Prediction method and system for gas injection pressure of newly developed oil field

Abstract

The invention relates to the technical field of oilfield development, and provides a prediction method and system for gas injection pressure of a newly developed oilfield. The method comprises the following steps: deploying gas injection wells at a preset distance from a set number of newly developed producing wells in a set range, injecting nitrogen into the gas injection wells, and driving crude oil in a stratum to move towards an oil drainage radius area of the set number of newly developed producing wells; the moment when the gas injection pressure wave and the front edge intersect with the oil drainage radius front edges of the newly developed producing wells with the set number is obtained to serve as the gas injection effective moment, and the flowing bottomhole pressure of the gas injection well before the gas injection effective moment and the flowing bottomhole pressure of the gas injection well after the gas injection effective moment are obtained through the Darcy law according to the original formation pressure; and finally, respectively solving the wellhead flow pressure of the gas injection well before the gas injection effect moment and the wellhead flow pressure of the gas injection well after the gas injection effect moment. According to the method, continuous prediction of the gas injection pressure of the gas injection well is achieved, and an important basis is provided for development index determination, displacement effect evaluation and surface pipeline type selection.

Description

Technical Field

[0001] This invention relates to the field of oilfield development technology, and in particular to a method and system for predicting gas injection pressure in newly developed oilfields. Background Technology

[0002] In the process of oilfield development, the prediction of gas injection pressure at the wellhead of gas injection wells is an important indicator for the preparation of gas injection development plans. The accurate prediction of this indicator is of great significance for the determination of development indicators, the evaluation of displacement effect, and the selection of surface pipelines.

[0003] Current methods for predicting injection pressure at the wellhead of gas injection wells have several drawbacks: 1. They are all designed for oilfields that have already been developed with water or gas injection. In these oilfields, well network control is generally well-established, and the distribution characteristics and seepage patterns of underground reservoirs are well understood. Based on this, injection pressure is predicted through direct calculation or indirect acquisition of relevant parameters. However, for newly developed oilfields, there are generally only a few exploratory and appraisal wells in the area. Calculations cannot be made using production parameters from production or injection wells, and the understanding of formation seepage patterns is limited to laboratory experiments, resulting in poor applicability to newly developed oilfields. 2. Existing technologies for predicting injection pressure in newly developed oilfields require pressure drop tests. This method necessitates shutting down the well and installing specialized equipment such as pressure gauges, which is cumbersome, costly, and has limited applicability. 3. Existing technologies do not consider the changes in injection pressure before and after the effective injection point. Due to these drawbacks, continuous prediction of injection pressure before and after the effective injection point in newly developed oilfields is not possible. Summary of the Invention

[0004] In view of this, the present invention provides a method and system for predicting gas injection pressure in newly developed oil fields, so as to solve the technical problem that the prior art cannot achieve continuous prediction of gas injection pressure before and after the effective time of gas injection in newly developed oil fields.

[0005] In a first aspect, the present invention provides a method for predicting gas injection pressure in newly developed oil fields, comprising:

[0006] S1. Select a set range and deploy a gas injection well at a preset distance from a set number of newly developed oil production wells. Inject nitrogen into the gas injection well so that the nitrogen diffuses into the formation through the bottom perforation of the gas injection well, driving the crude oil in the formation to move towards the drainage radius area of ​​the set number of newly developed oil production wells.

[0007] S2. The moment when the gas injection pressure wave front intersects with the drainage radius front of a set number of newly developed oil wells is taken as the gas injection effective moment. Based on the key parameters before and after the gas injection effective moment, the bottom flow pressure of the gas injection well before and after the gas injection effective moment is obtained from the original formation pressure using Darcy's law.

[0008] S3. Calculate the wellhead pressure of the gas injection well before the effective time of gas injection and the wellhead pressure of the gas injection well after the effective time of gas injection, respectively.

[0009] Furthermore, the key parameters before and after the effective gas injection point were obtained by interpreting electrical logging data from exploratory and appraisal wells and conducting coring experiments to determine the distribution of oil layer thickness and physical properties within the region.

[0010] The key parameters before the gas injection takes effect include: oil layer thickness before the gas injection takes effect, effective permeability before the gas injection takes effect, formation crude oil viscosity before the gas injection takes effect, volume factor before the gas injection takes effect, residual oil saturation before the gas injection takes effect, and water saturation before the gas injection takes effect.

[0011] The key parameters after the gas injection takes effect include: oil layer thickness, effective permeability, formation crude oil viscosity, volume factor, residual oil saturation, water saturation, bottom hole flowing pressure, and daily oil production of the oil well.

[0012] Furthermore, the formula for calculating the wellhead flowing pressure of the gas injection well before the effective time of gas injection is as follows:

[0013] p whi =p wfi -p G +p f -p v

[0014] Where, p whi p represents the wellhead flowing pressure before the gas injection takes effect. wfi p represents the bottom-hole flowing pressure of the gas injection well before the injection takes effect. G p represents the weight of the gas in the injection wellbore. f p represents the frictional resistance pressure drop. v This indicates the kinetic energy pressure drop.

[0015] Furthermore, the formula for calculating the bottom-hole flowing pressure of the gas injection well before the effective time of gas injection is as follows:

[0016]

[0017] Where, p wfi p represents the bottom-hole flowing pressure of the gas injection well before the injection takes effect. r q represents the original formation pressure before the gas injection takes effect. g Indicates the gas injection rate, μ g B represents the viscosity of gas under formation conditions.g k represents the volume coefficient of the injected gas. g h represents the effective gas permeability. g Indicates the gas injection depth, r g The radius of the precursor displacement, r, indicates the moment when the gas injection takes effect. w Indicates the diameter of the gas injection well;

[0018] p G =ρ m gHsinθ

[0019]

[0020]

[0021] Among them, P G p represents the weight of the gas in the wellbore before the injection becomes effective. f p represents the pressure drop due to frictional resistance before the gas injection takes effect. v ρ represents the kinetic energy pressure drop before the gas injection takes effect. m H represents density, θ represents injection depth, and f represents well inclination angle. m d represents the coefficient of frictional resistance, and v represents the differential. m d represents the flow velocity of the injected gas at any depth in the wellbore. z This indicates the position of each step in the wellbore.

[0022] Furthermore, the gravity of the wellbore gas, the frictional resistance pressure drop, and the kinetic energy pressure drop are obtained using an iterative method based on the fourth-order Runge-Kutta method.

[0023] Furthermore, the bottom-hole flowing pressure of the gas injection well after the gas injection takes effect is obtained based on the bottom-hole flowing pressure of the oil production well after the gas injection takes effect.

[0024] Furthermore, the formula for calculating the bottom-hole flowing pressure of the gas injection well after the gas injection takes effect is as follows:

[0025]

[0026] Where, p wfo This indicates the bottom hole flowing pressure (p) of the oil well after the gas injection takes effect. wfi1 q represents the bottom-hole flowing pressure of the injection well after the injection takes effect. g Indicates the injection rate after the injection takes effect, in μ. g B represents the viscosity of gas under formation conditions. g k represents the gas volume coefficient after the injection takes effect. g Indicates the effective gas permeability after the effect is achieved, h g Indicates the injection depth after the injection takes effect, r g1The displacement radius of the gas injection well is indicated by the time the gas injection becomes effective; q0 represents the oil drainage rate after the time the gas injection becomes effective, in μ. o B0 represents the crude oil viscosity under formation conditions, k0 represents the crude oil volume coefficient after the gas injection takes effect, h0 represents the crude oil effective permeability after the gas injection takes effect, r0 represents the oil layer thickness, and r0 represents the oil well drainage radius after the gas injection takes effect.

[0027] Furthermore, the formula for calculating the bottom hole flowing pressure of the oil well after the gas injection takes effect is as follows:

[0028]

[0029] Where, p i This indicates the original formation pressure after the gas injection takes effect.

[0030] Furthermore, obtaining the oil well drainage radius after the gas injection takes effect includes:

[0031] The injection displacement radius is obtained by considering the injected gas sweep volume, the gas injection working time of the injection well, formation parameters, and the gas injection volume.

[0032] Based on the aforementioned gas displacement radius, the time for gas injection to take effect is obtained;

[0033] The oil drainage radius of the production well after the gas injection takes effect is obtained by using the difference between the distance between the gas injection well and the production well and the displacement radius, as well as by using the time it takes for the gas injection to take effect.

[0034] Furthermore, the formula for calculating the wellhead flowing pressure of the gas injection well after the gas injection takes effect is as follows:

[0035] p whi1 =p wfi1 -p G +p f -p v

[0036] Where, p whi1 This indicates the wellhead flowing pressure (p) after the gas injection takes effect. wfi1 This indicates the bottom-hole flowing pressure of the injection well after the injection takes effect, where,

[0037] p G =ρ m gHsinθ

[0038]

[0039]

[0040] Secondly, the present invention provides a prediction system for gas injection pressure in newly developed oil fields, comprising: a selection module 401, an acquisition module 402, and a calculation module 403.

[0041] The selection module 401 is used to select a set range and deploy a gas injection well at a preset distance from a set number of newly developed oil production wells, inject nitrogen into the gas injection well, so that the nitrogen diffuses into the formation through the bottom perforation position of the gas injection well, driving the crude oil in the formation to move towards the oil drainage radius area of ​​the set number of newly developed oil production wells.

[0042] The acquisition module 402 is used to acquire the moment when the gas injection pressure wave front intersects with the oil drainage radius front of a set number of newly developed oil wells as the gas injection effective moment. Based on the key parameters before the gas injection effective moment and the key parameters after the gas injection effective moment, the bottom flow pressure of the gas injection well before the gas injection effective moment and the bottom flow pressure of the gas injection well after the gas injection effective moment are obtained from the original formation pressure through Darcy's law.

[0043] The calculation module 403 is used to calculate the wellhead pressure of the gas injection well before the gas injection effect time and the wellhead pressure of the gas injection well after the gas injection effect time, respectively, from the bottom flow pressure of the gas injection well before the gas injection effect time and the bottom flow pressure of the gas injection well after the gas injection effect time.

[0044] The formula for calculating the wellhead flowing pressure of the gas injection well before the effective time of gas injection is as follows:

[0045] p whi =p wfi -p G +p f -p v

[0046] Where, p whi p represents the wellhead flowing pressure before the gas injection takes effect. wfi p represents the bottom-hole flowing pressure of the gas injection well before the injection takes effect. G p represents the weight of the gas in the injection wellbore. f p represents the frictional resistance pressure drop. v This indicates the kinetic energy pressure drop.

[0047] Furthermore, the formula for calculating the bottom-hole flowing pressure of the gas injection well after the gas injection takes effect is as follows:

[0048]

[0049] Where, p wfo This indicates the bottom hole flowing pressure (p) of the oil well after the gas injection takes effect. wfi1 q represents the bottom-hole flowing pressure of the injection well after the injection takes effect. g Indicates the injection rate after the injection takes effect, in μ. g B represents the viscosity of gas under formation conditions. g k represents the gas volume coefficient after the injection takes effect. g Indicates the effective gas permeability after the effect is achieved, hg Indicates the injection depth after the injection takes effect, r g1 The displacement radius of the gas injection well is indicated by the time the gas injection becomes effective; q0 represents the oil drainage rate after the time the gas injection becomes effective, in μ. o B0 represents the crude oil viscosity under formation conditions, k0 represents the crude oil volume factor after the gas injection takes effect, h0 represents the crude oil effective permeability after the gas injection takes effect, r0 represents the oil layer thickness, and r represents the well drainage radius after the gas injection takes effect. w Indicates the diameter of the gas injection well.

[0050] Furthermore, the formula for calculating the bottom hole flowing pressure of the oil well after the gas injection takes effect is as follows:

[0051]

[0052] Furthermore, the formula for calculating the wellhead flowing pressure of the gas injection well after the gas injection takes effect is as follows:

[0053] p whi1 =p wfi1 -p G +p f -p v

[0054] Where, p whi1 This indicates the wellhead flowing pressure (p) after the gas injection takes effect. wfi1 This indicates the bottom-hole flowing pressure of the injection well after the injection takes effect, where,

[0055] p G =ρ m gHsinθ

[0056]

[0057]

[0058] Among them, P G p represents the weight of the gas in the wellbore before the injection becomes effective. f p represents the pressure drop due to frictional resistance before the gas injection takes effect. v ρ represents the kinetic energy pressure drop before the gas injection takes effect. m H represents density, θ represents injection depth, and f represents well inclination angle. m d represents the coefficient of frictional resistance, and v represents the differential. m d represents the flow velocity of the injected gas at any depth in the wellbore. z This indicates the position of each step in the wellbore.

[0059] The advantages of this invention compared to the prior art are:

[0060] 1. This invention uses parameters obtained from geological studies and laboratory data of newly developed oil fields to avoid the cumbersome operations of on-site testing and saves high time costs.

[0061] 2. This invention provides a technical basis for the continuous prediction of wellhead pressure of gas injection wells in newly developed oil fields by selecting locations to dig gas injection wells and injecting nitrogen.

[0062] 3. By screening parameters before and after the effective time of gas injection, this invention fully considers the changes in gas injection pressure before and after the effective time of gas injection, and realizes continuous prediction of gas injection pressure in gas injection wells.

[0063] 4. This invention provides an important basis for determining development indicators, evaluating displacement effects, and selecting surface pipelines by accurately predicting the wellhead pressure of gas injection wells. Attached Figure Description

[0064] To more clearly illustrate the technical solutions in this invention, the accompanying drawings used in the description of the embodiments or prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0065] Figure 1 This is a flowchart of a method for predicting gas injection pressure in newly developed oil fields, provided by an embodiment of the present invention.

[0066] Figure 2 This is a schematic diagram of the pressure profile changes of the gas injection well and the oil production well before the effective time of gas injection, provided in an embodiment of the present invention.

[0067] Figure 3 This is a schematic diagram of the pressure profile changes of the gas injection well and the oil production well after the gas injection takes effect, provided in an embodiment of the present invention.

[0068] Figure 4 This is a block diagram of a gas injection pressure prediction system for newly developed oil fields provided in an embodiment of the present invention. Detailed Implementation

[0069] In the following description, specific details such as particular system architectures and techniques are set forth for illustrative purposes and not for limitation, in order to provide a thorough understanding of the embodiments of the invention. However, those skilled in the art will understand that the invention can be implemented in other embodiments without these specific details. In other instances, detailed descriptions of well-known systems, apparatuses, circuits, and methods are omitted so as not to obscure the description of the invention with unnecessary detail.

[0070] The following will describe in detail, with reference to the accompanying drawings, a method for predicting gas injection pressure in newly developed oil fields according to the present invention.

[0071] Figure 1 This is a flowchart illustrating a method for predicting gas injection pressure in newly developed oil fields, provided by an embodiment of the present invention. Figure 1 As shown, the method for predicting gas injection pressure in newly developed oil fields includes:

[0072] S1. Select a set range and deploy a gas injection well at a preset distance from a set number of newly developed oil production wells. Inject nitrogen into the gas injection well so that the nitrogen diffuses into the formation through the bottom perforation of the gas injection well, driving the crude oil in the formation to move towards the drainage radius area of ​​the set number of newly developed oil production wells.

[0073] In the initial injection phase of the gas injection well, the gas diffuses into the formation through the perforation at the bottom of the well, driving the crude oil within the formation towards the drainage radius of the predetermined number of newly developed production wells. The pressure generated during this stage propagates to the drainage radius of the production wells; therefore, production in the production wells during this stage does not affect gas injection into the injection wells. The area around the injection well in this stage can be divided into two regions: the gas sweep zone in the inner region and the pressure conduction zone in the outer region. Within the gas sweep zone, the injected gas has already displaced the crude oil to deeper formations. In the pressure conduction zone, the crude oil is displaced and compressed by the injected gas, resulting in pressure propagation. The overall flow in the sweep zone conforms to Darcy's law.

[0074] S2. The moment when the gas injection pressure wave front intersects with the drainage radius front of a set number of newly developed oil wells is taken as the gas injection effective moment. Based on the key parameters before and after the gas injection effective moment, the bottom flow pressure of the gas injection well before and after the gas injection effective moment is obtained from the original formation pressure using Darcy's law.

[0075] The key parameters before and after the effective gas injection point were obtained by interpreting electrical logging data from exploratory and appraisal wells and conducting coring experiments to determine the distribution of oil layer thickness and physical properties within the region.

[0076] Among them, exploratory wells and appraisal wells are related to newly developed oil production wells. Exploratory wells are used to detect where there are oil production wells, while appraisal wells are used to determine whether the oil production well should be used for oil production.

[0077] The key parameters before the gas injection takes effect include: oil layer thickness before the gas injection takes effect, effective permeability before the gas injection takes effect, formation crude oil viscosity before the gas injection takes effect, volume factor before the gas injection takes effect, residual oil saturation before the gas injection takes effect, and water saturation before the gas injection takes effect.

[0078] The key parameters after the gas injection takes effect include: oil layer thickness, effective permeability, formation crude oil viscosity, volume factor, residual oil saturation, water saturation, bottom hole flowing pressure, and daily oil production of the oil well.

[0079] In one embodiment, the gas injection pressure of the injection well is dynamically changing. The distribution of oil layer thickness and physical properties is obtained through exploratory and appraisal wells within the region, and gas-liquid phase permeability experimental parameters are obtained through laboratory experiments. Based on formation seepage theory, the nodal method is used to calculate the wellhead injection pressure. The wellhead injection pressure calculation includes two stages: before and after the injection becomes effective. Because the influencing factors differ in these two stages, the calculation methods differ.

[0080] Before the injection well becomes effective, the area surrounding it can be considered as an infinitely distant formation. The injected gas flows through the formation pores, primarily influenced by geological factors and the well's operating regime. These geological factors include: the thickness of the perforated oil layer, porosity, permeability, and crude oil viscosity; the well's operating regime includes: injection rate and injection time. Thus, key parameters before the injection becomes effective are obtained.

[0081] After the gas injection takes effect, the injected gas drives crude oil to flow to the production well, replenishing the energy in the near-wellbore zone. Following this stage, the gas injection pressure is affected not only by geological factors and the operating conditions of the gas injection well, but also by the operating conditions of the production well, such as the daily oil production and the bottom hole production pressure differential. The operating conditions of the production well include the daily fluid production and the bottom hole production pressure differential. Thus, key parameters after the gas injection takes effect are obtained.

[0082] S3. Calculate the wellhead pressure of the gas injection well before the effective time of gas injection and the wellhead pressure of the gas injection well after the effective time of gas injection, respectively.

[0083] Figure 2 This is a schematic diagram of the pressure profile changes of the gas injection well and the oil production well before the effective time of gas injection, provided in an embodiment of the present invention.

[0084] Before the gas injection takes effect:

[0085] The formula for calculating the wellhead flowing pressure of the gas injection well before the effective time of gas injection is as follows:

[0086] p whi =p wfi -p G +p f -p v (1)

[0087] Where, p whi p represents the wellhead flowing pressure before the gas injection takes effect. wfi p represents the bottom-hole flowing pressure of the gas injection well before the injection takes effect. G p represents the gravity of the gas in the injection wellbore. f p represents the frictional resistance pressure drop. v This indicates the kinetic energy pressure drop.

[0088] In one embodiment, nitrogen is injected into the injection wellbore. The gas flows in a single phase, and the pressure change during the flow is divided into three parts: the pressure drop caused by the gas gravity in the injection wellbore, the pressure drop caused by frictional resistance, and the pressure drop caused by kinetic energy. For the injection well, the gas is injected downwards from the wellhead to the bottom of the well, and the injection process satisfies formula (1).

[0089] The formula for calculating the bottom-hole flowing pressure of the gas injection well before the effective time of gas injection is as follows:

[0090]

[0091] Where, p wfi p represents the bottom-hole flowing pressure of the gas injection well before the injection takes effect. r q represents the original formation pressure before the gas injection takes effect. g Indicates the gas injection rate, μ g B represents the viscosity of gas under formation conditions. g k represents the volume coefficient of the injected gas. g h represents the effective gas permeability. g Indicates the gas injection depth, r g The radius of the precursor displacement, r, indicates the moment when the gas injection takes effect. w Indicates the diameter of the gas injection well.

[0092] Where, p r μ, B, k, h, r g 、r w All are known, r g The flow pressure at the bottom of the injection well before the effect is obtained by multiplying the flow velocity of the injected gas at any depth in the wellbore by the injection time. Substituting the above parameter values ​​into formula (2) yields the bottom flow pressure of the injection well before the effect is achieved.

[0093] p G =ρ m gHsinθ (3)

[0094]

[0095]

[0096] Among them, P G p represents the weight of the gas in the wellbore before the injection becomes effective. fp represents the pressure drop due to frictional resistance before the gas injection takes effect. v ρ represents the kinetic energy pressure drop before the gas injection takes effect. m H represents density, θ represents injection depth, and f represents well inclination angle. m d represents the coefficient of frictional resistance, and v represents the differential. m d represents the flow velocity of the injected gas at any depth in the wellbore. z This indicates the position of each step in the wellbore.

[0097] Where, ρ m The density was obtained from a table; H was obtained from geological data; θ was obtained from drilling data; and f... m v is obtained by iterative calculation. m Based on the drilling data, the gas gravity in the injection wellbore, the frictional resistance pressure drop, and the kinetic energy pressure drop are calculated respectively. Then, the above parameter values ​​and the bottom flow pressure of the injection well before the effect are substituted into formula (1) to obtain the wellhead flow pressure of the injection well before the effect.

[0098] The gravity of the wellbore gas, the frictional resistance pressure drop, and the kinetic energy pressure drop are obtained using an iterative method based on the fourth-order Runge-Kutta method.

[0099] Figure 3 This is a schematic diagram showing the changes in pressure profiles of the gas injection well and the oil production well after the gas injection becomes effective, provided in an embodiment of the present invention.

[0100] After the gas injection takes effect:

[0101] The bottom-hole flowing pressure of the gas injection well after the gas injection becomes effective is obtained based on the bottom-hole flowing pressure of the oil production well after the gas injection becomes effective.

[0102] The formula for calculating the bottom-hole flowing pressure of the gas injection well after the gas injection takes effect is as follows:

[0103]

[0104] Where, p wfo This indicates the bottom hole flowing pressure (p) of the oil well after the gas injection takes effect. wfi1 q represents the bottom-hole flowing pressure of the injection well after the injection takes effect. g Indicates the injection rate after the injection takes effect, in μ. g B represents the viscosity of gas under formation conditions. g k represents the gas volume coefficient after the injection takes effect. g Indicates the effective gas permeability after the effect is achieved, h g Indicates the injection depth after the injection takes effect, r g1 The displacement radius of the gas injection well is indicated by the time the gas injection becomes effective; q0 represents the oil drainage rate after the time the gas injection becomes effective, in μ. oB0 represents the crude oil viscosity under formation conditions, k0 represents the crude oil volume coefficient after the gas injection takes effect, h0 represents the crude oil effective permeability after the gas injection takes effect, r0 represents the oil layer thickness, and r0 represents the oil well drainage radius after the gas injection takes effect.

[0105] The derivation of formula (6) is based on Darcy's formula. The bottom-hole flowing pressure of the gas injection well and the bottom-hole flowing pressure of the oil production well after the gas injection takes effect are calculated respectively. The formula is transformed as follows:

[0106] The formula for calculating the bottom-hole flowing pressure of the gas injection well after the gas injection takes effect is as follows:

[0107]

[0108] After the gas injection takes effect, the bottom flow pressure of the gas injection well, p i This indicates the formation pressure after the gas injection takes effect.

[0109] The formula for calculating the bottom hole flowing pressure of the oil well after the gas injection takes effect is as follows:

[0110]

[0111] Where, p i This indicates the original formation pressure after the gas injection takes effect.

[0112] The oil well drainage radius after the gas injection takes effect is obtained by measuring the difference between the injection-production distance and the displacement radius.

[0113] When the gas injection takes effect, the formation pressure after the gas injection takes effect is obtained by combining the above two equations (7) and (8) to obtain the bottom hole flowing pressure formula (6) of the oil well after the gas injection takes effect. In formula (6), only r o unknown,

[0114] Where L represents the distance between the gas injection well and the production well, and the distance between the gas injection well and the production well is equal to the sum of the gas injection pressure wave front and the oil drainage radius of the newly developed production well.

[0115] (b1) The gas injection displacement radius is obtained by considering the injected gas sweep volume, gas injection working time of the injection well, formation parameters and gas injection volume;

[0116] The formation parameters include: oil layer thickness, porosity, residual oil saturation, and bound water saturation.

[0117] The formula for calculating the injected gas wave volume is as follows:

[0118]

[0119] Where V represents the injected gas sweep volume, and h represents the oil layer thickness. S represents porosity.or S represents the residual oil saturation. wc This indicates the degree of bound water saturation.

[0120] The formula for calculating the cumulative gas injection volume is as follows:

[0121] Q g =q g .t g (10)

[0122] Among them, Q g q represents the cumulative gas injection volume. g Indicates the injection rate after the gas injection takes effect, t g This indicates the working time for gas injection in the injection well.

[0123] Since the cumulative injection volume is equal to the injected gas wave volume, combined with formulas (9) and (10), the injection displacement radius can be obtained as follows:

[0124]

[0125] (b2) Based on the gas injection displacement radius, the gas injection effective time t0 is obtained;

[0126] (b3) Obtain the oil drainage radius of the oil well after the gas injection takes effect by the difference between the distance between the gas injection well and the production well and the displacement radius, and by the time the gas injection takes effect.

[0127] Similarly, the oil drainage radius of an oil production well is:

[0128]

[0129] At the critical point where gas injection takes effect, the following conditions are met:

[0130] r o +r g1 =L (13)

[0131] Where L represents the distance between the gas injection well and the oil production well.

[0132] The combined formulas (11)-(13) are used to obtain the gas injection displacement radius, the gas injection effective time t0, and the oil well drainage radius after the gas injection effective time.

[0133] The formula for calculating the wellhead flowing pressure after the gas injection takes effect is as follows:

[0134] p wi1 =p wfi1 -p G +p f -p v (14)

[0135] Where, p whi1This indicates the wellhead flowing pressure (p) after the gas injection takes effect. wfi1 This indicates the bottom flow pressure of the injection well after the injection takes effect.

[0136] Once the gas injection takes effect, the formation pressure becomes uniform.

[0137] in,

[0138] p G =ρ m gHsinθ (15)

[0139]

[0140]

[0141] The gravity of the wellbore gas after the effective injection time, the frictional resistance pressure drop after the effective injection time, and the kinetic energy pressure drop after the effective injection time are all obtained by iterative method using the fourth-order Runge-Kutta method, which is the same as the method used before the effective injection time.

[0142] This invention avoids the cumbersome operations and high time costs of on-site testing by using parameters obtained from geological studies and laboratory data of newly developed oil fields; by selecting locations to drill gas injection wells and injecting nitrogen, it provides the technical premise for continuous prediction of wellhead pressure of gas injection wells in newly developed oil fields; by screening parameters before and after the effective injection time, it fully considers the changes in gas injection pressure before and after the effective injection time, and realizes continuous prediction of gas injection well pressure; through accurate prediction of wellhead pressure of gas injection wells, it provides an important basis for determining development indicators, evaluating displacement effects, and selecting surface pipelines.

[0143] Figure 4 This is a block diagram of a gas injection pressure prediction system for newly developed oil fields provided in an embodiment of the present invention.

[0144] Based on the same concept, the present invention also provides a prediction system for gas injection pressure in newly developed oil fields, comprising: a selection module 401, an acquisition module 402, and a calculation module 403.

[0145] The selection module 401 is used to select a set range and deploy a gas injection well at a preset distance from a set number of newly developed oil production wells, inject nitrogen into the gas injection well, so that the nitrogen diffuses into the formation through the bottom perforation position of the gas injection well, driving the crude oil in the formation to move towards the oil drainage radius area of ​​the set number of newly developed oil production wells.

[0146] In the initial injection phase of the gas injection well, the gas diffuses into the formation through the perforation at the bottom of the well, driving the crude oil within the formation towards the drainage radius of the predetermined number of newly developed production wells. The pressure generated during this stage propagates to the drainage radius of the production wells; therefore, production in the production wells during this stage does not affect gas injection into the injection wells. The area around the injection well in this stage can be divided into two regions: the gas sweep zone in the inner region and the pressure conduction zone in the outer region. Within the gas sweep zone, the injected gas has already displaced the crude oil to deeper formations. In the pressure conduction zone, the crude oil is displaced and compressed by the injected gas, resulting in pressure propagation. The overall flow in the sweep zone conforms to Darcy's law.

[0147] The acquisition module 402 is used to acquire the moment when the gas injection pressure wave front intersects with the oil drainage radius front of a set number of newly developed oil wells as the gas injection effective moment. Based on the key parameters before the gas injection effective moment and the key parameters after the gas injection effective moment, the bottom flow pressure of the gas injection well before the gas injection effective moment and the bottom flow pressure of the gas injection well after the gas injection effective moment are obtained from the original formation pressure through Darcy's law.

[0148] The key parameters before and after the effective time of gas injection were obtained by using electrical logging interpretation and coring experiments from exploration and appraisal wells to determine the distribution of oil layer thickness and physical properties within the region.

[0149] Among them, exploratory wells and appraisal wells are related to newly developed oil production wells. Exploratory wells are used to detect where there are oil production wells, while appraisal wells are used to determine whether the oil production well should be used for oil production.

[0150] The key parameters before the gas injection takes effect include: oil layer thickness before the gas injection takes effect, effective permeability before the gas injection takes effect, formation crude oil viscosity before the gas injection takes effect, volume factor before the gas injection takes effect, residual oil saturation before the gas injection takes effect, and water saturation before the gas injection takes effect.

[0151] The key parameters after the gas injection takes effect include: oil layer thickness, effective permeability, formation crude oil viscosity, volume factor, residual oil saturation, water saturation, bottom hole flowing pressure, and daily oil production of the oil well.

[0152] In one embodiment, the injection pressure of the injection well is dynamically changing. The distribution of oil layer thickness and physical properties is obtained through exploratory and appraisal wells within the region, and gas-liquid phase permeability experimental parameters are obtained through laboratory experiments. Based on formation seepage theory, the nodal method is used to calculate the wellhead injection pressure. The wellhead injection pressure calculation includes two stages: before and after the injection becomes effective. Because the influencing factors differ in these two stages, the calculation methods differ. Furthermore, the relationship between exploratory and appraisal wells and newly developed oil wells is crucial; exploratory and appraisal wells are used to determine whether an oil well should be used for production.

[0153] Before the injection well becomes effective, the area surrounding it can be considered as an infinitely distant formation. The injected gas flows through the formation pores, primarily influenced by geological factors and the well's operating regime. These geological factors include: the thickness of the perforated oil layer, porosity, permeability, and crude oil viscosity; the well's operating regime includes: injection rate and injection time. Thus, key parameters before the injection becomes effective are obtained.

[0154] After the gas injection takes effect, the injected gas drives crude oil to flow to the production well, replenishing the energy in the near-wellbore zone. Following this stage, the gas injection pressure is affected not only by geological factors and the operating conditions of the gas injection well, but also by the operating conditions of the production well, such as the daily oil production and the bottom hole production pressure differential. The operating conditions of the production well include the daily fluid production and the bottom hole production pressure differential. Thus, key parameters after the gas injection takes effect are obtained.

[0155] The calculation module 403 is used to calculate the wellhead pressure of the gas injection well before the gas injection effect time and the wellhead pressure of the gas injection well after the gas injection effect time, respectively, from the bottom flow pressure of the gas injection well before the gas injection effect time and the bottom flow pressure of the gas injection well after the gas injection effect time.

[0156] Before the gas injection takes effect:

[0157] The formula for calculating the wellhead flowing pressure of the gas injection well before the effective time of gas injection is as follows:

[0158] p whi =p wfi -p G +p f -p v (18)

[0159] Where, p whi p represents the wellhead flowing pressure before the gas injection takes effect. wfi p represents the bottom-hole flowing pressure of the gas injection well before the injection takes effect. G p represents the gravity of the gas in the injection wellbore. f p represents the frictional resistance pressure drop. v This indicates the kinetic energy pressure drop.

[0160] In one embodiment, nitrogen is injected into the injection wellbore. The gas flows in a single phase, and the pressure change during the flow is divided into three parts: the pressure drop caused by the gas gravity in the injection wellbore, the pressure drop caused by frictional resistance, and the pressure drop caused by kinetic energy. For the injection well, the gas is injected downward from the wellhead to the bottom of the well, and the injection process satisfies formula (18).

[0161] The formula for calculating the bottom-hole flowing pressure of the gas injection well before the effective time of gas injection is as follows:

[0162]

[0163] Where, p wfi p represents the bottom-hole flowing pressure of the gas injection well before the injection takes effect. r q represents the original formation pressure before the gas injection takes effect. g Indicates the gas injection rate, μ g B represents the viscosity of gas under formation conditions. g k represents the volume coefficient of the injected gas. g h represents the effective gas permeability. g Indicates the gas injection depth, r g The radius of the precursor displacement, r, indicates the moment when the gas injection takes effect. w This indicates the diameter of the gas injection well. Where p... r μ, B, k, h, r g 、r w All are known, r g The flow pressure at the bottom of the injection well before the effect is obtained by multiplying the flow velocity of the injected gas at any depth in the wellbore by the injection time. Substituting the above parameter values ​​into formula (2) yields the bottom flow pressure of the injection well before the effect is achieved.

[0164] in,

[0165] p G =ρ m gHsinθ (20)

[0166]

[0167]

[0168] Among them, P G p represents the weight of the gas in the wellbore before the injection becomes effective. f p represents the pressure drop due to frictional resistance before the gas injection takes effect. v ρ represents the kinetic energy pressure drop before the gas injection takes effect. m H represents density, θ represents injection depth, and f represents well inclination angle. m d represents the coefficient of frictional resistance, and v represents the differential. m d represents the flow velocity of the injected gas at any depth in the wellbore. z This indicates the position of each step in the wellbore.

[0169] Where, ρ m The density was obtained from a table; H was obtained from geological data; θ was obtained from drilling data; and f... m v is obtained by iterative calculation. m Based on the drilling data, the gas gravity in the injection wellbore, the frictional resistance pressure drop, and the kinetic energy pressure drop are calculated respectively. Then, the above parameter values ​​and the bottom flow pressure of the injection well before the effect are substituted into formula (18) to obtain the wellhead flow pressure of the injection well before the effect.

[0170] The gravity of the wellbore gas, the frictional resistance pressure drop, and the kinetic energy pressure drop are obtained using an iterative method based on the fourth-order Runge-Kutta method.

[0171] After the gas injection takes effect:

[0172] The bottom-hole flowing pressure of the gas injection well after the gas injection becomes effective is obtained based on the bottom-hole flowing pressure of the oil production well after the gas injection becomes effective.

[0173] The formula for calculating the bottom-hole flowing pressure of the gas injection well after the gas injection takes effect is as follows:

[0174]

[0175] Where, p wfo This indicates the bottom hole flowing pressure (p) of the oil well after the gas injection takes effect. wfi1 The bottom-hole flowing pressure (r) of the injection well indicates the moment when the gas injection takes effect. g1 The displacement radius of the gas injection well is indicated by the time the gas injection becomes effective; q0 represents the oil drainage rate after the time the gas injection becomes effective, in μ. o B0 represents the crude oil viscosity under formation conditions, k0 represents the crude oil volume coefficient after the gas injection takes effect, h0 represents the crude oil effective permeability after the gas injection takes effect, r0 represents the oil layer thickness, and r0 represents the oil well drainage radius after the gas injection takes effect.

[0176] The derivation of formula (23) is based on Darcy's formula. The bottom-hole flowing pressure of the gas injection well and the bottom-hole flowing pressure of the oil production well after the gas injection takes effect are calculated respectively. The formula is transformed as follows:

[0177] The formula for calculating the bottom-hole flowing pressure of the gas injection well after the gas injection takes effect is as follows:

[0178]

[0179] After the gas injection takes effect, the bottom flow pressure of the gas injection well, p i This indicates the formation pressure after the gas injection takes effect.

[0180] The formula for calculating the bottom hole flowing pressure of the oil well after the gas injection takes effect is as follows:

[0181]

[0182] The oil well drainage radius after the gas injection takes effect is obtained by measuring the difference between the injection-production distance and the displacement radius.

[0183] When the gas injection takes effect, the formation pressure after the gas injection takes effect is obtained by combining the above two equations (24) and (25) to obtain the bottom hole flowing pressure formula (23) of the oil well after the gas injection takes effect. In formula (23), only r o unknown,

[0184] Where L represents the distance between the gas injection well and the oil production well.

[0185] (b1) The gas injection displacement radius is obtained by considering the injected gas sweep volume, gas injection working time of the injection well, formation parameters and gas injection volume;

[0186] The formation parameters include: oil layer thickness, porosity, residual oil saturation, and bound water saturation.

[0187] The formula for calculating the injected gas wave volume is as follows:

[0188]

[0189] Where V represents the injected gas sweep volume, and h represents the oil layer thickness. S represents porosity. or S represents the residual oil saturation. wc This indicates the degree of bound water saturation.

[0190] The formula for calculating the cumulative gas injection volume is as follows:

[0191] Q g =q g .t g (27)

[0192] Among them, Q g q represents the cumulative gas injection volume. g Indicates the injection rate after the gas injection takes effect, t g This indicates the working time for gas injection in the injection well.

[0193] Since the cumulative injection volume is equal to the injected gas wave volume, combined with formulas (26) and (27), the injection displacement radius can be obtained as follows:

[0194]

[0195] (b2) Based on the gas injection displacement radius, the gas injection effective time t0 is obtained;

[0196] (b3) Obtain the oil drainage radius of the oil well after the gas injection takes effect by the difference between the distance between the gas injection well and the production well and the displacement radius, and by the time the gas injection takes effect.

[0197] Similarly, the oil drainage radius of an oil production well is:

[0198]

[0199] At the critical point where gas injection takes effect, the following conditions are met:

[0200] r o +r g1 =L (30)

[0201] Where L represents the distance between the gas injection well and the oil production well.

[0202] The combined formulas (28)-(30) are used to obtain the gas injection displacement radius, the gas injection effective time t0, and the oil well drainage radius after the gas injection effective time.

[0203] The formula for calculating the wellhead flowing pressure after the gas injection takes effect is as follows:

[0204] p wi1 =p wfi1 -p G +p f -p v (31)

[0205] Where, p whi1 This indicates the wellhead flowing pressure (p) after the gas injection takes effect. wfi1 This indicates the bottom flow pressure of the injection well after the injection takes effect.

[0206] Once the gas injection takes effect, the formation pressure becomes uniform.

[0207] in,

[0208] p G =ρ m gHsinθ (32)

[0209]

[0210]

[0211] The gravity of the wellbore gas after the effective injection time, the frictional resistance pressure drop after the effective injection time, and the kinetic energy pressure drop after the effective injection time are all obtained by iterative method using the fourth-order Runge-Kutta method, which is the same as the method used before the effective injection time.

[0212] All of the above-mentioned optional technical solutions can be combined in any way to form the optional embodiments of this application, and will not be described in detail here.

[0213] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0214] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A method for predicting gas injection pressure in newly developed oil fields, characterized in that, include: S1. Select a set range and deploy a gas injection well at a preset distance from a set number of newly developed oil production wells. Inject nitrogen into the gas injection well so that the nitrogen diffuses into the formation through the bottom perforation of the gas injection well, driving the crude oil in the formation to move towards the drainage radius area of ​​the set number of newly developed oil production wells. S2. The moment when the gas injection pressure wave front intersects with the drainage radius front of a set number of newly developed oil wells is taken as the gas injection effective moment. Based on the key parameters before and after the gas injection effective moment, the bottom flow pressure of the gas injection well before and after the gas injection effective moment is obtained from the original formation pressure using Darcy's law. S3. Calculate the wellhead pressure of the gas injection well before the effective time of gas injection and the wellhead pressure of the gas injection well after the effective time of gas injection, respectively.

2. The method for predicting gas injection pressure in newly developed oil fields according to claim 1, characterized in that, The key parameters before and after the effective gas injection point were obtained by interpreting electrical logging data from exploratory and appraisal wells and conducting coring experiments to determine the distribution of oil layer thickness and physical properties within the region. The key parameters before the gas injection takes effect include: oil layer thickness before the gas injection takes effect, effective permeability before the gas injection takes effect, formation crude oil viscosity before the gas injection takes effect, volume factor before the gas injection takes effect, residual oil saturation before the gas injection takes effect, and water saturation before the gas injection takes effect. The key parameters after the gas injection takes effect include: oil layer thickness, effective permeability, formation crude oil viscosity, volume factor, residual oil saturation, water saturation, bottom hole flowing pressure, and daily oil production of the oil well.

3. The method for predicting gas injection pressure in newly developed oil fields according to claim 1, characterized in that, The formula for calculating the wellhead flowing pressure of the gas injection well before the effective time of gas injection is as follows: p whi =p wfi -p G +p f -p v Where, p whi p represents the wellhead flowing pressure before the gas injection takes effect. wfi p represents the bottom-hole flowing pressure of the gas injection well before the injection takes effect. G p represents the weight of the gas in the injection wellbore. f p represents the frictional resistance pressure drop. v This indicates the kinetic energy pressure drop.

4. The method for predicting gas injection pressure in newly developed oil fields according to claim 3, characterized in that, The formula for calculating the bottom-hole flowing pressure of the gas injection well before the effective time of gas injection is as follows: Where, p wfi p represents the bottom-hole flowing pressure of the gas injection well before the injection takes effect. r q represents the original formation pressure before the gas injection takes effect. g Indicates the gas injection rate, μ g B represents the viscosity of gas under formation conditions. g k represents the volume coefficient of the injected gas. g h represents the effective gas permeability. g Indicates the gas injection depth, r g The radius of the precursor displacement, r, indicates the moment when the gas injection takes effect. w Indicates the diameter of the gas injection well; p G =ρ m gHsinθ Among them, P G p represents the weight of the gas in the wellbore before the injection becomes effective. f p represents the pressure drop due to frictional resistance before the gas injection takes effect. v ρ represents the kinetic energy pressure drop before the gas injection takes effect. m H represents density, θ represents injection depth, and f represents well inclination angle. m d represents the coefficient of frictional resistance, and v represents the differential. m d represents the flow velocity of the injected gas at any depth in the wellbore. z This indicates the position of each step in the wellbore.

5. The method for predicting gas injection pressure in newly developed oil fields according to claim 1, characterized in that, The gravity of the wellbore gas, the frictional resistance pressure drop, and the kinetic energy pressure drop are obtained using an iterative method based on the fourth-order Runge-Kutta method.

6. The method for predicting gas injection pressure in newly developed oil fields according to claim 1, characterized in that, The bottom-hole flowing pressure of the gas injection well after the gas injection becomes effective is obtained based on the bottom-hole flowing pressure of the oil production well after the gas injection becomes effective.

7. The method for predicting gas injection pressure in newly developed oil fields according to claim 1, characterized in that, The formula for calculating the bottom-hole flowing pressure of the gas injection well after the gas injection takes effect is as follows: Where, p wfo This indicates the bottom hole flowing pressure (p) of the oil well after the gas injection takes effect. wfi1 The bottom-hole flowing pressure (r) of the injection well indicates the moment when the gas injection takes effect. g1 The displacement radius of the gas injection well is indicated by the time the gas injection becomes effective; q0 represents the oil drainage rate after the time the gas injection becomes effective, in μ. o B0 represents the crude oil viscosity under formation conditions, k0 represents the crude oil volume coefficient after the gas injection takes effect, h0 represents the crude oil effective permeability after the gas injection takes effect, r0 represents the oil layer thickness, and r0 represents the oil well drainage radius after the gas injection takes effect.

8. The method for predicting gas injection pressure in newly developed oil fields according to claim 7, characterized in that, The formula for calculating the bottom hole flowing pressure of the oil well after the gas injection takes effect is as follows: Where, p i This indicates the original formation pressure after the gas injection takes effect.

9. The method for predicting gas injection pressure in newly developed oil fields according to claim 7, characterized in that, The determination of the well drainage radius after the gas injection takes effect includes: The injection displacement radius is obtained by considering the injected gas sweep volume, the gas injection working time of the injection well, formation parameters, and the gas injection volume. Based on the aforementioned gas displacement radius, the time for gas injection to take effect is obtained; The oil drainage radius of the production well after the gas injection takes effect is obtained by using the difference between the distance between the gas injection well and the production well and the displacement radius, as well as by using the time it takes for the gas injection to take effect.

10. The method for predicting gas injection pressure in newly developed oil fields according to claim 7, characterized in that, The formula for calculating the wellhead flowing pressure after the gas injection takes effect is as follows: p whi1 =p wfi1 -p G +p f -p v Where, p whi1 This indicates the wellhead flowing pressure (p) after the gas injection takes effect. wfi1 This indicates the bottom-hole flowing pressure of the injection well after the injection takes effect, where, p G =ρ m gHsinθ 11. A system for implementing the method for predicting gas injection pressure in a newly developed oil field according to any one of claims 1-10, characterized in that, include: Select module (401), obtain module (402), and calculate module (403). The selection module (401) is used to select a set range and deploy a gas injection well at a preset distance from a set number of newly developed oil production wells, inject nitrogen into the gas injection well, so that the nitrogen diffuses into the formation through the bottom perforation position of the gas injection well, driving the crude oil in the formation to move towards the oil drainage radius area of ​​the set number of newly developed oil production wells. The acquisition module (402) is used to acquire the moment when the gas injection pressure wave front intersects with the oil drainage radius front of a set number of newly developed oil wells as the gas injection effective moment. Based on the key parameters before the gas injection effective moment and the key parameters after the gas injection effective moment, the bottom flow pressure of the gas injection well before the gas injection effective moment and the bottom flow pressure of the gas injection well after the gas injection effective moment are obtained from the original formation pressure through Darcy's law. The calculation module (403) is used to calculate the wellhead pressure of the gas injection well before the gas injection effect time and the wellhead pressure of the gas injection well after the gas injection effect time, respectively, based on the bottom flow pressure of the gas injection well before the gas injection effect time and the bottom flow pressure of the gas injection well after the gas injection effect time.

12. The prediction system for gas injection pressure in newly developed oil fields according to claim 11, characterized in that, The formula for calculating the wellhead flowing pressure of the gas injection well before the effective time of gas injection is as follows: p whi =p wfi -p G +p f -p v Where, p whi p represents the wellhead flowing pressure before the gas injection takes effect. wfi p represents the bottom-hole flowing pressure of the gas injection well before the injection takes effect. G p represents the weight of the gas in the injection wellbore. f p represents the frictional resistance pressure drop. v This indicates the kinetic energy pressure drop.

13. The prediction system for gas injection pressure in newly developed oil fields according to claim 11, characterized in that, The formula for calculating the bottom-hole flowing pressure of the gas injection well after the gas injection takes effect is as follows: Where, p wfo This indicates the bottom hole flowing pressure (p) of the oil well after the gas injection takes effect. wfi1 q represents the bottom-hole flowing pressure of the injection well after the injection takes effect. g Indicates the injection rate after the injection takes effect, in μ. g B represents the viscosity of gas under formation conditions. g k represents the gas volume coefficient after the injection takes effect. g Indicates the effective gas permeability after the effect is achieved, h g Indicates the injection depth after the injection takes effect, r g1 The displacement radius of the gas injection well is indicated by the time the gas injection becomes effective; q0 represents the oil drainage rate after the time the gas injection becomes effective, in μ. o B0 represents the crude oil viscosity under formation conditions, k0 represents the crude oil volume factor after the gas injection takes effect, h0 represents the crude oil effective permeability after the gas injection takes effect, r0 represents the oil layer thickness, and r represents the well drainage radius after the gas injection takes effect. w Indicates the diameter of the gas injection well.

14. The prediction system for gas injection pressure in newly developed oil fields according to claim 13, characterized in that, The formula for calculating the bottom hole flowing pressure of the oil well after the gas injection takes effect is as follows:

15. The gas injection pressure prediction system for newly developed oil fields according to claim 13, characterized in that, The formula for calculating the wellhead flowing pressure after the gas injection takes effect is as follows: p whi1 =p wfi1 -p G +p f -p v Where, p whi1 This indicates the wellhead flowing pressure (p) after the gas injection takes effect. wfi1 This indicates the bottom-hole flowing pressure of the injection well after the injection takes effect, where, p G =ρ m gHsinθ Among them, P G p represents the weight of the gas in the wellbore before the injection becomes effective. f p represents the pressure drop due to frictional resistance before the gas injection takes effect. v ρ represents the kinetic energy pressure drop before the gas injection takes effect. m H represents density, θ represents injection depth, and f represents well inclination angle. m d represents the coefficient of frictional resistance, and v represents the differential. m d represents the flow velocity of the injected gas at any depth in the wellbore. z This indicates the position of each step in the wellbore.

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