A method for evaluating the water sensitivity degree of rocks based on mass change

Abstract

The present invention relates to a method for evaluating the water sensitivity of rocks based on mass change, and belongs to the field of oil and gas field development; it solves the problem that it takes a long time, has high labor costs, and is cumbersome to evaluate the water sensitivity of rocks using indoor experiments; its technical scheme is: according to Poiseuille's law, a relationship model between the water sensitivity index of rocks and the average pore radius is established; after water sensitivity, the clay absorbs water and expands, and the mass changes, and a relationship model between the mass change and the average pore radius is established; the mass change coefficient before and after water sensitivity is used to characterize the degree of mass change, and the mass change is dimensionlessly processed, and the relationship model between the water sensitivity index and the average pore radius and the relationship model between the mass change and the average pore radius are jointly established to establish a relationship model between the water sensitivity index and the mass change; then the pore volume of the rock sample, the volume of clay minerals, and the density change of the clay minerals before and after water sensitivity are tested, and then according to the mass of the rock sample before and after being fully saturated in salt water with different mineralizations, the water sensitivity index of the rock in salt water with different mineralizations can be calculated, and the water sensitivity of the rock can be quickly judged. The practical application effect of the present invention is good, the calculation result is reliable, and it is highly extensible.

Description

Technical Field

[0001] The invention relates to a method for evaluating rock water sensitivity based on mass change, and belongs to the field of oil and gas field development. Background Art

[0002] With the continuous development of oil and gas field development technology, people have gradually gained a full understanding of the damage caused by water sensitivity to reservoirs. Compared with the damage to oil and gas layers during drilling, the formation damage that may be caused during oil field development and production is more complex and superimposed. It almost runs through the entire process of oil field development, and the depth of damage is likely to affect the entire oil layer. Therefore, convenient and fast evaluation of rock water sensitivity has become the focus of oil and gas field development methods. Traditional indoor experiments to evaluate rock water sensitivity are time-consuming, labor-intensive, and cumbersome.

[0003] After extensive research, the patent number CN202310821256.8, "A method for evaluating the water sensitivity of tight sandstone reservoirs based on nuclear magnetic resonance technology", uses nuclear magnetic resonance, water sensitivity experiments and water sensitivity index calculations to establish a relationship chart between the nuclear magnetic resonance T2 geometric mean and the water sensitivity index to evaluate the water sensitivity of rocks; the patent number CN202011399022.1, "A method for evaluating water sensitivity, water locking and solid phase damage of oil and gas reservoirs", uses four gas permeability tests to calculate the water lock damage rate of the core, and uses the same core to quantitatively evaluate the water sensitivity, water lock and solid phase damage of the core.

[0004] In general, there are many methods for evaluating the water sensitivity of rocks at present, but they all have their own limitations. There are fewer methods that involve evaluating the water sensitivity of rocks based on mass changes. It is necessary to have a water sensitivity evaluation method that is easy to use and has high accuracy, considering the characteristics of rock mass changes before and after rock water sensitivity. Summary of the invention

[0005] The purpose of the present invention is to solve the problems of long time consumption, high labor cost and complicated process in evaluating the water sensitivity of rocks by indoor experiments. According to Poiseuille's law, the present invention establishes a relationship model between the water sensitivity index of rocks and the average pore radius, and then uses the change in mass of clay swelling after water sensitivity to establish a relationship model between mass change and average pore radius. The relationship model between the water sensitivity index and the average pore radius and the relationship model between mass change and average pore radius are combined to establish a relationship model between the water sensitivity index and the mass change. Then, the pore volume of the rock sample, the volume of clay minerals and the density change of the clay minerals before and after water sensitivity are tested. Then, according to the mass change of the rock sample after being fully saturated in salt water with different mineralization degrees, the water sensitivity index of the rock in salt water with different mineralization degrees is calculated. The water sensitivity index calculation can be quickly completed according to the change in rock mass, the calculation is simple, the practical application effect is good, the calculation result is reliable, and the generalizability is strong.

[0006] To achieve the above object, the present invention provides a method for evaluating rock water sensitivity based on mass change, which comprises the following steps:

[0007] First, based on Poiseuille's law, a model of the relationship between rock water sensitivity index and average pore radius is established;

[0008] Second, after water sensitivity, clay absorbs water and expands, and its mass changes. A model of the relationship between mass change and average pore radius is established.

[0009] Thirdly, the relationship model between water sensitivity index and average pore radius and the relationship model between mass change and average pore radius were combined to establish the relationship model between water sensitivity index and mass change.

[0010] Fourth, test the pore volume of the rock sample, the volume of clay minerals, and the density change of clay minerals before and after water sensitivity. Then, based on the mass of the rock sample before and after it is fully saturated in brine with different mineralization degrees, the water sensitivity index of the rock in brine with different mineralization degrees can be calculated to quickly determine the water sensitivity of the rock.

[0011] In the above-mentioned method for evaluating rock water sensitivity based on mass change, the step of establishing a relationship model between rock water sensitivity index and average pore radius is as follows:

[0012] First, Poiseuille's law was introduced, and the average pore radius before and after water sensitivity was used to characterize the permeability before and after water sensitivity.

[0013] Second, the permeability before and after water sensitivity represented by the average pore radius is substituted into the water sensitivity index expression to establish the relationship model between the rock water sensitivity index and the average pore radius: In the formula, I W is the water sensitivity index, the unit is dimensionless quantity, φ1 is the porosity before water sensitivity, the unit is dimensionless quantity, r1 is the average pore radius before water sensitivity, the unit is cm, φ2 is the porosity after water sensitivity, the unit is dimensionless quantity, r2 is the average pore radius after water sensitivity, the unit is cm.

[0014] In the above-mentioned method for evaluating rock water sensitivity based on mass change, the step of establishing a model of the relationship between mass change and average pore radius is as follows:

[0015] First, after water sensitivity, the clay absorbs water and expands, and the density decreases. The change in clay mass is the mass of clay minerals after water sensitivity minus the mass of clay minerals before water sensitivity, expressed as, Δm1 = ρ2(V1 + ΔV) - ρ1V1, where Δm1 is the mass change of clay before and after water sensitivity, in g, and ρ1 is the density of clay minerals before water sensitivity, in g / cm 3 , ρ2 is the density of clay minerals after water sensitivity, in g / cm 3 , V1 is the volume of clay minerals before water sensitivity, in cm 3, ΔV is the increased volume of clay minerals after water sensitivity, with the unit of cm 3 ;

[0016] Second, after water sensitivity, the clay absorbs water and swells, increasing its volume, which reduces the pore volume. As a result, the brine in the pores is expelled, causing the mass of the brine in the pores to decrease. The increase in the volume of clay minerals is equal to the decrease in the pore volume. Therefore, the change in the mass of the brine is Δm2 = ρ W ΔV. In the formula, Δm2 is the magnitude of the change in the mass of water in the pores before and after water sensitivity, with the unit of g, and ρ W is the density of the brine in the pores, with the unit of g / cm 3 , ΔV is the increased volume of clay minerals after water sensitivity, with the unit of cm 3 ;

[0017] Third, the decrease in the mass of the rock is equal to the decrease in the mass of the brine in the pores minus the increase in the mass of the clay minerals after water sensitivity. The relationship between the decrease in the mass of the rock and the pore volume is established as Δm = ρ W ΔV - [ρ2(V1 + ΔV) - ρ1V1]. In the formula, Δm is the magnitude of the change in the mass of the rock before and after water sensitivity, with the unit of g, and ρ W is the density of the brine in the pores, with the unit of g / cm 3 , ΔV is the increased volume of clay minerals after water sensitivity, with the unit of cm 3 , ρ1 is the density of the clay minerals before water sensitivity, with the unit of g / cm 3 , ρ2 is the density of the clay minerals after water sensitivity, with the unit of g / cm 3 , V1 is the volume of the clay minerals before water sensitivity, with the unit of cm 3 ;

[0018] Fourth, the degree of mass change is characterized by the mass change coefficient before and after water sensitivity, and Δm is made dimensionless. Δm = am1. In the formula, Δm is the magnitude of the change in the mass of the rock before and after water sensitivity, with the unit of g, a is the mass change coefficient before and after water sensitivity, with the unit of dimensionless, and m1 is the mass of the rock before water sensitivity, with the unit of g;

[0019] Fifth, based on the pore volume before and after water sensitivity, the average pore radius before and after water sensitivity is characterized, and the functional relationship between the average pore radius and the change in pore volume is established. Combining with the relationship between the decrease in the mass of the rock and the change in pore volume, a relationship model between mass change and average pore radius is established. The calculation formula for the average pore radius before water sensitivity is The calculation formula for the average pore radius after water sensitivity is In the formula, r1 is the average pore radius before water sensitivity, with the unit of cm, r2 is the average pore radius after water sensitivity, with the unit of cm, V0 is the initial pore volume of the rock, with the unit of cm 3 , V1 is the volume of the clay minerals before water sensitivity, with the unit of cm3 , N is the number of pores, unit is dimensionless quantity, Δm is the change of rock mass before and after water sensitivity, unit is g, ρ W is the density of the brine in the pores, in g / cm 3 , ρ1 is the density of clay minerals before water sensitivity, in g / cm 3 , ρ2 is the density of clay minerals after water sensitivity, in g / cm 3 .

[0020] In the above-mentioned method for evaluating the water sensitivity of rocks based on mass change, the establishment of the relationship model between the water sensitivity index and the mass change is to establish the relationship model between the water sensitivity index and the average pore radius by combining the relationship model between the water sensitivity index and the average pore radius and the relationship model between the mass change and the average pore radius, In the formula, I W is the water sensitivity index, unit is dimensionless quantity, V0 is the initial pore volume of rock, unit is cm 3 , a is the mass change coefficient before and after water sensitivity, unit is dimensionless, m1 is the mass of rock before water sensitivity, unit is g, V1 is the volume of clay minerals before water sensitivity, unit is cm 3 , ρ W is the density of the brine in the pores, in g / cm 3 , ρ1 is the density of clay minerals before water sensitivity, in g / cm 3 , ρ2 is the density of clay minerals after water sensitivity, in g / cm 3 .

[0021] In the above-mentioned method for evaluating the water sensitivity of rocks based on mass change, the step of calculating the water sensitivity index of rocks in salt water with different mineralization degrees is as follows:

[0022] First, test the rock porosity and rock volume, and calculate the rock pore volume V0 in cm 3 ;

[0023] Second, use X-ray diffraction to test the clay content of the rock and calculate the volume of clay minerals before water sensitivity, V1, in cm 3 ;

[0024] Third, use the actual clay samples in the target area to test the density ρ1 of the clay minerals before water sensitivity, in g / cm 3 , test the density of brine with different mineralization W , unit is g / cm 3 , use different mineralization degree salt water to soak clay samples, test the density ρ2 of clay after soaking with different mineralization degree, the unit is g / cm 3 ;

[0025] Fourth, measure the masses m1 and m2 of the rock sample before and after complete saturation in salt water with different salinities, in g. Calculate the mass change coefficient a before and after water sensitivity, and substitute it into the relationship model between the water sensitivity index and the mass change to calculate the water sensitivity index of the rock in salt water with different salinities, and quickly judge the water sensitivity degree of the rock.

[0026] In the above method for evaluating the water sensitivity degree of a rock based on mass change, the Poiseuille's law is that all factors affecting the pore structure of the rock affect the rock permeability, and the permeability can be expressed as In the formula, k is the rock permeability, in D; φ is the porosity, dimensionless; r is the average pore radius, in cm; τ is the tortuosity of the rock, dimensionless.

[0027] In the above method for evaluating the water sensitivity degree of a rock based on mass change, the expression for the mass change coefficient before and after water sensitivity is a is the mass change coefficient before and after water sensitivity, dimensionless; m1 is the mass of the rock before water sensitivity, in g; m2 is the mass of the rock after water sensitivity, in g.

[0028] In the above method for evaluating the water sensitivity degree of a rock based on mass change, the expression for the water sensitivity index is I W is the water sensitivity index, dimensionless; k1 is the permeability before water sensitivity, in D; k2 is the permeability before water sensitivity, in D.

[0029] In the above method for evaluating the water sensitivity degree of a rock based on mass change, the calculation of the water sensitivity index of the rock in salt water with different salinities and the quick judgment of the water sensitivity degree of the rock are as follows: I W ≤0.05, no water sensitivity; 0.05 < I W ≤0.30, weak water sensitivity; 0.30 < I W ≤0.50, moderately weak water sensitivity; 0.50 < I W ≤0.70, moderately strong water sensitivity; 0.70 < I W ≤0.90, strong water sensitivity; I W >0.90, extremely strong water sensitivity.

[0030] Compared with the prior art, the present invention has the following beneficial effects: (1) Combining theory with practice, the calculation results are more accurate and reliable; (2) The calculation process is simple and fast, and it is convenient to use; (3) It has strong popularization potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In the drawings:

[0032] Figure 1 is the technical roadmap of the method.

[0033] Figure 2 It is a water sensitivity index chart. DETAILED DESCRIPTION

[0034] The present invention is further described below in conjunction with the embodiments and drawings.

[0035] The present invention provides a method for evaluating the water sensitivity of rocks based on mass change. Figure 1 The technical roadmap of this method includes the following steps:

[0036] First, based on Poiseuille's law, a model of the relationship between rock water sensitivity index and average pore radius is established;

[0037] Second, after water sensitivity, clay absorbs water and expands, and its mass changes. A model of the relationship between mass change and average pore radius is established.

[0038] Thirdly, the relationship model between water sensitivity index and average pore radius and the relationship model between mass change and average pore radius were combined to establish the relationship model between water sensitivity index and mass change.

[0039] Fourth, test the pore volume of the rock sample, the volume of clay minerals, and the density change of clay minerals before and after water sensitivity. Then, based on the mass of the rock sample before and after it is fully saturated in brine with different mineralization degrees, the water sensitivity index of the rock in brine with different mineralization degrees can be calculated to quickly determine the water sensitivity of the rock.

[0040] Furthermore, the step of establishing the relationship model between rock water sensitivity index and average pore radius is as follows:

[0041] First, Poiseuille's law was introduced, and the average pore radius before and after water sensitivity was used to characterize the permeability before and after water sensitivity.

[0042] Second, the permeability before and after water sensitivity represented by the average pore radius is substituted into the water sensitivity index expression to establish the relationship model between the rock water sensitivity index and the average pore radius: In the formula, I w is the water sensitivity index, the unit is dimensionless quantity, φ1 is the porosity before water sensitivity, the unit is dimensionless quantity, r1 is the average pore radius before water sensitivity, the unit is cm, φ2 is the porosity after water sensitivity, the unit is dimensionless quantity, r2 is the average pore radius after water sensitivity, the unit is cm.

[0043] Furthermore, the step of establishing the relationship model between mass change and average pore radius is as follows:

[0044] First, after water sensitivity, the clay absorbs water and expands, and the density decreases. The change in clay mass is the mass of clay minerals after water sensitivity minus the mass of clay minerals before water sensitivity, expressed as, Δm1 = ρ2(V1 + ΔV) - ρ1V1, where Δm1 is the mass change of clay before and after water sensitivity, in g, and ρ1 is the density of clay minerals before water sensitivity, in g / cm3 , ρ2 is the density of clay minerals after water sensitivity, with the unit of g / cm 3 , V1 is the volume of clay minerals before water sensitivity, with the unit of cm 3 , ΔV is the increased volume of clay minerals after water sensitivity, with the unit of cm 3 ;

[0045] Second, after water sensitivity, the clay absorbs water and swells, increasing its volume, which reduces the pore volume. As a result, the brine in the pores is expelled, reducing the mass of the brine in the pores. The increase in the volume of clay minerals is equal to the decrease in pore volume. Therefore, the change in the mass of brine is Δm2 = ρ W ΔV, where Δm2 is the magnitude of the change in the mass of water in the pores before and after water sensitivity, with the unit of g, and ρ W is the density of brine in the pores, with the unit of g / cm 3 , ΔV is the increased volume of clay minerals after water sensitivity, with the unit of cm 3 ;

[0046] Third, the reduction in the mass of the rock is equal to the reduction in the mass of brine in the pores minus the increase in the mass of clay minerals after water sensitivity. The relationship between the reduction in the mass of the rock and the pore volume is established as Δm = ρ W ΔV - [ρ2(V1 + ΔV) - ρ1V1], where Δm is the magnitude of the change in the mass of the rock before and after water sensitivity, with the unit of g, and ρ W is the density of brine in the pores, with the unit of g / cm 3 , ΔV is the increased volume of clay minerals after water sensitivity, with the unit of cm 3 , ρ1 is the density of clay minerals before water sensitivity, with the unit of g / cm 3 , ρ2 is the density of clay minerals after water sensitivity, with the unit of g / cm 3 , V1 is the volume of clay minerals before water sensitivity, with the unit of cm 3 ;

[0047] Fourth, the degree of mass change is characterized by the mass change coefficient before and after water sensitivity, and Δm is made dimensionless, Δm = am1, where Δm is the magnitude of the change in the mass of the rock before and after water sensitivity, with the unit of g, a is the mass change coefficient before and after water sensitivity, with the unit of dimensionless, and m1 is the magnitude of the mass of the rock before water sensitivity, with the unit of g;

[0048] Fifth, based on the pore volume before and after water sensitivity, the average pore radius before and after water sensitivity is characterized, and the functional relationship between the average pore radius and the change in pore volume is established. Combining with the relationship between the reduction in the mass of the rock and the change in pore volume, a relationship model between mass change and average pore radius is established. The calculation formula for the average pore radius before water sensitivity is The calculation formula for the average pore radius after water sensitivity is Where r1 is the average pore radius before water sensitivity, in cm, r2 is the average pore radius after water sensitivity, in cm, and V0 is the initial pore volume of the rock, in cm 3 , V1 is the volume of clay minerals before water sensitivity, in cm 3 , N is the number of pores, unit is dimensionless quantity, Δm is the change of rock mass before and after water sensitivity, unit is g, ρ W is the density of the brine in the pores, in g / cm 3 , ρ1 is the density of clay minerals before water sensitivity, in g / cm 3 , ρ2 is the density of clay minerals after water sensitivity, in g / cm 3 .

[0049] Furthermore, the water sensitivity index and mass change relationship model is established by combining the water sensitivity index and average pore radius relationship model and the mass change and average pore radius relationship model to establish the water sensitivity index and mass change relationship model. In the formula, I W is the water sensitivity index, unit is dimensionless quantity, V0 is the initial pore volume of rock, unit is cm 3 , a is the mass change coefficient before and after water sensitivity, unit is dimensionless, m1 is the mass of rock before water sensitivity, unit is g, V1 is the volume of clay minerals before water sensitivity, unit is cm 3 , ρ W is the density of the brine in the pores, in g / cm 3 , ρ1 is the density of clay minerals before water sensitivity, in g / cm 3 , ρ2 is the density of clay minerals after water sensitivity, in g / cm 3 .

[0050] Furthermore, the step of calculating the water sensitivity index of rocks in salt water with different mineralization degrees is as follows:

[0051] First, test the rock porosity and rock volume, and calculate the rock pore volume V0 in cm 3 ;

[0052] Second, use X-ray diffraction to test the clay content of the rock and calculate the volume of clay minerals before water sensitivity, V1, in cm 3 ;

[0053] Third, use the actual clay samples in the target area to test the density ρ1 of the clay minerals before water sensitivity, in g / cm 3 , test the density of brine with different mineralization W , unit is g / cm 3 , use different mineralization degree salt water to soak clay samples, test the density ρ2 of clay after soaking with different mineralization degree, the unit is g / cm 3;

[0054] Fourth, measure the masses m1 and m2 of the rock sample before and after being completely saturated in salt water with different salinities, with the unit of g. Calculate the mass change coefficient a before and after water sensitivity, and substitute it into the relationship model between the water sensitivity index and the mass change to calculate the water sensitivity index of the rock in salt water with different salinities, and quickly judge the water sensitivity degree of the rock.

[0055] Further, the Poiseuille's law is that all factors affecting the pore structure of the rock affect the rock permeability, and the permeability can be expressed as In the formula, k is the rock permeability, with the unit of D, φ is the porosity, with the unit of dimensionless, r is the average pore radius, with the unit of cm, and τ is the tortuosity of the rock, with the unit of dimensionless.

[0056] Further, the expression of the mass change coefficient before and after water sensitivity is a is the mass change coefficient before and after water sensitivity, with the unit of dimensionless, m1 is the mass of the rock before water sensitivity, with the unit of g, and m2 is the mass of the rock after water sensitivity, with the unit of g.

[0057] Further, the expression of the water sensitivity index is I W is the water sensitivity index, with the unit of dimensionless quantity, k1 is the permeability before water sensitivity, with the unit of D, and k2 is the permeability before water sensitivity, with the unit of D

[0058] Further, the calculation of the water sensitivity index of the rock in salt water with different salinities and the quick judgment of the water sensitivity degree of the rock are as follows: I W ≤0.05, no water sensitivity; 0.05 < I W ≤0.30, weak water sensitivity; 0.30 < I W ≤0.50, moderately weak water sensitivity; 0.50 < I W ≤0.70, moderately strong water sensitivity; 0.70 < I W ≤0.90, strong water sensitivity; I W >0.90, extremely strong water sensitivity.

[0059] Taking different types of rocks as examples, use this series of rock samples to conduct water sensitivity degree evaluation experiments. The test results of the salt water density, the initial pore volume of the rock, the volume of clay minerals before water sensitivity, and the density of clay minerals before water sensitivity are shown in Table 1.

[0060] Table 1 Parameters before water sensitivity

[0061]

[0062] Test the density of clay minerals after water sensitivity and the change in rock mass before and after water sensitivity. Using the relationship model between the water sensitivity index and the mass change, calculate the water sensitivity index and obtain the judgment result of the water sensitivity degree of the rock, as shown in Table 2.

[0063] Table 2 Calculation results of water sensitivity index

[0064]

[0065] Draw a water sensitivity index graph, as Figure 2 shown, the greater the mass change coefficient before and after water sensitivity, the greater the water sensitivity index, and the greater the water sensitivity degree of the rock.

[0066] Compared with the prior art, the present invention has the following beneficial effects: (1) Combining theory with practice, the calculation results are more accurate and reliable; (2) The calculation process is simple and easy to use; (3) It has strong popularization.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the present invention can still be modified or equivalently replaced, and any modification or partial replacement without departing from the spirit and scope of the present invention shall be covered by the scope of the claims of the present invention.

Claims

1. A method for evaluating the water sensitivity degree of rocks based on mass change, characterized in that The method includes the following steps: S100. According to Poiseuille's law, the steps for establishing the relationship model between the rock water sensitivity index and the average pore radius are as follows. S101. Introduce Poiseuille's law and use the average pore radius before and after water sensitivity to characterize the permeability before and after water sensitivity. S102. Substitute the permeabilities before and after water sensitivity characterized by the average pore radius into the water sensitivity index expression, and establish the relationship model between the rock water sensitivity index and the average pore radius as In the formula, I W is the water sensitivity index, with the unit of dimensionless quantity; φ1 is the porosity before water sensitivity, with the unit of dimensionless quantity; r1 is the average pore radius before water sensitivity, with the unit of cm; φ2 is the porosity after water sensitivity, with the unit of dimensionless quantity; r2 is the average pore radius after water sensitivity, with the unit of cm. S200. After water sensitivity, the clay absorbs water and swells, and the mass changes. The steps to establish the relationship model between the mass change and the average pore radius are as follows. S201. After water sensitivity, the clay absorbs water and swells, and the density decreases. The change in the clay mass is the mass of the clay mineral after water sensitivity minus the mass of the clay mineral before water sensitivity. The expression is Δm1 = ρ2(V1 + ΔV) - ρ1V1. In the formula, Δm1 is the magnitude of the mass change of the clay before and after water sensitivity, with the unit of g; ρ1 is the density of the clay mineral before water sensitivity, with the unit of g / cm 3 , ρ2 is the density of the clay mineral after water sensitivity, with the unit of g / cm 3 , V1 is the volume of the clay mineral before water sensitivity, with the unit of cm 3 , ΔV is the increased volume of the clay mineral after water sensitivity, with the unit of cm 3 ; S202. After water sensitivity, the clay absorbs water and swells, increasing in volume, which reduces the pore volume, thereby discharging the brine in the pores and decreasing the mass of the brine in the pores. The increase in the volume of clay minerals is equal to the decrease in the pore volume. Therefore, the change in the mass of the brine is Δm2 = ρ W ΔV. In the formula, Δm2 is the magnitude of the change in the mass of water in the pores before and after water sensitivity, with the unit of g, and ρ W is the density of the brine in the pores, with the unit of g / cm 3 , and ΔV is the increased volume of clay minerals after water sensitivity, with the unit of cm 3 ; S203, the reduction in rock mass is equal to the reduction in the mass of brine in the pores minus the increase in the mass of clay minerals after water sensitivity. The relationship between the reduction in rock mass and the pore volume is established as Δm = ρ W ΔV - [ρ2(V1 + ΔV) - ρ1V1], where Δm is the change in rock mass before and after water sensitivity, with the unit of g, ρ W is the density of brine in the pores, with the unit of g / cm 3 , ΔV is the increased volume of clay minerals after water sensitivity, with the unit of cm 3 , ρ1 is the density of clay minerals before water sensitivity, with the unit of g / cm 3 , ρ2 is the density of clay minerals after water sensitivity, with the unit of g / cm 3 , V1 is the volume of clay minerals before water sensitivity, with the unit of cm 3 ; S204. Use the mass change coefficient before and after water sensitivity to characterize the degree of mass change, and perform dimensionless treatment on Δm. Δm = am1, where Δm is the magnitude of the rock mass change before and after water sensitivity, with the unit of g; a is the mass change coefficient before and after water sensitivity, with the unit of dimensionless; m1 is the magnitude of the rock mass before water sensitivity, with the unit of g. S205. Characterize the average pore radius before and after water sensitivity according to the pore volume before and after water sensitivity, establish the functional relationship between the average pore radius and the change in pore volume, and combine the relationship between the reduction in rock mass and the change in pore volume to establish a relationship model between mass change and average pore radius. The calculation formula for the average pore radius before water sensitivity is The calculation formula for the average pore radius after water sensitivity is In the formula, r1 is the average pore radius before water sensitivity, with the unit of cm; r2 is the average pore radius after water sensitivity, with the unit of cm; V0 is the initial pore volume of the rock, with the unit of cm 3 , V1 is the volume of clay minerals before water sensitivity, with the unit of cm 3 , N is the number of pores, with the unit of dimensionless quantity; Δm is the magnitude of the change in rock mass before and after water sensitivity, with the unit of g; ρ W is the density of brine in the pores, with the unit of g / cm 3 , ρ1 is the density of clay minerals before water sensitivity, with the unit of g / cm 3 , ρ2 is the density of clay minerals after water sensitivity, with the unit of g / cm 3 ; S300, the water sensitivity index and average pore radius relationship model and the mass change and average pore radius relationship model are combined to establish the water sensitivity index and mass change relationship model. In the formula, I W is the water sensitivity index, unit is dimensionless quantity, V0 is the initial pore volume of rock, unit is cm 3 , a is the mass change coefficient before and after water sensitivity, unit is dimensionless, m1 is the mass of rock before water sensitivity, unit is g, V1 is the volume of clay minerals before water sensitivity, unit is cm 3 , ρ W is the density of the brine in the pores, in g / cm 3 , ρ1 is the density of clay minerals before water sensitivity, in g / cm 3 , ρ2 is the density of clay minerals after water sensitivity, in g / cm 3 ; S400. Then, test the pore volume, clay mineral volume, and density change of the clay mineral before and after water sensitivity of the rock sample. According to the mass of the rock sample before and after being completely saturated in salt water with different salinities, the steps for calculating the water sensitivity index of the rock in salt water with different salinities and quickly judging the degree of rock water sensitivity are as follows. S401, Test the rock porosity and rock volume, calculate the rock pore volume V0, with the unit of cm 3 ; S402, use X-ray diffraction to test the clay content of the rock, calculate the volume V1 of clay minerals before water sensitivity, with the unit of cm 3 ; S403. Use the actual clay sample in the target area to measure the density ρ1 of clay minerals before water sensitivity, with the unit of g / cm 3 . Measure the density ρ W of brines with different salinities, with the unit of g / cm 3 . Immerse the clay sample in brines with different salinities and measure the density ρ2 of the clay after immersion in brines with different salinities, with the unit of g / cm 3 ; S404. Measure the mass m1 and m2 of the rock sample before and after being completely saturated in salt water with different salinities, with the unit of g. Calculate the mass change coefficient a before and after water sensitivity, and substitute it into the relationship model between the water sensitivity index and the mass change to calculate the water sensitivity index of the rock in salt water with different salinities and quickly judge the degree of rock water sensitivity.

2. The method for evaluating the water sensitivity degree of rock based on mass change according to claim 1, wherein: The Poiseuille's law states that all factors affecting the pore structure of rocks also affect rock permeability, and the permeability can be expressed as where k is the rock permeability in D, φ is the porosity in dimensionless, r is the average pore radius in cm, and τ is the tortuosity of the rock in dimensionless.

3. A method for evaluating the water sensitivity degree of rock based on mass change according to claim 1, characterized in that: The expression for the mass change coefficient before and after water sensitivity is where a is the mass change coefficient before and after water sensitivity, with the unit of dimensionless; m1 is the mass of the rock before water sensitivity, with the unit of g; and m2 is the mass of the rock after water sensitivity, with the unit of g.

4. A method for evaluating the water sensitivity degree of rock based on mass change according to claim 1, characterized in that: The water sensitivity index expression is as follows: I W is the water sensitivity index, with the unit of dimensionless quantity; k1 is the permeability before water sensitivity, with the unit of D; k2 is the permeability before water sensitivity, with the unit of D.

5. A method for evaluating the water sensitivity degree of rock based on mass change according to claim 1, characterized in that: The water sensitivity index of the rock in salt water with different salinities is calculated to quickly judge the water sensitivity degree of the rock as follows: I W ≤0.05, no water sensitivity; 0.05 < I W ≤0.30, weak water sensitivity; 0.30 < I W ≤0.50, moderately weak water sensitivity; 0.50 < I W ≤0.70, moderately strong water sensitivity; 0.70 < I W ≤0.90, strong water sensitivity; I W > 0.90, extremely strong water sensitivity.

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