected with the pump  100  back into the hot rock geothermal reservoir through the injection well  102  to sustain the process. Geothermal energy generation is considered a green technology, because little or no greenhouse gases are emitted into the atmosphere and the energy source is renewable. 
         [0008]    An Enhanced Geothermal System (EGS) is a man-made reservoir, created where there is sufficient underground hot rock but insufficient or little natural permeability or working fluid (F) saturation in the rock. EGS expands the geothermal energy domain into much deeper rock deposits by exploiting natural and artificial fracture systems/networks within the rock mass. Maintaining and/or creating such facture networks in complicated geological environments are critical to the successful development and long-term sustainability of the EGS. The EGS targets a huge energy source that amounts to 500 GWe in the western U.S. and 16,000 GWe in the entire U.S. Several demonstration projects are undergoing in the U.S. to validate different reservoir stimulation techniques. The ultimate reservoir will have a flow rate of 60 kg/s, a lifetime of 30 years along the drilling systems down to 10,000 meters deep at 374 Degrees Celsius. 
         [0009]    EGS reservoir stimulation technologies currently are adapted from the oil and natural gas industry including various hydrofracking methods with or without chemical additives. A potential drawback of using hydrofracking techniques is the lack of effective control in the creation of large fractures, which could result in by-pass of targeted fracture network or even fault movement in the rock formation. The loss of hydraulic medium can reduce heat exchange efficiency and increase the cost of the development of EGS. The use of chemicals along with the unpredictable fault movement may also adversely impact the environment. 
         [0010]    Cavitation is the process of the formation of vapors, voids, or bubbles due to pressure changes in a liquid flow as schematically illustrated in  FIG. 2 . The pressure wave propagation  200 , and eventual collapse of the bubbles  202  can cause local pressure changes in the working fluid (F), which can be transmitted to a target rock surface  204  either in the form of a shock wave  206 , or by micro jets  208 , depending on the bubble to surface distance. Pressure greater than 100,000 psi has been measured in a shock wave  206  resonating from cavitating bubbles  202 . It is generally understood that the cycle of formation and collapse of the bubbles that occurs, often at a high frequency, can generate dynamic stress on the surfaces of objects. Ultimately, the dynamic stress can contribute to the fatigue of the target surface, including micro-cracks that form and coalesce on the surface  204 , eventually leading to material removal known as cavitation damage. 
         [0011]    The operations of geothermal, oil and natural gas wells are expensive endeavors. Well site development and production activities involve vast capital investments in land and equipment as well as the support of highly specialized personnel. Due to these large investments, opportunities for in-situ research and development efforts in the geothermal, oil and natural gas industries may not be cost prohibitive. 
         [0012]    What are needed are apparatuses and methods for simulating a pulse-pressure cavitation technique (PPCT) in a laboratory environment. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    Disclosed are several examples of apparatuses and methods for simulating a pulse-pressure cavitation technique (PPCT) in a laboratory environment. 
         [0014]    Described in detail below is an apparatus for generating a pulsed pressure induced cavitation technique (PPCT) from a pressurized working fluid to simulate the hydrofracturing of a specimen when a working fluid and specimen are installed. In the apparatus, a pump is fluidly coupled to, and disposed downstream of, a reservoir and fluidly coupled to, and disposed downstream of, a control valve having an open position and a closed position, the pump capable of raising the pressure of a working fluid at the control valve. Also included is a test chamber for holding a specimen when a specimen is installed in the apparatus. The test chamber is fluidly coupled to, and disposed downstream of, the control valve and receives a working fluid from the control valve when the control valve is in the open position. Also included is a pressure regulator that is fluidly coupled to, and disposed downstream of, the test chamber and fluidly coupled to, and disposed upstream of, the reservoir. When the control valve is in the open position, it causes a working fluid to flow into the test chamber as a pressure pulse, causing cavitation to occur in a working fluid adjacent to a specimen when a specimen and a working fluid are installed in the apparatus. Other features and examples will be described in greater detail. 
         [0015]    Also described in detail below is an article or specimen for receiving a pulsed pressure induced cavitation technique (PPCT) from a pressurized working fluid as generated by a test apparatus. The specimen includes a shell body defined by a circular top surface, a circular bottom surface and a convex side surface joining the top and bottom. Also included in the shell body is an aperture defined by an opening in the top surface and an opening in the bottom surface. Also included is a core body defined by a top surface, a bottom surface and a convex side surface joining the top surface and bottom surface. The core body is disposed inside of the aperture in the shell body and the shell body and core body are made of rock materials. Other features and examples will be described in greater detail. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0016]    The apparatus and method may be better understood with reference to the following non-limiting and non-exhaustive drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating principles. In the figures, like referenced numerals may refer to like parts throughout the different figures unless otherwise specified. 
           [0017]      FIG. 1  is a simplified sectional schematic of a geothermal energy conversion plant. 
           [0018]      FIG. 2  is a simplified rendition of cavitation mechanics at a fluid and surface interface. 
           [0019]      FIG. 3  is a plan view of an exemplary apparatus for simulating a cavitation-based hydro-fracturing of a specimen. 
           [0020]      FIG. 4  is an illustration of another exemplary apparatus for simulating a cavitation-based hydro-fracturing of a specimen. 
           [0021]      FIG. 5  is an external view of an exemplary control valve for use with the apparatuses of  FIGS. 3 and 4 . 
           [0022]      FIG. 6  is a sectional view of the control valve of  FIG. 5 . 
           [0023]      FIG. 7  is an illustration of the internal elements of the control valve of  FIG. 5 . 
           [0024]      FIG. 8  is a sectional view of a test chamber as used with the control valve of  FIG. 5 . 
           [0025]      FIG. 9  is an illustration of an exemplary test chamber with instrumentation and dual control valves installed. 
           [0026]      FIG. 10  is an illustration of an exemplary heating device for use with the test chambers. 
           [0027]      FIG. 11  is an illustration of an exemplary specimen. 
           [0028]      FIG. 12  is an illustration of another exemplary specimen. 
           [0029]      FIG. 13  is an illustration of an exemplary specimen shell. 
           [0030]      FIG. 14  is an illustration of an exemplary specimen core for use with the shells of 
           [0031]      FIGS. 13 and 15 . 
           [0032]      FIG. 15  is an illustration of another exemplary specimen shell. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    With reference now to  FIG. 3 , an exemplary apparatus  300  for generating a pulsed pressure induced cavitation technique to simulate the hydrofracturing of a specimen will now be described in greater detail. A working fluid (F), such as water, hydraulic fluid, other fluid, or combination of fluids, is stored in a reservoir  302 . The reservoir  302  may be an open or closed vessel and may also include means for filtering, adding, removing, and/or monitoring the level of working fluid (F). A pump  304  draws the working fluid (F) from the upstream reservoir  302  and distributes it to one or more downstream control valves  306  at pressures less than or equal to approximately 300 psi (2068.4 kPa), greater than or equal to approximately 300 psi (2068.4 kPa), or greater than or equal to approximately 300 psi (2068.4 kPa) and less than or equal to approximately 2,000 psi (13789.5 kPa). The pump  304  may operate by compressed air or by an electric motor for example. An air operated liquid piston pump  304  from Haskel International, Inc. Burbank, Calif., 91502 is suitable for this particular application. 
         [0034]    The control valve  306  receives the pressurized working fluid (F) from the upstream pump  304  and delivers it to a downstream test chamber  308 . High speed compressed air or electric solenoid valves may be used for the control valve  306 . A programmable controller (not shown) is used to control the timing frequency of the opening and closing of the control valve  306  to suit each particular simulation. A laptop or desktop computer using LabVIEW software by National Instruments, or a similar controller and software product, may be used. In some examples, the controller may signal the control valve  306  to open and close at a predetermined open and close frequency and/or duration schedule. Frequencies less than or equal to approximately 300 cycles per minute, greater than or equal to approximately 300 cycles per minute, or greater than or equal to approximately 300 and less than or equal to approximately 60,000 cycles per minute may be used. In other examples, the controller may signal the control valve  306  to remain in the open position for a period of time. Although a single control valve  306  is illustrated in  FIGS. 3 and 4 , two or more control valves  306  may also be used as shown later in  FIG. 9 . 
         [0035]    The exemplary test chamber  308  receives the pressurized working fluid (F) from the upstream control valve  306 . In this embodiment, the test chamber  308  is a cylindrical shaped tube defining an internal cavitation chamber  310  for accepting a test specimen  312 . This example of a test chamber  308  has an upstream end cap  314  that is fluidly coupled to the upstream control valve  306 , a medial body  316  and a downstream end cap  318 . The term fluidly coupled refers to a system where the fluid is able to flow between one component and another. At least one of the end caps  314 ,  318  are removable from the body  316  to allow for loading and unloading of a specimen  312  into the cavitation chamber  310 . Corresponding threads  320  on the end caps  314 ,  318 , and body  316  cooperate to provide a fluid seal when assembled together (see  FIG. 8 ). The end caps  314 ,  318  and body  316  are machined from high-strength, corrosion-resistant material such as stainless steel for example. SAE  304  or SAE  316  stainless steel perform well in this application. Other suitable materials may also be used. 
         [0036]    A fluid pressure regulator  322  may be fluidly coupled between the test chamber  308  and the reservoir  302 . The pressure regulator  322  may be a diaphragm type, for example, and may contain an integral pressure gauge  324  for ensuring accurate adjustments to the fluid pressure in the system. As is typical in such regulators, a clockwise turn of the adjustment knob increases system pressure and a counterclockwise turn reduces system pressure. One or more pressure gauges  324  may be installed at different locations in the system to ensure proper working fluid (F) pressure. 
         [0037]    Referring now to  FIG. 4 , another exemplary apparatus  300  for generating a pulsed pressure induced cavitation technique to simulate the hydrofracturing of a specimen will now be described in greater detail. In this example, a pressure accumulator  326  may be fluidly coupled between the pump  304  and the control valve  306 . The pressure accumulator  326  may be a gas charged type, a bellows type, or other type of pressure accumulator known in the art. The pump  304  delivers the working fluid (F) to the accumulator  326 , raising its pressure, until the control valve  306  is opened. All other components and features of this exemplary apparatus  300  are as described above. 
         [0038]    Conduits  328  are used to fluidly couple each of the components together and direct the working fluid (F) between components. High pressure capacity conduits  328  made of stainless steel may be used. Suitable couplings such as flared end fittings, or AN style fittings may be used to join the conduits  328  to the individual components described above. 
         [0039]    Referring now to  FIGS. 5-8 , another exemplary control valve, also referred to as a rotary shutter valve  500 , will be described in greater detail. In this example, an outer housing  502  includes an upstream end  504 , an opposite downstream end  506 , and a medial portion  508  disposed between the two ends. The outer housing  502  is preferably made from two cylindrical-shaped segments that are joined together at a circumferential flange  510  to simplify assembly, cleaning, inspection, modification, and repair of the valve. The flange  510  is held together with a plurality of circumferentially spaced fasteners  512  such as rivets, clamps or threaded fasteners as shown. An O-ring type seal  514  engages a corresponding gland machined in one or both of the segments as illustrated in  FIG. 6 . The outer housing  502  is machined from a high strength, high temperature and corrosion resistant material such as stainless steel. SAE  304  or SAE  316  stainless steel performs well in this application. 
         [0040]    An inlet aperture  516  is defined by the outer housing  502  at its upstream end  504 . An integral boss  518  provides additional material for connecting a conduit  328  using fittings as described above. The inlet aperture  516  is fluidly coupled to a pressure chamber  520 , which is also defined by the outer housing  502  at its upstream end  504 . The working fluid (F) flows under pressure from the pump  304 , though the conduits  328  to the inlet aperture  516 , and into the pressure chamber  520 . The downstream end  506  of the outer housing  502  defines a pulse cavity  522 , which discharges the pressurized working fluid (F) from the rotary shutter valve  500  as a series of pressure pulses  200  into the test chamber  308  ( FIG. 8 ). 
         [0041]    The medial portion  508  of the outer housing  502  defines a bulkhead  524 , which separates the pressure chamber  520  from the pulse cavity  522 . The bulkhead  524  is preferably made integral with the outer housing  502 , but it may also be a separate component that is joined to the outer housing  502  by threads or other mechanical means such as welding. The bulkhead  524  defines one or more bulkhead apertures  526 , which fluidly couple the pressure chamber  520  with the pulse cavity  522 . In the example shown, two, equally spaced, circular bulkhead apertures  526  are used. In other examples, more or less apertures  526  of circular or other shapes are used. Also, apertures  526  with constant (shown), converging, or diverging cross sections from their upstream to downstream ends are contemplated. The upstream surface  528  of the bulkhead  524  is planar shaped and the downstream surface  530  is concave conical shaped in the example. The concave conical shape of the downstream surface helps direct the pressure waves  200 . Other shapes (e.g., concave spherical, concave parabolic) are contemplated for the bulkhead downstream surface  530  as well. 
         [0042]    A rotatable shutter  532  is disposed inside of the pressure chamber  520  and adjacent to the upstream surface  528  of the bulkhead  524 . The shutter  532  defines one or more windows  534  that generally conform in size, shape, and radial placement with the bulkhead apertures  526 . In the example shown, four, equally spaced, circular windows  534  are used. In other examples, more or less windows  534  of circular or other shapes and sizes are used. The shutter  532  is affixed to, or integral with, a shaft  536  that extends through the pressure chamber  520  and exits the outer housing  502  at its upstream end  504 . 
         [0043]    Thrust bearings  538  support the shaft  536  and fit in pockets machined in the bulkhead  524  and the upstream end  504  of the outer housing. Shoulders on the shaft  536  engage with the thrust bearings  538  to prevent the shaft  536  from moving axially, thus preventing the shutter  532  from contacting the bulkhead  524 , seizing, and/or causing destructive vibrations while rotating. An O-ring type seal  540  engages a corresponding gland machined in the radially outer surface of the shutter  532  and prevents leakage of the working fluid (F) from the gap between the shutter  532  and the outer housing  502 . A material such as polyurethane, aluminum, graphite or other strong, high temperature capable material may be used for the O-ring seal  540 . 
         [0044]    Extending outward from the upstream end  504  of the outer housing  502  is a mounting flange  542  for accepting a powering device  544 . The powering device  544  is affixed to the mounting flange  542  with one or more fasteners  546  such as rivets, bolts or screws. In the example shown, an electric motor is used as the powering device  544 , but a hydraulic motor, a pneumatic motor, or other such device would also work in this application. Electricity, air, or hydraulic fluid is supplied to the powering device  544  by wires or hoses respectively (not shown). 
         [0045]    A coupling  548  connects the powering device  544  to the shaft  536 . The coupling  548  may include threads, set screws, shear pins, keys, collets, and/or other connecting means. In order to protect the powering device  544  from damage, the coupling  548  is designed to fail if the shutter  532  and/or shaft  536  break, seize, or become otherwise jammed in the pressure chamber  520  for some reason. 
         [0046]    During operation of the rotary shutter valve  500 , the powering device  544  transfers rotation to the shaft  536  through the coupling  548 . The spinning shaft  536  rotates the shutter  532 , causing the windows  534  to alternately align with (unblock) and misalign with (block) the one or more bulkhead apertures  526 . The pressurized working fluid (F) in the pressure chamber  520  discontinuously flows through the apertures  526 , into the pulse cavity  522 , and out of the downstream end  506  as pressure pulses  200 . The pressure pulses cause cavitation to occur in the test chamber  308  and, in turn, introduce fractures and micro cracks in a test specimen  312  when a test specimen is installed. It is noted that the pulses  200  are controlled by the number and size of the bulkhead apertures  526 , the number and size of shutter windows  534 , the rotational speed of the shutter  532 , and the pressure of the working fluid (F). The shutter  532  can rotate at speeds less than or equal to approximately 300 revolutions per minute, greater than or equal to approximately 300 revolutions per minute, or greater than or equal to approximately 300 revolutions per minute and less than or equal to approximately 60,000 revolutions per minute (RPM). 
         [0047]    In this example, the test chamber  308  receives the pressurized working fluid (F) directly from the pulse cavity  522  of the rotary shutter valve  500 . The test chamber  308  is a cylindrical shaped tube defining an internal cavitation chamber  310  for accepting a test specimen  312 . This example has a medial body  316  and a downstream end cap  318 . The test chamber  308  is attached to the distal end  506  with threads or other features to allow for loading and unloading of the specimen  312 . Other features of the present test chamber  308  are as described in the earlier examples. 
         [0048]      FIG. 9  shows another example of a test chamber  308  including instruments  330  for monitoring the conditions inside the cavitation chamber  310  such as the temperature, pressure and flow rate of the working fluid (F). It is also noted that, in this particular embodiment, two control valves  306  are fluidly coupled to the test chamber  308  at the upstream end cap  314  with each valve  306  functioning as described above with respect to  FIGS. 3 and 4 . In this example, the working fluid (F) pressure pulses entering the test chamber  308  are directly controlled by the frequency and/or duration schedule(s) of the control valve(s), which may be programmed to open and close according to the same schedule or according to different schedules. 
         [0049]    Referring now to the example of  FIG. 10 , the test chamber  308  may be surrounded, at least partially, by a heating element  332  to simulate the elevated temperature found in a EGS reservoir, or an oil or gas well. In the example shown, a resistance heater  332  completely surrounds the test chamber  308 , but in other examples only a portion of the chamber is surrounded by a heater. In some examples, the heater is able to raise the temperature of the test chamber  308  and specimen  312  to a temperature less than or equal to approximately 50 degrees Celsius (122 Fahrenheit), greater than or equal to approximately 50 degrees Celsius (122 Fahrenheit), or greater than or equal to approximately 50 degrees Celsius (122 Fahrenheit) and less than or equal to approximately 250 degrees Celsius (482 Fahrenheit). 
         [0050]    Referring lastly to  FIGS. 11 and 12 , exemplary specimens  312  for evaluating pressure pulse cavitation in EGS, oil or gas well rock formations are shown. The specimens  312  are generally cylindrical in shape and defined by a circular top surface  334 , a circular bottom surface  336  and a convex side surface  338  extending between the top and bottom surfaces  334 ,  336 . The specimens  312  are comprised of rock or stone material from larger rock specimens of the type found in EGS reservoirs, oil or gas wells. They are machined or core drilled to shape and sized to fit within the test chamber  308 . 
         [0051]    In the example of  FIG. 11 , a blind hole  340  mimics a stimulation well. During testing, the hole  340  is filled with working fluid (F) and is subject to cavitation by controlling the opening frequency and duration of the control valve(s). In the example of  FIG. 12 , a series of artificial flaws  342  are included in the side surface  338 . Here, artificial flaws  342  such as cracks or fissures are introduced into the side surface  342  with a band saw, a water jet or other cutting device to simulate an existing crack structure and/or to assist the initiation of crack stimulation. 
         [0052]    In the examples of  FIGS. 13-15 , a shell body  344  is defined by a generally circular top surface  334 , a circular bottom surface  336  and a convex side surface  338  joining the top and bottom surfaces  334 ,  336 . An aperture  346 , having an interior surface  348 , is disposed through the shell body  344  and is defined by a circular opening in the top and bottom surfaces  334 ,  336 . A separate, core body  350  is defined by a circular top surface  334 , a circular bottom surface  336  and a convex side surface  338  joining the top and bottom surfaces  334 ,  336 . The aperture  346  of the shell  344  is sized to accept the core  350  therein. As in the previous examples, the specimens  312  are comprised of rock or stone material of the type found in EGS reservoirs, oil or gas wells. They are machined or core drilled to shape and sized to fit within the test chamber  308 . 
         [0053]    In the present examples, artificial flaws  342  (surface flaws or through thickness flaws) may be introduced into one or both of the shell  344  and core  350 . During testing, cavitating working fluid (F) is forced to flow along the interface between the shell  344  and the core  350 . Furthermore, by incorporating a  45  degree pitch spiral notch to the side surface  338  of the core  350  and/or a spiral through thickness notch to the side surface  338  of the shell  344 , these specimens  312  can also be used to evaluate the fracture toughness degradation during the EGS reservoir operation. 
         [0054]    Further information about a spiral-notch torsion test system (SNTT) may be found in U.S. Pat. No. 6,588,283, “Fracture Toughness Determination Using Spiral-Grooved Cylindrical Specimen and Pure Torsional Loading”, to Jy-An Wang and Kenneth C. Liu, the disclosure of which is hereby incorporated by reference. Additional information may also be found in “A New Test Method for Determining the Fracture Toughness of Concrete Materials” by J. A. Wang, K. C. Liu, D. N. in Cement and Concrete Research, Volume 40, Issue 3, March 2010, Pages 497-499, K. Scrivener editor. Such benchmark data can further provide the guideline on the stimulation pressure pulse design parameters and their effectiveness for generating crack growth. 
         [0055]    After simulation testing, the specimens  312  are examined in order to evaluate the fracture network caused by the pressure pulse cavitation technique by the apparatus  300 . It was found that two main mechanisms occur in cavitation erosion damage: high pressure shock waves created by the collapsing vapor bubbles, which can result in material fatigue and plastic deformation; and micro jet impingement resulting in asymmetrical collapse of the vapor bubble near the specimen  312  surface. It was also found that when the bubbles collapse due to external pressure, the working fluid (F) is accelerated toward the center of the bubble. Bubbles formed near solid surfaces have the largest potential to cause micro cracking of the specimen  312  surface. 
         [0056]    While this disclosure describes and enables several examples of a simulator apparatuses and methods for researching geothermal reservoir stimulation, other examples and applications are contemplated. Accordingly, the invention is intended to embrace those alternatives, modifications, equivalents, and variations as fall within the broad scope of the appended claims. The technology disclosed and claimed herein may be available for licensing in specific fields of use by the assignee of record.