Hypergravity experimental apparatus and method for natural gas hydrate exploitation by hydraulic fracturing

Abstract

A hypergravity experimental apparatus and method for natural gas hydrate exploitation by hydraulic fracturing are provided. A high-pressure vessel contains a hydrate reservoir model placed in a water bath environment connected to a water bath temperature control module. An effective stress control module applies stress control. A fracturing reinforcement exploitation module conducts fracturing, reinforcement, and exploitation experiments. A model multi-physical field monitoring module performs monitoring. A hydrate preparation module communicates with the high-pressure vessel. The high-pressure vessel, the effective stress control module, and the fracturing reinforcement exploitation module are all under hypergravity, and the hydrate preparation module and the water bath temperature control module are under normal gravity.

What is claimed is: 
     
         1 . A hypergravity experimental apparatus for natural gas hydrate exploitation by hydraulic fracturing, comprising:
 a high-pressure vessel, a water bath temperature control module, an effective stress control module, a fracturing reinforcement exploitation module, a hydrate preparation module, and a model multi-physical field monitoring module, the high-pressure vessel is provided with a hydrate reservoir model inside and is entirely placed in a water bath environment and connected to the water bath temperature control module, so that the hydrate reservoir model is placed in the water bath environment, the effective stress control module communicates with the inside of the high-pressure vessel and applies stress control to the hydrate reservoir model, the fracturing reinforcement exploitation module communicates with the inside of the high-pressure vessel and conducts fracturing, reinforcement, and exploitation experiments on the hydrate reservoir model, the model multi-physical field monitoring module is installed on the high-pressure vessel to monitor the hydrate reservoir model, and the hydrate preparation module communicates with the hydrate reservoir model inside the high-pressure vessel,   the high-pressure vessel, the effective stress control module, and the fracturing reinforcement exploitation module are all mounted within a centrifuge basket and operate under 1 g to 500 g hypergravity,   the high-pressure vessel is provided with a loading plate inside, the loading plate divides a space inside the high-pressure vessel into an upper chamber and a lower chamber, the lower chamber is provided with the hydrate reservoir model inside, the upper chamber is provided with an axial pressure fluid layer inside, the fracturing reinforcement exploitation module comprises a hydrate reservoir hydraulic fracturing module, an injection fluid switching and reservoir reinforcement module, and a hydrate exploitation module, the hydrate reservoir hydraulic fracturing module, the injection fluid switching and reservoir reinforcement module, and the hydrate exploitation module are all communicate with the hydrate reservoir model, and the effective stress control module communicates with the axial pressure liquid layer and is connected to the hydrate exploitation module.   
     
     
         2 . The hypergravity experimental apparatus for natural gas hydrate exploitation by hydraulic fracturing according to  claim 1 , wherein
 the water bath temperature control module comprises a water bath jacket, a constant temperature water bath, and a water bath circulation pump, the high-pressure vessel is placed in the water bath jacket with a guide groove, the water bath jacket, the constant temperature water bath, and the water bath circulation pump communicate in series for circulation, with the water bath circulation pump driving antifreeze liquid to circulate within the water bath jacket and the constant temperature water bath, and   the effective stress control module comprises an axial pressure pump, an injection pipeline, a back-pressure pump, and a buffer tank, an input end of the axial pressure pump communicates with a water storage container, an output end of the axial pressure pump communicates with the axial pressure fluid layer inside the high-pressure vessel through the injection pipeline, an output end of the back-pressure pump communicates with an upper portion of the buffer tank, and an lower end of the buffer tank is connected to a control end of the hydrate exploitation module.   
     
     
         3 . The hypergravity experimental apparatus for natural gas hydrate exploitation by hydraulic fracturing according to  claim 1 , wherein
 the hydrate reservoir hydraulic fracturing module comprises a piston temperature control container, an advection pump, a single-degree-of-freedom loading device, an injection pipeline, and an injection end head, an input end of the advection pump communicates with a water storage container, an output end of the advection pump communicates with one end of the piston temperature control container, the piston temperature control container is provided with fracturing fluid inside in advance, other end of the piston temperature control container communicates with an upper end of the injection pipeline, a lower end of the injection pipeline is inserted into a production well in the hydrate reservoir model and is installed with a specially designed injection end head, and a production well perforation corresponding to the injection end head is arranged on a side wall of the production well, and   an upper portion of the injection pipeline is installed on a loading arm of the single-degree-of-freedom loading device, the single-degree-of-freedom loading device drives the injection pipeline to move up and down, the injection end head comprises a top plate, a bottom plate, a cylindrical connecting rod and a sealing ring, the top plate is fixedly connected to the lower end of the injection pipeline, a through hole communicating with the lower end of the injection pipeline is formed in a middle of the top plate, the top plate and the bottom plate are fixedly connected by four cylindrical connecting rods, and the top plate and a periphery of the bottom plate are sealed and connected to an inner wall of the production well through the sealing ring,   the injection fluid switching and reservoir reinforcement module comprises two piston temperature control containers, the advection pump, the single-degree-of-freedom loading device, the injection pipeline, and the injection end head, the injection fluid switching and reservoir reinforcement module shares the advection pump, the single-degree-of-freedom loading device, the injection pipeline, and the injection end head with the hydrate reservoir hydraulic fracturing module, the two piston temperature control containers of the injection fluid switching and reservoir reinforcement module are connected in parallel to the piston temperature control container of the hydrate reservoir hydraulic fracturing module and are pre-filled with a gel breaker and a reinforcing agent.   
     
     
         4 . The hypergravity experimental apparatus for natural gas hydrate exploitation by hydraulic fracturing according to  claim 3 , wherein
 the piston temperature control container is mainly formed by a piston container, a pneumatic valve, and a semiconductor chip, the piston container communicates with the advection pump and the injection pipeline, the semiconductor chip is installed on the piston container, a piston plate is inside the piston container, the piston plate divides the piston container into upper and lower chambers, the lower chamber communicates with the output end of the advection pump through the pneumatic valve, a required substance is pre-added in the upper chamber, and the upper chamber communicates with the upper end of the injection pipeline.   
     
     
         5 . The hypergravity experimental apparatus for natural gas hydrate exploitation by hydraulic fracturing according to  claim 1 , wherein
 the hydrate exploitation module comprises an production pipeline, a solid separation meter, a back-pressure valve, and a liquid-gas separation collection and metering module, an input end of the solid separation meter communicates with a top portion of an production well through the production pipeline, an output end of the solid separation meter communicates with an inlet of the back-pressure valve, and an outlet of the back-pressure valve communicates with the liquid-gas separation collection and metering module.   
     
     
         6 . The hypergravity experimental apparatus for natural gas hydrate exploitation by hydraulic fracturing according to  claim 1 , wherein
 the model multi-physical field monitoring module comprises a sensor installed on the high-pressure vessel, an acoustic emission probe and a sapphire endoscope tube on an inner wall of the high-pressure vessel, and a model monitoring and data collection module, the sensor comprises a thermocouple, a pressure sensor, an earth pressure gauge, a resistivity probe, and a strain gauge, etc., the acoustic emission probe is horizontally arranged towards an production well, the sapphire endoscope tube is vertically inserted into the hydrate reservoir model and parallel to the production well, an endoscope camera is installed inside the sapphire endoscope tube, and the sensor, the acoustic emission probe, and the endoscope camera are all connected via sensor signal lines to the model monitoring and data collection module outside the centrifuge basket for communication,   the injection fluid switching and reservoir reinforcement module and one pneumatic valve connected to a pneumatic valve/an input end of a solid separation meter in a piston temperature control container of the hydrate reservoir hydraulic fracturing module are both connected to an electromagnetic valve group, the electromagnetic valve group is communicatively connected via an electromagnetic valve group signal line to an external model monitoring and data collection module, and the model monitoring and data collection module remotely controls the electromagnetic valve group to further control opening and closing of each the pneumatic valve.   
     
     
         7 . A method for natural gas hydrate exploitation by hydraulic fracturing applied to the hypergravity experimental apparatus and filed exploitation according to  claim 1 , comprising:
 S 1 : hydrate formation process   preparing, by the hydrate preparation module, a hydrate in the high-pressure vessel to prepare the hydrate reservoir model;   S 2 : effective stress control process   mounting the high-pressure vessel, the effective stress control module, and the fracturing reinforcement exploitation module of the apparatus to the centrifuge basket, starting a centrifuge, through high-speed rotation of the centrifuge, applying an ng hypergravity field to reach an ng hypergravity state, injecting, by a axial pressure pump, liquid from an axial pressure liquid injection pipeline in a water storage container to into the axial pressure liquid layer in the high-pressure vessel to increase pressure, and transmitting, through the loading plate, a pressure of the axial pressure liquid layer to the hydrate reservoir model, and converting an overlying total stress σ 0  applied by the axial pressure pump into a total stress of the hydrate reservoir model,   applying, through a back-pressure pump and a buffer tank, back-pressure to a back-pressure valve to control opening, closing, and an opening level of a valve core, so that the back-pressure valve communicates with the hydrate reservoir model via an production pipeline, and indirectly controlling an effective stress thereof by controlling the total stress and a pore water pressure of the hydrate reservoir model; and   S 3 : hydraulic fracturing process   driving, by a single-degree-of-freedom loading device, an injection pipeline and an injection end head to move downward to a position below where the production pipeline communicates with to an production well and within the production well in the hydrate reservoir model before reservoir hydraulic fracturing is performed,   opening only a pneumatic valve corresponding to a piston container I when the reservoir hydraulic fracturing is performed, remotely controlling a advection pump to draw liquid from the water storage container, and injecting water at a constant flow speed to push a piston plate inside the piston container I, so that fracturing fluid in an upper chamber of the piston container I flows at a constant flow speed through the injection pipeline to reach the injection end head, flows out from the injection end head into the production well, and is ejected out from an production well perforation to enter the hydrate reservoir model to form a hydraulic fracturing fracture and to implement hydraulic fracturing;   S 4 : reservoir reinforcement process   opening only the pneumatic valve corresponding to a piston container II after hydraulic fracturing is completed, remotely controlling the advection pump to draw liquid from the water storage container, injecting water at a constant flow speed to push the piston plate inside the piston container II, so that a gel breaker in an upper chamber of the piston container II flows at a constant flow speed through the injection pipeline to reach the injection end head, flows out from the injection end head into the production well, flows out from the production well perforation to enter a crack of the formed hydraulic fracturing fracture,   opening only the pneumatic valve corresponding to a piston container III instead subsequently, remotely controlling the advection pump to draw liquid from the water storage container, and injecting water at a constant flow speed to push the piston plate inside the piston container III, so that a reinforcing agent in an upper chamber of the piston container III flows at a constant flow speed through the injection pipeline to reach the injection end head, and flows out from the injection end head into the production well, flows out of the production well perforation to enter the crack of the hydraulic fracturing fracture that is injected with the gel breaker to form a chemically-modified reinforcement zone,   forming a continuous porous surface structure supporting fracture surface under an action of water and salt in a pore water of the hydrate reservoir model for several hours of hydration after a reinforcing agent is injected;   S 5 : hydrate exploitation process   driving, by the single-degree-of-freedom loading device, the injection pipeline and the injection end head to move upwards to a top portion of the production well and above where the production pipeline communicates with the production well, so that the production pipeline communicates with the production well at the chemically-modified reinforcement zone, and   decomposing hydrate in the hydrate reservoir model to generate gas which enters the production pipeline through the production well, so that only a controlled pressure of the back-pressure valve enters a liquid-gas separation collection and metering module for liquid-gas separation and metering after a solid is separated by a solid separation meter to complete the experiment; and   S 6 : monitoring process   collect and analyzing, by the model multi-physical field monitoring module, data to obtain a hydraulic fracturing-exploitation condition.   
     
     
         8 . The method for natural gas hydrate exploitation by hydraulic fracturing applied to the hypergravity experimental apparatus and filed exploitation according to  claim 7 , wherein
 in the hydraulic fracturing process, fracture initiation pressure strength is obtained according to the following equation:   
       
         
           
             
               
                 
                   p 
                   f 
                 
                 = 
                 
                   
                     
                       a 
                       f 
                     
                     ⁢ 
                     
                       v 
                       f 
                     
                     ⁢ 
                     μ 
                   
                   + 
                   
                     
                       
                         
                           
                             K 
                             0 
                           
                           ⁢ 
                           
                             
                               tan 
                               2 
                             
                             ( 
                             
                               
                                 π 
                                 / 
                                 4 
                               
                               + 
                               
                                 ψ 
                                 / 
                                 2 
                               
                             
                             ) 
                           
                         
                         - 
                         1 
                       
                       
                         
                           
                             tan 
                             2 
                           
                           ( 
                           
                             
                               π 
                               / 
                               4 
                             
                             + 
                             
                               ψ 
                               / 
                               2 
                             
                           
                           ) 
                         
                         - 
                         1 
                       
                     
                     ⁢ 
                     
                       ρ 
                       ′ 
                     
                     ⁢ 
                     ngz 
                   
                   + 
                   
                     
                       2 
                       ⁢ 
                       
                         N 
                         p 
                       
                       ⁢ 
                       
                         c 
                         h 
                       
                       ⁢ 
                       
                         tan 
                         ⁡ 
                         ( 
                         
                           
                             π 
                             / 
                             4 
                           
                           + 
                           
                             ψ 
                             / 
                             2 
                           
                         
                         ) 
                       
                     
                     
                       
                         
                           tan 
                           2 
                         
                         ( 
                         
                           
                             π 
                             / 
                             4 
                           
                           + 
                           
                             ψ 
                             / 
                             2 
                           
                         
                         ) 
                       
                       - 
                       1 
                     
                   
                 
               
               , 
             
           
         
         wherein p f  represents the fracture initiation pressure strength of hydraulic fracturing, μ and v f  represent a viscosity and a flow speed of the fracturing fluid, respectively, g is gravitational acceleration, n is a multiple of gravitational acceleration, K 0  and ψ represent a sediment lateral pressure coefficient and an internal friction angle, respectively, ρ′ is a sediment buoyant density, z represents a reservoir burial depth, N p  is a pore characteristic constant related to hydrate saturation, c h  is a cohesion of hydrate-bearing sediment, and a f  is a constant parameter related to permeability of the hydrate-bearing sediment and flow conductivity of splitting fracture. 
       
     
     
         9 . The method for natural gas hydrate exploitation by hydraulic fracturing applied to the hypergravity experimental apparatus and filed exploitation according to  claim 7 , wherein
 in the hydrate formation process, the effective stress control process, the hydraulic fracturing process, the reservoir reinforcement process by injecting the gel breaker and the reinforcing agent, and the subsequent hydrate exploitation process, real-time monitoring of temperature, pore pressure, total stress, resistivity, and reservoir deformation is performed through sensors comprising a thermocouple, a pressure sensor, an earth pressure gauge, a resistivity probe, and a strain gauge, respectively, and   in the hydraulic fracturing process, real-time monitoring of fracture splitting strength and a splitting location of hydraulic fracture inside the hydrate reservoir model is performed through an acoustic emission probe, and an actual fracture propagation process, a fracture location, a fracture opening level, and particle migration and deformation response of the hydrate reservoir model are observed in real-time through an endoscope camera inside a sapphire endoscope tube.   
     
     
         10 . The method for natural gas hydrate exploitation by hydraulic fracturing applied to the hypergravity experimental apparatus and filed exploitation according to  claim 7 , wherein
 in the method, in the hydraulic fracturing process, acoustic signal data collected by the acoustic emission probe is processed according to the following equations to obtain a rise time t r  and an average frequency AF of an acoustic signal:   
       
         
           
             
               
                 
                   
                     
                       t 
                       r 
                     
                     = 
                     
                       
                         t 
                         d 
                       
                       / 
                       
                         A 
                         e 
                       
                       ⁢ 
                           
                       and 
                     
                   
                 
               
               
                 
                   
                     
                       AF 
                       = 
                       
                         NF 
                         / 
                         
                           t 
                           f 
                         
                       
                     
                     , 
                   
                 
               
             
           
         
         wherein the rise time t r  is a ratio of a delay t d  from a moment of acoustic signal starts to a maximum amplitude thereof to an amplitude A e , and the average frequency AF is a ratio of a number of effective fracture acoustic signals NF to total duration of hydraulic fracturing t f , and 
         after the hydraulic fracturing begins, determination is made based on the real-time obtained rise time t r  and the average frequency AF to determine different stages of hydraulic fracturing: 
         when the average frequency AF increases by greater than a predetermined threshold compared to a value before hydraulic fracturing and a rate of change of the average frequency AF between adjacent moments is lower than a predetermined slope threshold, while the rise time t r  increases by less than a predetermined threshold compared to a value before hydraulic fracturing and a rate of change of the rise time t r  between adjacent moments is lower than the predetermined slope threshold, it is considered to be in a tensile fracture stage, and 
         when the average frequency AF continuously increases and the rate of change of the average frequency AF between adjacent moments is greater than the predetermined slope threshold, while the rise time t r  continuously increases and the rate of change of the rise time t r  between adjacent moments is greater than the predetermined slope threshold, it is considered to be in a main fracture formation and propagation stage.