Method for determining the skin factor of drilling fluid invasion in horizontal wells of sandstone reservoirs

Abstract

A method for determining the skin coefficient of drilling fluid invasion in a horizontal well of a sandstone reservoir. The method comprises the following steps: the first step is to establish an elliptical cone-shaped physical model of the horizontal well borehole and the reservoir damage zone; the second step is to test the dynamic damage filtration loss of the core drilling fluid, establish the function of the unit length filtration rate and time, and calculate the equivalent invasion radius of the drilling fluid; the third step is to test the linear permeability of the core before and after the drilling fluid damage, establish the function of the permeability recovery rate and the measurement position, and calculate the radial permeability after the damage around the wellbore; the fourth step is to calculate the equivalent invasion radius and radial permeability of the drilling fluid according to the obtained equivalent invasion radius and radial permeability. ， Determine the skin coefficient of drilling fluid invasion in horizontal wells. This method takes into account the influence of radial seepage on reservoir damage and the influence of reservoir permeability and the change of drilling fluid invasion radius along the damage zone on the skin coefficient of drilling fluid invasion, so it can more truly reflect the damage suffered by the reservoir caused by the invasion of drilling fluid.

Description

Technical field:

[0001] The invention relates to a method for determining the invasion skin factor of drilling fluid in a sandstone reservoir horizontal well, which is applied in the field of reservoir damage evaluation. Background technology:

[0002] With the continuous advancement of oil and gas field mining technology, horizontal well oil production technology has been more and more widely used in the development of thin sandstone oil and gas layers. During the horizontal well drilling process, the invasion of drilling fluid causes the reservoir to suffer varying degrees of damage, reducing the reservoir's production capacity. For this reason, technicians use the horizontal well drilling fluid invasion skin coefficient to characterize the degree of drilling fluid damage to the reservoir. Obtaining an accurate horizontal well drilling fluid invasion skin coefficient can not only optimize the drilling fluid system, but also has important significance for evaluating the effect of horizontal well acidizing production increase.

[0003] Patent application number CN202010844468.4, "A method for evaluating reservoir damage caused by drilling fluid in tight sandstone gas reservoirs", evaluates the degree of reservoir damage by using the reservoir damage ratio, while actual reservoir damage is not only reflected in changes in permeability, but also in changes in invasion radius. Patent application number CN201710823011.3, "A system for evaluating reservoir damage under simulated downhole conditions", uses this system to simulate the process of drilling fluid damage to the reservoir under actual downhole working conditions, but the system fails to reflect a specific method for evaluating the degree of damage.

[0004] In summary, the existing evaluation technology still has the following problems: First, the calculation of reservoir permeability and drilling fluid invasion radius is carried out under indoor linear seepage conditions, not radial seepage conditions of actual well conditions. Among them, Ferguson CK established an empirical formula for drilling fluid loss rate and time (see Formula 2-1); Second, the impact of changes in reservoir permeability and drilling fluid invasion radius along the damaged area on the evaluation of reservoir damage degree is not considered simultaneously. Summary of the invention:

[0005] In order to solve the technical problems mentioned in the background technology, the present invention provides a method for determining the skin coefficient of drilling fluid invasion in horizontal wells of sandstone reservoirs. The method takes into account the influence of radial seepage on reservoir damage and the influence of changes in reservoir permeability and drilling fluid invasion radius along the damage zone on the drilling fluid invasion skin coefficient. The drilling fluid invasion skin coefficient determined by this method has higher accuracy.

[0006] The technical solution of the present invention is: the method for determining the invasion skin coefficient of drilling fluid in a horizontal well of a sandstone reservoir is characterized in that the method comprises the following steps:

[0007] The first step is to establish an elliptical cone-shaped physical model of the wellbore and reservoir damage zone in the horizontal section of the horizontal well. The path is as follows:

[0008] The volume calculation formula of the elliptical cone physical model is shown in formula 1-1.

[0009]

[0010] Where:

[0011] V cone —Volume of elliptical cone physical model, cm 3 ;

[0012] a H,min —The minimum horizontal axis length in the elliptical cone physical model, cm;

[0013] a V,min —The minimum vertical axis length in the elliptical cone physical model, cm;

[0014] a H,max —The maximum horizontal axis length in the elliptical cone physical model, cm;

[0015] a V,max —The maximum vertical axis length in the elliptical cone physical model, cm;

[0016] L—horizontal section length of horizontal well, cm;

[0017] The anisotropy coefficient β is defined as:

[0018]

[0019] Where:

[0020] β—anisotropy coefficient, dimensionless;

[0021] K H —Horizontal permeability, 10 -3 μm 2 ;

[0022] K V —Vertical permeability, 10 -3 μm 2 ;

[0023] The ratio of the horizontal axis length to the vertical axis length:

[0024] a H =β·a V (1-3)

[0025] Where:

[0026] a H —The length of the horizontal axis in the elliptical cone physical model, cm;

[0027] a V—length of the vertical axis in the elliptical cone physical model, cm;

[0028] Since the minimum horizontal axis length a in the elliptical cone physical model H,min or the minimum vertical axis length a V,min is the wellbore radius r w Available,

[0029]

[0030] Where:

[0031] r w —wellbore radius, cm;

[0032] After considering the rock pore volume in the reservoir damage area, the volume calculation formula of the elliptical cone physical model is simplified to:

[0033]

[0034] Where:

[0035] φ—porosity, dimensionless;

[0036] The second step is to test the dynamic damage loss of core drilling fluid and establish the function of unit length loss rate and time, as shown in formula 2-1. Then, use formula 2-6 to calculate the equivalent invasion radius r of drilling fluid. s ′;

[0037] q m =ae -bt (2-1)

[0038] Where:

[0039] q m —Filtration rate per unit length, cm 3 / (cm·h);

[0040] a—filtration coefficient, cm 3 / (cm·h);

[0041] b—filtration loss index, 1 / h;

[0042] t—drilling fluid invasion time, h;

[0043] The relationship between the experimental data and the volume of the elliptical cone physical model is shown in Equation 2-2:

[0044]

[0045] Where:

[0046] V—Drilling fluid loss volume during the experiment, cm 3 ;

[0047] Separate variables and integrate equation 2-2 to obtain equation 2-3:

[0048]

[0049]

[0050] Where:

[0051] L e —Equivalent length of horizontal well, cm;

[0052] ROP—mechanical drilling rate, cm / h;

[0053] Using formula 2-3, we can calculate a H,max , substituting into formula 1-5, we can get the volume of the elliptical cone physical model, which is equivalent to a cylinder, and using formula 2-5 to calculate the equivalent invasion radius r of the drilling fluid s ′;

[0054]

[0055] Where:

[0056] r s ′—drilling fluid equivalent invasion radius, cm;

[0057] The third step is to establish the function of permeability recovery rate and measurement position (Formula 3-1) by testing the linear permeability of the core before and after drilling fluid damage. This function is applied to the core damage around the wellbore, and the radial permeability K after wellbore damage is calculated using (Formula 3-2). rR ;

[0058]

[0059] Where:

[0060] c—damage coefficient, dimensionless;

[0061] d—damage index, dimensionless;

[0062] x—drilling fluid linear invasion depth, cm;

[0063] l d — Maximum depth of core damage, cm;

[0064]

[0065] in:

[0066] f K (r) = f K (x) (3-3)

[0067] Where:

[0068] K rR —Radial permeability after damage around the wellbore, 10 -3 μm 2 ;

[0069] K—original permeability of core, 10 -3 μm 2 ;

[0070] R—maximum equivalent radius of the damage area, cm;

[0071] r w —Borehole radius;

[0072] Step 4: Based on the drilling fluid equivalent invasion radius r obtained from steps 1 to 3, s ′, radial permeability K rR , use (Formula 4-1) to calculate the horizontal well drilling fluid invasion skin coefficient;

[0073]

[0074] in:

[0075]

[0076] Where:

[0077] S—Horizontal well drilling fluid invasion skin coefficient, dimensionless;

[0078] K—original permeability of core, 10 -3 μm 2 ;

[0079] K rR —Radial permeability after damage around the wellbore, 10 -3 μm 2 ;

[0080] r s ′—drilling fluid equivalent invasion radius, cm;

[0081] r′ w —Equivalent wellbore radius;

[0082] β—anisotropy coefficient, dimensionless;

[0083] r w —Borehole radius.

[0084] The present invention has the following beneficial effects: the method takes into account the influence of radial seepage on reservoir damage and the influence of changes in reservoir permeability and drilling fluid invasion radius along the damage zone on the drilling fluid invasion skin coefficient. The drilling fluid invasion skin coefficient determined by the method has higher accuracy and can more truly reflect the different degrees of damage suffered by the reservoir due to the invasion of drilling fluid. Description of the drawings:

[0085] Figure 1 This is a schematic diagram of the drilling fluid invasion structure of a horizontal well;

[0086] Figure 2 A schematic diagram of the structure of an elliptical cone-shaped physical model of a horizontal wellbore and a reservoir damage zone for establishing a horizontal well using the method of the present invention;

[0087] Figure 3 A relationship diagram of the drilling fluid invasion skin factor along the horizontal well section length calculated by the present invention by taking into account the reservoir permeability and the drilling fluid invasion radius change;

[0088] Figure 4 When the present invention is specifically implemented, only the relationship diagram of the drilling fluid invasion skin factor calculated by the change of the drilling fluid invasion radius along the length of the horizontal well section is considered;

[0089] Figure 5 When the present invention is specifically implemented, only the relationship diagram of the drilling fluid invasion skin factor calculated by the change of reservoir permeability along the length of the horizontal well section is considered. Specific implementation method:

[0090] The present invention is further described below in conjunction with the accompanying drawings: The present invention provides a method for determining the invasion skin coefficient of drilling fluid in a horizontal well of a sandstone reservoir, and the method is performed according to the following steps:

[0091] Step (1): Establish an elliptical cone-shaped physical model of the horizontal wellbore and reservoir damage zone, such as Figure 1 , Figure 2 As shown, the volume calculation formula of the elliptical cone physical model is shown in formula 1-1.

[0092]

[0093] Where:

[0094] V cone —Volume of elliptical cone physical model, cm 3 ;

[0095] a H,min —The minimum horizontal axis length in the elliptical cone physical model, cm;

[0096] a V,min—The minimum vertical axis length in the elliptical cone physical model, cm;

[0097] a H,max —The maximum horizontal axis length in the elliptical cone physical model, cm;

[0098] a V,max —The maximum vertical axis length in the elliptical cone physical model, cm;

[0099] L—horizontal section length of horizontal well, cm;

[0100] Anisotropy coefficient according to Muskat definition It can be obtained that β is 0.25. Muskat first expressed the reservoir permeability as the geometric mean of horizontal permeability and vertical permeability. Later, this correction method was widely used to derive the production of horizontal wells in anisotropic reservoirs, namely

[0101] From this, we can get the minimum horizontal axis length a in the elliptical cone physical model H,min is the wellbore radius r w .

[0102] Considering the rock pore volume in the reservoir damage area, the volume calculation formula of the elliptical cone physical model is simplified to:

[0103]

[0104] Where:

[0105] φ—porosity, dimensionless;

[0106] Step (2): Test the dynamic damage loss of core drilling fluid. The basic parameters of the formation and core are shown in Table 1. The function of unit length loss rate and time is established (Formula 2-1). The results are shown in Table 2.

[0107] q m =ae -bt (2-1)

[0108] Where:

[0109] q m —Filtration rate per unit length, cm 3 / (cm·h);

[0110] a—filtration coefficient, cm 3 / (cm·h);

[0111] b—filtration loss index, 1 / h;

[0112] t—drilling fluid invasion time, h;

[0113] The relationship between the experimental data and the volume of the elliptical cone physical model is shown in Equation 2-2:

[0114]

[0115] Where:

[0116] V—Drilling fluid loss volume during the experiment, cm 3 ;

[0117] Separate variables and integrate equation 2-2 to obtain equation 2-3:

[0118]

[0119] in:

[0120]

[0121] Where:

[0122] L e —Equivalent length of horizontal well, cm;

[0123] ROP—mechanical drilling rate, cm / h,

[0124] Using formula 2-3, we can calculate a H,max , substituting into formula 1-5, we can get the volume of the elliptical cone physical model, which is equivalent to a cylinder. Using formula 2-5, we can calculate the equivalent invasion radius r of the drilling fluid s ′, the results are shown in Table 4.

[0125]

[0126] Step (3): By testing the linear permeability of the core before and after drilling fluid damage, a function of permeability recovery rate and measurement position (Formula 3-1) is established. This function is applied to the core damage around the wellbore, and the radial permeability K is calculated using (Formula 3-2): rR , the results are shown in Table 3.

[0127]

[0128] Where:

[0129] c—damage coefficient, dimensionless;

[0130] d—damage index, dimensionless;

[0131] x—drilling fluid linear invasion depth, m;

[0132] l d — Maximum depth of core damage, m;

[0133]

[0134] in:

[0135] f K (r) = f K (x) (3-3)

[0136] Where:

[0137] K rR —Radial permeability after damage around the wellbore, 10 -3 μm 2 ;

[0138] K—original permeability of core, 10 -3 μm 2 ;

[0139] R—maximum equivalent radius of the damage area, cm;

[0140] r w —Borehole radius;

[0141] Step (4): Use (Formula 4-1) to calculate the horizontal well drilling fluid invasion skin coefficient. The result is shown in Figure 3 , Figure 4 and Figure 5 And Table 4.

[0142]

[0143] in:

[0144]

[0145] Where:

[0146] S—Horizontal well drilling fluid invasion skin coefficient, dimensionless;

[0147] K—original permeability of core, 10 -3 μm 2 ;

[0148] K rR —Radial permeability after damage around the wellbore, 10 -3 μm 2 ;

[0149] r s ′—drilling fluid equivalent invasion radius, cm;

[0150] r′ w —Equivalent wellbore radius;

[0151] β—anisotropy coefficient, dimensionless;

[0152] r w —Borehole radius.

[0153] Table 1 Basic parameters

[0154] Porosity 0.1 Core length, cm 30 Core diameter, cm 2.5 <![CDATA[Original core permeability, ×10 -3 μm 2 > 2 Anisotropy coefficient 0.25 Borehole radius, cm 6.985 Mechanical drilling speed cm / h 30 Horizontal well horizontal section length, cm 30000

[0155] Table 2 Experimental data

[0156]

[0157] Table 3 Fitting data

[0158]

[0159]

[0160] Table 4 Calculation of the invasion skin coefficient of drilling fluid in horizontal wells

[0161]

[0162] Muskat first expressed reservoir permeability as the geometric mean of horizontal permeability and vertical permeability. Later, this correction method was widely used to derive the production of horizontal wells in anisotropic reservoirs, namely Muskat M.The flow of fluids through porous media[J].Journal of AppliedPhysics,1937,8(4):274-282.

[0163] Ferguson CK established an empirical formula for the relationship between drilling fluid filtration rate and time (see formula 2-1). Ferguson CK, Klotz J A. Filtration from mud during drilling [J]. Journal of Petroleum Technology, 1954, 6 (02): 30-43.

[0164] Results evaluation: Reservoir contamination experiments were conducted using three different types of drilling fluid systems: water-based drilling fluid, water-in-oil drilling fluid, and oil-in-water drilling fluid. Figure 3 (The drilling fluid invasion radius is fixed) It can be seen that the water-in-oil drilling fluid system causes more serious damage to the reservoir than the oil-in-water drilling fluid system; Figure 4 (The ratio of the original permeability of the core to the permeability after damage around the wellbore has been determined) It can be seen that the oil-in-water drilling fluid system causes more serious damage to the reservoir than the water-in-oil drilling fluid system; Figure 5It can be seen that the skin coefficients of drilling fluid invasion are 4.67, 1.26, and 1.29, respectively. The water-in-oil drilling fluid system is more suitable for the reservoir, but the damage degree caused by the water-in-oil drilling fluid system and the oil-in-water drilling fluid system to the reservoir is the same. Therefore, it is inaccurate to evaluate the reservoir damage degree only by the reservoir permeability or the drilling fluid invasion radius. The above two factors need to be considered comprehensively. It can be seen from Table 4 that after the reservoir is contaminated by water-based drilling fluid, the maximum equivalent damage radius is 23.05 cm, so the acidification radius is at least 23.05 cm.

Claims

1. A method for determining the invasion skin factor of drilling fluid in a horizontal well of a sandstone reservoir, characterized in that: The method comprises the following steps: The first step is to establish an elliptical cone-shaped physical model of the wellbore and reservoir damage zone in the horizontal section of the horizontal well. The path is as follows: The volume calculation formula of the elliptical cone physical model is shown in formula 1-1. Where: V cone —Volume of elliptical cone physical model, cm 3 ; a H,min —The minimum horizontal axis length in the elliptical cone physical model, cm; a V,min —The minimum vertical axis length in the elliptical cone physical model, cm; a H,max —The maximum horizontal axis length in the elliptical cone physical model, cm; a V,max —The maximum vertical axis length in the elliptical cone physical model, cm; L—horizontal section length of horizontal well, cm; The anisotropy coefficient β is defined as: Where: β—anisotropy coefficient, dimensionless; K H —Horizontal permeability, 10 -3 μm 2 ; K V —Vertical permeability, 10 -3 μm 2 ; The ratio of the horizontal axis length to the vertical axis length: a H =β·a V (1-3) Where: a H —The length of the horizontal axis in the elliptical cone physical model, cm; a V —length of the vertical axis in the elliptical cone physical model, cm; Since the minimum horizontal axis length a in the elliptical cone physical model H,min or the minimum vertical axis length a V,min is the wellbore radius r w , can be obtained, Where: r w —wellbore radius, cm; After considering the rock pore volume in the reservoir damage area, the volume calculation formula of the elliptical cone physical model is simplified to: Where: φ—porosity, dimensionless; The second step is to test the dynamic damage loss of core drilling fluid and establish the function of unit length loss rate and time, as shown in Formula 2-1. Then, use Formula 2-5 to calculate the equivalent invasion radius r' of drilling fluid. s ; q m =ae -bt (2-1) Where: q m —Filtration rate per unit length, cm 3 / (cm·h); a—filtration coefficient, cm 3 / (cm·h); b—filtration loss index, 1 / h; t—drilling fluid invasion time, h; The relationship between the experimental data and the volume of the elliptical cone physical model is shown in Equation 2-2: Where: V—Drilling fluid loss volume during the experiment, cm 3 ; Separate variables and integrate equation 2-2 to obtain equation 2-3: in: Where: L e —Equivalent length of horizontal well, cm; ROP—mechanical drilling rate, cm / h; Use formula 2-3 to calculate a H,max , substituting into formula 1-5, we can get the volume of the elliptical cone physical model, which is equivalent to a cylinder, and using formula 2-5 to calculate the equivalent invasion radius r′ of the drilling fluid s ; Where: r′ s —Equivalent invasion radius of drilling fluid, cm; The third step is to establish the function of permeability recovery rate and measurement position (Formula 3-1) by testing the linear permeability of the core before and after drilling fluid damage. This function is applied to the core damage around the wellbore, and the radial permeability K after wellbore damage is calculated using (Formula 3-2). rR ; Where: c—damage coefficient, dimensionless; d—damage index, dimensionless; x—drilling fluid linear invasion depth, cm; l d — Maximum depth of core damage, cm; in: f K (r)=f K (x) (3-3) Where: K rR —Radial permeability after damage around the wellbore, 10 -3 μm 2 ; K—original permeability of core, 10 -3 μm 2 ; R—maximum equivalent radius of the damage area, cm; r w —Borehole radius; Step 4: Based on the drilling fluid equivalent invasion radius r′ obtained from steps 1 to 3 s , radial permeability K rR , use (Formula 4-1) to calculate the horizontal well drilling fluid invasion skin coefficient; in: Where: S—Horizontal well drilling fluid invasion skin coefficient, dimensionless; K—original permeability of core, 10 -3 μm 2 ; K rR —Radial permeability after damage around the wellbore, 10 -3 μm 2 ; r′ s —Equivalent invasion radius of drilling fluid, cm; r′ w —Equivalent wellbore radius; β—anisotropy coefficient, dimensionless; r w —Borehole radius.

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