Increased Gas Production from Hydrates by Combining

Sep 10, 2012 - (solid lines) and the corresponding numerical simulation. (dashed lines). ..... Proceedings of the 6th International Conference on Gas ...
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Increased Gas Production from Hydrates by Combining Depressurization with Heating of the Wellbore S. Falser,*,† S. Uchida,‡ A. C. Palmer,† K. Soga,‡ and T. S. Tan† †

Department of Civil and Environmental Engineering, National University of Singapore, Singapore 117576, Singapore Department of Engineering, University of Cambridge, Cambridge CB2 1PZ, United Kingdom

‡

ABSTRACT: To extract gas from hydrate reservoirs, it has to be dissociated in situ. This endothermic dissociation process absorbs heat energy from the formation and pore fluid. The heat transfer governs the dissociation rate, which is proportional to the difference between the actual temperature and the equilibrium temperature. This study compares three potential gas production schemes from hydrate-bearing soil, where the radial heat transfer is governing. Cylindrical samples with 40% porefilling hydrate saturation were tested. The production tests were carried out over 90 min by dissociating the hydrate from a centered miniature wellbore, by either lowering the pressure to 6, 4, or 6 MPa with simultaneous heating of the wellbore to 288 K. All tests were replicated by a numerical simulation. With additional heating at the same wellbore pressure, the gas production from hydrates could, on average, be increased by 1.8 and 3.6 times in the simulation and experiments, respectively. If the heat influx from the outer boundary is limited, a simulation showed that the specific heat of the formation is rapidly used up when the wellbore is only depressurized and not heated. experimentally.9,10 In situ combustion and microwave heating are also possible.11,12 Contrary to the depressurization method, the hydrate dissociation rate and the gas production rate from hydrates do not necessarily evolve in parallel; extensive heating with little pressure reduction can dissociate the hydrate, but the pressure gradient might still be insufficient for the well to flow. Kurihara et al.13 compared the theoretical energy efficiency of different production methods, such as depressurizing, depressurizing and heating the wellbore, hot-water huff and puff, hotwater and methanol huff and puff, and hot-water flooding by numerical simulations. They concluded that, on the basis of the boundary conditions of the eastern Nankai Trough, the economics of additional wellbore heating at a constant 50 °C are almost the same as for depressurization only. In this study, it is hypothesized that a lower heating temperature is sufficient to increase gas production from hydrates. To test this hypothesis, small-scale experimental production schemes with different bottom hole pressures (BHPs) and simultaneous heating of the wellbore have been conducted. Results show that gas production at 6 MPa BHP can be increased by additional heating from 282 to 288 K. The results are replicated numerically by a fully coupled numerical code.14

1. INTRODUCTION Hydrates are crystalline solids formed when gas and water interact at favorable high-pressure and low-temperature conditions. Gas hydrates exist in abundance in nature, and with the most common hydrate-forming gas being methane, they are a very attractive source of energy for the future.1−3 To harvest methane gas, hydrate has to be dissociated in situ to enable the production of the free gas. This dissociative phase change can be carried out by depressurization, heating, inhibitor injection, or a combination. The dissociation of methane hydrates is endothermic and requires 52.4 kJ/(mol of CH4) of enthalpy,4 which makes it heavily dependent upon heat transfer from the surroundings. Several different production methods have been proposed. The field tests at the Mallik site in the Canadian Northwest Territories have shown that depressurization is a more effective production method than pure thermal stimulation by hot-water circulation.5 Dissociation by depressurization is however limited by heat transfer.6 In numerous small-scale depressurization tests, Oyama et al.7 show that the dissociation rate increases exponentially with the reduction of the pore pressure. This is because the temperature quickly drops to the pressurecorresponding equilibrium temperature and, therefore, increases the driving sensible heat/latent heat ratio of the sample. However, enhanced depressurization has its drawbacks: reduced pore pressure leads to increased effective stress in the produced formation, which, in turn, increases vertical straining and, hence, the possibility of ground settlements.8 It also requires a higher downhole pumping rate, making the process less energy efficient. Significant temperature reductions can also lead to hydrate reformation or pore water freezing, both reducing the permeability of the formation and, hence, the gas recovery rate. Alternatively, thermal stimulation, such as hot-water, steam, and inhibitor injection, has been proposed and investigated © 2012 American Chemical Society

2. METHODOLOGY 2.1. Laboratory Experiments. Tests were conducted on artificial methane hydrate samples using a rigid wall pressure vessel, as shown in Figure 1. The cylindrical soil samples had a diameter of 180 mm and an initial height of around 195 mm. The excess-water hydrate formation technique was used to obtain pore-filling hydrates with saturations of approximately 40% in a water-saturated environment, which represent uniformly heterogeneous hydrate deposits in nature. Received: June 25, 2012 Revised: August 4, 2012 Published: September 10, 2012 6259

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Figure 1. Schematic overview of hydrate-testing apparatuses (left) and hydrate-testing vessel in operation (right).18 The formation steps were the following: (1) Dry Toyoura sand (D50 = 250 μm, and Cu = 1.68) was filled in the pressure vessel at a dry density of 1.60 g/cm3. (2) Thermocouples were placed in a horizontal plane at half of the height of the sample at 1, 4, 7, 10, 14, and 18 wellbore radii from the axis; the wellbore radius is 5 mm. (3) Vertical effective stress of 2.37 MPa was applied to the sample. (4) The sample was vacuumed to about −0.15 MPa gauge pressure for 30 s to remove residual air from its pore space and supply piping. (5) Methane gas was filled into the sample, and its pressure was raised according to the desired hydrate saturation (e.g., P = 6.8 MPa and T = 305 K for Sh = 0.4). (6) The pressure was increased further to 15 MPa by injecting water. (7) The sample was cooled to about 276 K. (8) The pressure during the hydrate formation process was maintained between 10 and 16 MPa by water injection. (9) At the end of hydrate formation, the hydrate-bearing soil sample was fully water-saturated, under the effective stress of 2.37 MPa and the pore pressure of about 14.5 MPa. The required gas pressure to inject methane gas into the soil model was calculated by the Peng−Robinson equation of state,15 accounting for the reduced porosity of the sample after stress application, the methane solubility in water, and the hydration number of 6 for structure I hydrates. The assumption of full gas conversion was thought to be legitimate for dwell periods of about 70 h at about 15 MPa and 276 K. The completion of hydrate formation was indicated by no further decline in the gas pressure and a constant temperature during the pressure increase by water, showing the absence of free gas. Pressurizing by water compressed the gas into bubbles inside the pore space, from which gas−water interface hydrates were grown under stability conditions. The resulting hydrate was pore-filling or load-bearing and not matrix-cementing, which has been shown by resonance column testing of artificially prepared methane hydrate specimens created in the same manner.16 The same study also showed that, for saturations Sh ≤ 0.4, the hydrate is uniformly distributed in the pore space. The radial density profile in Figure 2 was obtained by calibrated γ-ray transmissivity measurements in axial direction. It confirms the uniformity of the sample after hydrate formation, in which the density raised from the original dry density of 1.60 g/cm3 with increasing hydrate formation and, hence, the diminution of free methane. Upon complete formation, the hydrate samples were dissociated from a miniature wellbore with a radius rwb of 5 mm located on their cylinder axis (see Figure 1). The wellbore was a fixed component of the apparatus around which the sample was formed. Thus, there have been no disturbances to the grain skeleton between hydrate formation and dissociation. Dissociation was carried out by a pressure reduction

Figure 2. Radial density profile of the sample after completed hydrate formation. or a combination of depressurization and resistivity heating. The wellbore or BHP was controlled by a spring-loaded regulator valve. The pore fluid flow was radial toward the uniformly depressurized wellbore. Gravitational effects in the partially water- and gas-saturated zone have shown to be negligibly small for the stated conditions.17 The heating temperature was achieved by running 60 V direct current (DC) through the 240 Ω heater and was controlled by a solid-state relay regulating the current supply. Thermocouples were placed at radial distances r/rwb = 1, 4, 7, 10, 14, and 18 wellbore radii from the axis (the absolute distances were 5, 20, 35, 70, and 90 mm from the axis). The heating temperature was controlled by the thermocouple at r/r0 = 1, which at steady state fluctuated within 1 K around the set temperature with a frequency of about 1 min. Therefore, only the mean temperature history is shown at the wellbore. The extracted gas and water were separated by gravity before the gas was metered and the water production was recorded by weighing. The experimental gas production rate was throttled between the separator and the flow meter in order not to exceed the upper range of the meter of 0.8 SL/ min. Larger flow rates were therefore measured by a continuously weighted water displacement device. The vertical effective stress was applied through a weight-loaded hydraulic piston, which maintained the vertical effective stress level independent of the straining. The environmental temperature of the sample was controlled by glycol circulation around the pressure vessel and an air-conditioned enclosure and was maintained constant throughout the testing. A more detailed description of the apparatus can be found by Falser et al.18 Three different dissociation tests were conducted: (1) ΔP6 depressurization to 6 MPa BHP without heating, (2) ΔP 4 depressurization to 4 MPa BHP without heating, and (3) ΔP6 + ΔT depressurization to 6 MPa BHP and heating from 282 to 288 K. 6260

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Each experimental test was duplicated to check repeatability. The wellbore temperature and pressure conditions before and during testing are shown in the methane hydrate phase diagram (Figure 3). In

equilibrium temperature as the outer boundary (insulated, no heat from the outer boundary) were performed (subscript n,ins) to show this boundary effect on the gas production. The parameters used in the simulation are listed in Table 2: The absolute permeability k0 was chosen as 350 mD.7 Changes during dissociation were modeled by the following semi-empirical power law in eq 1.20 The relative permeability for two-phase flow and the capillary pressure relationship are modeled by the van Genuchten21 relations. Table 3 summarizes the physical parameters used in this study. Further details of the models implemented in the numerical code, such as dissociation kinetics and hydrate-dependent elastoplastic soil model, can be found from the study by Klar et al.14 k = k 0(1 − S h)2

(1)

where Sh is the hydrate saturation.

3. RESULTS AND DISCUSSION The production period for each test was 90 min after the methane hydrate phase boundary conditions were reached. The wellbore pressure was kept constant during that period, despite some small fluctuation because of intermittent production rates. Figure 4 shows the wellbore pressure (top) and the accumulated gas volume (bottom) for each experimental test (solid lines) and the corresponding numerical simulation (dashed lines). When ΔP6 is compared to ΔP6 + ΔT, a manifold increase in gas production can be observed if the wellbore is simultaneously heated during depressurization. This substantial difference is caused by the difference in dissociation drive; the endothermic hydrate dissociation in sediments is clearly a heat-transfer-limited process. The larger the difference between the initial in situ temperature and the equilibrium temperature for the corresponding wellbore pressure, the faster dissociation takes place, leading to a higher production rate. The mechanism of this dissociation drive can be comprehended by the temperature histories shown for each test in Figure 5. The equilibrium temperatures Teq in Figure 5 are calculated from the measured pore pressure histories shown in the upper graph in Figure 4, which are almost constant within the sample, except for small pressure gradients when the wellbore is flowing. In ΔP6, the difference between the boundary and equilibrium temperatures is just 1 K, which means little heat energy can be obtained from the sediment and the temperature gradient between the well and the boundary temperature, which is kept constant at 282 K at 18 wellbore radii, is small. As a result, the conditions are at equilibrium throughout the testing period of 90 min, leading to an intermittent gas production rate, as shown in Figure 9. In ΔP4, where a larger pressure reduction was applied, the temperature difference is 5 K. Hence, the immediate

Figure 3. Wellbore pressure and temperature conditions during the production tests. ΔP6 + ΔT, the miniature well was first heated until the set temperature was reached, and afterward, the pressure was reduced at about 2 MPa/ min. No hydrate had been dissociated before the pressure was reduced. The slight increase in the temperature during pressure reduction in ΔP6 and ΔP4 is due to the decrease in gas solubility and, hence, additional hydrate formation.19 The temperature change at the wellbore in ΔP4 and ΔP6 + ΔT was of comparable inverse magnitude, and its significance for hydrate dissociation is discussed in section 3. A total of six experimental tests and six numerical simulations (subscript n) were conducted. The properties of the samples and initial and boundary conditions are given in Table 1 and are similar to those found in the α field of the Nankai Trough.13 The final sample porosity depended upon its compaction during stress application and varied slightly between the different test runs. 2.1. Numerical Simulation. The numerical simulations of the experiments were carried out in one radial dimension, using a fully coupled thermo-hydromechanical code for hydrate-bearing sediments by Klar et al.14 The boundary conditions at the wellbore varied between insulated sample temperature for depressurization and constant temperature when the wellbore is heated. The outer boundary was modeled with a steel element representing the wall of the pressure vessel, at which the outer side of the environmental temperature was controlled in two different ways: in the first simulations, the temperature was kept constant to simulate the laboratory experiments (subscript n,exp). Because a constant temperature boundary is an experimental limitation compared to in situ conditions, additional numerical simulations with the methane hydrate

Table 1. Properties of Hydrate-Bearing Test Samples test

ΔP6

ΔP6

ΔP6,n

ΔP4

ΔP4

ΔP4,n

ΔP6 + ΔT

ΔP6 + ΔT

ΔP6 + ΔTn

porosity, n hydrate saturation, Sh initial pressure, P0 (MPa) initial and boundary temperature, T0 (K) bottom hole pressure, BHP (MPa) heating temperature, Twell (K) constant environmental temperature (K) depressurization rate, dP/dt (MPa/min) vertical effective stress, σz′ (MPa) initial sample height (mm)

0.375 0.40 14.56 282.0 5.97

0.383 0.38 14.58 282.0 5.86

0.38 0.40 15 282.0 6.0

0.397 0.40 14.82 282.0 4.24

0.385 0.40 14.83 282.1 3.88

0.38 0.40 15 282.0 4.0

282 −1.22 2.37 202

282 −2.38 2.37 198

282 −1.73 2.37 200

282 −1.93 2.37 198

282 −1.35 2.37 206

282 −2.07 2.37 200

0.378 0.42 14.44 282.2 5.95 288.15 282 −2.5 2.37 206

0.395 0.41 14.52 282.3 5.87 288.15 282 −1.56 2.37 207

0.38 0.40 15 282.0 6.0 288.15 282 −1.73 2.37 200

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Table 2. Species Properties Used in the Numerical Simulation Cp k μ ρ

specific heat capacity (T = 293 K) thermal conductivity viscosity (T = 293 K) density

(J kg−1 K−1) (W m−1 K−1) (Pa s) (g/cm3)

viscosity ratio hydrate number phase equilibrium pressure

Kw Kg P0 a b c μw/μg Nh Peq

(GPa) (kPa)

(kPa)

steel

water

methane

hydrate

800 3.92

500 16 7.8

2157 0.034 1.25 × 10−5 16 g/mol

2010 0.62

2.60

4189 0.56 0.001 1.0

0.90

environment leads to a progressive deviation from equilibrium, resulting in a more sustained production rate. In ΔP6 + ΔT, the same little natural heat energy as in ΔP6 is available but the flux from the heated wellbore leads to a constant disequilibrium zone in its vicinity. Because of this, a similar sustained production rate to that in ΔP4 is achieved. The initial temperature spike at around −5 min is due to the test procedure: the miniature wellbore was first heated, and only once the set 288 K had been reached, the pressure was decreased to 6 MPa. The initially high pore−water flux toward the well equilibrated the temperature just before dissociation commenced. As shown in Figure 4, the numerical simulations (subscript n,exp) confirm the production trends in each test and the produced volumes agree within 25%. The differences are caused by small deviations of the relative permeability evolution of gas to water, which change as dissociation progresses, and therefore, the relative saturations shift. Adjusting it in repetitive simulations would lead to a better convergence but remains of little use without a physical measurement to use as a benchmark. Further discrepancies arise from differences in temperature evolutions at the boundaries. The left column in

Table 3. Physical Flow and Dissociation Parameters Used in the Numerical Simulation water bulk modulus gas bulk modulus Van Genuchten parameters

sand

2 Pg (ideal gas) 10 0.92 0.5 0.5 1050 − 3.31T 6 exp(40.234 − 8860/T)

dissociation drive is larger because more heat energy is available from the formation and the constant temperature environment. At the outer boundary (18 r/rwb), where the temperature outside the pressure vessel is maintained constant at 282 K by fluid circulation, the temperature of the sample does not drop to the equilibrium temperature but remains below the set boundary temperature throughout the testing period, suggesting that the dissociation process was not fully completed after 90 min. The comparably enhanced heat flux from the

Figure 4. (Top) Pore pressure development during production tests with 4 MPa BHP (ΔP4), 6 MPa BHP (ΔP6), and 6 MPa BHP combined with wellbore heating to 288 K (ΔP6 + ΔT) tests. (Bottom) Accumulative gas production of the same tests. Experimental results are shown in solid lines, and numerical simulations are shown in dashed lines. 6262

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Figure 6 shows the temperature profile in the radial direction at different times (10, 50, and 90 min). The heating of the well affected a zone extending to about 8−10 wellbore radii from the center after 90 min, which is in good agreement with the simulation. In ΔP4, on the other hand, where the radial temperature gradient is largest at the outer boundary, the measured heat flux from the outer boundary was smaller than in the numerical simulation. Hence, the produced gas volumes in the depressurization experiments are smaller than those calculated from the simulations. The accumulative water production histories in Figure 7 show that the extracted

Figure 7. Accumulative water production in liters during the depressurization tests ΔP4 and ΔP6.

water volume increased with a decreased wellbore pressure. In ΔP4, the produced water volume after 90 min was 0.79 L (66% of the total water in place), while in ΔP6 and ΔP6 + ΔT, only 0.53 L (45%) and 0.51 L (43%) were extracted. The trends show that almost all of the extracted water is produced during the depressurizing of the wellbore between −5 and 5 min. The

Figure 5. Temperature evolutions of ΔP6 (top), ΔP4 (center), and ΔP6 + ΔT (bottom) at different wellbore radii r/rwb. The dashed lines represent the methane hydrate phase equilibrium temperature Teq.

Figure 6. Temperature and methane hydrate saturation profiles for constant outer boundary temperature conditions after 10, 50, and 90 min of dissociation. Experimental data and numerical simulations are shown as bullets and dashed lines, respectively. The color for ΔP6 is blue. The color for ΔP6 + ΔT is red. The color for ΔP4 is green. 6263

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Figure 8. Produced gas after 90 min of controlled methane hydrate dissociation (experiments are labeled with tests 1 and 2, and simulations are labeled with experimental boundary conditions with subscript n,exp). The total production simulations (subscript n,∞) took 600 min in ΔP6,n,∞, 228 min in ΔP6 + ΔTn,∞, and 202 min in ΔP4,n,∞ for completion. All produced gas volumes are given in liters at standard conditions (SL), and the recovery factor is the ratio between the volume recovered and the gas volume in place.

depressurization cases ΔP6 and ΔP4, the dissociation front moves from the outer boundary toward the center, because all of the required heat energy is supplied from the environment held constantly at 282 K. In ΔP6 + ΔT, the hydrate is simultaneously dissociated from the wellbore and the outer boundary but at a higher rate from the wellbore as a result of the higher dissociation drive. The production rate depends upon the rate of dissociation. In the laboratory tests, the BHP is controlled by a spring-loaded regulator with a defined cracking pressure. The intermittency in the measured production rate for ΔP6 is due to the free gas being removed faster from the sample than its being dissociated from the hydrate for the given wellbore geometry. In the numerical simulation, the rate is defined solely by the amount of gas reaching the wellbore. The gas production rates for each test are given in Figure 9; the calculated production rate in general matches well with the trend of the experiments. As one of the unique features of gas production from hydrates, the ΔP4 tests show that the production rate is high at the beginning but then decreases rapidly. This is because the large dissociation drive can be obtained from the initial temperature difference right after depressurization, but afterward, it reduces as the formation temperature drops by the endothermic hydrate dissociation process. For the insulated outer boundary conditions, the production rate peaks at the beginning, identical to the simulated rate with the fixed temperature boundary conditions, but then ceases altogether for the depressurization schemes and is reduced to a fraction for the wellbore heating. This implies that the dissociation process is first governed by the latent heat of the formation and later by the heat energy transferred to the dissociating region, by either conduction or fluid convection from outer non-dissociated regions or conduction from the heated wellbore. Similar to the tests presented here, in a full-scale production scheme from a uniform hydrate reservoir, the radial heat transfer may govern the dissociation rate and, hence, the recovery. One of the unavoidable differences lies in the radial outer boundary condition; while in the laboratory tests, the temperature was kept constant at the fixed boundary at 18

local maxima in the graphs are caused by the flow dynamics in the gas−water separator. In ΔP6 + ΔT, only the total volume could be measured because of data recording inconsistencies during the test. Figure 8 shows the produced volumes and recovery factors of the experimental tests and their numerical simulations (subscript n,exp), as well as the total gas recovery (subscript n,∞). The recovery factor is defined as the ratio between the produced gas and the total initial gas in the hydrate in place. The total producible gas volume for the three different recovery methods has been determined by simulations. Hydrate dissociation stops when the available heat from the environment and specific heat from the formation have been exhausted or are no longer sufficient for the endothermic process, while the remaining gas from dissociated hydrates is still being produced. Two experimental tests were carried out for each production scheme, and the close agreement of their results confirms their repeatability. By comparing the production volumes in the simulations after 90 min with the total recovered gas, one notes the following: In ΔP6, the production continues over 10 h and the recovery is increased by 3 times to about 50%. In ΔP4, the production ceases much earlier, after about 3.4 h, and the recovery is only increased by about 20% to a total recovery of 80%, which shows that a large proportion of the hydrate was dissociated and recovered within the first 90 min of testing. In ΔP6 + ΔT, the gas volume recovered after 90 min is doubled within 4 h, giving a gas recovery of 62%. This shows again the heat transfer dependency of the dissociation process; the dissociation rate is proportional to the dissociation drive, which is the difference between the boundary temperature and the equilibrium temperature.17 In production schemes with a larger thermal dissociation drive (approximately ΔT = 5 K for ΔP4 and ΔT = 6 K at the wellbore for ΔP6 + ΔT), a larger proportion of gas can be recovered from hydrates in a shorter time than those with a comparably smaller thermal dissociation drive (approximately ΔT = 1 K for ΔP6). The hydrate saturation profiles after 10, 50, and 90 min of gas production are given in the right column of Figure 6. The numerically obtained saturation profiles confirm that, in the 6264

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Figure 10. Accumulated gas production for the methane hydrate equilibrium temperature as outer insulated boundary conditions (subscript ins) and experimental outer boundary conditions (subscript exp).

Figure 9. Production rates in SL per minute. Experimental data are shown in solid lines, and numerical simulations with constant (282 K) and hydrate equilibrium outer boundary conditions are shown in dashed and dashed-dotted lines, respectively.

wellbore radii, in a reservoir, the temperature at the dissociation front will always be close to the hydrate equilibrium temperature, depending upon the pressure gradient and, consequently, the effect of forced convection through the pore fluid into the dissociating region. To determine the lower bound of the effect of the environmental temperature, a numerical simulation has been conducted in which the outer boundary of the same geometry was kept at the hydrate equilibrium temperature for the respective wellbore pressure. This limits the heat flux from the environment, and hence, most of the required heat for dissociation has to be absorbed from the formation and pore fluid (for depressurization) or obtained through conduction from the heated wellbore. The production curves in Figure 10 confirm this hypothesis: if the available sensible heat is approximated with the specific heat of the sand only (800 J kg−1 K−1), which is reasonable because a large proportion of the water was very rapidly drained in the experiments, it amounts to about 37.4 kJ in ΔP4,n,ins. Assuming that all of the energy is used for dissociation, it frees 16 L of CH4 from hydrates, which is in good agreement with the recovered 11.4 L if the residual gas in the pore space is taken into account. It further shows that this sensible heat from the formation is used up by the hydrate dissociation very rapidly, whereas a heated wellbore (ΔP6 + ΔTn,ins) leads to a slower but more sustained production rate. The production data for this insulated case is given in Figure 11; without any significant heat flux from the outer boundary, the depressurization scheme at 6 MPa BHP leads to only a small amount of gas production, despite the theoretical 12.2 kJ of specific heat available from the sediment. This suggests that

Figure 11. Produced gas after 90 min of controlled methane hydrate dissociation with insulated outer boundary conditions compared to the constant temperature outer boundary conditions of the experiments.

the available dissociation energy in the form of the latent heat of the formation and the recovered volume is not strictly correlated, because some fraction of the freed gas remains trapped in the pore space. This phenomenon is enhanced in the experiments because of the impermeable outer boundary and, hence, the absence of a pressure gradient. The presence of gas dissociated from hydrates can induce a pressure gradient as a result of capillary pressure and enhanced gas permeability. However, in the case of ΔP6,n,ins, the amount of trapped gas is so small that both the capillary pressure and gas permeability are not large enough to provide sufficient flow. The temperature and saturation profiles for the insulated outer boundary case are given in Figure 12. In the depressurization schemes, the temperatures are at equilibrium throughout, whereas the zone affected by the wellbore heating remains unchanged in comparison to the previous boundary conditions. In line with the latent heat that governed dissociation in the ΔP4,n,ins case, its hydrate saturation is uniformly reduced to 35% and remains constant over the entire testing period. In ΔP6 + ΔTn,ins on the other hand, the dissociation front extending over about 6 wellbore radii moves progressively outward from the wellbore, suggesting that also, in this insulated case, a combination of reduced pressure and 6265

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Figure 12. Temperature and methane hydrate saturation profiles for insulated outer boundary temperature conditions after 10, 50, and 90 min of dissociation. The color for ΔP6 is blue. The color for ΔP6 +ΔT is red. The color for ΔP4 is green.

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wellbore heating is a more efficient dissociation driver in the long term compared to a pure depressurization.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +65-91307307. E-mail: [email protected].

4. CONCLUSION

Notes

The authors declare no competing financial interest.

The novel experimental results with a heated miniature wellbore were successfully replicated by numerical simulations. The experimental tests showed that gas production during the period of 90 min at the same wellbore pressure of 6 MPa was on average increased by 3.6 times when the wellbore was heated as compared to sole depressurization. The numerical simulation showed a 1.8 times increase in the produced gas. This difference is due to deviations in the numerically approximated relative gas permeability as well as small discrepancies in heat fluxes from the boundaries between the experiments and simulations. Choosing a lower wellbore pressure to produce a similar gas volume within the testing period of 90 min results in a substantially increased upfront production rate but which then declines because of endothermic cooling. Hence, a heated wellbore results in a more sustained dissociation drive compared to a lower wellbore pressure. The dissociation process of hydrates in saturated sediments is governed by the radial heat flux from the environment into the dissociating zone or, in the case of a heated wellbore, from both directions. In natural conditions, pore water feeds additional heat energy into the dissociating region by forced convection. The boundary conditions in a production scenario from a semiinfinite reservoir may, hence, lie between the presented constant outer boundary temperature and the insulated case. The effect of heat convection depends upon the geomechanical and thermodynamic properties of the larger reservoir and is therefore difficult to assess and account for in small-scale experiments. The results showed further that the specific heat of the formation was consumed rapidly and, thus, contributed only at a very early stage to the dissociation process.

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ACKNOWLEDGMENTS The financial support from the Agency of Science, Technology and Research (A*STAR) and the Maritime and Port Authority of Singapore (MPA) (MCE/99/003 SERC-072-135-0026) is gratefully acknowledged.

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REFERENCES

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dx.doi.org/10.1021/ef3010652 | Energy Fuels 2012, 26, 6259−6267