Device and method for measuring magnitude of seepage force and its influence on effective stress of formation

A method for measuring the magnitude of a seepage force and its influence on effective stress of a formation, comprising the following steps:

• (1) taking underground cores with a drilling tool, selecting a core with a complete structure and processing it into a cylindrical sample with a diameter of 50 cm and a height of 100 cm, placing the sample in a confining pressure loading chamber, marking 11 points at equal intervals from top to bottom on an outer wall of the sample as measuring points, placing an upper permeable pressure-bearing steel sheet and a lower permeable pressure-bearing steel sheet having the radius as the sample on the top and bottom of the sample, respectively, and then sleeving the sample into a transparent rubber sleeve;
• (2) turning on a vacuum pump to vacuumize the sample for two hours, then turning off the vacuum pump, and recording an axial initial data L00, L01, L02 . . . , L10, and a lateral initial data D00, D01, D02 . . . , D10 of respective measuring points of the sample through a non-contact three-dimensional deformation measuring system;
• dividing the 11 measuring points into 5 segments, calculating an axial initial amount of each segment λ1 (λ1=L01−L00), and then calculating by analogy λ2, λ3, λ4 and λ5, and calculating a lateral initial amount of each segment γ1 (γ1=(D01+D00)/2), and then calculating by analogy γ2, γ3, γ4 and γ5;
• (3) starting a confining pressure loading system to inject fluid into the confining pressure loading chamber, and simultaneously opening an exhaust valve and then closing the exhaust valve until the fluid fills the confining pressure loading chamber, loading a confining pressure to a predetermined value, and keeping the confining pressure unchanged;
• (4) starting an axial pressure servo motor to apply a bias voltage to the sample to a predetermined value, and keeping the bias voltage constant;
• (5) recording an axial displacement amount l0, l1, l2 . . . , l10, and a lateral displacement amount d0, d1, d2 . . . , d10 of respective measuring points of the sample when the sample is stabilized after deformation, the calculation method being the same as in step (2), and calculating an axial deformation amount L1, L2, L3, L4, L5, a lateral deformation amount D1, D2, D3, D4, D5, and a value F1 of the pressure testing device at this time by analogy;
• obtaining therefore an axial strain ε1 and a lateral strain ε3 of respective segments of the sample under non-porous pressure loading which are respectively:
• ε1′=(L1−λ1)/λ1, ε1″=(L2−λ2,)/λ2, and ε1′″, ε1″″, ε1′″″ can be obtained in the same way;
• and ε3′=(D1−γ1)/γ1, ε3″=(D2−γ2)/γ2, ε3′″, ε3″″ and ε3′″″ can be obtained in the same way;
• according to the formula
• ɛ
• 1
• =
• (
• σ
• 1
• -
• 2
• ⁢
• μσ
• 3
• )
• E
• ,
• ɛ
• 3
• =
• [
• σ
• 3
• -
• μ
• ⁡
• (
• σ
• 1
• +
• σ
• 3
• )
• ]
• E
• ,
• the effective stress of the five segments of the sample can be calculated in sequence, namely (σ1′, σ3′), (σ1″, σ3″, (σ1′″, σ3′″), (σ1″″, σ3″″), and (σ1′″″, σ3′″″), where E is Young's modulus and is Poisson's ratio;
• (6) starting the pore pressure loading system to inject fluid into the sample, so as to increase the injection pressure, and recording a pore pressure value P1 at an inlet end of the sample;
• (7) recording a pressure P2 in a water storage chamber and a value F2 of the pressure testing device by a flow pressure sensor after a water pressure in the water storage chamber is stabilized, which gradually increases due to the hysteresis of fluid flowing in the sample when adjusting the pore pressure loading system to inject fluid into the sample;
• (8) obtaining a seepage force measurement value of the core by J=F1−F2;
• (9) calculating a seepage force according to a seepage force calculation formula J′=rw·i·V, where
• i
• =
• h
• 1
• -
• h
• 2
• l
• ,
• P1=rw·h1, P2=rw·h2, where rw is volume weight of water, i is pressure gradient, V is sample volume, and h1 and h2 are water heads corresponding to P1 and P2, respectively;
• (10) obtaining the seepage force of J under the corresponding axial pressure, confining pressure and flow pressure gradient of the sample when
• 
• J
• ′
• -
• J
• J
• 
• <
• 0.01
• ;
• otherwise, repeating step (1) to step (9);
• (11) recording and calculating an axial deformation data L1′, L2′ . . . , L5′, and a lateral deformation data D1′, D2′ . . . , D5′ of respective segments corresponding to respective measuring points of the sample, the calculation method being the same as in step (2);
• (12) obtaining therefore an axial strain ε11 and a lateral strain ε33 of respective segments of the sample in the process of pore pressure loading which are respectively:
• ε11′=(L1′−λ1)/λ1, ε11″=(L2′−λ2)/λ2, and ε11′″, ε11″″, ε11′″″ can be obtained in the same way;
• and ε33′=(D1′−γ1)/γ1, ε33″=(D2′−γ2)/γ2, and ε33′″, ε33″″, ε33′″″ can be obtained in the same way;
• similarly, the effective stress of each of the five measuring segments in the presence of fluid in the pore can be calculated in sequence by formula
• ɛ
• 1
• =
• (
• σ
• 1
• -
• 2
• ⁢
• μσ
• 3
• )
• E
• ,
• ɛ
• 3
• =
• [
• σ
• 3
• -
• μ
• ⁡
• (
• σ
• 1
• +
• σ
• 3
• )
• ]
• E
• ,
• which are respectively (σ11′, σ33′), (σ11″, σ33″), (σ11′″, σ33′″), (σ11″″, σ33″″), and (σ11′″″, σ33′″″);
• and
• (13) curve-fitting an increment of the effective stress of respective measuring segments corresponding to respective measuring points generated by the seepage force, by the following formula, respectively:
• σaxial′=σ11′−σ1′, σradical′=σ33′−σ3′;
• σaxial″=σ11″−σ1″, σradical″=σ33″−σ3″;
• σaxial′″=σ11′″−σ1′″, σradical′″=σ33′″−σ3′″;
• σaxial″″=σ11″″−σ1″″, σradical″″=σ33″″−σ3″″;
• σaxial′″″=σ11′″″−σ1′″″, σradical′″″=σ33′″″−σ3′″″.
• according to the effective stress obtained in step (13), so as to determine an effective stress change law of at different positions the sample under the seepage force of J.