Shunshi Fn ,Ki Guo ,Ynhong Wng ,**,Xuemei Lng ,N Wei ,Qingping Li

a Key Lab of Enhanced Heat Transfer and Energy Conservation,Ministry of Education,School of Chemistry and Chemical Engineering,South China University of Technology,China

b State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Southwest Petroleum University,No.8 Xindu Avenue,Xindu District,Chengdu,610500,PR China

c State Key Laboratory of Natural Gas Hydrate,Beijing,100028,China

ABSTRACT Natural gas hydrate is a kind of clean energy with huge reserves,and the saturation(volume percentage of hydrate in pore space of sediments)is the key parameter for determining whether the reservoir is worthy of exploitation.In this work,rapid hydrate dissociation by the combination of heat injection and NaCl inhibitor addition was studied,and an on-site evaluation method for hydrate saturation in sediment samples was proposed by using a core sampler to transfer hydrate samples under pressure.The results showed that the average gas production rate per unit volume was increased significantly to reach 7.22 L/Lr·min-1 by the injection of NaCl aqueous solution with 50.9 °C,which was attributed to the increase of the chemical potential to further accelerate the rate of hydrate dissociation in the presence of NaCl.Furthermore,for the measurement of methane hydrate samples saturation with a volume of 673 cm3(which contained 1.4 mol hydrates with the saturation of 58%),hydrate saturation could be accurately achieved within 30 min with a relative error lower than 11.7% This work may provide new thoughts for on-site saturation evaluation and rapid dissociation of hydrate samples during natural gas hydrate exploitation.

Keywords:Natural gas hydrate Saturation Rapid on-site measurement NaCl solution Rapid dissociation

1.Introduction

Natural gas hydrate(NGH)is a crystalline compound,which requires water,gas,temperature,pressure,time,and space to form[1].These conditions occur globally in permafrost regions and seafloor sediments[2].More than 230 NGH potential deposits have been confirmed globally.Hydrate-bearing regions have huge reserves of natural gas resources,ranging from 3114 to 7,634,000 TCM[3].In China,over 20 research organizations are working on NGH researches.NGH in the marine settings of the South China Sea and the permafrost of the Qinghai-Tibet plateau are estimated to be 65 TCM and 0.12-240 TCM,respectively[4].

Hydrate saturation is one of the most important indexes to estimate the development value of hydrate reservoir[5].At present,the determination methods of hydrate saturation mainly include resistivity logging,seismic wave testing,and chloride concentration of pore water,etc[6-11].As for hydrate samples drilled from the Northern Cascadia margin in Canada,gas hydrate saturation is determined by combining pore water chemistry and in situ downhole log measurement in a transect[12].The average value of gas hydrate saturation may be as high as 30%-40%or as low as 5%-10%depending on the no-gas hydrate reference salinity[13].Gas hydrate saturation sampled from Sites 994,995,and 997 in Blake ridge are 2.2%,3.3%,and 2.2%,respectively,through P- and V- wave testing.While it is calculated to be 1.3%,1.8%,and 2.4% via chloride concentration of pore water.It is further calculated to be 1.9%,3.0%,and 3.4% through resistivity logging[14].There are big differences among the results of the three methods.Two highly gas hydratesaturated regions with the average saturation of 50% and 54% at Mount Elbert are reported,which is evaluated from nuclear magnetic resonance(NMR),P- and S-wave velocity,and electrical resistivity logs[15].It also suggests that the salinity of pore water can be used to determine whether there is gas hydrate,but it can't be used to measure the saturation.Gas hydrate obtained from site SH2 in the South China Sea is studied through different methods which consist of resistivitylogging,P-wave velocity,chloride concentration of pore water,and decompression dissociation[16].Results from those methods show similar values,the average gas hydrate saturation is around 20%[17].Qian et al.[18]analyzed P-wave and Swave velocity logs acquired at Sites W17 during GMGS3(China's Geological Survey/Guangzhou Marine Geological Survey in the South China Sea),where gas hydrate saturation is 30% ～40%.It is found that the interference of free gas and water must be considered in the estimation of gas hydratesaturation.Researchers hold that the calculation of gas hydrate saturation needs ajoint analysis of P-wave velocity and resistivity.In addition,it also proposed that hydrate morphology affects the estimated hydrate saturation in the South China Sea[19].Elastic-wave velocities can't provide accurate hydrate saturation estimates,which means empirical and theoretical models must be unified and modified for better prediction[20].

For sediment samples with different saturation,hydrate decomposition can be achieved by pressure reduction[21,22],heat injection to increase temperature[23,24],and inhibitor injection[25,26].The rate of gas production per unit reservoir range was 0.154 L/Lr·min-1by using the technology of pressure reduction for seafloor sediments in the South China Sea and the secondary formation of hydrate was avoided and the efficiency of gas recovery was improved during the gradual depressurization[27-29].The average rate of gas production was 0.144-1.634 L/min,and the injection temperature was 25-50°C for hydrate with initial saturation of 34%[30].The combination of pressure reduction and heat injection was applied to promote hydrate dissociation and the rate of gas production per unit reservoir was increased to 0.43 L/Lr·min-1[31].However,there are still some challenges of easier freezing and secondary hydrate formation.

Although indirect methods were widely adopted by researchers,the low accuracy in saturation estimation was nonnegligible.In this paper,a rapid on-site evaluation method for hydrate saturation was proposed.To verify the accuracy and determination duration,a series of simulation experiments were performed,which included hydrate synthesis,transfer,and saturation estimation.Then,the combination of heat injection and NaCl inhibitor addition was applied to accelerate hydrate dissociation and the rate of gas production was studied.

2.Materials and methods

2.1.Experimental apparatus

In this work,a novel apparatus for the rapid hydrate dissociation and determination of hydrate saturation was presented.Fig.1 shows the diagram of the test system for hydrate saturation.The test system includes a high-pressure autoclave,a syringe pump,gas receivers,and a data acquisition system.The high-pressure autoclave(φ80 mm,0-20.0 MPa)is stainless steel cabin with a ball valve.Pressure sensors(Senex,0-25 MPa)and temperature sensors(PT100)are placed at one end of the autoclave,all of them connect to the data acquisition(Agilent 34970A).The syringe pump is used to inject fluid into the dissociation autoclave,accelerating hydrate decomposition.Gas receivers(24.2 L)are used for gas collection,where pressure change reflects gas flow variance.

2.2.Experimental procedures

2.2.1.Hydrate sample transfer with pressurization

Before the hydrate saturation was measured,the hydrate sample needs to be transferred from the sampling core holder to the measurement apparatus to simulate the on-site scenario.Fig.2 shows the typical process of pressure core transferring into the saturation measurement device.It is divided into four steps.First,make the high-pressure autoclave connect with an equally pressurized cutting tool and pressure core storage chamber using ball valves and quick clamps,as shown in Fig.2(a).Second,push the core into the cutter for sub-sampling(L ＜200 mm,φ ＜80 mm),then load it into the high-pressure autoclave,as shown in Fig.2(b)Third,return the remaining core to the storage chamber,then close the ball valves,as shown in Fig.2(c).At last,disconnect the high-pressure autoclave with the cutter,as shown in Fig.2(d).After completion of the hydrate core sample transfer,then connect it with the hydrate saturationtest systemproposed in Fig.1.Duringthe testing process,open the gas outlet,water outlet &inlet,and start the syringe pump,hydrate sample will enter the rapid dissociation stage,where hydrate saturation could be determined in a short time.

2.2.2.Rapid hydrate dissociation and saturation evaluation method

The experiment is mainly divided into hydrate synthesis,sample transfer,and saturation test.The hydrate sample was synthesized in the high-pressure autoclave with a certain saturation.It was then transferred to the dissociation system,where hydrate dissociated and the saturation could be measured.Compared to the saturation of synthesized samples in the high-pressure autoclave,the accuracy of the hydrate saturation test system could be obtained.

During the process of hydrate formation,firstly,the quartz sand and deionized water were put into the high-pressure autoclave several times to make them evenly distributed with the amount of 920 g and 240 g,respectively,where they were put into.Then,purge the autoclave with methane for 5 min,close the outlet valve,continue to inject methane,and pressurize it to 7-8 MPa.Next,start the air bath,cool it down to 0-4°C.Hydrate gradually formed in the autoclave.After a certain time,the pressure in the autoclave was almost no longer reduced.The hydraulic cylinder was used to supply the confining pressure of the synthesized hydrate core,attempting to be similar to the mechanical conditions of the actual coring samples,where the hydrate was in close contact with quartz sand and other hydrates,as shown in Fig.3(a).After a period of continuous stabilization,a gas hydrate sample with a certain saturation has been synthesized.

Synthesized hydrate sample could be transferred to the dissociation system to determine the saturation.During the process of hydrate saturation determination,first of all,empty the excess free methane in the high-pressure autoclave,close the ball valve.Then,remove the hydraulic cylinder on the ball valve side,replace the dissociation autoclave and connect it to the gas and water outlet pipeline.Next,rotate the whole device 180°to make the dissociation autoclave was at the bottom.Start the hydraulic cylinder at the side of the high-pressure autoclave again,push the synthesized sample out of it and drop into the dissociation autoclave,as shown in Fig.3(b).Finally,open the gas outlet,wateroutlet,and water inlet,and start the syringe pump,hydrate sample would enter the rapid dissociation stage,where hydrate saturation could be determined in a short time.Compared to the saturation of the synthesized sample in the highpressure autoclave with it transferred to the dissociation autoclave,the accuracy of the hydrate saturation test system could be obtained.

This paper adopted the definition approach to determine gas hydrate saturation,namely,hydrate volume is a percentage of total pore volume in sediments.The calculation formula of gas hydrate saturation is shown in Equation(1).

The operation procedure of rapid hydrate dissociation was similar to that in the procedure of rapid hydrate saturation evaluation.The cumulative amount of gas production and the rate of gas production were calculated to study the process of hydrate dissociation by the determination of pressure and temperature.

2.2.3.Accuracy and test speed analysis method

Through in situ synthesis and transfer dissociation experiments,the accuracy of hydrate saturation was measured by the definition method and the test system was verified.

The saturation of the synthesized hydrate in the high-pressure autoclave during formation process can be calculated by temperature and pressure changes,as shown in Equation(2)and Equation(3).

By releasing the pressure in the high-pressure autoclave,the synthesized sample was transferred to the dissociation autoclave to dissociate.The amount of the hydrate transferred to the dissociation autoclave can be calculated,concurrently with the hydrate saturation in the dissociation autoclave before hydrate dissociation,as shown in Equation(4)and Equation(5).

Hydrate saturation determined via the test system is calculated by Equation(6)and Equation(7).Otherwise,the associated software can be applied to calculate the saturation.

By comparing the measured saturation in the dissociation autoclave with the synthetic saturation in the high-pressure autoclave,test error of the hydrate saturation test system can be obtained,calculated by Equation(8).

3.Rapid dissociation of hydrate-bearing sediment sample by injection of NaCl solution

The estimation of hydrate saturation is conducive to the development of hydrate reservoir.Moreover,the rate of hydrate dissociation with a saturation of 28.17%-34.02%can be enhanced in the presence of hot NaCl aqueous solution.Fig.4(a)shows the process of hydrate dissociation with 3.5 wt%NaCl aqueous solution at an injection temperature of 5.1°C.The hydrate sample with column shape formed was compact and evenly dispersed in white quartz sand,as illustrated in Fig.4(a).After the injection of 3.5 wt%NaCl aqueous solution for about 10 s,hydrate was decomposed intensely,and released decimeter bubbles and the liquid flowed upward together due to immediately dispersion of the injected NaCl solution by gas,as depicted in Fig.4(b).When hydrate was completely immersed by the liquid,more large bubbles at the level of centimeters were formed,which was attributed to intense dissociation of the hydrate during this period,as shown in Fig.4(c).In the stable dissociation stage of hydrate,the rate of gas generation was decreased,and the bubbles became smaller,making it difficult to observe the bubbles clearly in the dispersed quartz sand,as revealed in Fig.4(d)and(e).Only a small number of centimeterlevel bubbles were generated by aggregation around the top gas distributor,as shown in Fig.4(d).Until the hydrate was completely decomposed,unconsolidated sand was settled to the bottom of the decomposition kettle,and the gas distributor fell to the bottom of the decomposition kettle,as illustrated in Fig.4(f).At the same time,the pressure in the gas collecting tank was not increased.

The hydrate dissociation was affected by the injection temperatures of NaCl aqueous solution,as described in Fig.5.The cumulative amount of gas production was gradually increased with the hydrate dissociation,as shown in Fig.5(a).When hydrate was completely decomposed,the total amount of gas production reached a stable maximum value.The total amount of gas released of 24.70 L was obtained in 4.33 min at the injection temperature of 50.9°C.Fig.5(b)shows the curve of gas production rate with time at different injection temperatures in the hydrate dissociation process.The rate of gas production was increased to reach the maximum value at first and then decreased gradually to 0 L/min.The maximum rates of gas production were 4.56 L/min and 8.60 L/min at the injection temperatures of 5.1°C and 50.9°C.When the injection temperature increased from 5.1°C to 30.9°C,the average rate of gas production increased from 1.15 L/min to 3.85 L/min.The average rate of gas production was 5.57 L/min at 50.9°C and increased by 4.84 times compared with that at 5.1°C.In addition,the average rate of gas production per unit volume was 7.22 L/Lr·min-1by the injection of NaCl aqueous solution with 50.9°C and higher than the experimental results of Song et al.[27-31].The chemical potential was increased and the heat and mass transfer were enhanced to further promote hydrate dissociation in the presence of NaCl.Therefore,the combination of heat injection and inhibitor(NaCl)addition,as an effective method,can be applied in the rapid dissociation process of methane hydrate.

4.Rapid saturation evaluation and error analysis in the simulated hydrate-bearing sediment sample

Specific calculation and analysis were carried out for an experiment(Experiment 4 in Table 1).The apparent volume of the synthesized hydrate sample was 673 mL(φ 69×180 mm),which meant that the residual volume in the high-pressure autoclave was 1296 mL.

Table 1 Experimental results analysis of hydrate saturation rapid determination through definition method.

Fig.6 shows the temperature and pressure change during the process of methane hydrate formation.The high-pressure autoclave was filled with 18.9°C methane until the pressure increase to 7.23 MPa.After 24 h,the temperature and pressure in the autoclave were maintained stable,which became 0.4°C and 4.50 MPa,respectively.The results show that the amount of synthesized hydrate in the autoclave was 1.56 mol with a saturation of 68.7%.While hydrate saturation was calculated to be 61.6% after the process of pressure release and sample transfer.

After the hydrate was synthesized and the pressure in the highpressure autoclave was stable,it entered the stage of sample transfer and saturation measurement.Vent the high-pressure autoclave to the atmospheric pressure,and transfer the synthesized hydrate sample to the dissociation autoclave.Then fluid was injected into the dissociation autoclave by a syringe pump,hydrate sample gradually dissociated.Fig.7 shows temperature and pressure changes during the process of methane hydratedissociation with 33°C water injection.With hydrate dissociation,thepressurechangeinthetotalgasreceivers(24.2 L+1.7 L=25.9 L)tended to be gentle until it did not increase.It was considered to be the complete dissociation point of the hydrate sample.At this time,the total pressure in the dissociation autoclave varied from 0.1 MPa to 0.23 MPa(ΔP=0.13 MPa),which suggested the total dissociation duration was 6.3 min.During the process of water injection dissociation,the average temperature in the dissociation autoclave maintained at 14°C.

The results showed that,during six test experiments,hydrate saturation was measured to be in the range of 44.0%-63.0%,with a relative error of 3.4%-11.7%.It is further indicated that the definition method and the test system have pretty good accuracy for the determination of hydrate saturation.The specific conditions and results are listed in Table 1.

This paper designs a hydrate saturation test system that achieves hydrate saturation determination within 30 min successfully.Test speed is further improved to 2-10 times.Meanwhile,the relative error of hydrate saturation is less than 11.7%.In addition,it seems that there is no strong regularity among gas production rate,water injection temperature,or hydrate saturation,which is caused by the inconsistency of hydrate occurrence.

According to the experimental process and results analysis,there are two main factors for test error.On the one hand,the estimated error of the dissociation amount in the process of pressure releasing and transferring is 2%-10%.However,it does not exist in the application of actual hydrate saturation measurement.It indicates that this error can be avoided.On the other hand,the estimated error of the physical property constant is 0.5%-3.0%.The four parameters of sediment density,hydrate density,hydration number,and volumetric water content reported in the literature,are used in this paper,instead of the actual measured values.

5.Conclusions

The increase of liquid injection temperature was found conducive to hydrate dissociation,and the average gas production rate per unit volume was increased significantly to reach 7.22 L/Lr·min-1by the injection of NaCl aqueous solution with 50.9°C.Therefore,a rapid and precise evaluation method of hydrate saturation was proposed for potential application in the on-site scenario of natural gas hydrate exploitation.This method was able to give a good representation of gas hydrate saturation within 30 min,which increased the test speed 2-10 times faster than conventional indirect methods.Moreover,the error analysis was carried out during six experiments,which showed the relative error of hydrate saturation is less than 11.7%.This work can contribute to the efficient exploitation of natural hydrate resources by rapidly and accurately evaluating hydrate reservoir quality and enhancing hydrate dissociation.

Declaration of competing interests

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.

Acknowledgments

This study has been supported by the National Natural Science Foundation of China(21736005),and the National Key R&D Program of China(2016YFC0304006 and 2017YFC0307302).

Appendix