CN106951666B - Method for calculating temperature field of drilling shaft of marine natural gas hydrate layer - Google Patents
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- 238000005553 drilling Methods 0.000 title claims abstract description 101
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Abstract
本发明公开了一种海洋天然气水合物层钻井井筒温度场计算方法，包括：（1）获取海洋天然气水合物层钻井参数和初始条件；（2）进行空间节点划分；（3）建立海洋天然气水合物层钻井井筒温度场计算模型；（4）钻柱中井口处的流体温度即为计算时刻的钻井液注入温度T p(0) (n)，环空中井口处的流体温度T a(0) (n)即为从环空中返出流体的温度，假设T a(0) (n)的值；（5）根据节点i处钻柱中流体温度T p(i) (n)和环空中流体温度T a(i) (n)，计算下一节点i+1处的井筒温度，得到钻柱中井底处的流体温度T p(k) (n)和环空中井底处的流体温度T a(k) (n)；（6）检验是否满足计算误差。本发明原理可靠，便于操作，能够为判断海洋天然气水合物层稳定状态、钻井液流变性、计算井筒流动参数，进而保障海洋钻井施工安全提供理论依据。
The invention discloses a method for calculating the temperature field of a drilling wellbore of a marine natural gas hydrate layer, comprising: (1) obtaining drilling parameters and initial conditions of the marine natural gas hydrate layer; (2) dividing space nodes; (4) The fluid temperature at the wellhead in the drill string is the drilling fluid injection temperature T p (0) ( n ) at the time of calculation, and the fluid temperature at the wellhead in the annulus is T a (0) ( n ) is the temperature of the fluid returning from the annulus, assuming the value of T a (0) ( n ) ; (5) according to the fluid temperature T p ( i ) ( n ) in the drill string at node i and the fluid temperature in the annulus temperature T a ( i ) ( n ) , calculate the wellbore temperature at the next node i + 1, and obtain the fluid temperature T p ( k ) ( n ) at the bottom hole in the drill string and the fluid temperature at the bottom hole in the annulus T a ( k ) ( n ) ; (6) Check whether the calculation error is satisfied. The invention is reliable in principle and easy to operate, and can provide a theoretical basis for judging the stable state of the marine natural gas hydrate layer, the rheological properties of drilling fluid, and calculating wellbore flow parameters, thereby ensuring the safety of marine drilling construction.
Description
Technical Field
The invention belongs to the technical field of marine drilling, and particularly relates to a marine natural gas hydrate layer drilling technical method, in particular to a method for calculating a temperature field of a wellbore of a marine natural gas hydrate layer drilling.
Background
The natural gas hydrate is a crystalline compound which stably exists under the conditions of low temperature and high pressure, has extremely rich resource amount, is mainly distributed in land permafrost zones and submarine sediments at the periphery of land margins, and has the marine natural gas hydrate resource amount which is about more than 100 times of that of the land permafrost zones. The marine natural gas hydrate layer is in a low-temperature and high-pressure environment on the seabed, and a stable occurrence state is kept. With the development of the ocean oil and gas drilling technology, the number of ocean drilling wells is increasing, and in the drilling process of the ocean drilling wells, the drilling fluids sequentially drill to a stratum, a natural gas hydrate layer and the stratum, wherein the drilling fluids are injected into a shaft from a well opening, namely a sea level, in a drill string, flow over a mud line, namely a well section where seawater is located, flow to the position of the mud line, namely a seabed, flow out of the annular space from a drill bit through the well section below the mud line, namely the well section where the stratum is located, and then flow back to the well opening through the annular space, and flow out of the shaft from the well opening.
However, when the marine drilling well meets the natural gas hydrate layer, the drill bit is positioned in the marine natural gas hydrate layer, and the temperature of the drilling fluid flowing in the shaft is high, so that the original temperature field of the marine natural gas hydrate layer can be changed, the stable state of the hydrate is further influenced, the decomposition and gasification of the hydrate are caused, and the safety of the drilling well construction is seriously threatened. On the other hand, in the process of drilling the ocean natural gas hydrate layer, hydrate drilling cutting particles entering the annular space of the well barrel are different from rock debris particles in the conventional ocean drilling, and are decomposed due to the fact that the temperature of the well barrel rises and the pressure of the well barrel is reduced in the process of upward return of the annular drilling fluid, the decomposition and heat absorption effects of the hydrate drilling cutting particles further influence the temperature change of the well barrel, the temperature field of the whole well barrel is changed accordingly, and the rheological property of the drilling fluid and the flowing parameter change of the well barrel are further influenced. Therefore, accurate calculation of the temperature field of the drilling shaft of the marine natural gas hydrate layer has important guiding significance for judging the stable state of the marine natural gas hydrate layer, the rheological property of drilling fluid and calculating the flow parameters of the drilling shaft so as to ensure the construction safety of marine drilling.
At present, the temperature field of a drilling shaft of an ocean natural gas hydrate layer is researched less at home and abroad, the existing research mainly aims at the temperature field of the drilling shaft in the ocean drilling process, and the temperature field of the drilling shaft, which is generated when hydrate drilling cutting particles in annular space return upwards along with drilling fluid and are decomposed when drilling on the ocean natural gas hydrate layer, is not researched. The patent CN103226641A discloses a deepwater gas-liquid two-phase flow circulation temperature and pressure coupling calculation method, which comprises the steps of calculating the node temperature and pressure data of drilling fluid in a drill string and drilling fluid in an annulus in an iterative manner according to the sequence of the drilling fluid in the drill string and the drilling fluid in the annulus, and finally obtaining a simulation result of the temperature and pressure of a wellbore of the deepwater gas-liquid two-phase flow, wherein the method can be applied to the calculation of the temperature field of the wellbore in the process of marine drilling, but cannot accurately reflect the temperature field of the wellbore in the process of drilling the; patent CN102943620A discloses a pressure-controlled drilling method based on drilling annulus wellbore multiphase flow calculation, in the solution of wellbore annulus multiphase flow control equation set, the required wellbore temperature is reflected, but the specific solution method is not proposed, and cannot be directly applied to the calculation of wellbore temperature field in the drilling of marine natural gas hydrate layer. Therefore, a shaft temperature field calculation method for drilling a sea natural gas hydrate layer is urgently needed, the influence of decomposition of hydrate drilling cutting particles in an annular space is considered when the sea natural gas hydrate layer is drilled, and theoretical basis is provided for judging the stable state of the sea natural gas hydrate layer, the rheological property of drilling fluid and calculating shaft flow parameters.
Disclosure of Invention
The invention aims to provide a method for calculating a temperature field of a drilling shaft of an ocean natural gas hydrate layer, which has reliable principle and convenient operation, can provide theoretical basis for judging the stable state of the ocean natural gas hydrate layer, the rheological property of drilling fluid and calculating the flow parameter of the drilling shaft so as to ensure the safety of ocean drilling construction, and has wide market prospect.
In order to achieve the technical purpose, the invention adopts the following technical scheme.
Acquiring drilling parameters and initial conditions of an ocean natural gas hydrate layer, and dividing space nodes on the basis; considering the influence of decomposition and heat absorption of natural gas hydrate drilling cutting particles, establishing a temperature field calculation model of a drilling shaft of an ocean natural gas hydrate layer to obtain temperature change in a calculation grid; then, calculating the temperature of the shaft at the next node according to the temperature at the known node, and iteratively calculating the temperature of the shaft according to the sequence from the well head to the well bottom of the nodes in the drill string and the annulus at the same time until the temperature of the well bottom meets the calculation error, and ending the iteration; and the wellbore temperatures at all the nodes obtained by iterative calculation are the wellbore temperature field of the drilling of the marine natural gas hydrate layer.
A method for calculating a temperature field of a drilling shaft of an ocean natural gas hydrate layer sequentially comprises the following steps:
(1) acquiring drilling parameters and initial conditions of the marine natural gas hydrate layer according to drilling design and reservoir parameters, wherein the drilling parameters comprise: well bore structure, drilling tool combination, pumping parameters, seawater depth, seawater temperature, formation temperature, hydrate abundance in reservoir, mechanical drilling rate and well depth, wherein the initial conditions comprise: calculating the drilling fluid injection temperature T at a timep(0) (n)Annulus wellhead pressure pa(0) (n)。
(2) Performing space node division, wherein according to the drilling parameters of the marine natural gas hydrate layer in the step (1), the space domain is the whole shaft, and the axial serial numbers of the nodes are sequentially increased from 0 from the well mouth to the well bottom; the node serial number at the well head is 0, the serial number of any calculation node in the well shaft is represented by i, the serial number of the next calculation node is represented by i +1, and the serial number of the node at the well bottom is k.
(3) According to the theory of heat transfer and the law of conservation of energy, a calculation model of the temperature field of the drilling shaft of the marine natural gas hydrate layer is established, and the expression of temperature change in a calculation grid is as follows (Gaoyonghai, Sunpojiang, WangShi, and the like; calculation and analysis of the temperature field of the deep water drilling shaft [ J ]. Chinese university of Petroleum journal (Nature science edition), 32 (2003): 58-62):
in the drill string:
encircling in the air:
well sections above the mudline (seabed), i.e. i.DELTA.h < HseaThe method comprises the following steps:
the well section below the mud line (at the seabed), i.e. i.delta.h is more than or equal to HseaThe method comprises the following steps:
in the formula: i is a computing node;
n is the calculation time;
delta h is the length of a calculation grid, and is set to be 1m in the calculation;
Hseais the depth of the seawater, m;
ΔTp(i) (n)、ΔTa(i) (n)calculating the temperature change in the grid, K, in the drill string and the annulus respectively;
ρp(i)、ρa(i)calculating the density of the mixed fluid at the node i in the drill string and the annulus respectively in kg/m3；
vp(i)、va(i)Calculating the flow velocity m/s of the mixed fluid at the node i in the drill string and the annulus respectively;
cp(i)、ca(i)calculating the specific heat capacity of the mixed fluid at a node i in a drill column and a well casing in an annulus respectively, wherein J/(kg & K);
Dpi、Dpo、Dri、Dcirespectively the inner diameter of a drill column, the outer diameter of the drill column, the inner diameter of a marine riser and the inner diameter of a casing pipe, m;
Qap(i)heat exchange between the annulus and fluid at a computing node i in the drill string, W;
Qwa(i)、Qsa(i)respectively calculating heat exchange between fluids at a node i in the sea water and the annulus and between the stratum and the annulus, and W;
Qfp(i)、Qfa(i)calculating heat, W, generated by flow friction at node i in the drill string and in the annulus, respectively;
Qh(i)the heat absorbed by the hydrate decomposition at node i, W, is calculated in the annulus.
(4) When the well mouth position i is equal to 0, the fluid temperature at the well mouth position in the drill column is the drilling fluid injection temperature T at the moment of calculationp(0) (n)Known parameters according to the initial conditions in step (1); and the fluid temperature T at the wellhead in the annulusa(0) (n)That is, the temperature of the fluid returning from the annulus is unknown, assuming Ta(0) (n)Set the assumed range: t is more than or equal to 273Ka(0) (n)≤Tp(0) (n)。
(5) According to the sequence of the nodes in the drill string and the annular space from the well head to the well bottom at the same time, according to the temperature T of the fluid in the drill string at the node ip(i) (n)And annular space intermediate fluid temperature Ta(i) (n)And calculating the temperature of the well bore at the next node i + 1:
in the formula: t isp(i) (n)、Ta(i) (n)Respectively calculating the temperature K of fluid in the drill column and in the annular space at the node i;
Tp(i+1) (n)、Ta(i+1) (n)the fluid temperature, K, in the drill string and in the annulus at node i +1 is calculated, respectively.
According to the formulas (4) and (5), the fluid temperature T at the bottom of the well in the drill string is obtained from the iterative calculation of the well head to the node k at the bottom of the wellp(k) (n)And the fluid temperature T at the bottom of the well in the annulusa(k) (n)。
(6) And (5) according to the fluid temperature at the bottom of the well in the drill string and the annular space obtained by iterative calculation, comparing whether the calculation error is met:
in the formula: t isp(k) (n)Is the fluid temperature at the bottom of the well in the drill string, K;
Ta(k) (n)is the fluid temperature at the bottom of the well in the annulus, K;
and gamma is the calculation error of the fluid temperature at the bottom of the well, and 1K is taken.
And (5) if the formula (6) is satisfied, satisfying the calculation error, and iteratively calculating the fluid temperature in the drill string and in the annulus at all the nodes through the step (5) to obtain the well drilling shaft temperature field of the marine natural gas hydrate layer. If the formula (6) does not work, the calculation error is not satisfied, and the temperature T of the fluid at the well head in the annular space in the step (4) needs to be calculateda(0) (n)And (5) re-assuming, and repeating the step (5) iterative calculation again until the formula (6) is established.
In the step (3), heat exchange Q is carried out between the annular space and the fluid at the calculation node i in the drill stringap(i)Heat exchange Q between seawater and fluid at calculation node i in annuluswa(i)Computing the heat exchange Q between fluids at the node i in the stratum and the annulussa(i)Heat Q generated by flow friction at calculation node i in the drill stringfp(i)Calculating heat Q generated by flow friction at node i in annulusfa(i)(Z.M.Wang,X.N.Hao,X.Q.Wang et al.Numerical simulation on deepwater drillingwellbore temperature and pressure distribution[J]Petroleum Science and technology,28(2010): 911-h(i)(E.D.Sloana,F.Fleyfelb.Hydrate dissociation enthalpy and guest size[J]The calculation method for FluidPhase Equilibria,76(1992): 123-:
heat exchange Q between fluid at calculation node i in annular space and drill stringap(i)Is calculated as follows
Heat exchange Q between seawater and fluid at calculation node i in annuluswa(i)Is calculated as follows
Heat exchange Q between fluids at calculation node i in stratum and annulussa(i)Is calculated as follows
The comprehensive heat exchange coefficient U of the annular fluid and the stratum in the formula (9) at the calculation node isa(i)Is calculated as follows
Calculating heat Q generated by flow friction at node i in drill stringfp(i)Is calculated as follows
Calculating heat Q generated by flow friction at node i in annulusfa(i)Is calculated as follows
Calculating heat Q absorbed by decomposition of hydrate at node i in annulush(i)Is calculated as follows
In formulae (10) to (13):
Tw(i) (n)、Ts(i) (n)respectively calculating the seawater temperature and the formation temperature at the node i, and K;
Dro、Dco、Dcso、Dcsithe outer diameter of the riser, the outer diameter of the casing, the outer diameter of the cement sheath and the inner diameter of the cement sheath are m;
αf1(i)、αf2(i)、αf3(i)calculating the forced convection heat transfer coefficient at the node i on the inner surface of the drill string, the inner surface of the marine riser and the inner surface of the casing pipe respectively, wherein W/(m) is2·K)；
αm1(i)、αm2(i)The convective heat transfer coefficients of the outer winding surfaces at the position of a calculated node i on the outer surface of the drill string and the outer surface of the marine riser are respectively W/(m)2·K)；
λp(i)、λr(i)、λc(i)、λcs(i)、λs(i)Respectively calculating the heat conductivity coefficients W/(m.K) of the drill string, the marine riser, the casing, the cement sheath and the stratum at a node i;
Usa(i)is the comprehensive heat exchange coefficient W/(m) of the annular air fluid and the stratum at a calculation node i2·K)；
TDIs a transient heat transfer function without dimension;
vp(i)、va(i)respectively calculating the flow velocity m/s of the mixed fluid in the drill column and the annular space at a node i;
mp(i)、ma(i)respectively calculating the mass flow rate kg/s of the mixed fluid in the drill column and the annular space at the node i;
Daothe calculation node is the inner diameter D of the riser in the well section above the mud lineriThe well section below the mud line is the inner diameter D of the casingci，m；
ZgeqThe natural gas compression factor is a natural gas compression factor at a node under the conditions of temperature and pressure in the annular space, and has no dimension;
r is a general gas constant, J/(mol. K);
peq(i) (n)and calculating the natural gas hydrate phase equilibrium pressure Pa at the node i.
Compared with the prior art, the invention has the following remarkable advantages:
(1) according to the method, the shaft temperature field is obtained through iterative calculation according to the sequence from the wellhead to the shaft bottom of the nodes in the drill column and the annulus, the calculation is convenient and fast, the error is small, and the shaft temperature field can be accurately and quickly calculated.
(2) The method can realize the calculation of the temperature field of the shaft of the marine natural gas hydrate layer during the drilling of the marine natural gas hydrate layer, and provides a theoretical basis for judging the stable state of the marine natural gas hydrate layer, the rheological property of the drilling fluid and calculating the flow parameters of the shaft so as to ensure the construction safety of marine drilling.
Drawings
FIG. 1 is a schematic diagram of heat exchange in a wellbore drilled in a marine natural gas hydrate formation.
FIG. 2 is a graph of example calculated results of a drilling wellbore temperature field for a marine natural gas hydrate formation.
Detailed Description
The present invention will be described in further detail below with reference to the drawings by taking the drilling of a hydrate layer of actual marine natural gas in a certain area as an example, but the present invention is not limited to the following examples.
The heat exchange schematic diagram of the marine natural gas hydrate layer drilling shaft is shown in fig. 1, seawater is arranged outside the shaft above a mud line, and a stratum is arranged outside the shaft below the mud line. Drilling fluid is injected from the drill string at the wellhead and exchanges heat with the fluid in the annulus through the drill string. After reaching the bottom of the well, the drilling fluid carries natural gas hydrate drilling cutting particles to return upwards from the annular space, and on one hand, the fluid in the annular space and the fluid in the drill string exchange heat; on the other hand, the fluid in the annulus of the lower well section above the mud line exchanges heat with the stratum through the casing and the cement sheath, and the fluid in the annulus of the upper well section above the mud line exchanges heat with the seawater through the marine riser; in the upward returning process, the natural gas hydrate drilling cutting particles are decomposed and absorb heat along with the temperature rise and the pressure reduction of the shaft, so that the temperature field of the shaft is influenced.
A method for calculating a temperature field of a drilling shaft of an ocean natural gas hydrate layer sequentially comprises the following steps:
(1) acquiring drilling parameters and initial conditions of an ocean natural gas hydrate layer according to drilling design and reservoir parameters: outer diameter D of marine riserroOuter diameter D of sleevecoAre all 0.508 m; drill string outside diameter Dpo0.127m, bit diameter 0.445 m; the well depth H is 1600m, the discharge capacity of the pumped drilling fluid is 30l/s, and the density rho of the pumped drilling fluidp(0)Is 1030kg/m3(ii) a Depth H of sea waterseaIs 1500 m; the sea surface temperature is 298K; the temperature gradient of the stratum is 3 ℃/100 m; the abundance of hydrates in the reservoir is 70%; the mechanical drilling speed is 10 m/h; calculating the drilling fluid injection temperature T at a timep(0) (n)298K; annulus wellhead pressure pa(0) (n)Is 101300 Pa.
(2) Performing space node division, wherein according to the drilling parameters of the marine natural gas hydrate layer in the step (1), the space domain is the whole shaft, and the axial serial numbers of the nodes are sequentially increased from 0 from the well mouth to the well bottom; the node number at the well head is 0, and the node number at the well bottom is 1600.
(3) According to the theory of heat transfer and the law of conservation of energy, based on a temperature field calculation model of the drilling shaft of the marine natural gas hydrate layer, the temperature change in the grid is calculated: delta T in drill stringp(i) (n)And Δ T in the annulusa(i) (n)。
(4) The well head is the node 0, and when calculation is carried out, the fluid temperature at the well head in the drill column is the drilling fluid injection temperature T at the moment of calculationp(0) (n)298K; and the fluid temperature T at the wellhead in the annulusa(0) (n)I.e. the temperature of the fluid returning from the annulus is an unknown parameter, making an assumption Ta(0) (n)According to the assumed range: t is more than or equal to 273Ka(0) (n)≤Tp(0) (n)Assume 292K and perform the calculation.
(5) According to the sequence of the nodes in the drill string and the annular space from the well head to the well bottom at the same time, according to the temperature T of the fluid in the drill string at the node ip(i) (n)And annular space intermediate fluid temperature Ta(i) (n)And calculating the temperature of the well bore at the next node i + 1: temperature T of fluid in drill stringp(i+1) (n)And annular space intermediate fluid temperature Ta(i+1) (n)(ii) a Iteratively calculating to the bottom of the well from the well head to obtain the fluid temperature T at the bottom of the well in the drill columnp(k) (n)281.06K, fluid temperature T at the bottom of the well in the annulusa(k) (n)＝281.41K。
(6) According to the well bore temperature calculation method in the step (5), the obtained fluid temperature T at the well bottom in the drill stringp(k) (n)Fluid temperature T at the bottom of the well in the annulusa(k) (n)And comparing whether the calculation error gamma is 1K or not according to the calculation error gamma:
the calculation error is satisfied.
Thus, the temperature T of the fluid at the well-head in the annulusa(0) (n)At 292K, the fluid temperature at the bottom of the well meets the calculation error through calculation, and the fluid temperatures in the drill string and in the annulus at all the nodes obtained through iterative calculation in the step (5) are the temperature field of the drilling well bore of the marine natural gas hydrate layer, as shown in fig. 2 (fig. 2 is a calculation result diagram of an example of the temperature field of the drilling well bore of the marine natural gas hydrate layer).
Claims (3)
1. A method for calculating a temperature field of a drilling shaft of an ocean natural gas hydrate layer sequentially comprises the following steps:
(1) acquiring drilling parameters and initial conditions of the marine natural gas hydrate layer according to drilling design and reservoir parameters, wherein the drilling parameters comprise: well bore structure, drilling tool combination, pumping parameters, seawater depth, seawater temperature, formation temperature, hydrate abundance in reservoir, mechanical drilling rate and well depth, wherein the initial conditions comprise: calculating the drilling fluid injection temperature T at a timep(0) (n)Annulus wellhead pressure pa(0) (n)；
(2) Carrying out space node division, wherein a space domain is the whole shaft, the axial serial numbers of the nodes are sequentially increased from 0 to the bottom of the well from the well, the serial number of the node at the well is 0, the serial number of any calculation node in the shaft is represented by i, the serial number of the next calculation node is represented by i +1, and the serial number of the node at the bottom of the well is k;
(3) establishing a calculation model of the temperature field of the drilling shaft of the marine natural gas hydrate layer, wherein the expression of the temperature change in a calculation grid is as follows:
in the drill string:
encircling in the air:
the well section above the mud line is well,
the well section below the mud line is provided with a mud line,
in the formula: i is a calculation node and i is a calculation node,
n is the time of the calculation,
Δ h is the calculation grid length, which is set to 1m in the calculation,
Hseathe depth of the seawater, m,
ΔTp(i) (n)、ΔTa(i) (n)the temperature changes in the computational grid, K,
ρp(i)、ρa(i)calculating the density of the mixed fluid at the node i in the drill string and the annulus respectively in kg/m3，
vp(i)、va(i)The flow velocity of the mixed fluid at node i, m/s,
cp(i)、ca(i)the specific heat capacity of the mixed fluid at the position of a calculated node i in a drill string and a well casing in an annular space is J/(kg. K),
Dpi、Dpo、Dri、Dcirespectively the inner diameter of a drill stem, the outer diameter of the drill stem, the inner diameter of a riser and the inner diameter of a casing pipe, m,
Qap(i)for heat exchange between the annulus and the fluid at computation node i in the drill string, W,
Qwa(i)、Qsa(i)respectively, heat exchange between seawater and fluids at a calculation node i in the annulus and between the stratum and the fluid at the calculation node i in the annulus, W,
Qfp(i)、Qfa(i)the heat generated by flow friction at node i, W,
Qh(i)calculating the heat quantity W absorbed by the decomposition of the hydrate at the node i in the annulus;
heat exchange Q between fluid at calculation node i in annular space and drill stringap(i)The calculation is as follows:
heat exchange Q between seawater and fluid at calculation node i in annuluswa(i)The calculation is as follows:
heat exchange Q between fluids at calculation node i in stratum and annulussa(i)The calculation is as follows:
calculating heat Q generated by flow friction at node i in drill stringfp(i)The calculation is as follows:
calculating heat Q generated by flow friction at node i in annulusfa(i)The calculation is as follows:
calculating heat Q absorbed by decomposition of hydrate at node i in annulush(i)The calculation is as follows:
in the formula:
Tw(i) (n)、Ts(i) (n)respectively calculating the seawater temperature, the formation temperature, K,
Dro、Dco、Dcso、Dcsirespectively the outer diameter of the riser, the outer diameter of the casing, the outer diameter of the cement sheath and the inner diameter of the cement sheath, m,
αf1(i)、αf2(i)、αf3(i)calculating the forced convection heat transfer coefficient at the node i on the inner surface of the drill string, the inner surface of the marine riser and the inner surface of the casing pipe respectively, wherein W/(m) is2·K)，
αm1(i)、αm2(i)The convective heat transfer coefficients of the outer winding surfaces at the position of a calculated node i on the outer surface of the drill string and the outer surface of the marine riser are respectively W/(m)2·K)，
λp(i)、λr(i)、λc(i)、λcs(i)、λs(i)Respectively are the heat conductivity coefficients of a drill stem, a marine riser, a casing, a cement sheath and a stratum at a calculation node i, W/(m.K),
Usa(i)is the comprehensive heat exchange coefficient W/(m) of the annular air fluid and the stratum at a calculation node i2·K)，
TDIs a transient heat transfer function, has no dimension,
vp(i)、va(i)the flow velocity of the mixed fluid in the drill string and the annular space at the calculation node i, m/s,
mp(i)、ma(i)mass flow, kg/s,
Daothe calculation node is the inner diameter D of the riser in the well section above the mud lineriThe well section below the mud line is the inner diameter D of the casingci，m，
ZgeqIs a natural gas compression factor at a node under the conditions of temperature and pressure in the annular space, has no dimension,
r is a general gas constant, J/(mol. K),
peq(i) (n)calculating the natural gas hydrate phase equilibrium pressure Pa at the node i;
(4) when the well head i is 0, the fluid temperature at the well head in the drill string is the drilling fluid injection temperature T at the moment of calculationp(0) (n)For known parameters, the temperature T of the fluid at the well head in the annulusa(0) (n)I.e. the temperature of the fluid returning from the annulus is an unknown parameter, assuming Ta(0) (n)A value of (d);
(5) according to the pitch in the drill string and in the annulusThe order of the points from the well head to the well bottom at the same time is determined by the temperature T of the fluid in the drill string at the point ip(i) (n)And annular space intermediate fluid temperature Ta(i) (n)And calculating the temperature of the well bore at the next node i + 1:
in the formula: t isp(i) (n)、Ta(i) (n)The temperature of the fluid in the drill string and in the annular space at the node i, K,
Tp(i+1) (n)、Ta(i+1) (n)the temperature of the fluid in the drill string and in the annular space at the node i +1, K,
from the well head to the well bottom node k, obtaining the fluid temperature T at the well bottom in the drill columnp(k) (n)And the fluid temperature T at the bottom of the well in the annulusa(k) (n)；
(6) If the following equation is satisfied:
in the formula: t isp(k) (n)The temperature of the fluid at the bottom of the well in the drill string, K,
Ta(k) (n)the temperature of the fluid at the bottom of the well in the annulus, K,
gamma is the error in the calculation of the fluid temperature at the bottom of the well,
and (5) iteratively calculating the fluid temperature in the drill string and in the annulus at all the nodes, namely the temperature field of the well bore of the marine natural gas hydrate layer.
2. The method for calculating the temperature field of the wellbore of the marine natural gas hydrate layer well drilling as claimed in claim 1, wherein the assumption in the step (4) is thatTa(0) (n)A value of (d), with a hypothetical range of 273K ≦ Ta(0) (n)≤Tp(0) (n)。
3. The method for calculating the temperature field of the marine natural gas hydrate formation drilling well bore according to claim 1, wherein the calculation error gamma of the fluid temperature at the bottom of the well in the step (6) is 1K.
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