Method for dynamic imbibition capacity of shale

An experimental test method for dynamic imbibition capacity of shale, comprising the following steps:

• (1) core preparation:
• the core is made of the downhole rock column in the shale reservoir section or outcrop in the same formation;
• the imbibition area A of the core is calculated according to the end size of the core, the core length is the experimental measurement length L, and the standard core is dried in the dryer to a constant weight;
• (2) physical parameter test of the shale:
• the porosity φ of the dried core mentioned in Step (1) is tested by a porosity tester with helium as the working medium;
• and
• (3) the experimental loading conditions are determined according to the formation stress, the formation temperature and hydraulic fracturing parameters, which determines the confining pressure loaded in the experiment with the formulas (1) to (4), the reservoir temperature is the experimental temperature, and the fluid injection pressure is determined by the formula (5);
• σ′z=σz−αPp (1);
• σ′H=σH−αPp (2);
• σ′h=σh−αPp (3);
• σC=(σ′z+σH′+σh′)/3 (4);
• Pinj=PISI−Pp (5);
• in the formulas, σ′z is the vertical effective stress, in MPa;
• σ′H is the maximum horizontal effective principal stress, in MPa;
• σ′h the minimum horizontal effective principal stress, in MPa;
• σz is the vertical stress, in MPa;
• σH is the maximum horizontal principal stress, in MPa;
• σh the minimum horizontal principal stress, in MPa;
• α is the effective stress coefficient, in decimal;
• σc is the experimental confining pressure, in MPa;
• Pinj is the fluid injection pressure, in MPa;
• PISI is the bottom-hole instantaneous shut-in pressure for the hydraulic fracturing, in MPa;
• PP is the formation pore pressure, in MPa;
• (4) the fracturing fluid is prepared according to the fracturing fluid formula on the construction site, and poured into the intermediate container of the constant-speed and constant-pressure pump;
• (5) the core after the porosity test in Step (2) is put into the core holder, and loaded with an initial confining pressure by the confining pressure pump;
• (6) the reaction kettle, the core and the core holder are heated by the heater to the experimental temperature determined in Step (3);
• (7) the air in the pipe and the reaction kettle is completely pumped out by the vacuum pump, then the inlet valve of the vacuum pump is closed, and the fracturing fluid in the intermediate container in Step (4) is pumped by the constant-speed and constant-pressure pump into the reaction kettle;
• (8) the linear velocity of the fracturing fluid on the fracture surface is calculated with the formula (6) according to the fracturing displacement of the shale gas well, the shale reservoir thickness, and the hydraulic fracture width;
• the loading velocity is calculated with the formula (7) according to the relationship between the linear velocity and the rotation speed;
• the motor is switched on to load the rotor to the calculated rotation speed;
• v
• =
• Q
• h
• 
• w
• ;
• (
• 6
• )
• n
• =
• v
• 2
• 
• π
• 
• r
• ;
• (
• 7
• )
• in the formulas, v is the linear velocity of fluid, in m/s, Q is the fracturing displacement of the shale gas well, in m3/min, h is the thickness of the reservoir where the shale gas well is located, in m, w is the hydraulic fracture width, in m, n is the loading speed, in rad/min, and r is the rotor radius, in m;
• (9) the injection pressure of the constant-speed and constant-pressure pump is set according to the loading pressure of the confining pressure pump that is determined in Step (3) and the determined injection pressure;
• the control computer of the constant-speed and constant-pressure pump records the injected fluid volume once every three minutes;
• the cumulative volume of each time node is the dynamic imbibition amount of the shale at that time;
• the test time is the average single-stage fracturing time of the shale gas well;
• (10) according to the dynamic imbibition amount measured in Step (9), the dynamic imbibition saturation I is defined to characterize the dynamic imbibition amount, that is, the percentage I of the dynamic imbibition amount to the core pore volume, expressed as follows:
• I
• =
• V
• ϕ
• 
• 
• AL
• ×
• 1
• 
• 0
• 
• 0
• 
• %
• ;
• (
• 8
• )
• in the formula:
• I is the dynamic imbibition saturation, in %, Vis the dynamic imbibition amount, in cm3, A is the imbibition area, in cm2, L is the core length, in cm, and φ is the porosity, in %.