Methods for determining the true thickness of strata

Abstract

This invention relates to a method for determining the true thickness of a formation. The method includes the following steps: determining the vertical depth corresponding to the intersection of the plumb line at the entry point on the top surface of the formation and the bottom surface of the formation at the exit point; using the obtained vertical depths Z1 and Z3 of the entry point A and A0, calculating the vertical thickness h = |Z3 - Z1| of the formation at the entry point; and calculating the true thickness H of the formation at the entry point A based on the true dip angle, true dip direction, and h of the top surface of the formation. This invention's method for determining formation thickness utilizes the true dip angle, true dip direction, true dip angle, and true dip direction of the bottom surface of the formation to calculate the true thickness of the formation at the entry point. The calculated true thickness is closer to the actual formation thickness with smaller errors, improving the accuracy of geological research. Furthermore, the determination process does not require consideration of the direction of the wellbore trajectory relative to the formation strike, making the process simple and efficient.

Description

Technical Field

[0001] This invention relates to a method for determining the true thickness of a formation. Background Technology

[0002] Currently, the main types of oil and gas fields under exploration and development include conventional oil and gas, represented by clastic rocks and carbonate rocks, and unconventional oil and gas, represented by tight oil and gas and shale oil and gas. For conventional oil and gas, the main approach is to improve the recovery rate of formations rich in remaining oil and gas through drilling techniques such as highly deviated directional wells and horizontal wells. For unconventional oil and gas, the main approach is to use drilling techniques such as long horizontal wells and branched horizontal wells, as well as well factory models, to conduct three-dimensional development of sweet spots and sweet spot sections of formations, in order to increase production, improve efficiency, and reduce costs, thereby achieving the profitable development of low-grade and low-abundance oil and gas resources.

[0003] Strata are the collective term for layered rocks formed in a certain geological era, while oil and gas reservoirs refer to strata containing oil and natural gas. The upper and lower interfaces of strata are called bedding planes; the upper interface is the top surface or upper bedding plane, and the lower interface is the bottom surface or lower bedding plane.

[0004] Whether it's conventional or unconventional oil and gas resources, in most cases, the oil-rich strata are not horizontal but inclined, meaning they exhibit strike, dip, and dip angle. In practice, we frequently use concepts such as true dip, true dip angle, apparent dip, apparent dip angle, apparent thickness, straightened thickness, vertical thickness, and true thickness.

[0005] The true dip direction of a stratum refers to the angle between the vertical projection of the stratum's downward dip direction onto the horizontal plane and true north on a cross-section perpendicular to the strike. The true dip angle refers to the angle between the top or bottom of a stratum and the horizontal line on a cross-section perpendicular to the strike. The apparent dip direction of a stratum refers to the angle between the vertical projection of the stratum's downward dip direction onto the horizontal plane and true north on a cross-section not perpendicular to the strike. The apparent dip angle refers to the angle between the top or bottom of a stratum and the horizontal line on a cross-section not perpendicular to the strike. For example... Figure 1 As shown, the dip direction is the true dip direction of the strata, and the dip angle is the true dip angle of the strata.

[0006] like Figure 2As shown, apparent formation thickness refers to the path length of the wellbore through the entire formation. For example, TB1 and TB2 are the apparent formation thicknesses of two wells, respectively. Formation straightening thickness refers to the absolute value of the difference between the vertical depth of the wellbore through the top and bottom surfaces of the formation. For example, TB11 and TB22 are the formation straightening thicknesses of two wells, respectively. Formation vertical thickness refers to the absolute value of the difference between the vertical depth of the entry point and the vertical depth of this intersection point, obtained by drawing a vertical line from the entry point. For example, TO. True formation thickness refers to the vertical distance between the top and bottom surfaces of the formation, such as TB. Therefore, apparent formation thickness, formation straightening thickness, and formation vertical thickness cannot accurately reflect the true formation thickness. True formation thickness = formation vertical thickness * cosα (where α is the true dip angle of the formation). Therefore, formation vertical thickness is greater than or equal to formation true thickness.

[0007] In exploration and development practice, the true thickness of the formation where oil and gas accumulate is a very important parameter for studying sedimentary patterns, evaluating oil and gas resources, calculating reserves and predicting production capacity. At the same time, it is also crucial for the geological guidance of horizontal well drilling in exploration and development.

[0008] Currently, there are two main technical methods for determining the true thickness of a formation using known parameters such as wellbore trajectory and formation attitude (dip, dip angle):

[0009] Method 1: Completely ignore the formation attitude, directly use well inclination data to calculate the vertical depth of the entry point and exit point, and obtain the absolute value of the difference between the two, which is the formation straightening thickness, and take the formation straightening thickness as the true formation thickness.

[0010] from Figure 2 As can be seen, when a deviated well drills through inclined formations, the wellbore trajectory direction is the same as the formation dip, i.e., in the forward direction. Therefore, the calculated formation straightening thickness is greater than the actual true formation thickness. Figure 2 The TB11 shown is greater than TB; when the drilling trajectory direction is opposite to the formation dip (i.e., reverse), the calculated formation straightening thickness is less than the actual true formation thickness, such as... Figure 2 The TB22 shown is less than TB. Therefore, the formation straightening thickness cannot reflect the true formation thickness.

[0011] Exploration and development practices show that, in most cases, the strata are not horizontal but inclined, meaning they exhibit both dip and dip angle. Identifying the straightened strata thickness as the true strata thickness will result in significant errors, leading to serious misjudgments in geological understanding by technical personnel and negatively impacting high-quality exploration and profitable development.

[0012] Method 2: A typical example is the paper "Calculation Method of True Formation Thickness in High-Angle Wellbores" published by Jiang Ming on June 20, 2011, in *Information Systems Engineering* (International Serial No. 1001-2362, Domestic Serial No. 12-1158 / N). The specific calculation process is as follows:

[0013] First, the line connecting the wellbore's entry and exit points in the formation (a spatial straight line) is used as a virtual borehole. Based on the spatial coordinates of the entry and exit points, the spatial length, planar length, vertical length, dip, dip angle, and apparent dip angle of the formation of the virtual borehole are calculated. Then, considering the formation attitude, it is determined which of the four scenarios it belongs to: the forward and reverse scenarios when the virtual borehole is perpendicular to the formation strike, and the forward and reverse scenarios when the virtual borehole is not perpendicular to the formation strike. Finally, the apparent thickness of the formation is calculated using trigonometric function formulas for the corresponding scenario, and the true thickness of the formation is ultimately calculated.

[0014] This method fully considers the wellbore trajectory and formation attitude, but it assumes that the top and bottom surfaces of the formation at the drilling and exit points have exactly the same attitude, i.e., the same dip angle and dip direction. Exploration and development practice shows that in most cases, the top and bottom surfaces of the formation are not completely consistent; that is, there are certain differences in their dip direction and dip angle. Therefore, when the top and bottom surfaces of the formation are not completely consistent, the calculated true formation thickness still has a significant error compared to the actual formation thickness, affecting the accuracy of geological research. Furthermore, the process of determining the true formation thickness using this method is also relatively complex. Summary of the Invention

[0015] The purpose of this invention is to provide a method for determining the true thickness of a formation, which solves the technical problem that current methods for determining the thickness of a formation have large errors.

[0016] The method for determining the true thickness of a formation in this invention adopts the following technical solution:

[0017] The method for determining the true thickness of a formation includes the following steps:

[0018] Establish a three-dimensional coordinate system in XYZ space. The drilling entry point A(X1, Y1, Z1) on the top surface of the formation, the drilling exit point B(X2, Y2, Z2) on the bottom surface of the formation, the true dip angle of the top surface of the formation, the true dip angle of the top surface of the formation, and the true dip angle of the bottom surface of the formation are known.

[0019] Step (1) Calculate the vertical depth corresponding to the intersection of the plumb line at the entry point A on the top surface of the stratum and the bottom surface of the stratum where the exit point B is located. The specific method is as follows:

[0020] Project A and B vertically onto the XY horizontal plane, which are A1 and B1 respectively. Project A along the vertical line onto the bottom surface of the stratum where the exit point B is located, which is A0(X1, Y1, Z3). Use the true dip angle and true dip direction of the bottom surface of the stratum where the exit point B is located to obtain the vertical depth value Z3 of A0.

[0021] Step (2): Using the vertical depth Z1 of the entry point A and the vertical depth Z3 of A0, calculate the vertical thickness h of the stratum at the entry point A: h = |Z3-Z1|.

[0022] Step (3): Calculate the true thickness H of the stratum at point A based on the true dip angle, true dip direction, and h of the top surface of the stratum.

[0023] Beneficial effects: The formation thickness determination method of the present invention uses the true dip angle, true dip direction, true dip angle, and true dip direction of the formation top surface to calculate the vertical thickness of the formation at the entry point, and then calculates the true thickness of the formation at the entry point. The calculated true thickness of the formation is closer to the actual formation thickness with a smaller error, which improves the accuracy of geological research. Moreover, the determination process does not need to consider the relationship between the wellbore trajectory direction and the formation strike, i.e., forward or reverse, making the determination process simple and efficient.

[0024] Furthermore, the formula for calculating the true thickness H of the stratum at the entry point A is: H = h·cosα; where: α—the true dip angle of the top surface of the stratum.

[0025] Furthermore, the method for determining the vertical depth value Z3 of A0 is as follows: On the XY plane, starting from B1 and facing A1, along the same or opposite direction as the true dip of the stratum floor where the exit point B is located, using the true dip angle of the stratum floor at the exit point B, draw the vertical depth contour lines of the stratum floor on the XY plane, and perform linear extrapolation of the vertical depth at equal steps until the vertical depth contour lines exceed A1; A1 lies between two adjacent vertical depth contour lines, and the vertical depth value Z3 of A0 is obtained on the plane using the contour line linear interpolation method. This reduces the amount of calculation and improves the determination efficiency.

[0026] Furthermore, at least three vertical depth contour lines are set for linear extrapolation with equal step sizes to improve calculation accuracy.

[0027] Furthermore, 3 to 8 vertical depth contour lines are set for linear extrapolation with equal step size.

[0028] Furthermore, the method for determining the vertical depth value Z3 of A0 is as follows: On the XY plane, starting from B1 and facing A1, along the same or opposite direction as the true dip of the stratum floor where the exit point B is located, using the true dip angle of the stratum floor at the exit point B, draw the vertical depth contour lines of the stratum floor on the XY plane, and perform linear extrapolation of the vertical depth at equal steps until the vertical depth contour lines exceed A1; A1 lies between two adjacent vertical depth contour lines, and the vertical depth value Z3 of A0 is obtained on the plane using the contour line linear interpolation method. This reduces the amount of calculation and improves the determination efficiency. Attached Figure Description

[0029] Figure 1 It is a three-dimensional schematic diagram of stratigraphic occurrence elements.

[0030] Figure 2 This is a schematic diagram of the borehole trajectory of an inclined well penetrating an inclined stratum (perpendicular to the strike of the stratum).

[0031] Figure 3 This is a flowchart of the method for determining the true thickness of strata in a specific embodiment 1 of the present invention.

[0032] Figure 4 This is a schematic diagram of the entry and exit points in a specific embodiment 1 of the method for determining the true thickness of strata according to the present invention.

[0033] Figure 5 This is a schematic diagram of a linear extrapolation profile with equal steps of vertical depth in a specific embodiment 1 of the method for determining the true thickness of strata of the present invention (perpendicular to the strike of the strata).

[0034] Figure 6 This is a schematic diagram of the vertical depth equal step linear extrapolation plane in specific embodiment 1 of the method for determining the true thickness of the strata of the present invention (in the same or opposite direction as the true dip of the strata).

[0035] Figure 7 This is a schematic diagram showing the cosine function relationship between the true thickness of the formation and its vertical thickness in a specific embodiment 1 of the method for determining the true thickness of the formation of the present invention (at the entry point A).

[0036] Figure 8 This is a linear extrapolation plan view of the vertical depth of well P8 in a specific embodiment 1 of the method for determining the true thickness of formations of the present invention (to obtain the vertical depth of A0).

[0037] Figure 9 This is a specific embodiment 1 of the method for determining the true thickness of formations in this invention, showing a formation straightening thickness contour map determined by method 1 described in the background art for an oilfield.

[0038] Figure 10This is a contour map of the true thickness of a formation determined by method 2 described in the background art in a specific embodiment 1 of the method for determining the true thickness of a formation in this invention.

[0039] Figure 11 This is a contour map of the true formation thickness determined by the method of the present invention in a specific embodiment 1 of the method for determining the true formation thickness of an oil field. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention; that is, the described embodiments are merely some embodiments of the invention, and not all embodiments. The components of the embodiments of the invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0041] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0042] It should be noted that relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0043] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0044] Specific embodiment 1 of the method for determining the true thickness of strata in this invention:

[0045] In this embodiment, well P8 is used as an example for explanation. Figures 3 to 4As shown, a three-dimensional coordinate system in XYZ space is established. Given the entry point A (X1, Y1, Z1) at the top of the formation and the exit point B (X2, Y2, Z2) at the bottom of the formation, the specific steps for determining the true thickness of the formation in well P8 are as follows:

[0046] Step (1): Given the wellhead coordinates, elevation, core height, well inclination, formation dip at the top of the formation, formation dip angle at the top of the formation, formation dip angle at the bottom of the formation, and formation dip angle at the bottom of the formation, calculate the vertical depth corresponding to the intersection point A0(X1, Y1, Z3) of the plumb line at the entry point A and the bottom of the formation where the exit point B is located. The specific method is as follows:

[0047] like Figure 8 As shown, using the wellhead coordinates (x-axis 54332.5, y-axis 60528.3), elevation 45.9m, core height 9.0m, and well inclination data (including well depth, inclination angle, and azimuth), see the well inclination data table on page 8, the spatial coordinates of the entry point A and exit point B are calculated as follows: A (X1 = 53971.1, Y1 = 60754.2, Z1 = -3673.5) and B (X2 = 53948.8, Y2 = 60769.8, Z2 = -3740.1). These coordinates are then vertically projected onto the horizontal plane as A1 (X1 = 53971.1, Y1 = 60754.2) and B1 (X2 = 53948.8, Y2 = 60769.8), respectively.

[0048] Using seismic, geological, and well logging data, the true dip of the stratum at point B was determined to be 297 degrees and the true dip angle to be 30.0 degrees. For example... Figures 5 to 8 As shown, on the XY plane, starting from B1 and facing A1, along the opposite direction of the true dip of the stratum bottom surface at point B (297 degrees), using the true dip angle of 30.0 degrees at point B, a linear extrapolation is performed with equal steps of 5m for vertical depth. This involves drawing vertical depth contour lines on the XY plane and performing linear extrapolation with equal steps until the vertical depth contour lines exceed A1, where A1 lies between two adjacent vertical depth contour lines. On the XY plane, the vertical depth of point A0 (-3724.5m) is determined using the linear interpolation method of vertical depth contour lines. This method involves dividing the vertical depth contour lines between two adjacent stratum bottom surfaces at point A1 into several equal parts, and then calculating the vertical depth represented by point A1. Typically, about five vertical depth contour lines are suitable for extrapolation. In other embodiments, the vertical depth contour lines for linear extrapolation with equal step size can be set to more than 3 as needed, preferably 3 to 8.

[0049] Table 1. P8 Well Deviation Data

[0050]

[0051] Step (2): Using the vertical depth of the entry point A of well P8 and the vertical depth of the intersection point A0 of its vertical line and the bottom surface of the formation where the exit point B is located, the vertical thickness of the formation at the entry point A is calculated. That is, using the vertical depth Z1 = -3673.5m of the entry point A and the vertical depth Z3 = -3724.5m of point A0 obtained in step (1), the vertical thickness of the formation at the entry point h = |Z3-Z1| = 51.0m is calculated.

[0052] Step (3): Utilizing the relationship that the true thickness of the formation and the vertical thickness of the formation are cosine functions of the true dip angle, the true thickness of the formation at well point A in well P8 is determined. The specific method is as follows:

[0053] The true dip angle α of the top surface of the stratum at the entry point A, determined using seismic, geological, and well logging data, is 18.7 degrees. Then, using the vertical stratum thickness h = 51.0 m obtained in step (2) above, as... Figure 7 Based on the relationship that the true thickness H and vertical thickness h of the stratum at entry point A are cosine functions of its true dip angle, the true thickness H of the stratum at entry point A is calculated using the formula: H = h·cosα. The true thickness H is determined to be 48.3 m, where: H—true thickness of the stratum; h—vertical thickness of the stratum; α—true dip angle of the stratum.

[0054] This embodiment also provides data from three wells, P2, P5, and P7. Using the same method described above, the true formation thickness of the other three wells can be determined. The data from the four wells are shown in the table below.

[0055] Table 2. Data table of examples for determining the true thickness of formations from four wells in a certain oilfield.

[0056]

[0057] As can be seen from the example data table, there is a significant difference between the formation straightening thickness determined by the traditional method 1 described in the background art and the true formation thickness determined by this invention. Conversely, the true formation thickness determined by method 2 described in the background art is essentially consistent with the true formation thickness determined by this invention. Based on the obtained data, thickness contour maps can be compiled as follows: Figures 9 to 11 , among them Figure 9 and Figure 11 The difference is significant, and Figure 10 and Figure 11 They are basically the same. Mining practice shows that... Figure 9 This could lead to serious misjudgments in geological understanding by technical personnel, Figure 10 This would lead to biases in the geological understanding of technicians; only by... Figure 11 It can accurately reflect the actual planar distribution of the true thickness of strata, which is conducive to the formation of correct geological understanding by technical personnel.

[0058] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. The scope of patent protection of the present invention shall be determined by the claims. Similarly, any equivalent structural changes made based on the description and drawings of the present invention shall also be included within the scope of protection of the present invention.

Claims

1. A method for determining the true thickness of a formation, characterized in that, Includes the following steps: Establish a three-dimensional coordinate system in XYZ space. The drilling entry point A (X1, Y1, Z1) on the top surface of the formation, the drilling exit point B (X2, Y2, Z2) on the bottom surface of the formation, the true dip angle of the top surface of the formation, the true dip angle of the top surface of the formation, and the true dip angle of the bottom surface of the formation are known. Step (1) Calculate the vertical depth corresponding to the intersection of the plumb line at the entry point A on the top surface of the stratum and the bottom surface of the stratum where the exit point B is located. The specific method is as follows: Project A and B vertically onto the XY horizontal plane, resulting in A1 and B1 respectively. On the XY plane, starting from B1 and facing A1, draw vertical depth contour lines of the stratum bottom surface in the same or opposite direction as or opposite to the true dip angle of the stratum bottom surface at the exit point B. Then, perform linear extrapolation of the vertical depth in equal steps until the vertical depth contour lines exceed A1. A1 is located between two adjacent vertical depth contour lines. On the plane, use the linear interpolation method of contour lines to obtain the vertical depth value Z3 of A0(X1, Y1, Z3) projected from the vertical line onto the stratum bottom surface at the exit point B. Step (2): Using the vertical depth Z1 of the entry point A and the vertical depth Z3 of A0, calculate the vertical thickness h of the stratum at the entry point A: h = |Z3-Z1|. Step (3): Calculate the true thickness H of the stratum at point A based on the true dip angle, true dip direction, and h of the top surface of the stratum.

2. The method for determining the true thickness of a formation according to claim 1, characterized in that, The formula for calculating the true thickness H of the formation at point A is: H = h • cosα; where: α — the true dip angle of the top surface of the formation.

3. The method for determining the true thickness of a formation according to claim 1, characterized in that, Set at least three vertical depth contour lines for linear extrapolation with equal step size.

4. The method for determining the true thickness of a formation according to claim 3, characterized in that, Set 3 to 8 vertical depth contour lines for linear extrapolation with equal step size.

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