Method for determining reasonable well inclination angle and safe drilling fluid density window for directional wells

Abstract

The present invention relates to a method for determining a reasonable well inclination angle and a safe drilling fluid density window of a directional well. The method mainly starts with analyzing the stress of the well wall by using an elastic mechanics method. Firstly, aiming at the well wall stability problem of the directional well, the influence of the additional stress of fluid seepage, the circulating pressure and the fluctuating pressure during the drilling process on the bottom hole pressure is considered. A prediction model for the safe drilling fluid density window at each stage in the drilling process of a directional well with an arbitrary well inclination angle is established. Then, the physical and mechanical characteristic parameters of the target formation rock and general drilling operation data are obtained by means of field data or indoor experiments. The obtained parameters are then brought into the model for solving. Afterwards, the obtained results are analyzed and corrected in combination with the situations that may occur in each stage of the construction. The obtained results are consistent with the actual situation, providing a reliable basis for on-site design and construction, and having a good application prospect.

Description

Technical Field

[0001] The invention belongs to the field of geological exploration and development of sandstone oil and gas reservoirs, can be used for drilling operations in the process of geological exploration and oil and gas reservoir development, and specifically relates to a method for determining a reasonable well inclination angle and a safe drilling fluid density window of a directional well. Background Art

[0002] In the entire oil and gas industry, drilling is an indispensable and important link, and the corresponding cost is also very high, often accounting for more than half of the cost of oil and gas exploration and development. In the process of drilling directional wells, especially high-angle wells, wellbore instability often occurs, causing wellbore block falling, necking, drill bit sticking or wellbore rupture, which seriously affects the drilling speed and safety, delays the drilling time, increases the drilling cost, and also brings great difficulties to the subsequent oil production operations and reservoir protection. Therefore, it is necessary for us to conduct in-depth research on the stress state of the wellbore surrounding rock at each stage during the drilling of directional inclined wells at any angle, and then establish an accurate prediction model for the reasonable well inclination angle and safe drilling fluid density window of directional wells, so as to reduce the occurrence of wellbore instability and achieve the purpose of safe, fast and high-quality drilling.

[0003] Literature research shows that predecessors have conducted a lot of research on the design of safe drilling fluid density window to maintain wellbore stability in directional wells, but most of the existing results do not take into account the effects of fluid seepage additional stress, circulation pressure, pumping pressure when pulling out of the hole, and excitation pressure when drilling down. Therefore, this paper specifically tackles the problem of reservoir wellbore stability and analyzes in detail the effects of different additional pressures under various working conditions during directional well drilling, hoping to establish a more accurate model of reasonable well inclination angle and safe drilling fluid density window for directional wells, and play a certain guiding role in future production practice. Summary of the invention

[0004] The purpose of the present invention is to provide a method for determining a reasonable well inclination angle and a safe drilling fluid density window of a directional well, so as to solve the problem of inaccurate prediction of a reasonable well inclination angle and a safe drilling fluid density window of a directional well in the existing method.

[0005] Based on the above purpose, a technical solution for determining a reasonable well inclination angle and a safe drilling fluid density window for a directional well is as follows:

[0006] (1) Considering the influence of fluid seepage additional stress, circulation pressure and fluctuating pressure during drilling on bottom hole pressure, a safe drilling fluid density window prediction model is established under various working conditions (static well, circulation, drilling, drilling) during directional well drilling with arbitrary well inclination angle.

[0007] (2) Obtain the physical and mechanical properties of the target formation rock and general drilling operation data through field data or indoor experiments.

[0008] (3) The parameters and data obtained in step (2) are brought into the model for solution, and the reasonable well inclination angle and safe drilling fluid density window under different working conditions are obtained respectively, and the reasons for the upward or downward movement of the reasonable well inclination angle and the safe drilling fluid density window are analyzed.

[0009] (4) In order to reduce the number of times the drilling fluid density is adjusted and to reduce the risk of drilling accidents, the well inclination angle and drilling fluid density during the entire directional well drilling process are determined to be within a more reasonable range, that is, the overlap area of ​​the reasonable well inclination angle and the safe drilling fluid density window during the static well, circulation, drilling and drilling states under this condition.

[0010] (5) The reasonable well inclination angle and safe drilling fluid density window obtained in step (4) are further corrected to cope with the angle deviation that may occur at any time.

[0011] The beneficial effects of the above method are as follows:

[0012] The method of the present invention mainly starts with analyzing the stress conditions of the well wall by using the elastic mechanics method. Firstly, aiming at the well wall stability problem of the directional well, the influence of the additional stress of fluid seepage, the circulating pressure and the fluctuating pressure during the drilling process on the bottom hole pressure is considered, and a prediction model of the safe drilling fluid density window at each stage in the drilling process of the directional well with any well inclination angle is established. Then, taking the loose sandstone reservoir of Shengtuo Oilfield as an example, the physical and mechanical parameters of the well wall rock are applied, and the pore pressure of the target reservoir given in the field logging data and general construction operation data are calculated, so as to obtain the reasonable well inclination angle and the safe drilling fluid density window of the directional well of the target reservoir, and the analysis and correction are carried out in combination with the possible situations in each stage of the construction. The obtained results are basically consistent with the actual situation of the previous drilling of the reservoir in the block, which provides a reliable basis for on-site design and construction, and has a good application prospect.

[0013] Furthermore, in step (1), the process of establishing the safe drilling fluid density window prediction model under various working conditions (static well, circulation, pulling out, and drilling down) during directional well drilling at any well inclination angle is as follows:

[0014] Assuming that the reservoir is an isotropic homogeneous elastic body, the single-phase fluid flows in the uniformly distributed pore space, ignoring the temperature field effect, and not considering the influence of tectonic stress, the formula for the stress concentration of the wellbore rock is derived under the condition of a directional well with a well inclination angle of α:

[0015] The pressure exerted by the overlying rock is 10 -6 ρgH, considering the reservoir pore pressure P f The effective vertical pressure acting on the rock is F:

[0016] F=10 -6 ρgH-Pf (1)

[0017] Where: ρ——average density of overlying rock, kg / m 3 ;

[0018] g——acceleration due to gravity, m / s 2 ;

[0019] H——reservoir depth, m;

[0020] P f ——Reservoir pore pressure, MPa.

[0021] F produces two components:

[0022] The component force F1 parallel to the wellbore axis (z direction) is:

[0023] F1=F·cosα (2)

[0024] Where: α is the well inclination angle, degrees.

[0025] The component force F2 perpendicular to the wellbore axis (x direction) is:

[0026] F2=F·sinα (3)

[0027] The x and y lateral pressures generated by F1 on the wellbore rock are:

[0028]

[0029] Where: v is the Poisson's ratio of rock, dimensionless;

[0030] v / (1-v)——lateral pressure coefficient, dimensionless.

[0031] The y lateral pressure generated by F2 on the wellbore rock is:

[0032]

[0033] therefore,

[0034] The actual stress in the x direction is: x =F2+F3 (7)

[0035] The actual stress in the y direction is: y =F3+F4 (8)

[0036] Combining formula (1) - formula (8), we can get

[0037]

[0038]

[0039] Since the rock Poisson's ratio v is between 0.15 and 0.4, the lateral pressure coefficient v / (1-v) is always less than 1, so it can be fully proved that σ x >σ y .

[0040] According to the theory of classical elastic mechanics, the circumferential stress at any radius r in the formation around the wellbore is:

[0041]

[0042] Where: r w ——Wellbore radius, m;

[0043] r——the distance between any point of the formation and the well axis, m;

[0044] θ——Any direction and σ x The angle with respect to the direction (x-axis), degrees.

[0045] From formula (11), we can see that at the well wall (r = r w ), cos2θ=-1, that is, θ=90° or 270° when σ reaches the maximum value σ max , cos2θ=1, that is, θ=0° or 180°, σ reaches the minimum value σ min :

[0046]

[0047]

[0048] Considering the additional stress induced by the fluid flow in the pores at any point in the formation as σ l :

[0049]

[0050] Where: P is the bottom hole pressure, MPa.

[0051] φ——porosity, dimensionless.

[0052] At the well wall r = r w , then

[0053]

[0054] When P>P f When l is a positive value, that is, σ l Same direction as P, when P<P f When l is a negative value, that is, σ l The opposite of P.

[0055] During the drilling process, if the density of the drilling fluid is too high or too low, it will lead to instability and damage of the wellbore. Therefore, in order to avoid the instability of the wellbore, the density of the drilling fluid should be limited to an appropriate range, that is, the safe drilling fluid density window, and the maximum and minimum values ​​of the safe drilling fluid density window can be obtained according to the formation fracture judgment criteria and formation collapse judgment criteria respectively.

[0056] In general, formation rupture is mainly caused by the excessive density of the drilling fluid in the well, which causes the maximum effective principal stress on the wellbore rock to exceed the tensile strength of the rock. At this time, the maximum effective principal stress on the wellbore rock points to the formation along the direction of the minimum circumferential stress of the wellbore, and acts on the wellbore rock in the form of tensile stress. The formation rupture judgment criteria are as follows:

[0057]

[0058] Where: T is the tensile strength of the wellbore rock, MPa.

[0059] The formation collapse is mainly caused by the low density of drilling fluid in the well, which makes the maximum effective principal stress of the well wall rock exceed the compressive strength of the rock. At this time, the maximum effective principal stress of the well wall rock points to the wellbore along the direction of the maximum circumferential stress of the well wall, and acts on the well wall rock in the form of compressive stress. The formation collapse judgment criteria are as follows:

[0060]

[0061] Where: C is the compressive strength of the wellbore rock, MPa.

[0062] In drilling operations, bottom hole pressure mainly comes from drilling fluid static column pressure, annular circulation pressure and fluctuating pressure. Under different conditions, these pressure sources may appear or disappear, and the bottom hole pressure will change accordingly. Therefore, understanding the accurate composition of bottom hole pressure under different working conditions is the basis for determining the reasonable inclination angle of directional wells and the safe drilling fluid density window.

[0063] In the static well state, the bottom hole pressure P is equal to the static column pressure P of the drilling fluid. w ,Right now

[0064] P=P w =10 -6 ρ 静井 gH (18)

[0065] According to the stratum rupture judgment criteria and stratum collapse judgment criteria, there are

[0066]

[0067]

[0068] Under the circulation state, the bottom hole pressure P is equal to the static column pressure P of the drilling fluid. w Plus the annular circulation pressure P xh ,Right now

[0069] P=P w +P xh =10 -6 ρ 循环 gH+P xh (twenty one)

[0070] at this time,

[0071]

[0072]

[0073] During actual operations, the movement of the tubing will cause the flow of fluid in the wellbore, thereby generating additional pressure in the well. This form of additional pressure is generally called surge pressure, which includes the pumping pressure generated during the drilling process and the excitation pressure generated during the drilling process.

[0074] During the drilling process, the annular circulation pressure P xh =0, the bottom hole pressure P is equal to the static column pressure P of the drilling fluid w Subtract the pumping pressure P cj ,Right now

[0075] P=P w -P cj =10 -6 ρ 起钻 gH-P cj (twenty four)

[0076] at this time,

[0077]

[0078]

[0079] During the drilling process, the annular circulation pressure P xh =0, the bottom hole pressure P is equal to the static column pressure P of the drilling fluid w Plus the excitement pressure P jd ,Right now

[0080] P=P w +P jd =10 -6 ρ 下钻 gH+P jd (27)

[0081] at this time,

[0082]

[0083]

[0084] Furthermore, in step (2), there are two main methods for obtaining rock mechanical parameters: one is to obtain rock mechanical parameters using geophysical logging data, and the other is to conduct actual measurements on rock samples in the laboratory.

[0085] Because the mechanical parameters obtained from the sonic logging data need to be corrected through core experiments, and the elastic parameters need to be converted from dynamic to static before they can be applied to field practice, the present invention recommends the use of indoor test methods. The specific methods are: the volumetric method is used to determine the density and porosity of the rock sample, the uniaxial compression deformation test is used to determine the compressive strength, elastic modulus and Poisson's ratio of the rock sample, and the splitting method (also known as the Brazilian test method) is used to determine the tensile strength of the rock sample.

[0086] The criteria for selecting experimental rock samples are: the rock samples come from representative formation rocks in the target drilling area, the cores must be able to carry out physical and mechanical experiments of a certain intensity, and they must be homogeneous, shaped, and not too loose. According to the requirements of the experiment, when sampling, the rock blocks must be drilled from the entire core in the direction of the well depth, and processed into standard rock samples of 25mm×50mm, with both ends ground flat, the dimensions accurately measured, the volume calculated, and the weight numbered.

[0087] Furthermore, in step (3), the parameters and data obtained in step (2) are brought into the model established in step (1) to solve and obtain ρ at any well inclination angle. max静井 , min静井 , max循环 , min循环 , max起钻 , min起钻 , max下钻 , min下钻 , plot the results in the same figure, where ρ max静井 , min静井 The well inclination angle corresponding to the intersection of the two lines is the maximum well inclination angle that can be designed for the directional well in the static well state under this condition. If this angle is exceeded, wellbore instability will be inevitable. In addition, the area surrounded by the two lines on the left side of this point is the reasonable well inclination angle and safe drilling fluid density window for the directional well in the static well state under this condition. Similarly, the reasonable well inclination angle and safe drilling fluid density window for the directional well in the circulation, drilling and drilling states under this condition can be obtained.

[0088] The results show that the safe drilling fluid density window during the drilling process is higher than the safe drilling fluid density window in the static well state, higher than the safe drilling fluid density window during the drilling process, and higher than the safe drilling fluid density window in the circulating state. This also explains why: when the static well state is converted to the circulating state, that is, during the drilling process, the drilling fluid density needs to be appropriately reduced to prevent the application of the circulating pressure from crushing the formation; when the drilling is stopped, the drilling fluid density needs to be increased in time, and when the drilling starts, the drilling fluid density needs to be increased additionally, in order to prevent the collapse of the well wall rock; before starting the drilling operation in the static well state, the drilling fluid density needs to be gradually reduced to prevent the formation from being crushed due to the generation of agitated pressure.

[0089] Furthermore, in step (4), the general steps of oil field on-site drilling are: static well - drilling down - short static well - drilling - stop drilling (short static well) - starting drilling - short static well - drilling down - short static well - drilling - stop drilling (short static well) - starting drilling - short static well - drilling down - short static well - drilling... drilling completion (static well), that is, the safe drilling fluid density window will experience a process of decrease-increase-decrease-increase... decrease-increase-decrease, and remain stable during the period from the end of the drilling operation to the start of the completion construction.

[0090] Considering that the switching of construction status during on-site drilling is very frequent, in order to reduce the number of times the drilling fluid density is adjusted and to reduce the risk of drilling accidents, the present invention proposes to determine the well inclination angle and the drilling fluid density within a more reasonable range from the beginning, that is, the overlap area of ​​the reasonable well inclination angle and the safe drilling fluid density window during the static well, circulation, tripping and running of the directional well under this condition, that is, to set ρ min起钻 is defined as the lower limit of the safe drilling fluid density window during the entire drilling process, ρ max循环 It is defined as the upper limit of the safe drilling fluid density window during the entire drilling process.

[0091] In step (5), considering that it is almost impossible to drill stably along a given angle during the actual drilling process, a slight angle deviation is inevitable during the process, which can generally be controlled within 4°. Therefore, in order to ensure the smooth progress of the drilling process, the reasonable well inclination angle and the safe drilling fluid density window obtained in step (4) need to be further corrected to cope with the angle deviation that may occur at any time. The correction method is: the maximum value of the lower limit and the minimum value of the upper limit of the safe drilling fluid density window within the preset well inclination angle range of α±4° are used as the lower limit and upper limit of the safe drilling fluid density window of the directional well, respectively.

[0092] The technical effects of the present invention are as follows:

[0093] The method of the present invention fully considers the influence of ground stress concentration, additional stress of fluid seepage, circulating pressure and fluctuating pressure during drilling on the stress condition of well wall rock when establishing a prediction model for a reasonable well inclination angle and a safe drilling fluid density window of a directional well. The calculation result is more accurate, has strong applicability, and is more suitable for guiding production.

[0094] The method of the present invention is to min起钻 Determined as the lower limit of the safe drilling fluid density window during the entire drilling process, ρ max循环 Determining the upper limit of the safe drilling fluid density window during the entire drilling process can reduce the number of drilling fluid density adjustments and reduce the risk of drilling accidents.

[0095] The method of the present invention uses the maximum value of the lower limit and the minimum value of the upper limit of the safe drilling fluid density window within the preset well inclination angle range of α±4° as the lower limit and upper limit of the safe drilling fluid density window of the directional well, respectively, to offset the influence of the angle deviation during the drilling process, and improve the accuracy of the reasonable well inclination angle and the safe drilling fluid density window of the directional well.

[0096] In summary, the method for determining the reasonable well inclination angle and safe drilling fluid density window of directional wells recommended by the present invention is more in line with the actual situation, can better guide the design of the well inclination angle and the selection of drilling fluid density on site, reduce the occurrence of well wall instability, and thus achieve the purpose of safe, fast and high-quality drilling. BRIEF DESCRIPTION OF THE DRAWINGS

[0097] Figure 1 Schematic diagram of derivation of the formula for well wall rock in-situ stress concentration in an embodiment of the present invention;

[0098] Figure 2 is a schematic diagram of the force analysis of the well wall rock in the embodiment of the present invention;

[0099] Figure 3 is a schematic diagram of rock deformation in a uniaxial compression experiment in an embodiment of the present invention;

[0100] Figure 4 is a stress-strain result diagram of a uniaxial compression test of rock in an embodiment of the present invention;

[0101] Figure 5 Schematic diagram of measuring the tensile strength of a rock sample by the splitting method (also called the Brazilian test method) in an embodiment of the present invention;

[0102] Figure 6 It is a diagram of the upper and lower limits of the safe drilling fluid density window of a directional well with different well inclination angles under various working conditions in the embodiment of the present invention;

[0103] Figure 7 It is a schematic diagram of the safe drilling fluid window and the corrected safe drilling fluid window during the whole drilling process of directional wells with different well inclination angles in an embodiment of the present invention. DETAILED DESCRIPTION

[0104] The specific technical solution of the present invention is explained in conjunction with embodiments.

[0105] The main producing layer in the Shengli Oil Production Plant area of ​​Shengtuo Oilfield is a weakly cemented sandstone formation. It is now necessary to design a suitable well inclination angle and a safe drilling fluid density window to ensure that the wellbore instability will not occur during the drilling process.

[0106] The specific process is:

[0107] Step 1: Derivation of the calculation formula for safe drilling fluid density window for directional wells with arbitrary well inclination angles

[0108] The present invention assumes that the reservoir is an isotropic homogeneous elastic body, the single-phase fluid flows in the uniformly distributed pore space, the temperature field effect is ignored, and the influence of tectonic stress is not considered. The formula for the in-situ stress concentration of the wellbore rock is derived under the condition of a directional well with a well inclination angle of α. The schematic diagram of the in-situ stress concentration of the wellbore rock involved is shown in FIG. Figure 1 shown.

[0109] The pressure exerted by the overlying rock is 10 -6 ρgH, considering the reservoir pore pressure P f The effective vertical pressure acting on the rock is F.

[0110] F=10 -6 ρgH-P f (1)

[0111] Where: ρ——average density of overlying rock, kg / m 3 ;

[0112] g——acceleration due to gravity, m / s 2 ;

[0113] H——reservoir depth, m;

[0114] P f ——Reservoir pore pressure, MPa.

[0115] F produces two components:

[0116] The component force F1 parallel to the wellbore axis (z direction) is:

[0117] F1=F·cosα (2)

[0118] Where: α is the well inclination angle, degrees.

[0119] The component force F2 perpendicular to the wellbore axis (x direction) is:

[0120] F2=F·sinα (3)

[0121] The x and y lateral pressures generated by F1 on the wellbore rock are:

[0122]

[0123] Where: v is the Poisson's ratio of rock, dimensionless;

[0124] v / (1-v)——lateral pressure coefficient, dimensionless.

[0125] The y lateral pressure generated by F2 on the wellbore rock is:

[0126]

[0127] therefore,

[0128] The actual stress in the x direction is: x =F2+F3 (7)

[0129] The actual stress in the y direction is: y =F3+F4 (8)

[0130] Combining formula (1) - formula (8), we can get

[0131]

[0132]

[0133] Since the rock Poisson's ratio v is between 0.15 and 0.4, the lateral pressure coefficient v / (1-v) is always less than 1, so it can be fully proved that σ x >σ y .

[0134] According to the theory of classical elastic mechanics, the circumferential stress at any radius r in the formation around the wellbore is:

[0135]

[0136] Where: r w ——Wellbore radius, m;

[0137] r——the distance between any point of the formation and the well axis, m;

[0138] θ——Any direction and σ x The angle with respect to the direction (x-axis), degrees.

[0139] From formula (11), we can see that at the well wall (r = r w ), cos2θ=-1, that is, θ=90° or 270° when σ reaches the maximum value σ max , cos2θ=1, that is, θ=0° or 180°, σ reaches the minimum value σmin ,

[0140]

[0141]

[0142] Considering the additional stress induced by the fluid flow in the pores at any point in the formation as σ l :

[0143]

[0144] Where: P is the bottom hole pressure, MPa.

[0145] φ——porosity, dimensionless.

[0146] At the well wall r = r w , then

[0147]

[0148] When P>P f When l is a positive value, that is, σ l Same direction as P, when P<P f When l is a negative value, that is, σ l The opposite of P.

[0149] In summary, the force diagram on the well wall is as follows: Figure 2 shown.

[0150] During the drilling process, if the density of the drilling fluid is too high or too low, it will lead to instability and damage of the wellbore. Therefore, in order to avoid the instability of the wellbore, the density of the drilling fluid should be limited to an appropriate range, that is, the safe drilling fluid density window, and the maximum and minimum values ​​of the safe drilling fluid density window can be obtained according to the formation fracture judgment criteria and formation collapse judgment criteria respectively.

[0151] In general, formation rupture is mainly caused by the excessive density of the drilling fluid in the well, which causes the maximum effective principal stress on the wellbore rock to exceed the tensile strength of the rock. At this time, the maximum effective principal stress on the wellbore rock points to the formation along the direction of the minimum circumferential stress of the wellbore, and acts on the wellbore rock in the form of tensile stress. The formation rupture judgment criteria are as follows:

[0152]

[0153] Where: T is the tensile strength of the wellbore rock, MPa.

[0154] The formation collapse is mainly caused by the low density of drilling fluid in the well, which makes the maximum effective principal stress of the well wall rock exceed the compressive strength of the rock. At this time, the maximum effective principal stress of the well wall rock points to the wellbore along the direction of the maximum circumferential stress of the well wall, and acts on the well wall rock in the form of compressive stress. The formation collapse judgment criteria are as follows:

[0155]

[0156] Where: C is the compressive strength of the wellbore rock, MPa.

[0157] In drilling operations, bottom hole pressure mainly comes from drilling fluid static column pressure, annular circulation pressure and fluctuating pressure. Under different conditions, these pressure sources may appear or disappear, and the bottom hole pressure will change accordingly. Therefore, understanding the accurate composition of bottom hole pressure under different working conditions is the basis for determining the reasonable inclination angle of directional wells and the safe drilling fluid density window.

[0158] In the static well state, the bottom hole pressure P is equal to the static column pressure P of the drilling fluid. w ,Right now

[0159] P=P w =10 -6 ρ 静井 gH (18)

[0160] According to the stratum rupture judgment criteria and stratum collapse judgment criteria, there are

[0161]

[0162]

[0163] Under the circulation state, the bottom hole pressure P is equal to the static column pressure P of the drilling fluid. w Plus the annular circulation pressure P xh ,Right now

[0164] P=P w +P xh =10 -6 ρ 循环 gH+P xh (twenty one)

[0165] at this time,

[0166]

[0167]

[0168] During actual operations, the movement of the tubing will cause the flow of fluid in the wellbore, thereby generating additional pressure in the well. This form of additional pressure is generally called surge pressure, which includes the pumping pressure generated during the drilling process and the excitation pressure generated during the drilling process.

[0169] During the drilling process, the annular circulation pressure P xh =0, the bottom hole pressure P is equal to the static column pressure P of the drilling fluid w Subtract the pumping pressure P cj ,Right now

[0170] P=P w -P cj =10 -6 ρ 起钻 gH-P cj (twenty four)

[0171] at this time,

[0172]

[0173]

[0174] During the drilling process, the annular circulation pressure P xh =0, the bottom hole pressure P is equal to the static column pressure P of the drilling fluid w Plus the excitement pressure P jd ,Right now

[0175] P=P w +P jd =10 -6 ρ 下钻 gH+P jd (27)

[0176] at this time,

[0177]

[0178]

[0179] Step 2: Determination of physical and mechanical property parameters of loose sandstone reservoir in Shengtuo Oilfield

[0180] The physical and mechanical properties of rocks mainly include the physical parameters (density, porosity, etc.), elastic parameters (elastic modulus, Poisson's ratio, etc.) and strength parameters (tensile strength, compressive strength, etc.) of rocks. With these parameters, the physical and mechanical properties of rocks can be evaluated, which lays a good foundation for further research on wellbore stability technology. In order to obtain these parameters, this paper selected six representative cores of loose sandstone reservoirs in Shengtuo Oilfield to make standard rock samples, and used the volume product method to determine their physical parameters under different states. After that, half of the rock samples saturated with simulated formation crude oil were selected for uniaxial compression tests to determine compressive strength, elastic modulus and Poisson's ratio, and the other half were subjected to Brazilian splitting tests to determine tensile strength. The results provided data support for subsequent calculations.

[0181] Preparation of experimental rock samples:

[0182] According to the specific conditions of Shengli Oil Production Plant in Shengtuo Oilfield, the selection criteria for experimental rock samples are: the rock samples come from representative oil-bearing sandstone in the main oil-producing layer, and the sandstone core must be able to carry out physical and mechanical experiments of a certain strength, and must be homogeneous, formed, and not too loose. According to the requirements of the experiment, when sampling, the rock block must be drilled from the entire rock core in the direction of the well depth, and processed into a standard rock sample of 25mm×50mm, with both ends ground flat, the size accurately measured, the volume calculated, and the weight number. The detailed information of the rock sample is shown in Table 1 below.

[0183] Table 1 Experimental rock sample information

[0184]

[0185] Determination of rock sample density and porosity by volumetric method:

[0186] The principle of the volumetric method is to obtain the volume of the specimen by measuring the cross-sectional area and height of the specimen, and then weigh the mass of the specimen under various states (natural, dried, saturated), and calculate the density of the rock block by the ratio of the mass of the specimen to the volume of the specimen. The fluid used to force the saturation of the specimen is simulated formation oil with a density of 0.85g / cm 3 This method can be used to determine the natural density, dry density, saturated density and porosity of rock blocks.

[0187] The density and porosity of rock blocks are calculated as follows:

[0188]

[0189]

[0190]

[0191]

[0192] In the formula

[0193] V--specimen volume, cm 3 ;

[0194] m0--natural mass of the specimen, g;

[0195] m d --Forced saturation mass of the specimen, g:

[0196] m s --Dry mass of specimen, g;

[0197] ρ0--Natural density of specimen block g / cm 3 ;

[0198] ρ d --Dry density of specimen, g / cm 3 ;

[0199] ρ s --Saturation density of specimen block, g / cm 3 ;

[0200] ρ l -Simulated formation fluid density, g / cm 3 ;

[0201] φ-porosity, dimensionless.

[0202] Table 2. Rock sample density and porosity experimental records

[0203]

[0204] Uniaxial compression deformation test to determine the compressive strength, elastic modulus and Poisson's ratio of rock samples:

[0205] The uniaxial compression test is the most common method to study rock deformation. This paper selects rock samples No. 1-3 for uniaxial compression test. During the experiment, it is necessary to paste a resistance wire sheet on the side of the rock sample after forced saturation with simulated formation oil in order to observe the deformation. The sample is pressurized with a press. The axial strain and lateral strain of the sample are measured under any axial pressure. Assuming the length of the sample is l and the diameter is d, the sample is axially shortened by Δl and laterally expanded by Δd under the action of load P. The deformation diagram of the rock uniaxial compression test is shown in Figure 1. Figure 3 As shown, the axial strain and lateral strain of the specimen are:

[0206]

[0207]

[0208] Assuming that the rock obeys Hooke's law (linear elastic material), the stress-strain results of the rock uniaxial compression test are as follows: Figure 4The compressive strength, elastic modulus and Poisson's ratio of the rock are given by:

[0209]

[0210]

[0211]

[0212] Table 3. Rock sample uniaxial compression deformation test record

[0213]

[0214] The splitting method (also known as the Brazilian test method) is used to determine the tensile strength of rock samples.

[0215] The splitting method, also known as radial fracturing, was proposed by Brazilian scholar Hondros, so it is often called the Brazilian experimental method. This experimental method applies a relative compressive linear load in the diameter direction of the rock sample that is forced to be saturated with simulated formation oil to cause it to fail, such as Figure 5 Then, according to the analytical solution of the concentrated force acting on the Boursinesq semi-infinite body, the maximum tensile stress acting on the diameter surface parallel to the load direction of the specimen when the specimen is destroyed is obtained, that is, the tensile strength of the rock is obtained, and the calculation formula is as follows:

[0216]

[0217] Table 4. Experimental record of rock sample splitting method (also known as Brazilian test method)

[0218]

[0219] Step 3:

[0220] According to the results obtained in step 2, the main production layer in the Shengli Oil Production Plant area of ​​Shengtuo Oilfield is a weakly cemented sandstone formation, the reservoir burial depth H≈2000m, and the average density of the overlying rock layer ρ≈2240kg / m 3 , porosity φ≈0.3, Poisson's ratio v≈0.3, rock tensile strength T≈4Mpa, and compressive strength C≈26MPa. Combined with the on-site geological and logging data, it can be seen that the original formation pore pressure P of the main reservoir f ≈20MPa, g≈9.8m / s 2 According to previous drilling experience in this area, the circulating pressure during drilling in a 2000m reservoir is generally controlled at around 6MPa, and the pumping pressure and agitation pressure generated during tripping are both about 3Mpa.

[0221] Substituting the physical and mechanical parameters of the loose sandstone reservoir of Shengtuo Oilfield and the drilling construction parameters into equations (19) to (29) in step 1, the safe drilling fluid density window of directional wells with different well inclination angles under various working conditions during the drilling process can be obtained. The results are shown in Table 5 below:

[0222] Table 5. Upper and lower limits of safe drilling fluid density window for directional wells with different well inclination angles under various working conditions

[0223]

[0224] The above results are plotted on the same graph to obtain the upper and lower limits of the safe drilling fluid density window for directional wells with different well inclination angles under various working conditions. Figure 6 As shown in the figure, the safe drilling fluid window and the corrected safe drilling fluid window diagram of directional wells with different well inclination angles during the whole drilling process can be obtained. Figure 7 shown.

[0225] From the calculation results reflected in the chart, it can be seen that under conventional drilling technology conditions, the maximum drilling angle of directional inclined wells in the loose sandstone reservoir of Shengtuo Oilfield cannot exceed 34°, otherwise wellbore instability will be inevitable. And under the limited condition that the well inclination angle does not exceed 34°, the safe drilling fluid density window will also change with the conversion of different construction states during the drilling process. When the construction state changes, the site needs to make corresponding adjustments to the drilling fluid density in time to ensure that the drilling fluid density is within the safe drilling fluid density window under the construction state.

[0226] Depend on Figure 7 It can be seen that the safe drilling fluid density window during the drilling process is higher than the safe drilling fluid density window in the static well state, higher than the safe drilling fluid density window during the drilling process, and higher than the safe drilling fluid density window in the circulating state. This also explains why: when the static well state is converted to the circulating state, that is, during the drilling process, the drilling fluid density needs to be appropriately reduced to prevent the application of the circulating pressure from crushing the formation; when the drilling is stopped, the drilling fluid density needs to be increased in time, and when the drilling starts, the drilling fluid density needs to be increased additionally, in order to prevent the collapse of the well wall rock; before starting the drilling operation in the static well state, the drilling fluid density needs to be gradually reduced to prevent the formation from being crushed due to the generation of agitated pressure.

[0227] The general steps of drilling on site in an oil field are: quiet well-drilling-temporary quiet well-drilling-stop drilling (temporary quiet well)-start drilling-temporary quiet well-drilling-temporary quiet well-drilling-stop drilling (temporary quiet well)-start drilling-temporary quiet well-drilling-temporary quiet well-drilling... drilling completion (quiet well), that is to say, the safe drilling fluid density window will experience a process of decrease-increase-decrease-increase... decrease-increase-decrease, and remain stable during the period from the end of the drilling operation to the start of completion construction.

[0228] Taking a vertical well as an example, the drilling fluid density should be controlled at 799.27 kg / m 3 ~2003.12kg / m 3 The density of drilling fluid should be controlled at 493.14 kg / m 3 ~1697.00kg / m 3 During the drilling process, the density of the drilling fluid should be controlled at 952.33 kg / m 3 ~2156.18kg / m 3 During the drilling process, the density of the drilling fluid should be controlled at 646.20 kg / m 3 ~1850.06kg / m 3 As the well inclination angle increases, the safe drilling fluid density window at each stage of the drilling operation will decrease. The closer the well inclination angle is to 34°, the smaller the fluctuation range of the drilling fluid density at each stage will be, and finally it will approach a fixed value, which is obviously not conducive to drilling construction.

[0229] Step 4:

[0230] Considering that the switching of construction status is very frequent during on-site drilling, in order to reduce the number of drilling fluid density adjustments and the risk of drilling accidents, the well inclination angle and drilling fluid density are usually determined to be within a more reasonable range at the beginning, that is, ρ min起钻 is defined as the lower limit of the safe drilling fluid density window during the entire drilling process, ρ max循环 The upper limit of the safe drilling fluid density window during the entire drilling process is set. In this way, the maximum directional well drilling angle allowed to be designed in the work area should be controlled within 20° (less than 20°). The limited range of the safe drilling fluid density window for directional wells with an inclination angle of 0° to 20° is shown in Table 6 below:

[0231] Table 6. Safe drilling fluid density window for directional wells with a well inclination angle of 0° to 20°

[0232]

[0233] Step 5:

[0234] It is worth noting that it is almost impossible to drill stably along a given angle during the actual drilling process. It is inevitable that a small angle deviation will occur during the process, which can generally be controlled within 4°. Therefore, in order to ensure the smooth progress of the drilling process, the well inclination angle should be further limited to within 16° (less than 16°) at the beginning of the design. Unless there is a very stable directional drilling technology, it is not advisable to design a directional well with a higher well inclination angle. In addition, the limited range of the safe drilling fluid density window of a directional well with a well inclination angle of 0° to 16° during actual drilling also needs to be corrected to cope with the angle deviation that may occur at any time. The correction method is: the maximum value of the lower limit and the minimum value of the upper limit of the safe drilling fluid density window within the preset well inclination angle range of α±4° are used as the lower limit and upper limit of the safe drilling fluid density window of the directional well, respectively.

[0235] Taking a directional well with a preset well inclination angle of 3° as an example, the well inclination angle range that may be formed after an uncontrollable deviation occurs during the drilling process is -1° to 7° (note that there is no essential difference in effect between -1° and 1° in the calculation method of this article). The maximum value of the lower limit of the safe drilling fluid density window within this range is obtained when the deviation well inclination angle is 7°, and the result is 1246.80 kg / m 3 The minimum value of the upper limit is obtained when the deviation angle is 0°, and the result is 1697.00 kg / m 3 Therefore, the corrected safe drilling fluid density window for a directional well with a preset well inclination angle of 3° is 1246.80 kg / m 3 ~1697.00kg / m 3 Similarly, the limited range of the safe drilling fluid density window after the directional inclined well maintenance certificate for well inclination angles within 0° to 16° is shown in Table 7 below:

[0236] Table 7. Safe drilling fluid density window for directional wells with a corrected well inclination angle of 0° to 16°

[0237]

[0238] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for determining a reasonable well inclination angle and a safe drilling fluid density window for a directional well, characterized by: The steps include: (1) Considering the effects of fluid seepage additional stress, circulation pressure, and fluctuating pressure during tripping on bottom hole pressure, a safe drilling fluid density window prediction model is established under various working conditions during directional well drilling with arbitrary well inclination angles; each working condition includes static well, circulation, tripping, and tripping; The prediction model of safe drilling fluid density window under various working conditions during directional well drilling at any well inclination angle is as follows: According to the stratum fracture and stratum collapse judgment criteria are: Where: ρ is the average density of the overlying rock, kg / m 3 ; g is the acceleration due to gravity, m / s 2 ; H is the reservoir depth, m; P f is the reservoir pore pressure, MPa; α is the well inclination angle, °; v is the rock Poisson's ratio, dimensionless; σ min Minimum circumferential stress of wellbore, MPa; σ max Maximum circumferential stress of wellbore, MPa; ρ min静井 The lower limit of safe drilling fluid density in static well state, kg / m 3 ; ρ max静井 The upper limit of safe drilling fluid density in static well state, kg / m 3 ; P xh is the circulation pressure, MPa; ρ min循环 is the lower limit of safe drilling fluid density under circulation state, kg / m 3 ; ρ max循环 The upper limit of safe drilling fluid density under circulation condition, kg / m 3 ; P cj is the pumping pressure, MPa; ρ min起钻 The lower limit of safe drilling fluid density for drilling rig, kg / m 3 ; ρ max起钻 The upper limit of safe drilling fluid density under the drilling state, kg / m 3 ; P jd is the excitation pressure, MPa; ρ min下钻 The lower limit of safe drilling fluid density for drilling rig, kg / m 3 ; ρ max下钻 The upper limit of safe drilling fluid density under the drilling state, kg / m 3 ; φ is porosity, dimensionless; T is the tensile strength of the wellbore rock, MPa; C is the compressive strength of the wellbore rock, MPa; (2) Obtain the physical and mechanical property parameters of the target formation rock and general drilling operation data through field data or indoor tests; (3) The parameters and data obtained in step (2) are brought into the model for solution, and the reasonable well inclination angle and safe drilling fluid density window under different working conditions are obtained respectively, and the reasons causing the reasonable well inclination angle and safe drilling fluid density window to move up or down are analyzed; (4) In order to reduce the number of drilling fluid density adjustments and the risk of drilling accidents, the well inclination angle and drilling fluid density during the entire directional well drilling process are determined to be within a more reasonable range, that is, the overlap area of ​​the reasonable well inclination angle and the safe drilling fluid density window during the static well, circulation, drilling and drilling under this condition; (5) The reasonable well inclination angle and safe drilling fluid density window obtained in step (4) are further corrected to cope with the angle deviation that may occur at any time.

2. The method for determining the reasonable well inclination angle and the safe drilling fluid density window of a directional well according to claim 1, characterized in that: In step (2), the method for obtaining rock mechanical parameters uses an indoor test method, specifically: the density and porosity of the rock sample are determined by the volumetric product method, the compressive strength, elastic modulus and Poisson's ratio of the rock sample are determined by the uniaxial compression deformation test, and the tensile strength of the rock sample is determined by the splitting method; The criteria for selecting experimental rock samples are: the rock samples come from representative formation rocks in the target drilling area. According to the requirements of the experiment, the rock blocks should be drilled from the whole rock core in the direction of the well depth during sampling, and processed into standard rock samples of 25mm×50mm, with both ends ground flat, the dimensions accurately measured, the volume calculated and weighed and numbered.

3. The method for determining the reasonable well inclination angle and safe drilling fluid density window of a directional well according to claim 1, characterized in that: In step (3), the parameters and data obtained in step (2) are brought into the model established in step (1) to solve and obtain ρ at any well inclination angle. max静井 , min静井 , max循环 , min循环 , max起钻 , min起钻 , max下钻 , min下钻 , plot the results in the same figure, where ρ max静井 , min静井 The well inclination angle corresponding to the intersection of the two lines is the maximum well inclination angle that can be designed when the directional well is in a static well state under this condition; the area enclosed by the two lines on the left side of this point is the reasonable well inclination angle and safe drilling fluid density window when the directional well is in a static well state under this condition; similarly, the reasonable well inclination angle and safe drilling fluid density window are obtained when the directional well is in circulation, drilling and drilling under this condition.

4. The method for determining the reasonable well inclination angle and safe drilling fluid density window of a directional well according to claim 1, characterized in that: In step (4), the well inclination angle and the drilling fluid density are determined to be within a reasonable range, that is, the overlap area of ​​the reasonable well inclination angle and the safe drilling fluid density window during the static well, circulation, drilling and drilling conditions of the directional well under this condition, that is, ρ min起钻 is defined as the lower limit of the safe drilling fluid density window during the entire drilling process, ρ max循环 It is defined as the upper limit of the safe drilling fluid density window during the entire drilling process.

5. The method for determining the reasonable well inclination angle and safe drilling fluid density window of a directional well according to claim 1, characterized in that: The reasonable well inclination angle and safe drilling fluid density window obtained in step (4) are corrected by taking the maximum value of the lower limit and the minimum value of the upper limit of the safe drilling fluid density window within the preset well inclination angle range of α±4° as the lower limit and upper limit of the safe drilling fluid density window of the directional well, respectively.

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