Sensitivity analysis of reservoir risk in marine gas hydrate drilling

Wntong Sun ,Jun Pei ,N Wei **,Jinzhou Zho ,Jin Xue ,Shouwei Zhou ,Liehui Zhng ,Bjørn Kvmme ,Qingping Li ,Hito Li ,d,Lin Jing ,Cho Zhng ,Cong Li

a State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,Chengdu,610500,China

b State Key Laboratory of Natural Gas Hydrate,Beijing,100027,China

c CNOOC Research Institutes Limited Liability Company,Beijing,100027,China

d Southern Marine Science and Engineering Guangdong Laboratory(Zhanjiang),Zhanjiang,524013,China

ABSTRACT

Keywords:Marine gas hydrate Drilling Temperature rising Reservoir risk Sensitivity analysis

1.Introduction

At present,natural gas hydrate,a new type of clean energy,has attracted more and more attention.Natural gas hydrate is a white clathrate crystalline compound formed by the interaction of methane and other hydrocarbon gases with water under certain temperature and pressure[1].It is mainly distributed in the surface sediments or sedimentary rocks of the land slope,islands and basins in the permafrost regions with high latitude or high altitude and the marginal areas of ocean with depth of 300-3000 m,and it can also be distributed on the seabed surface[2,3].It is estimated that 20.7%of the land and 90%of the ocean floor[4]have favorable conditions for the formation of gas hydrate,and the temperature and pressure in these areas keep gas hydrate stable.Since the 1970s,natural gas hydrate has been found directly or indirectly in many regions through programs and projects such as deep-sea drilling programs,ocean drilling programs,etc.around the world.It is estimated that the total amount of natural gas hydrate resources are about 2.1 × 1016m3[5-8],which is about twice the carbon content of proven traditional fossil energy.More than 90%of gas hydrate is distributed in the ocean.

The regions in the world where gas hydrate trial production has been carried out mainly include[9-11]:Messoyakha gas field in Siberia[12,13],Mallik permafrost region in Canada[14,15],Mount Elbert well No.1 in the North Slope,Alaska[16,17],Aichi-ken Sea Area in Japan[18,19]and Shenhu Area in South China Sea[20,21].To exploit gas hydrate,we must first drill wells.Some scholars have carried out research on marine gas hydrate reservoir drilling.Wang et al.[22]established the wellbore multi-phase flow equation and numerical calculation method during drilling overflow in marine gas hydrate reservoir,and analyzed the overflow law and the change of bottom hole pressure.Wei et al.[23-27]studied the wellbore multi-phase flow law of drilling through marine gas hydrate reservoir,and obtained the sensitivity effect of different drilling parameters.They also analyzed the law of cuttings migration in horizontal wells in marine gas hydrate reservoir drilling[28].Merey[29]analyzed the measured drilling parameters during drilling through gas hydrate reservoir and the engineering and reservoir parameters in the drilling process in frozen soil and the ocean,and believed that borehole enlargement often occurs in marine gas hydrate reservoir drilling.Sahu et al.[30]reviewed the drilling of natural gas hydrate reservoir,and summarized the progress in hydrate drilling design,drilling tools,drilling fluid,logging and so on.Yan et al.[31]analyzed the creep rule of natural gas hydrate reservoir and obtained the variation characteristics of wellbore size during drilling of marine gas hydrate reservoir.Huang et al.[32]studied the effects of different drilling fluid circulation rates on the reservoir during the drilling of hydrate bearing sediments.Chaturvedi et al.[33]investigated the low-temperature rheology of drilling fluid containing new additives and presented drilling fluid additives suitable for drilling in marine gas hydrate formation.

Compared with conventional oil and gas reservoirs,marine gas hydrate is often buried shallow[34],and hydrate reservoirs are loose and unstable,without tight caprock.In marine gas hydrate reservoir drilling,the drilling fluid is continuously injected from the drill string to the bottom of the well,which is very easy to change the original temperature state of hydrate reservoir.When the reservoir temperature rises to the phase equilibrium temperature under reservoir pressure,the hydrate will decompose and release a large amount of gas.This decomposition is often uncontrollable.The decomposition gas will further escape and expand,and the reservoir will break and collapse due to its weak cementation characteristic,which will easily lead to a series of other potential risks.Therefore,it is very important to figure out the risks during marine gas hydrate reservoir drilling.Based on the wellbore temperature analysis of drilling in marine gas hydrate formation,this study will further investigate the rule of reservoir risk sensitivity.

2.Theoretical model

As shown in Fig.1,during marine gas hydrate reservoir drilling,there is continuous heat exchange between drilling fluid,drill string,mixed fluid in annulus,marine riser and seawater in the well section above the seabed mudline,and heat exchange also occurs between drilling fluid,drill string,mixed fluid in annulus,casing,cement sheath and formation in the well section below the seabed mudline.At the same time,the fluid flow in the wellbore will carry heat and generate heat by friction with the wall.In addition,the heat of phase change should also be considered in multi-phase flow in annular space.

The gas hydrate reservoir can remain stable under the original temperature and pressure.Affected by drilling,when the hydrate reservoir temperature increases to the phase equilibrium temperature under reservoir pressure,the hydrate in the reservoir will decompose and pose potential risks,namely,the“decomposition risk area”in Fig.2.The decomposition front facies is the position of hydrate phase equilibrium,so hydrate phase equilibrium is important to evaluate the risks of marine gas hydrate reservoir drilling.

2.1.Temperature equations in annulus

According to the first law of thermodynamics,the total energy in a flow control body is constant[35].As shown in Fig.3,the microelement section of the wellbore with length dz is taken as the control body in annulus.The annulus fluid flows in from crosssection S1 and flows out from cross-section S2.The heat transfer process in annulus is analyzed as follows.

The heat flowing into the control body in dt time is:

The heat flowing out of the control body in dt time is:

The heat exchange of the control body in the radial direction in dt time is:

The heat energy in the control body at time t is:

The heat energy in the control body at time t+dt is:

Therefore,it can be concluded that:

namely

where Tais the temperature in the annulus,K;ρmis the density of mixed fluid in the annulus,kg/m3;vmis the velocity of mixed fluid in the annulus,m/s;cmis the specific heat capacity of mixed fluid in the annulus,J/(kg·K);Sais the annular cross-sectional area,m2;qpais the heat exchange between fluids in the annulus and the drill string,J;qsais the heat exchange between the mixed fluid in annulus and outside of wellbore,J;qfais the heat generated by the friction between the mixed fluid in the annulus and the wall,J;qhais the heat released or absorbed by hydrate phase transition in the annulus,J.qpa,qsa,qfa,qhaand Saare solved as follows:

where Tpis the temperature in drill string,K;Tsis the temperature outside of wellbore,K;hpais the integrated heat transfer coefficient between annulus and drill string,W/(m2·K);hsais the integrated heat transfer coefficient between annulus and outside of wellbore,W/(m2·K);daiis the inner diameter of annulus,equal to the outer diameter of drill string,m;daois the outer diameter of annulus,m;fmis a flow friction coefficient,dimensionless;nhis the molar quantity of hydrate phase transition in unit volume annulus,mol/m3;Δqhis the hydrate phase transition heat,W/mol.Δqhis calculated by Clausius Clapeyron equation[36]as follows:

where Z is the gas compression factor,dimensionless.

Combining Equations(7)-(13),the temperature equation in the annulus can be obtained as follows:

2.2.Temperature equation of inner drill string

Using the same method as the annular temperature equation,the temperature equation in the drill string can be obtained as follows:

2.3.Calculation of integrated heat transfer coefficient

2.3.1.The integrated heat transfer coefficient between annulus and drill string(hpa)

hpa is calculated as follows:

where U1is the convective heat transfer coefficient between the fluid in the drill string and the inner surface of the drill string,W/(m2·K);U2is the thermal conductivity of drill string,W/(m·K);U3is the convective heat transfer coefficient between the mixed fluid in annulus and the outer surface of the drill string,W/(m2·K);dpiis the inner diameter of drill string,m.

2.3.2.The integrated heat transfer coefficient between annulus and outside of wellbore(hsa)

(1)Well section above the mudline

In the well section above the mudline,the outside of the wellbore is seawater.In Equation(9),the temperature outside the wellbore represents the seawater temperature,hsais the integrated heat transfer coefficient between annulus and seawater:

where U4is the convective heat transfer coefficient between the mixed fluid in annulus and the inner surface of the marine riser,W/(m2·K);U5is the thermal conductivity of the marine riser,W/(m·K);U6is convective heat transfer coefficient between seawater and outer surface of marine riser,W/(m2·K);drois the outer diameter of the riser,m.Sea water temperature from top to bottom can be divided into mixing layer,thermocline layer and constant temperature layer.According to relevant studies,the temperature of the mixing layer and thermocline layer is calculated as follows[37,38]:

The temperature of constant temperature layer is calculated as follows[37,38]:

where Tseais the seawater temperature,K;Tsea0is the ocean surface temperature,K;hwis the sea depth,m;h is the well depth,m.

(2)Well section below the mudline

In the well section below the mudline,the outside of the wellbore is stratum.In Equation(9),the temperature outside the wellbore represents the temperature of outer surface of wellbore,hsais the integrated heat transfer coefficient between annulus and the outer surface of wellbore:

whereis the convective heat transfer coefficient between the mixed fluid in annulus and inner surface of the casing,W/(m2·K);is the thermal conductivity of the cement sheath,W/(m·K);dcois outer diameter of the casing,m;dcsois outer diameter of cement sheath,m;dcsiis inner diameter of cement sheath,m.Equation(20)shows the integrated heat transfer coefficient calculation model of the mixed fluid in annulus and the outer surface of the wellbore in a single-layer casing structure.In the actual process,under the condition of multi-layer casing structure,the corresponding heat exchange terms can be accumulated according to the equation.

The heat exchange between the stratum and the outer surface of wellbore is:

where qfsis the heat exchange between the stratum and the outer surface of wellbore;J is the thermal conductivity of the stratum,W/(m·K);TDis dimensionless temperature;Tf is the original stratum temperature,K.TDis calculated as follows[39]:

where tDis dimensionless time;t is the drilling time,s;ρfis the density of stratum,kg/m3;cf is the specific heat capacity of stratum,J/(kg·K);rsis the wellbore radius,m.

Tfis calculated as follows:

where Thwis the seabed temperature,K;GTis the geothermal gradient,K/m.

According to the first law of thermodynamics,the heat exchange between the stratum and the outer surface of wellbore is equal to that between the mixed fluid in annulus and the outer surface of wellbore:

Combining Equations(9),(21)and(24),Equation(25)can be obtained as follows:

2.4.Phase equilibrium model of natural gas hydrate

Since the empirical model of methane hydrate phase equilibrium established by researchers in the Natural Gas Hydrate Laboratory of China University of Petroleum(East China)based on experimental data has been verified[40],we choose this model to describe the phase equilibrium of natural gas hydrate in this study:

where peqis the phase equilibrium pressure of methane hydrate at temperature T,MPa.

3.Numerical calculation method

Based on the finite difference method,the wellbore temperature in marine gas hydrate reservoir drilling is solved iteratively.The calculation domain is the node in annulus,drill string,seawater and surrounding stratum,and the sequence is from wellhead to the bottom hole.The difference scheme of annular temperature equation is:

The difference scheme of drill string temperature equation is:

To carry out numerical calculation,as shown in Equations(29)-(32),some definite conditions are set as follows:the temperature in the drill string at the wellhead is equal to that of the injected drilling fluid;the temperature in the drill string at the bottom hole is equal to that in the annulus;seawater temperature and far-end stratum temperature do not change with time,but are only functions related to well depth.

4.Calculation results and discussions

Basic parameters are as follows:the depth of sea water is 1000 m,the density of sea water is 1030 kg/m3,the sea surface temperature is 303.15 K,the geothermal gradient is 0.03 K/m,the formation pressure coefficient is 1.03-1.06,the buried depth of hydrate reservoir is 150-200 m below the seabed,the reservoir porosity is 60%,and the hydrate saturation in the pores is 60%.The drilling parameters of the example well are as follows:it is a vertical well,the well depth is 1200 m,drilling fluid displacement is 60 L/s,drilling fluid density is 1030kg/m3,drilling fluid viscosity is 0.003 Pa s,the injection temperature of drilling fluid is 303.15 K,the mechanical drilling rate is 5 m/h,the bit diameter is 0.311 m,the drill collar diameter is 0.228 m,the drill pipe diameter is 0.139 m,the well structure and basic thermophysical parameters are shown in Table 1 and Table 2 respectively.

Table 1 The well structure and casing parameters of the example well.

Table 2 Basic thermophysical parameters of the example well.

4.1.Influence of drilling fluid displacement

In the process of marine gas hydrate reservoir drilling,under the condition of drilling fluid displacement of 30 L/s,40 L/s,50 L/s,60 L/s,70 L/s and 80 L/s,the temperatures in annulus and drill string are calculated,as shown in Fig.4.

In Fig.4,the initial temperature represents the temperature distribution of seawater and the original temperature distribution of stratum,and the phase equilibrium temperature represents the equilibrium temperature of natural gas hydrate corresponding to the pressure distribution of seawater and stratum.With the increase of drilling fluid displacement from 30 L/s to 80 L/s,the temperatures both in annulus and drill string increase,and the increase amplitude increases continuously from wellhead to the bottom hole.The wellhead temperature changes slightly.The bottom hole temperature increases from 282.13 K to 291.93 K,the minimum temperature in annulus increases from 281.58 K to 290.10 K,and the minimum temperature in drill string increases from 281.88 K to 290.70 K.The major reasons for situation aboveare as follows:with the increase of drilling fluid displacement,the heat carried by the drilling fluid injected into the wellbore through the drill string increases per unit time.Besides,the time of drilling fluid flowing through the control unit in the annulus and the time of heat exchange time between the drilling fluid and the seawater and the formation are both shortened.Meanwhile,the friction between drilling fluid and the inner surface of wellbore increases,generating more heat.

When the drilling fluid displacement is 30 L/s,40 L/s and 50 L/s,the annulus temperature of the well section in the gas hydrate reservoir is lower than the phase equilibrium temperature of the hydrate.Under such conditions,drilling has little influence on the hydrate in the reservoir.When the displacement of drilling fluid is 60 L/s,70 L/s and 80 L/s,the annulus temperature of well section or some well sections in gas hydrate reservoir is higher than the hydrate phase equilibrium temperature.Under such conditions,hydrate in the reservoir is easy to decompose.Through the heat exchange analysis between wellbore and hydrate reservoir,the temperature distributions of hydrate reservoir at 0 m,0.1 m,0.2 m,0.3 m,0.4 m,0.5 m,0.6 m,0.7 m,0.8 m,0.9 m,1 m,2 m,3 m,4 m,5 m away from wellbore are obtained,as shown in Figs.5-7.

It can be concluded from Figs.5-7:when the drilling fluid displacement is 60 L/s,temperature in only a small well section is higher than the hydrate phase equilibrium temperature,indicating that under this condition,only a small amount of hydrate will decompose affected by drilling,which has little effect on the stability of hydrate reservoir.When the drilling fluid displacement is 70 L/s,the location of hydrate phase equilibrium temperature is at about 0.3 m away from the wellbore,indicating that the hydrate in the reservoir less than 0.3 m away from the wellbore may be decomposed,and the stability of the hydrate reservoir in this range will be damaged.When the drilling fluid displacement is 80 L/s,the location of hydrate phase equilibrium temperature is at about 0.7 m away from the wellbore,indicating that the hydrate in the reservoir less than 0.7 m away from the wellbore may be decomposed,and then the stability of the hydrate reservoir will be damaged in a large range.Based on the above analysis,it can be concluded that in marine gas hydrate reservoir drilling,increasing the drilling fluid displacement will increase the amplitude of temperature rise of hydrate reservoirs,and the influence range on the stability of hydrate reservoirs will extend outward from the wellbore,thereby increasing the safety risks.

4.2.Influence of drilling fluid density

In the process of marine gas hydrate reservoir drilling,under the condition of drilling fluid density of 1030 kg/m3,1130 kg/m3and 1230 kg/m3,the temperatures in annulus and drill string are calculated,as shown in Fig.8.

It can be seen from Fig.8 that with the increase of drilling fluid density from 1030 kg/m3to 1230 kg/m3,the temperatures both in annulus and drill string increase,and the increase amplitude increases continuously from wellhead to the bottom hole.The wellhead temperature changes slightly.The bottom hole temperature increases from 287.96 K to 289.40 K,the minimum temperature in annulus increases from 286.77 K to 288.22 K,and the minimum temperature in drill string increases from 287.27 K to 288.77 K.The major reasons for situation above are as follows:with the increase of drilling fluid density,the mass and heat carried by drilling fluid per unit length in the wellbore increase.Meanwhile,the friction between drilling fluid flow and inner surface of wellbore increases,generating more heat.

Under the conditions of 3 drilling fluid densities,the annulus temperature is higher than the hydrate phase equilibrium temperature in well section or part of well section in hydrate reservoir.Then the temperature distributions of hydrate reservoir at different distances from wellbore are obtained,as shown in Figs.5,9 and 10.

It can be concluded from Figs.5,9 and 10 that:when the drilling fluid density is 1030 kg/m3,only the temperature in a small part of well section is higher than the hydrate phase equilibrium temperature.When the drilling fluid density is 1130 kg/m3,the location of hydrate phase equilibrium temperature is at about 0.1 m away from the wellbore.When the drilling fluid density is 1230 kg/m3,the location of hydrate phase equilibrium temperature is at about 0.2 m away from the wellbore.Based on the above analysis,it can be concluded that in marine gas hydrate reservoir drillings,increasing the drilling fluid density will increase the amplitude of temperature rise of hydrate reservoirs,and the influence range on the stability of hydrate reservoirs will extend outward from the wellbore,thereby increasing the safety risks.

In Fig.13,when the drilling fluid injection temperature is 298.15 K,the annulus temperature in reservoir section is lower than the hydrate phase equilibrium temperature,so drilling in such condition has little influence on hydrate in the reservoir.When the drilling fluid injection temperature is 303.15 K or 308.15 K,the annulus temperature of the well section or part of the well section in hydrate reservoir is higher than the hydrate phase equilibrium temperature.As shown in Figs.5 and 15,the temperature distributions of hydrate reservoir at different distances from wellbore are obtained.

It can be concluded from Figs.5 and 15 that:when the drilling fluid injection temperature is 303.15 K,only the temperature in a small part of well section is higher than the hydrate phase equilibrium temperature.When the drilling fluid injection temperature is 308.15 K,the location of hydrate phase equilibrium temperature is between 0.1 m and 0.2 m away from the wellbore.Based on the above analysis,it can be concluded that in marine gas hydrate reservoir drillings,higher drilling fluid injection temperature will increase the temperature of hydrate reservoir and pose greater threats to the stability of hydrate reservoir.Cooling drilling fluid can help ensure the stability of hydrate reservoir during drilling.

4.5.Influence of seawater depth

In the process of marine gas hydrate reservoir drilling,under the condition of seawater depth of 800 m,1000 m and 1200 m(the corresponding well depths are 1000 m,1200 m and 1400 m respectively),the temperatures in annulus and drill string are calculated,as shown in Fig.16.

It can be seen from Fig.16 that with the increase of seawater depth from 800 m to 1200 m,the temperatures both in annulus and drill string decrease,and the decrease amplitude increases continuously from wellhead to the bottom hole.The annulus wellhead temperature decreases slightly,the bottom hole temperature decreases from 291.32 K to 285.31 K,the annulus minimum temperature decreases from 290.06 K to 284.14 K,and the drill string minimum temperature decreases from 290.67 K to 284.55 K.The major reasons for the situation above are as follows:as the seawater depth increases,the seabed and stratum temperatures both decrease.Meanwhile,the corresponding well depth increases,making the time for circulation of drilling fluid in wellbore and heat transfer time between drilling fluid,seawater and stratum longer.Thus,more heat is transferred outside the wellbore,which increases the decrease amplitude of wellbore temperature.

In Fig.16,when the seawater depth is 1200 m,the annulus temperature in reservoir section is lower than the hydrate phase equilibrium temperature,so drilling in such condition has little influence on hydrate in the reservoir.When the seawater depth is 1000 m or 800 m,the annulus temperature of the well section or part of the well section in hydrate reservoir is higher than the hydrate phase equilibrium temperature.As shown in Figs.5 and 17,the temperature distributions of hydrate reservoir at different distances from wellbore are obtained.

It can be concluded from Figs.5 and 17 that:when the seawater depth is 800 m,the location of hydrate phase equilibrium temperature is at nearly 2 m away from the wellbore,drilling in such condition has huge influence on the stability of the reservoir.When the seawater depth is 1000 m,only the temperature in a small part of the well section is higher than the hydrate phase equilibrium temperature.Based on the above analysis,it can be concluded that in marine gas hydrate reservoir drilling is more likely to increase the temperature of shallow depth reservoir,thus increasing the risk.For reservoirs in deep depth,drilling makes the reservoir temperature rise slightly,and has little influence on the stability of hydrate reservoir.

4.6.Risk analysis of different factors

According to temperature variations of wellbore and hydrate reservoir under different drilling fluid displacement,density,viscosity and injection temperature,and seawater depth,combined with hydrate phase equilibrium conditions,the maximum gas production volume of hydrate decomposition and the maximum solid volume of collapse after decomposition are obtained under different influencing factors,as shown in Table 3.It can be seen from Table 3 that with the increase of drilling fluid displacement,density,viscosity and injection temperature,or the decrease of seawater depth,the decomposed hydrate in the reservoir and the decomposed gas from hydrate caused by drilling will increase.Under the calculation conditions,the maximum gas production volume of hydrate decomposition caused by drilling reaches 9867.527 m3,and the maximum solid volume that may collapse after hydrate decomposition reaches 63.924 m3,which is easy to cause a series of risks.

Table 3 Risk analysis of drilling in hydrate reservoir under different influencing factors.

Based on the above analysis,we can see that the drilling fluid displacement,density and viscosity should be properly adjusted in marine natural gas hydrate reservoir drilling,so as to avoid the high bottom hole temperature and the rise of reservoir temperature,which will lead to hydrate decomposition and cause safety risks.Lowering the injection temperature of drilling fluid helps ensure drilling safety.For reservoirs in shallow seawater depth,measures such as using lower drilling fluid displacement,density,and viscosity or cooling drilling fluid shall be adopted to avoid breaking the stable state of hydrate in the reservoir and ensure drilling safety.

5.Conclusions

Aiming at reservoir risk in marine gas hydrate drilling,we establish the theoretical model and numerical calculation method of wellbore temperature field,analyze the influence on wellbore temperature of drilling fluid displacement,density,viscosity and injection temperature,and seawater depth.Then the sensitivity laws of reservoir risk in marine gas hydrate drilling are obtained.

With the increase of drilling fluid displacement and density,the temperature in annulus and drill string increases,and the increase amplitude increases from the wellhead to the bottom hole.In such condition,the wellhead temperature changes slightly,and the bottom hole temperature increases.With the increase of drilling fluid viscosity,the temperature in the upper annulus section decreases slightly,the temperature in the lower annulus and drill string increases,and the annulus wellhead temperature decreases while the bottom hole temperature increases.As the increase of drilling fluid injection temperature,the temperature in annulus and drill string increases,and the increase amplitude decreases from wellhead to bottom hole.The temperature in annulus increases along with the bottom hole temperature.As the seawater depth increases,the temperature in annulus and drill string decreases,and the decrease amplitude increases from the wellhead to the bottom hole.The annulus wellhead temperature decreases slightly,and the bottom hole temperature also decreases.

When the displacement,density,viscosity or injection temperature of drilling fluid increases,it will magnify the temperature increasing amplitude of hydrate reservoir and extend the scope of influence on the stability of hydrate reservoir,thus causing greater risks.To avoid these risks,the displacement,density,viscosity or injection temperature of drilling fluid should be adjusted in the drilling process.Drilling is more likely to increase the reservoir temperature in shallow seawater depth and posing greater risks.Therefore,measures such as using lower drilling fluid displacement,density,and viscosity or cooling drilling fluid shall be adopted to avoid breaking the stable state of hydrate in the reservoir and ensure drilling safety.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The research was supported by the 111 Project(D21025),National Key Research and Development Program(2019YFC0312300),National Natural Science Foundation Item of China(U20B6005-05,51874252 and 5177041544),Open Fund Project of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation(PLN2021-02 and PLN2021-03),the Fund of Southern Marine Science and Engineering Guangdong Laboratory(Zhanjiang)(ZJW-2019-03).