CN103775058B - Method for determining heat loss of shaft - Google Patents

Abstract

The invention provides a method for determining heat loss of a shaft, which comprises the following steps: reading in calculation parameters; calculating the thermal resistance values of all parts in the shaft in the radius direction and the upper limit value of the radial heat loss of the shaft unit; setting an initial value of the radial heat loss of the shaft unit; setting a first current value and a second current value; circularly calculating the current total thermal resistance value and the third current value, and updating the first current value and the second current value until the updated second current value is greater than or equal to the depth of the shaft; and summing the obtained third current values, and determining the summation result as the heat loss of the wellbore. The method adopted by the invention is used for calculating the heat loss of the shaft sections, so that the calculated heat loss result of the shaft is more accurate.

Description

Method for determining heat loss of shaft

Technical Field

The invention relates to the field of steam injection oil extraction, in particular to a method for determining heat loss of a shaft.

Background

In the oil extraction technology, crude oil is extracted directly from an oil well, and the viscosities of different crude oils are generally different. Wherein, the thickened oil refers to crude oil with higher content of asphaltene and colloid and higher viscosity. Practice has shown that the viscosity of thick oil generally changes significantly with temperature. The viscosity of the thick oil generally decreases along with the increase of the temperature, so the exploitation and the transportation of the super thick oil generally need to reduce the viscosity of the super thick oil by utilizing a thermal means, for example, the exploitation of the super thick oil is carried out by adopting a steam injection exploitation technology. At present, the main mode of thick oil recovery is steam injection recovery, which comprises two modes of steam huff and puff and steam flooding, the heat of injected wet saturated steam is utilized to heat an oil layer, the viscosity of thick oil is reduced, and the thick oil is conveyed to the ground in a self-spraying and mechanical lifting mode.

The profile structure of the steam-injected oil recovery wellbore is shown in fig. 1, after steam is injected into the wellbore, heat convection and heat radiation exist in the annular space layer due to heat conduction among the oil pipe wall, the casing wall and the cement sheath in the wellbore, so that heat in the wellbore flows to the stratum along the radial direction, and heat loss is generated.

In the steam injection oil extraction process, the heat loss in the shaft directly determines whether steam or saturated water is injected into the bottom of the shaft, so that the heating effect is determined. The Liaohe oil field is used as a main thickened oil production base in China, the energy consumption of a steam injection system accounts for 80% of the total energy consumption of thickened oil production, and the energy consumption of heat loss of steam injection pipelines such as a shaft and the like accounts for 26.36% of the total energy consumption of thickened oil production. Therefore, the calculation of the heat loss of the shaft has very important significance, and measures for reducing the heat loss can be provided based on the calculated heat loss of the shaft, so that the heating effect of steam injection oil recovery is improved.

In the calculation method of the heat loss of the shaft, the calculation method of the prior art is as follows: setting a total heat transfer coefficient, calculating parameters such as oil pipe temperature and casing pipe temperature according to the total heat transfer coefficient, correcting the total heat transfer coefficient according to the calculated parameters such as the oil pipe temperature and the casing pipe temperature, continuously iterating, finishing iteration when the total heat transfer coefficients obtained by two adjacent calculations are close, determining the total heat transfer coefficient obtained by the last calculation as the total heat transfer coefficient of a shaft, and calculating the heat loss of the shaft according to the determined total heat transfer coefficient of the shaft. However, the above calculation method continues to calculate the whole depth of the wellbore as a whole, and does not consider that factors such as steam temperature in the wellbore and the like change with the depth of the wellbore, and accordingly, wellbore heat loss also changes, so the calculation result of the above method is inaccurate. Meanwhile, after the calculation method is iterated for three or four times, the phenomenon that the temperature of the outer wall of the heat insulation layer is lower than the temperature of the inner wall of the sleeve can occur, and if the phenomenon does not accord with the natural law, iteration is not converged, and iterative calculation cannot be continued.

Disclosure of Invention

The invention aims to provide a method for determining heat loss of a shaft, so as to realize accurate calculation result of the heat loss of the shaft.

The invention provides a method for determining heat loss of a shaft, which comprises the following steps:

s1: reading in calculation parameters;

s2: calculating the thermal resistance values of all layers except the annular part in the radius direction in the shaft and the upper limit value of the radial heat loss of the shaft unit according to the parameters;

s3: setting an initial value of the radial heat loss of the shaft unit according to the upper limit value;

s4: taking the initial value as a first current value; taking the position of the wellhead calculated to the position of the preset step length below the position of the wellhead calculated as a second current value;

s5: calculating a current total thermal resistance value based on the first current value, the second current value, and the thermal resistance values of the layers except for the ring space portion in S2;

s6: determining a third current value based on the current total thermal resistance value; the third current value is the wellbore unit radial heat loss of the second current value;

s7: updating the first current value in S6 to the third current value; increasing the second current value in S6 by a predetermined step size, and updating the second current value to the value increased by the predetermined step size;

s8: circularly executing S5-S7 until the updated second current value in S7 is greater than or equal to the depth of the shaft;

s9: and summing the third current values obtained by executing the S6 each time, wherein the summation result is determined as the heat loss of the wellbore.

Calculating the current total thermal resistance value in the step S5 specifically includes:

s51: calculating the average temperature, average pressure and average temperature of saturated steam of the second current value;

s52: calculating a temperature at a radially different position from the second current value based on the result of S51;

s53: calculating a second current value of annulus natural convection heat transfer coefficient and annulus radiation heat transfer coefficient according to the result of S52;

s54: calculating an annulus heat convection heat resistance value according to the annulus natural convection heat transfer coefficient and the annulus radiation heat transfer coefficient;

s55: and calculating the current total thermal resistance value according to the annular thermal convection thermal resistance value and the thermal resistance values of all layers except the annular part in the S2.

In S51, the average formation temperature, the average pressure, and the average saturated steam temperature at the second current value are calculated as follows:

the calculation formula of the average temperature of the stratum is as follows:

T_e＝(b_k+b_k-1)/2

in the above formula, b_kThe temperature of the stratum at the k-time step below the wellhead is shown, and the unit is centigrade; b_k-1The formation temperature at (k-1) times the step below the wellhead is expressed in degrees centigrade; b_kThe calculation formula of (2) is as follows:

b_k＝(b_k-1+a1×dl)

in the above formula, b_k-1The formation temperature at (k-1) times the step below the wellhead is expressed in degrees centigrade; a1 is the ground temperature gradient with the unit of centigrade per meter; dl represents a predetermined step size in meters; initial value of formation temperature b₀Is the surface temperature in degrees centigrade;

the calculation formula of the average pressure is as follows:

$\frac{dp}{dl}=\frac{\[{\mathrm{\ρ}}_l{H}_l+{\mathrm{\ρ}}_g(1-H_l)\]gsin\mathrm{\θ}+\frac{\mathrm{\λ}Gv}{2DA}}{1-\frac{\[{\mathrm{\ρ}}_l{H}_l+{\mathrm{\ρ}}_g(1-H_l)\]{\mathrm{vv}}_{sg}}{p}}$

in the above formula, p represents the pressure of the mixture, pascal; rho_lExpressed as liquid phase density, kg/cubic meter; rho_gExpressed as gas phase density, kg/cubic meter; h_lExpressed as liquid holdup, cubic meters per cubic meter; g represents the acceleration of gravity, meter per square second; theta represents the included angle between the pipeline and the horizontal direction, and degree; λ represents the on-way resistance coefficient of two-phase flow, and the unit is 1; g represents the mass flow of the mixture, kg/sec; v represents the flow velocity of the mixture, m/s; v. of_sgRepresenting the reduced velocity of the gas phase, m/s; d represents the diameter of the pipeline, meter; a represents the sectional area of the pipeline, square meter;

the calculation method of the average temperature of the saturated steam comprises the following steps:

T_s＝195.94P^0.225-17.8

in the above formula, T_sRepresents the saturated steam average temperature, degrees centigrade; p represents the mean pressure, pascal.

Calculating a temperature at a radially different location at a second current value in said S52, including:

temperature T of inner wall of oil pipe_tiComprises the following steps: t is_ti＝T_s-R₁Q_k/dl

Temperature T of outer wall of oil pipe_toComprises the following steps: t is_to＝T_ti-R₂Q_k/dl

Temperature T of inner wall of heat insulation pipe_iComprises the following steps: t is_i＝T_to-R₃Q_k/dl

Outer wall temperature T of heat-insulating pipe_oComprises the following steps: t is_o＝T_i-R₄Q_k/dl

Cement sheath external temperature T_hComprises the following steps: t is_h＝T_e+R₈Q_k/dl

Temperature T of outer wall of casing_coComprises the following steps: t is_co＝T_h+R₇Q_k/dl

Temperature T of inner wall of casing_ciComprises the following steps: t is_ci＝T_co+R₆Q_k/dl

In the above formula, the units of the temperature are all centigrade degrees; r₁Represents the thermal convection resistance, R, between the steam and the inner wall of the oil pipe₂Represents the heat conduction resistance value, R, between the inner wall and the outer wall of the oil pipe₃Indicating the thermal conductivity resistance, R, of the insulating layer₄Heat transfer resistance value, R, representing wall of heat insulation pipe₆Representing the thermal resistance, R, of the heat conduction of the casing wall₇Indicating the thermal conductivity resistance, R, of the cement sheath₈Representing the thermal conductivity resistivity of the formation, which is the thermal resistivity calculated at S2The units are (meter Kelvin)/watt; q_kThe first current value, kilojoules/hour, at which S5 was executed for the kth cycle; t is_eRepresents formation temperature, in known quantities, in degrees celsius; t is_sIs the injected steam temperature, in known amounts, in degrees celsius; dl represents a predetermined step size, meter.

Calculating a second current value of annulus natural convection heat transfer coefficient and annulus radiative heat transfer coefficient in S53, including:

annular radiation heat transfer coefficient h_rThe calculation formula of (a) is as follows:

$h_r={\mathrm{\δF}}_{tci}(T_o^{*2}+T_{ci}^{*2})+(T_o^{*}+T_{ci}^{*})$

wherein,

$T_o^{*}=T_o+273.15,T_{ci}^{*}=T_{ci}+273.15$

in the above formula: t is_oThe temperature of the outer wall of the heat insulation pipe is expressed in centigrade; t is_ciIs the temperature of the inner wall of the casing in centigrade and is Stefan-Boltzmann constant, the value of which is 2.189 × 10^-8Watt/(meter kelvin); f_tciThe effective coefficient of radiation from the outer wall surface of the oil pipe or the heat insulation pipe to the inner wall surface of the casing is calculated by the following formula:

$\frac{1}{F_{tci}}=\frac{1}{{\mathrm{\ϵ}}_o}+\frac{r_{to}}{r_{ci}}(\frac{1}{{\mathrm{\ϵ}}_{ci}}-1)$

wherein,_othe blackness of the outer wall of the heat insulation pipe is a known amount;_cithe blackness of the inner wall of the sleeve is a known quantity; r is_toThe radius of the outer wall of the oil pipe is meter, and the known quantity is obtained by measurement; r is_ciRadius of the inner wall of the casing, meter, known quantity obtained by measurement;

annular natural convection heat transfer coefficient h_cThe calculation formula of (a) is as follows:

$h_c=\frac{0.049{\left(G_r{P}_r\right)}^{0.33}{P}_r^{0.074}{K}_{ha}}{r_{o}ln\frac{r_{ci}}{r_o}}$

in the above formula, G_rIs the Grashof number; p_rIs the prandtl number; g_rAnd P_rThe calculation formula of (a) is as follows:

$G_r=\frac{{(r_{ci}-r_o)}^3{\mathrm{g\ρ}}_{an}^2\mathrm{\β}(T_o-T_{ci})}{U_{an}^2}$

$P_r=\frac{C_{an}{U}_{an}}{K_{ha}}$

wherein: r is_oIs the radius of the outer wall of the insulated pipe, meter is a known measured quantity, β is the gas volume expansion coefficient of annular fluid, K_haThe thermal conductivity of the annular fluid, watts/(meter-kelvin); g is gravity acceleration, meter/square second; rho_anFor annular fluid at mean temperature T_an(iv) density of units per cubic meter; u shape_anFor annular fluid at mean temperature T_anViscosity in millipascal-seconds; mean temperature T_an＝(T_s+T_ci) 2, kelvin; c_anFor annular flowBody at average temperature T_anHeat capacity in joules/(cubic meters kelvin).

Calculating an annular heat convection thermal resistance value in the step S54 according to the following formula:

$R_5=\frac{1}{2\mathrm{\π}(h_c+h_r)r_o}$

wherein R is₅Represents the annular heat convection heat resistance value, (meter Kelvin)/tile; h is_rThe annular radiation heat transfer coefficient is W/(square meter Kelvin); h is_cThe heat transfer coefficient of natural convection of the annular space is watt/(square meter Kelvin); r is_oThe radius of the outer wall of the heat insulation pipe is meter.

In the step S55, the current total thermal resistance value is calculated by the following method:

R＝R₁+R₂+R₃+R₄+R₅+R₆+R₇+R₈

in the above formula, R represents the current total thermal resistance value, R₁Represents the thermal convection resistance, R, between the steam and the inner wall of the oil pipe₂Represents the heat conduction resistance value, R, between the inner wall and the outer wall of the oil pipe₃Indicating the thermal conductivity resistance, R, of the insulating layer₄Heat transfer resistance value, R, representing wall of heat insulation pipe₅Indicates the annular heat convection thermal resistance, R₆Representing the thermal resistance, R, of the heat conduction of the casing wall₇Indicating the thermal conductivity resistance, R, of the cement sheath₈Representing a thermal conductivity resistivity of the formation; the units are (meter)Kelvin)/Watt.

The step S6 of determining a third current value based on the current total thermal resistance value specifically includes:

s61: calculating the current heat loss according to the current total heat resistance value;

s62: judging whether the current heat loss is larger than the upper limit value or not; if the current heat loss is smaller than or equal to the upper limit value, taking the current heat loss as a third current value; if the current heat loss is greater than the upper limit value, the initial value in S3 is taken as a third current value.

And S61, calculating the current heat loss according to the current total thermal resistance value, wherein the calculation method specifically comprises the following steps:

$Q_k^{\′}=\frac{T_s-T_h}{R}dl$

in the above formula, Q'_kRepresents the current heat loss, T, calculated when the k-th cycle was executed at S61_sThe temperature of injected steam is in centigrade; t is_hThe temperature outside the cement sheath is set in centigrade; r represents the current total thermal resistance value, (meter Kelvin)/watt; dl represents a predetermined step size, meter.

The upper limit value of the radial heat loss of the shaft unit is obtained by calculation through the following formula:

$Q_m=\frac{T_s-T_e}{R_1+R_2+R_3+R_4+R_5+R_6+R_7+R_8}dl$

in the above formula, dl represents a predetermined step size, meter; r₁Represents the thermal convection resistance, R, between the steam and the inner wall of the oil pipe₂Represents the heat conduction resistance value, R, between the inner wall and the outer wall of the oil pipe₃Indicating the thermal conductivity resistance, R, of the insulating layer₄Heat transfer resistance value, R, representing wall of heat insulation pipe₅Indicates the annular heat convection thermal resistance, R₆Representing the thermal resistance, R, of the heat conduction of the casing wall₇Indicating the thermal conductivity resistance, R, of the cement sheath₈The heat conduction resistance of the stratum is expressed in the unit of (meter-Kelvin)/watt; t is_sFor injection of steam temperature, T_eThe formation temperature is given in degrees celsius.

The selection range of the preset step length is as follows: 0< dl < h; dl represents a predetermined step size, meter; h represents the depth of the wellbore below the surface, in meters.

The value of the initial value of the radial heat loss of the shaft unit is 0-Q_mSaid Q is_mIs the upper limit value of the wellbore unit radial heat loss calculated in S2.

The value of the initial value of the radial heat loss of the shaft unit is further selected to be 0.9Q_m。

According to the method for determining the heat loss of the shaft, the factors that the heat loss at different depths in the shaft is uneven are considered, and the heat loss of the shaft is calculated by adopting a sectional method, so that the calculated heat loss result of the shaft is more accurate; meanwhile, the radial heat flow heat loss of the shaft unit is used as an iteration variable, a numerical value slightly smaller than the upper limit value of the radial heat flow heat loss of the shaft unit is selected as an iteration initial value, the upper limit value of the radial heat flow heat loss of the shaft unit is also considered when the radial heat flow heat loss of the shaft unit is corrected, and therefore the phenomenon that the calculation is stopped after the total heat transfer coefficient with low precision is obtained through a few iterations in the conventional algorithm can be avoided, and the iteration process is converged.

Drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced below, it is obvious that the drawings in the following description are only some embodiments described in the present application, and for those skilled in the art, other drawings can be obtained according to the drawings without any creative effort.

FIG. 1 is a schematic representation of a wellbore profile configuration of the present application;

FIG. 2 is a flow chart of a wellbore heat loss determination method of the present application;

FIG. 3 is a flow chart of a method for calculating a current total thermal resistance value for a wellbore heat loss determination method of the present application;

FIG. 4 is a flow chart of a third current value determined in the wellbore heat loss determination method of the present application.

Detailed Description

In order to make those skilled in the art better understand the technical solutions in the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are only a part of the embodiments of the present application, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

FIG. 1 is a schematic cross-sectional view of a steam-injected production wellbore. As shown in figure 1, an inner pipe, a heat insulation layer, an outer pipe, an annular space layer, a casing, a cement sheath and a stratum are arranged from the center of the shaft along the radial direction outwards in sequence. The temperature is highest after steam injection at the central location, and the temperature of the layers radially outward from the central location of the wellbore also increases due to heat transfer between the layers in the wellbore. The temperature decreases gradually from the center of the wellbore to the edge in the radial direction of the wellbore.

FIG. 2 is a flow chart of a wellbore heat loss determination method of the present application. As shown in fig. 2, the method of the present invention comprises:

s1: reading in calculation parameters.

During oil recovery, parameters related to heat loss from the wellbore mainly include three aspects:

the first is the structure of the wellbore and its associated physical property parameters, such as: radius r of inner wall of oil pipe_ti(ii) a Radius r of outer wall of oil pipe_to(ii) a Inner diameter r of casing_ci(ii) a Radius r of inner wall of heat insulation pipe_i(ii) a Radius r of outer wall of heat-insulating pipe_o(ii) a Outer diameter r of sleeve_co(ii) a Thermal conductivity K of the thermal insulation layer_ins(ii) a Coefficient of thermal conductivity K of cement sheath_cem(ii) a Radius of borehole r_h(ii) a And the like.

The second is the thermophysical parameters of the formation, such as: surface temperature T_e(ii) a Formation heat conductivity coefficient K_e(ii) a And the like.

The third is the wellhead injection parameters, such as: pressure of injected steam P_s(ii) a Steam-injection dryness X_i(ii) a Steam injection rate M_s(ii) a Steam injection time t; and the like.

In this step, for example, the above-mentioned parameters relating to the heat loss of the borehole are read in.

S2: and calculating the thermal resistance values of all layers except the annular part in the radius direction in the shaft and the upper limit value of the radial heat loss of the shaft unit according to the parameters.

The method comprises the following steps of firstly calculating the thermal conduction resistance or the thermal convection resistance of each part of a shaft. Specifically, because the heat loss of the shaft is caused by heat convection and heat conduction, according to the steam injection oil extraction principle, the calculation method of the thermal resistance value of each part in the shaft is as follows in sequence:

thermal convection resistance R between steam and inner wall of oil pipe₁The calculation formula of (2) is as follows:

$R_1=\frac{1}{2{\mathrm{\&pi;h}}_f{r}_{ti}}---\left(1\right)$

wherein h is_fIs the water film heat transfer coefficient, watts/(square meter kelvin), a known quantity; r is_tiIs the radius of the inner wall of the tubing, meter, which is a known quantity obtained by measurement.

Thermal conduction resistance R between inner wall and outer wall of oil pipe₂The calculation formula of (2) is as follows:

$R_2=\frac{1}{2{\mathrm{\&pi;K}}_{tub}}ln\frac{r_{to}}{r_{ti}}---\left(2\right)$

wherein, K_tubThe heat conductivity coefficient of the oil pipe is shown as watt/(square meter Kelvin), which is a known quantity; r is_toIs the tubing outer wall radius, meter, is a known quantity measured.

Thermal conduction resistance R of thermal insulation layer₃The calculation formula of (2) is as follows:

$R_3=\frac{1}{2{\mathrm{\&pi;K}}_{ins}}ln\frac{r_i}{r_{to}}---\left(3\right)$

wherein, K_insThe thermal conductivity of the thermal insulation layer, watt/(square meter Kelvin), is a known quantity; r is_iRadius of the inner wall of the insulated pipe, meter, known quantity obtained for measurement。

Thermal conduction resistance R of heat insulation pipe wall₄The calculation formula of (2) is as follows:

$R_4=\frac{1}{2{\mathrm{\&pi;K}}_{tub}}ln\frac{r_o}{r_i}---\left(4\right)$

wherein r is_oIs the radius of the outer wall of the insulated pipe, meter, which is a known quantity measured.

Thermal annular convection resistance R₅The calculation formula of (2) is as follows:

$R_5=\frac{1}{2\mathrm{\&pi;}(h_c+h_r)r_o}---\left(5\right)$

wherein h is_rIs the annular radiation heat transfer coefficient, watt/(square meter Kerr)Text); h is_cThe heat transfer coefficient is the natural convection heat transfer coefficient of the annular space, and is watt/(square meter Kelvin); both of the above parameters are unknown.

Thermal conduction resistance R of casing wall₆The calculation formula of (2) is as follows:

$R_6=\frac{1}{2{\mathrm{\&pi;K}}_{cas}}ln\frac{r_{co}}{r_{ci}}---\left(6\right)$

wherein, K_casThe casing thermal conductivity, watts/(square meter kelvin), is a known quantity; r is_ciRadius of the inner wall of the casing, meter, known quantity obtained by measurement; r is_coIs the casing outer wall radius, meter, a known quantity measured.

Thermal conduction resistance R of cement sheath₇The calculation formula of (2) is as follows:

$R_7=\frac{1}{2{\mathrm{\&pi;K}}_{cem}}ln\frac{r_h}{r_{co}}---\left(7\right)$

wherein, K_cemThe cement sheath thermal conductivity is given as the known value of Watt/(square meter Kelvin); r is_hIs the borehole radius, meter, a known quantity obtained by measurement.

Thermal conduction resistance R of the formation₈The calculation formula of (2) is as follows:

$R_8=\frac{f\left(t\right)}{2{\mathrm{\&pi;K}}_e}---\left(8\right)$

wherein, K_eIs the formation thermal conductivity, watts/(square meter kelvin), a known quantity;

in equation (8), f (t) is a time-varying heat transfer function, and is calculated by the following equation:

$f\left(t\right)=ln\left(\frac{2\sqrt{at}}{r_h}\right)-0.29---\left(9\right)$

in the formula (9), a is the average heat dissipation coefficient of the stratum, and is a known quantity in square meters per day; t represents the steam injection time and day, and is taken according to the actual days in working; r is_hIs the borehole radius, meter, a known quantity obtained by measurement.

In the above equations (1) to (8), only the equation (5) is used to calculate the required parametric annular radiation heat transfer coefficient h_rAnd annular natural convection heat transfer coefficient h_cIs unknown and cannot be directly calculated to obtain R₅The rest can be directly calculated to obtain the resistance value of the thermal resistance.

In the radial direction of a shaft, setting the radial heat loss of a shaft unit on the preset step length dl as Q, wherein the unit is kilojoule/hour, and according to the principle of steam injection oil extraction, the Q value of each part in the radial direction of the shaft satisfies the following formula:

$Q=\frac{T_s-T_{ti}}{R_1}dl=\frac{T_{ti}-T_{to}}{R_2}dl=\frac{T_{to}-T_i}{R_3}dl=\frac{T_i-T_o}{R_4}dl=\frac{T_o-T_{ci}}{R_5}dl=\frac{T_{ci}-T_{co}}{R_6}dl=\frac{T_{co}-T_h}{R_7}dl=\frac{T_h-T_e}{R_8}dl---\left(10\right)$

in the formula (10), T_sIs the temperature of the injected steam; t is_tiThe temperature of the inner wall of the oil pipe is used as the temperature of the inner wall of the oil pipe; t is_toThe temperature of the outer wall of the oil pipe is used; t is_iThe temperature of the inner wall of the heat insulation pipe; t is_oThe temperature of the outer wall of the heat insulation pipe; t is_hThe temperature outside the cement sheath; t is_coIs the temperature of the outer wall of the casing; t is_ciIs the temperature of the inner wall of the casing; t is_eIs the formation temperature; t is_sAnd T_eThe measured values are known quantities, and the rest are parameters needing to be calculated, and the unit is centigrade degrees; dl represents a predetermined step size, meter.

From equation (10), the following two equations can be obtained:

T_o＝T_s-(R₁+R₂+R₃+R₄)Q/dl (11)

T_ci＝T_e+(R₆+R₇+R₈+R₈)Q/dl (12)

the heat loss at the well head is generally at a maximum, if there is no heat loss in the annulus, i.e. R₅If the value of (2) is 0, the radial heat loss of the shaft unit at the well opening is the maximum radial heat loss of the shaft unit, and can be used as the upper limit value of the radial heat loss of the shaft unit. According to the principle, the radial heat loss of the shaft unit at the wellhead is calculated by using the formula (11) and the formula (12) and is used as an upper limit value Q of the radial heat loss of the shaft unit_mThe calculation formula is as follows:

$Q_m=\frac{T_s-T_e}{R_1+R_2+R_3+R_4+R_5+R_6+R_7+R_8}dl---\left(13\right)$

s3: and setting an initial value of the radial heat loss of the shaft unit according to the upper limit value.

Q for initial value of radial heat loss of shaft unit₀And (4) showing. The value of the radial heat loss of the shaft unit is less than the upper limit value Q_mThus initial value Q of the wellbore unit radial heat loss₀Is 0 to Q_mAccording to experimental experience, Q can be taken₀Has a value of 0.9Q_m. Using k and Q, respectively_kRepresenting the wellbore unit radial heat loss after the kth cycle and the kth iteration.

S4: taking the initial value as a first current value; the wellhead position to its lower predetermined step position will be calculated as the second current value.

The Q is added₀As the first nonce value. Position of well head part₀Is represented by₀0. The predetermined step size is expressed in dl, the range of dlComprises the following steps: 0<dl<h. The dl may be taken to be 10 meters. Will calculate the wellhead position l₀To its lower predetermined step dl position as the second current value. l_kAfter representing the kth iteration, the radial heat loss Q of the shaft unit is calculated_kThe longitudinal position of (a).

S5: the current total thermal resistance value is calculated based on the first current value, the second current value, and the thermal resistance values of the layers other than the ring space portion in S2.

From S2, the thermal resistance value R required for calculating the total thermal resistance value₁-R₈In (3), R also needs to be calculated₅Calculating R₅Then, the total thermal resistance can be further calculated. Fig. 3 is a flowchart of calculating the current total thermal resistance value in the wellbore heat loss determination method according to the present application, and as shown in fig. 3, this step calculates the current total thermal resistance value based on the first current value, the second current value, and the thermal resistance values of the layers other than the annular space portion in S2. The method specifically comprises the following steps:

s51: the average temperature of the formation, the average pressure, and the average temperature of the saturated steam are calculated for the second current value.

To calculate the second current value of the temperature at each radial location in the wellbore, the second current value of the average temperature of saturated steam in the wellbore, the average temperature of the formation, and the average pressure are calculated.

The second current value of the formation average temperature T_eThe following formula was used for calculation:

T_e＝(b_k+b_k-₁)/2 (14)

in the above formula, b_kThe temperature of the stratum at the k-time step below the wellhead is shown, and the unit is centigrade; b is b_k-1The formation temperature at (k-1) times the step below the wellhead is expressed in degrees centigrade; b_kThe calculation formula of (2) is as follows:

b_k＝(b_k-1+a1×dl) (15)

in the above formula, b_k-1Represents the position below the well head at (k-1) times the step lengthLayer temperature in degrees celsius; a1 is the ground temperature gradient with the unit of centigrade per meter; dl represents a predetermined step size in meters; initial value of formation temperature b₀Is the surface temperature in degrees centigrade;

the second current value of the formation mean pressure P is calculated as follows:

$\frac{dp}{dl}=\frac{\&lsqb;{\mathrm{\&rho;}}_l{H}_l+{\mathrm{\&rho;}}_g(1-H_l)\&rsqb;gsin\mathrm{\&theta;}+\frac{\mathrm{\&lambda;}Gv}{2DA}}{1-\frac{\&lsqb;{\mathrm{\&rho;}}_l{H}_l+{\mathrm{\&rho;}}_g(1-H_l)\&rsqb;{\mathrm{vv}}_{sg}}{p}}---\left(16\right)$

in formula (16), p represents the pressure (absolute) of the mixture, pascal; rho_lExpressed as liquid phase density, kg/cubic meter; rho_gExpressed as gas phase density, kg/cubic meter; h_lExpressed as liquid holdup, cubic meters per cubic meter; g represents the acceleration of gravity, meter per square second; theta represents the included angle between the pipeline and the horizontal direction, and degree; λ represents the on-way resistance coefficient of two-phase flow, and the unit is 1; g represents the mass flow of the mixture, kg/sec; v represents the flow velocity of the mixture, m/s; v. of_sgRepresenting the reduced velocity of the gas phase, m/s; d represents the diameter of the pipeline, meter; a represents the cross-sectional area of the pipe, square meter. The values of the above physical quantities are known.

Calculating the average temperature T of saturated steam in the shaft at the second current value according to the average pressure_sThe calculation formula is as follows:

T_s＝195.94P^0.225-17.8 (17)

in equation (17), P represents the formation mean pressure at the second current value, pascal.

S52: the temperature at the radially different position of the second current value is calculated from the result of S51.

First, R is calculated according to S2₁、R₂、R₃、R₄、R₆、R₇、R₈And a first current value to calculate a temperature at a radially different location from the second current value, the thermometer at each locationThe calculation method comprises the following steps:

temperature T of inner wall of oil pipe_tiComprises the following steps: t is_ti＝T_s-R₁Q_k/dl (18)

Temperature T of outer wall of oil pipe_toComprises the following steps: t is_to＝T_ti-R₂Q_k/dl (19)

Temperature T of inner wall of heat insulation pipe_iComprises the following steps: t is_i＝T_to-R₃Q_k/dl (20)

Outer wall temperature T of heat-insulating pipe_oComprises the following steps: t is_o＝T_i-R₄Q_k/dl (21)

Cement sheath external temperature T_hComprises the following steps: t is_h＝T_e+R₈Q_k/dl (22)

Temperature T of outer wall of casing_coComprises the following steps: t is_co＝T_h+R₇Q_k/dl (23)

Temperature T of inner wall of casing_ciComprises the following steps: t is_ci＝T_co+R₆Q_k/dl (24)

In the above formulas (18) to (24), the units of temperature are all degrees centigrade; r₁、R₂、R₃、R₄、R₆、R₇、R₈The calculated thermal resistance for S2 is in units of (meter Kelvin)/Watt; q_kThe first current value in the k-th cycle.

S53: and calculating the natural convection heat transfer coefficient and the radiation heat transfer coefficient of the annulus according to the result of the S52.

Since there is both thermal convection and thermal radiation in the annular space layer, the thermal resistance R of the annular space layer is calculated₅When in use, the temperature T of the outer wall of the heat insulation pipe is required to be firstly determined_oAnd temperature T of inner wall of casing_ciTo calculate the heat transfer coefficient h of natural convection of annular space_cAnd annular heat transfer coefficient h_r。

Annular radiation heat transfer coefficient h_rThe calculation formula of (a) is as follows:

$h_r={\mathrm{\&delta;F}}_{tci}(T_o^{*2}+T_{ci}^{*2})+(T_o^{*}+T_{ci}^{*})---\left(25\right)$

wherein,

$T_o^{*}=T_o+273.15,T_{ci}^{*}=T_{ci}+273.15---\left(26\right)$

$\frac{1}{F_{tci}}=\frac{1}{{\mathrm{\&epsiv;}}_o}+\frac{r_{to}}{r_{ci}}(\frac{1}{{\mathrm{\&epsiv;}}_{ci}}-1)---\left(27\right)$

in the formula (25), the constant is Stefan-Boltzmann constant, and the value is 2.189 × 10^-8Watt/(meter kelvin); f_tciThe effective coefficient of radiation from the outer wall surface of the oil pipe or the heat insulation pipe to the inner wall surface of the casing is calculated by a formula (27);_othe blackness of the outer wall of the heat insulation pipe is a known amount;_ciis the blackness of the inner wall of the casing, in known amounts. Generally, the ratio of the radiation power of a gray body to that of a black body at the same temperature is defined as the blackness of an object, or the emissivity of an object, at a certain temperature, and has a unit of 1.

In the formula (26), the reaction mixture is,represents the temperature of the outer wall of the heat-insulating pipe under the Kelvin temperature scale,indicating the temperature of the inner wall of the casing on the kelvin scale.

Annular natural convection heat transfer coefficient h_cThe calculation formula of (a) is as follows:

$h_c=\frac{0.049{\left(G_r{P}_r\right)}^{0.33}{P}_r^{0.074}{K}_{ha}}{r_{o}ln\frac{r_{ci}}{r_o}}---\left(28\right)$

in the above formula, G_rIs the Grashof number; p_rIs the Prandtl number. G_rAnd P_rThe calculation formula of (a) is as follows:

$G_r=\frac{{(r_{ci}-r_o)}^3{\mathrm{g\&rho;}}_{an}^2\mathrm{\&beta;}(T_o-T_{ci})}{U_{an}^2}---\left(29\right)$

$P_r=\frac{C_{an}{U}_{an}}{K_{ha}}---\left(30\right)$

in the formula(28) In (1) to (30): k_haThe thermal conductivity of the annular fluid, watts/(meter-kelvin); g is gravity acceleration, meter/square second; rho_anFor annular fluid at mean temperature T_an(iv) density of units per cubic meter; u shape_anFor annular fluid at mean temperature T_anViscosity in millipascal-seconds; mean temperature T_an＝(T_s+T_ci) 2, kelvin; c_anFor annular fluid at mean temperature T_anThe specific heat capacity is Joule/(cubic meter Kelvin), β is the gas volume expansion coefficient of the annular fluid, and in the parameters, g is a known quantity, and other parameters are measured or calculated and are known quantities.

S54: and calculating the annulus heat convection thermal resistance value according to the annulus natural convection heat transfer coefficient and the annulus radiation heat transfer coefficient.

The annular natural convection heat transfer coefficient h calculated by S52_cAnd annular heat transfer coefficient h_rSubstituting the formula (5) for calculation to obtain the annular thermal convection resistance R₅。

S55: and calculating the current total thermal resistance value according to the annular thermal convection thermal resistance value and the thermal resistance values of all layers except the annular part in the S2.

Annular heat convection resistance R calculated according to S54₅And the other partial thermal resistances calculated at S2, the total thermal resistance R of the wellbore can be calculated at the second current value.

Specifically, the calculation formula of the total thermal resistance R is:

R＝R₁+R₂+R₃+R₄+R₅+R₆+R₇+R₈(31)

s6: a third current value is determined based on the current total thermal resistance value.

The total thermal resistance value calculated according to equation (31) may be used to determine the wellbore unit radial heat loss at the second current value, and the calculated wellbore unit radial heat loss as the third current value. Fig. 4 is a flowchart of determining a third current value in the method for determining wellbore heat loss according to the present application, as shown in fig. 4, the step specifically includes:

s61: and calculating the current heat loss according to the current total heat resistance value.

According to the basic thermodynamic heat transfer formula, the formula for calculating the heat loss according to the total thermal resistance value is as follows:

$Q_k^{\&prime;}=\frac{T_s-T_h}{R}dl---\left(32\right)$

in the formula (32), Q'_kRepresenting the calculated current heat loss, the numerator part representing the total temperature loss, and the denominator part representing the total thermal resistance; t is_sThe temperature of injected steam is in centigrade; t is_hThe temperature outside the cement sheath is given in degrees centigrade.

S62: judging whether the current heat loss is larger than the upper limit value or not; if the current heat loss is smaller than or equal to the upper limit value, taking the current heat loss as a third current value; if the current heat loss is greater than the upper limit value, the initial value in S3 is taken as a third current value.

This step is used to determine the current heat loss Q'_kWhether the heat loss is larger than the upper limit value Q of the radial heat loss of the shaft unit_mIf the current heat loss Q'_kLess than or equal to the upper limit value Q_mComparing the current heat loss Q'_kAs a third nonce; if the current heat loss Q'_kIf the value is larger than the upper limit value and does not conform to the natural law, the initial value in S3 is determinedThe start value being the third current value, i.e. Q_mAs the third nonce value.

S7: updating the first current value in S6 to the third current value; the second current value in S6 is increased by a predetermined step size, and the second current value is updated to the value after the predetermined step size is increased.

Updating the first current value in S6 to a third current value, Q'_kA value of (d); the second current value in S6 is increased by a predetermined step dl, i.e., (lk + dl), and the second current value is updated to the value after the predetermined step is added.

S8: and circularly executing S5-S7 until the updated second current value in S7 is greater than or equal to the depth of the shaft.

And circularly executing S5-S7, calculating the heat loss of the shaft with the length of dl each time of circular calculation, and when the updated second current value in S7 is greater than or equal to the depth of the shaft, indicating that all the lengths of the shaft are calculated, not performing circular calculation.

S9: and summing the third current values obtained by executing the S6 each time, wherein the summation result is determined as the heat loss of the wellbore.

After the loop calculation process is completely finished, the third current value obtained in the step S6 in each execution loop is summed, and the summation result is determined as the heat loss of the shaft.

According to the method for determining the heat loss of the shaft, the factors that the heat loss at different depths in the shaft is uneven are considered, and the heat loss of the shaft is calculated by adopting a sectional method, so that the calculated heat loss result of the shaft is more accurate; meanwhile, the radial heat flow heat loss of the shaft unit is used as an iteration variable, a numerical value slightly smaller than the upper limit value of the radial heat flow heat loss of the shaft unit is selected as an iteration initial value, the upper limit value of the radial heat flow heat loss of the shaft unit is also considered when the radial heat flow heat loss of the shaft unit is corrected, and therefore the phenomenon that the calculation is stopped after the total heat transfer coefficient with low precision is obtained through a few iterations in the conventional algorithm can be avoided, and the iteration process is converged.

While the present invention has been described with respect to the embodiments, those skilled in the art will appreciate that there are numerous variations and permutations of the present invention without departing from the spirit of the invention, and it is intended that the appended claims cover such variations and modifications as fall within the true spirit of the invention.

Claims (13)

1. A method of determining wellbore heat loss, comprising:

s1: reading in calculation parameters;

s2: calculating the thermal resistance values of all layers except the annular part in the radius direction in the shaft and the upper limit value of the radial heat loss of the shaft unit according to the parameters;

s3: setting an initial value of the radial heat loss of the shaft unit according to the upper limit value;

s4: taking the initial value as a first current value; taking the position of the wellhead calculated to the position of the preset step length below the position of the wellhead calculated as a second current value;

s5: calculating a current total thermal resistance value based on the first current value, the second current value, and the thermal resistance values of the layers except for the ring space portion in S2;

s6: determining a third current value based on the current total thermal resistance value; the third current value is the wellbore unit radial heat loss of the second current value;

s7: updating the first current value in S6 to the third current value; increasing the second current value in S6 by a predetermined step size, and updating the second current value to the value increased by the predetermined step size;

s8: circularly executing S5-S7 until the updated second current value in S7 is greater than or equal to the depth of the shaft;

s9: and summing the third current values obtained by executing the S6 each time, wherein the summation result is determined as the heat loss of the wellbore.

2. The method for determining wellbore heat loss according to claim 1, wherein the step of calculating the current total thermal resistance value in S5 specifically comprises:

s51: calculating the average temperature, average pressure and average temperature of saturated steam of the second current value;

s52: calculating a temperature at a radially different position from the second current value based on the result of S51;

s53: calculating a second current value of annulus natural convection heat transfer coefficient and annulus radiation heat transfer coefficient according to the result of S52;

s54: calculating an annulus heat convection heat resistance value according to the annulus natural convection heat transfer coefficient and the annulus radiation heat transfer coefficient;

s55: and calculating the current total thermal resistance value according to the annular thermal convection thermal resistance value and the thermal resistance values of all layers except the annular part in the S2.

3. The method for determining wellbore heat loss of claim 2, wherein the second current value of the average formation temperature, the average pressure, and the average saturated steam temperature is calculated in S51 as follows:

the calculation formula of the average temperature of the stratum is as follows:

T_e＝(b_k+b_k-1)/2

in the above formula, b_kThe temperature of the stratum at the k-time step below the wellhead is shown, and the unit is centigrade; b_k-1The formation temperature at (k-1) times the step below the wellhead is expressed in degrees centigrade; b_kThe calculation formula of (2) is as follows:

b_k＝(b_k-1+a1×dl)

in the above formula, b_k-1The formation temperature at (k-1) times the step below the wellhead is expressed in degrees centigrade; a1 is the ground temperature gradient with the unit of centigrade per meter; dl represents a predetermined step size in meters; initial value of formation temperature b₀Is the surface temperature in degrees centigrade;

the calculation formula of the average pressure is as follows:

in the above formula, p represents the pressure of the mixture, pascal; rho_lExpressed as liquid phase density, kg/cubic meter; rho_gExpressed as gas phase density, kg/cubic meter; h_lExpressed as liquid holdup, cubic meters per cubic meter; g represents the acceleration of gravity, meter per square second; theta represents the included angle between the pipeline and the horizontal direction, and degree; λ represents the on-way resistance coefficient of two-phase flow, and the unit is 1; g represents the mass flow of the mixture, kg/sec; v represents the flow velocity of the mixture, m/s; v. of_sgRepresenting the reduced velocity of the gas phase, m/s; d represents the diameter of the pipeline, meter; a represents the sectional area of the pipeline, square meter;

the calculation method of the average temperature of the saturated steam comprises the following steps:

T_s＝195.94P^0.225-17.8

in the above formula, T_sRepresents the saturated steam average temperature, degrees centigrade; p represents the mean pressure, pascal.

4. The method of claim 2, wherein calculating the temperature at the radially different location for the second current value in S52 comprises:

temperature T of inner wall of oil pipe_tiComprises the following steps: t is_ti＝T_s-R₁Q_k/dl

Temperature T of outer wall of oil pipe_toComprises the following steps: t is_to＝T_ti-R₂Q_k/dl

Temperature T of inner wall of heat insulation pipe_iComprises the following steps: t is_i＝T_to-R₃Q_k/dl

Outer wall temperature T of heat-insulating pipe_oComprises the following steps: t is_o＝T_i-R₄Q_k/dl

Cement sheath external temperature T_hComprises the following steps: t is_h＝T_e+R₈Q_k/dl

Temperature T of outer wall of casing_coComprises the following steps: t is_co＝T_h+R₇Q_k/dl

Temperature T of inner wall of casing_ciComprises the following steps: t is_ci＝T_co+R₆Q_k/dl

In the above formula, the units of the temperature are all centigrade degrees; r₁Represents the thermal convection resistance, R, between the steam and the inner wall of the oil pipe₂Represents the heat conduction resistance value, R, between the inner wall and the outer wall of the oil pipe₃Indicating the thermal conductivity resistance, R, of the insulating layer₄Heat transfer resistance value, R, representing wall of heat insulation pipe₆Representing the thermal resistance, R, of the heat conduction of the casing wall₇Indicating the thermal conductivity resistance, R, of the cement sheath₈Expressing the heat conduction thermal resistance values of the stratum, wherein the thermal resistance values are all calculated by S2 and the unit is (meter Kelvin)/watt; q_kThe first current value, kilojoules/hour, at which S5 was executed for the kth cycle; t is_eRepresents formation temperature, in known quantities, in degrees celsius; t is_sIs the injected steam temperature, in known amounts, in degrees celsius; dl represents a predetermined step size, meter.

5. The method for determining wellbore heat loss of claim 2, wherein calculating the second current value of annulus natural convection heat transfer coefficient and annulus radiation heat transfer coefficient in S53 comprises:

annular radiation heat transfer coefficient h_rThe calculation formula of (a) is as follows:

$h_r={\mathrm{\&delta;F}}_{tci}(T_o^{*2}+T_{ci}^{*2})+(T_o^{*}+T_{ci}^{*})$

wherein,

$T_o^{*}=T_o+273.15,T_{ci}^{*}=T_{ci}+273.15$

in the above formula: t is_oThe temperature of the outer wall of the heat insulation pipe is expressed in centigrade; t is_ciIs the temperature of the inner wall of the casing in centigrade and is Stefan-Boltzmann constant, the value of which is 2.189 × 10^-8Watt/(meter kelvin); f_tciThe effective coefficient of radiation from the outer wall surface of the oil pipe or the heat insulation pipe to the inner wall surface of the casing is calculated by the following formula:

$\frac{1}{F_{tci}}=\frac{1}{{\mathrm{\&epsiv;}}_o}+\frac{r_{to}}{r_{ci}}(\frac{1}{{\mathrm{\&epsiv;}}_{ci}}-1)$

wherein,_othe blackness of the outer wall of the heat insulation pipe is a known amount;_cithe blackness of the inner wall of the sleeve is a known quantity; r is_toThe radius of the outer wall of the oil pipe is meter, and the known quantity is obtained by measurement; r is_ciRadius of the inner wall of the casing, meter, known quantity obtained by measurement;

annular natural convection heat transfer coefficient h_cThe calculation formula of (a) is as follows:

$h_c=\frac{0.049{\left(G_r{P}_r\right)}^{0.33}{P}_r^{0.074}{K}_{ha}}{r_{o}ln\frac{r_{ci}}{r_o}}$

in the above formula, G_rIs the Grashof number; p_rIs the prandtl number; g_rAnd P_rThe calculation formula of (a) is as follows:

$G_r=\frac{{(r_{ci}-r_o)}^3{\mathrm{g\&rho;}}_{an}^2\mathrm{\&beta;}(T_o-T_{ci})}{U_{an}^2}$

$P_r=\frac{C_{an}{U}_{an}}{K_{ha}}$

wherein: r is_oIs the radius of the outer wall of the insulated pipe, meter is a known measured quantity, β is the gas volume expansion coefficient of annular fluid, K_haThe thermal conductivity of the annular fluid, watts/(meter-kelvin); g is gravity acceleration, meter/square second; rho_anFor annular fluid at mean temperature T_an(iv) density of units per cubic meter; u shape_anFor annular fluid at mean temperature T_anViscosity in millipascal-seconds; mean temperature T_an＝(T_s+T_ci) 2, kelvin; c_anFor annular fluid at mean temperature T_anHeat capacity in joules/(cubic meters kelvin).

6. The method of claim 2, wherein the calculating of the annulus thermal convection resistance at S54 is performed according to the following equation:

$R_5=\frac{1}{2\mathrm{\&pi;}(h_c+h_r)r_o}$

wherein R is₅Represents the annular heat convection heat resistance value, (meter Kelvin)/tile; h is_rThe annular radiation heat transfer coefficient is W/(square meter Kelvin); h is_cThe heat transfer coefficient of natural convection of the annular space is watt/(square meter Kelvin); r is_oThe radius of the outer wall of the heat insulation pipe is meter.

7. The method for determining heat loss from a wellbore of claim 2, wherein the step of calculating the current total thermal resistance value in S55 comprises:

R＝R₁+R₂+R₃+R₄+R₅+R₆+R₇+R₈

in the above formula, R represents the current total thermal resistance value, R₁Represents the thermal convection resistance, R, between the steam and the inner wall of the oil pipe₂Represents the heat conduction resistance value, R, between the inner wall and the outer wall of the oil pipe₃Indicating the thermal conductivity resistance, R, of the insulating layer₄Heat transfer resistance value, R, representing wall of heat insulation pipe₅Indicates the annular heat convection thermal resistance, R₆Representing the thermal resistance, R, of the heat conduction of the casing wall₇Indicating the thermal conductivity resistance, R, of the cement sheath₈Representing a thermal conductivity resistivity of the formation; the units are (meter Kelvin)/watt.

8. The method for determining wellbore heat loss according to claim 1, wherein the step S6 of determining the third current value based on the current total thermal resistance value specifically comprises:

s61: calculating the current heat loss according to the current total heat resistance value;

s62: judging whether the current heat loss is larger than the upper limit value or not; if the current heat loss is smaller than or equal to the upper limit value, taking the current heat loss as a third current value; if the current heat loss is greater than the upper limit value, the initial value in S3 is taken as a third current value.

9. The method for determining heat loss from a wellbore of claim 8, wherein the step S61 is to calculate the current heat loss according to the current total thermal resistance value by:

$Q_k^{\&prime;}=\frac{T_s-T_h}{R}dl$

in the above formula, Q'_kRepresents the current heat loss, T, calculated when the k-th cycle was executed at S61_sThe temperature of injected steam is in centigrade; t is_hThe temperature outside the cement sheath is set in centigrade; r represents the current total thermal resistance value, (meter Kelvin)/watt; dl represents a predetermined step size, meter.

10. The method of claim 1, wherein the upper radial heat loss limit of the wellbore unit is calculated by the following equation:

$Q_m=\frac{T_s-T_e}{R_1+R_2+R_3+R_4+R_5+R_6+R_7+R_8}dl$

in the above formula, dl represents a predetermined step size, meter; r₁Represents the thermal convection resistance, R, between the steam and the inner wall of the oil pipe₂Represents the heat conduction resistance value, R, between the inner wall and the outer wall of the oil pipe₃Indicating the thermal conductivity resistance, R, of the insulating layer₄Heat transfer resistance value, R, representing wall of heat insulation pipe₅Indicates the annular heat convection thermal resistance, R₆Representing the thermal resistance, R, of the heat conduction of the casing wall₇Indicating the thermal conductivity resistance, R, of the cement sheath₈The heat conduction resistance of the stratum is expressed in the unit of (meter-Kelvin)/watt; t is_sFor injection of steam temperature, T_eThe formation temperature is given in degrees celsius.

11. The method of claim 1, wherein the predetermined step size is selected from the group consisting of: 0< dl < h; dl represents a predetermined step size, meter; h represents the depth of the wellbore below the surface, in meters.

12. The method of claim 1, wherein the initial value of the radial heat loss of the wellbore unit is in the range of 0-Q_mSaid Q is_mIs the upper limit value of the wellbore unit radial heat loss calculated in S2.

13. The method of claim 12, wherein the wellbore unit is radially oriented to determine heat loss from the wellboreThe value of the initial value of the heat loss is further selected to be 0.9Q_m。