Effectiveness of Low-Dosage Hydrate Inhibitors and their Rheological

Aug 19, 2014 - Management of hydrate flow assurance issues has been a major problem in offshore and deepwater–oil and gas production. Hydrate flow ...
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Effectiveness of Low-Dosage Hydrate Inhibitors and their Rheological Behavior for Gas Condensate/Water Systems Afzal Memon and Heng-Joo Ng* Schlumberger, 9450 17th Avenue NW, Edmonton, Alberta T6N 1M9, Canada ABSTRACT: Management of hydrate flow assurance issues has been a major problem in offshore and deepwater−oil and gas production. Hydrate flow assurance strategies include the use of low dosage hydrate inhibitors (LDHIs) such as kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs) among others. The effectiveness of AAs should be studied in detail before embarking on using such chemicals in the field. In the current work, a systematic AA laboratory screening study has been undertaken on a gas condensate field. The rheological measurement of hydrate slurry in the presence of AAs under various operating conditions provided important insight into AA screening. Hydrate slurries were formed in a fully visual sapphire pressure−volume−temperature (PVT) cell in the presence of various AA chemicals. The hydrate slurry was then transferred to a high pressure rheometer for viscosity measurements at various shear rates. The effectiveness of these chemicals and their rheological behavior were evaluated as a function of pressure, water cut, and shear rates under various operating scenarios, that is, flowing conditions and shutdown/restart conditions. The result of this hydrate flow assurance testing indicated that hydrate slurry in the presence of the AAs behaved as a non-Newtonian fluid with a shear thinning effect. The water cut and pressure had an impact on the effectiveness of the AAs.

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INTRODUCTION Flow assurance is one of the major issues facing the offshore oil and gas exploration and production (E&P) industry. Hydrate flow assurance issues are the largest concern by an order of magnitude because the formation of hydrate is usually very rapid in comparison to wax, asphaltene, and scale.1 Gas hydrate is composed of gas molecules entrapped by water molecules.2 Remediation of hydrate flow assurance after hydrate plugs are formed can take days to months, thus increasing the cost of downtime in hydrocarbon production. The cost of loss of production particularly in offshore environments can severely impact the profitability of oil and gas production companies. The hydrate flow assurance issue can also pose health, safety, and environment (HSE) risk when encountered during production. Hydrate flow assurance strategies include the prevention or avoidance of hydrate formation by shifting the hydrate formation pressure−temperature (P−T) phase envelope toward the left. This condition can be achieved by using thermodynamic hydrate inhibitors such as methanol, monoethylene glycol (MEG), and so forth. Crossing the hydrate phase boundary can also be avoided by maintaining the temperature of hydrocarbon fluid through thermal insulation or heat treatment. Both of these avoidance strategies; that is, the use of thermodynamic inhibitors and thermal management can be effective but are not very practical for offshore environments due to the associated high costs of such strategies. Hydrate risk management strategies can be more economical or desirable for such cases where the hydrate formation is allowed but the flow of hydrate particles is ensured through nonthermodynamic © XXXX American Chemical Society

chemical inhibitors in low dosage. The type of LDHIs are KHIs and AAs. The KHIs inhibit the hydrate plug formation for a longer period of time whereas the AAs modify the hydrate particles by altering the agglomeration of hydrate particles.3,4 Application of KHIs is also restricted by the degree of subcooling. Use of AAs to manage hydrate flow assurance riskmanagement has been on the increase and is more commonly being accepted by the industry. The viscosity of hydrate/water/oil slurries and parent water/ oil emulsion samples are measured in the past.5 Webb et al.6 have systematically studied the viscosity and rheological behavior of hydrate/water/oil emulsions, yield stress as a function of annealing time and effect of shear on hydrate slurry with an oil/water system. Yan et al.7 and Peng et al.8 have studied the flow characteristics and rheological properties of natural gas hydrate slurry for gas + diesel oil/condensate + water systems in the presence of AA. The apparent viscosity of hydrate slurry was derived from hydrate tests in a flow loop apparatus under flowing and shut-down/restart scenarios at a constant pressure of 2.1 MPa. They had observed shear thinning behavior of hydrate slurry from the shear rate of 40 to 360 s−1. Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: June 26, 2014 Accepted: August 4, 2014

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The main objective of this laboratory case study was to test the effectiveness of various AA-LDHI chemicals by mimicking the live flowline and riser conditions in a high pressure PVT cell. The hydrate AA-LDHI effectiveness runs were performed by mimicking two types of protocols: • Flowing experiments. • Shutdown/restart experiments. Based on the visual observation of mixing hydrate slurry in the presence of AA-LDHI chemicals, the effectiveness of the AA-LDHI was judged with “pass” or “fail” criteria as described in the following section. For all pass runs with the AA-LDHI chemical, the hydrate slurry viscosity was further measured as a function of shear rate with a high-pressure rheometer system. Hydrate slurry viscosity measurement with AA for this gas condensate/water system would provide key input to dynamic multiphase flow simulation with OLGA for the pressure/ temperature/water cut conditions where AA was effective in keeping hydrate in the transportable slurry form. In the current work, apparent hydrate slurry viscosity was measured using high pressure rheometer system at various shear rates ranging from 250 to 1000 s−1, water cut volume fractions w up to 0.5 and pressures up to 12.5 MPa.

Table 1. Compositions of Condensate Used in This Work, As Mole Fractions m

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EXPERIMENTAL SECTION AA-LDHI Effectiveness Testing. The methodology and testing protocol used for the AA-LDHI evaluation is briefly described here. First, a stabilized condensate sample was conditioned and homogenized at 323.15 K for 3−4 h. The molecular composition of stabilized condensate sample is presented in Table 1. The stabilized condensate was very waxy in nature; hence, it was conditioned above the wax appearance temperature (WAT) or the cloud point. The conditioning at this temperature may have vaporized some light components from stabilized condensate; however, the conditioning was necessary to maintain the validity of the stabilized condensate sample with respect to wax precipitation. Two AA-LDHI chemicals, a and b were tested in this work. The operator also planned to inject wax inhibitor/pour point depressant (PPD) to reduce the pour point below the seabed temperature of 278.15 K. Hence, 1000 mg/L of wax PPD was also added to the stabilized condensate sample to study the effectiveness of the AA-LDHI chemicals in the presence of the PPD. The required volume of “doped” stabilized condensate was displaced in to the sapphire PVT cell at 323.15 K and atmospheric pressure. The sapphire PVT cell setup is shown in Figure 1. The sapphire PVT cell is rated to 16.65 MPa and 373.15 K. A donut-shaped mixer was installed inside the sapphire PVT cell, which is magnetically coupled with an outer magnetic ring. The outer magnetic ring is connected to an electrical motor with crank-shaft mechanism for up−down movement. This mechanism provides a strong mixing of live fluid inside the PVT cell. The pressure can be maintained inside of the sapphire PVT cell by a piston connected with hydraulic fluid on the other side. The pressure inside the PVT cell can also be maintained by connecting the gas cylinder to the top side of PVT cell. The pressure in the high-pressure cylinder is maintained by an automatic pump. Before performing the hydrate AA-LDHI effectiveness tests, various viscosity reference standards were transferred to the sapphire PVT cell to check the maximum viscosity fluid that could be effectively mixed with the magnetically coupled donutshaped mixers. Some “slippage” of the mixer was observed for

component

m

propane i-butane n-butane i-pentane n-pentane hexane methyl cyclopentane benzene cyclohexane heptane methyl cyclohexane toluene haptanes ethylbenzene m-xylene and p-xylene o-xylene nonanes decanes undecanes dodecanes tridecanes tetradecanes pentadecanes hexadecanes heptadecanes octadecanes nonadecanes icosanes plus

0.0052 0.0055 0.0159 0.0170 0.0217 0.0484 0.0159 0.0109 0.0198 0.0559 0.0327 0.0182 0.0705 0.0035 0.0152 0.0058 0.0614 0.0704 0.0526 0.0457 0.0462 0.0387 0.0376 0.0296 0.0261 0.0262 0.0233 0.1800

Figure 1. High pressure sapphire PVT cell used for AA-LDHI effectiveness testing and hydrate slurry generation with various protocols.

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all reference runs when viscosity exceeded 40 mPa·s. The mixer completely stopped mixing (moving up/down) or failed at a viscosity of 75 mPa·s. A total of 1.25% of specific AA-LDHI by volume and 1.84% sodium chloride (NaCl) by weight were mixed with water in a separate high pressure cylinder. The specified volume of brine and AA-LDHI mixture was added to achieve the necessary water cut (by volume) with doped condensate in the sapphire PVT cell at 323.15 K and atmospheric pressure. The sapphire PVT cell was then pressurized with a specified hydrocarbon gas to the test pressure (5/10/12.5 MPa). The molecular composition of hydrocarbon gas used in this work is presented in Table 2. The sapphire cell contents were then equilibrated at Table 2. Compositions of Natural Gas Used in This Work, As Mole Fractions m component

m

nitrogen methane ethane propane i-butane n-butane i-pentane n-pentane

0.0042 0.8699 0.0774 0.0313 0.0051 0.0082 0.0020 0.0020

Figure 2. Examples of pass and fail runs for hydrate slurry in the presence of AA-LDHI for flowing experiments.

rheometer for slurry viscosity measurements. The laboratory setup of the sapphire PVT cell connected to the HP rheometer used in this study is shown in Figure 3. The HP rheometer,

the measurement pressure and 323.15 K for an appropriate duration with the mixer turned ON to saturate the doped condensate, water and AA-LDHI mixtures at the measurement pressure. The cell and contents were cooled to 278.15 K at a rate of 5 K per hour with sapphire PVT cell mixer is an ON condition. During cooling, the system pressure was maintained constant by keeping the PVT cell connected to the pressurized gas at the test pressure, which avoided gas depletion in the headspace. All LDHI effectiveness runs were performed at 278.15 K. The maximum subcooling tested in work is about 15 K at the highest measurement pressure of 12.5 MPa. For the flowing experiments, the contents in the sapphire PVT cell were stabilized and held at the test pressure and 278.15 K for 8 h with the mixer in the ON condition. Visual observations and photographs were taken during the cooling and stabilization periods. During and after 8 h of stabilization at 278.15 K to mimic the flowing experiments (mixer turned ON), the sapphire PVT cell was checked to determine if the hydrate slurry could be mixed. Based on the visual observation, the runs were deemed to be pass or fail runs. Essentially the fail runs would have the slurry viscosity greater than 75 mPa·s. For shutdown/restart experiments, the contents were stabilized at the test pressure and 278.15 K for 8 h with the mixer in an OFF condition. After 8 h of static stabilization at 278.15 K with mixer turned OFF, the mixer was then turned ON to mimic the restart conditions if the hydrate slurry could be mixed after the shut-in period. Visual observations were made, and videos and photographs were taken during the cooling, stabilization and restart periods. If the hydrate slurry could be mixed upon mixer restart, the run was deemed to be a “pass” run. The example pictures of “pass” and “fail” runs of the hydrate slurry in the presence of AA-LDHI for dynamic runs are shown in Figure 2. Rheological Measurements of Hydrate Slurry. For all pass runs, with both AA-LDHI chemicals, the hydrate slurry was transferred through a 1/8 in. line to high pressure (HP)

Figure 3. High pressure (HP) rheometer setup connected to the sapphire PVT cell used for hydrate slurry viscosity measurements.

pressure rated to 40.78 MPa and a temperature range of T = (263.15 to 473.15) K, consists of the ATS rheometer drive unit in addition to a HP rheometer cell (Figure 4). The inner cylinder (bobbin), which has only one contact point through a sapphire ball bearing at the top of the high-pressure cell, is driven by magnetic coupling. The uncertainty of the viscosity measurement of the HP rheometer system is ±10%. The hydrate slurry densities were also measured gravimetrically by transferring the content from PVT cell to a 75 cm3 pycnometer at test conditions. The uncertainty of gravimetric high pressure density measurements is ±0.5%.

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RESULTS AND DISCUSSION Rheological Measurements with Chemical a. For all pass hydrate runs with chemical a, the hydrate slurry was transferred to the HP rheometer and the slurry viscosity was measured at the shear rates of 300, 500, 750, and 1000 s−1 and at 278.15 K with a gas cap on top of it to maintain the test pressure. As shown in Figure 5, the hydrate slurry created at water cut volume fraction w = 0.25 and 12.5 MPa with flowing experiments protocol was first subjected to an increase in shear rate from 300 to 1000 s−1 in the HP rheometer. As the shear rate increased, the viscosity decreased illustrating a shear thinning behavior. Moreover, the hydrate slurry viscosity C

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Figure 6. Hydrate slurry apparent viscosity μapp as a function of shear rate γ̇ at various water cuts and pressures with chemical a for pass runs only. Blue ◆, water cut volume fraction w = 0.25, pressure = 12.5 MPa, flowing experiment; black ×, water cut volume fraction w = 0.4, pressure = 5 MPa, flowing experiment; red ○, water cut volume fraction w = 0.25, pressure = 12.5 MPa, shutdown/restart experiment.

experiments protocol. While mimicking the shutdown/restart scenario, the hydrate slurry was maintained at 278.15 K for 8 h without mixing. This process of shutdown (mixer OFF) for 8 h created more viscous hydrate slurry than the flowing experiments (mixer ON) protocol under the same water cut and test (pipeline) pressure. Lower hydrate slurry viscosity with flowing experiments protocol can be attributed to the shearing effect due to mixing at 278.15 K for 8 h with mixer ON. On the basis of these results, all subsequent runs of hydrate AA-LDHI testing and its rheological measurements were performed by mimicking the shutdown/restart experiments protocol. AA-LDHI Effectiveness Testing with Chemical a. An AA-LDHI run at water cut volume fraction w = 0.25 and 12.5 MPa was observed to be a pass run under the flowing experiments procedure with chemical a. The hydrate testing run with water cut volume fraction w = 0.4 was observed to be a pass at 5 MPa. The other two runs with a water cut volume fraction w = 0.4 at 10 and 12.5 MPa were observed to be fail runs under the flowing experiments procedure. After changing the hydrate testing protocol from flowing experiments to shutdown/restart experiments, two runs with water cut volume fraction of w = 0.25 and w = 0.4 at 12.5 MPa were performed. These tests were carried out to check the effect of change in hydrate testing protocol on the effectiveness of chemical a. The same results were obtained with two repeat runs with shutdown/restart experiments protocol confirming that the change in the hydrate testing protocols did not influence the effectiveness of chemical a. AA-LDHI Effectiveness Testing with Chemical b. All hydrate testing runs with chemical b were performed using the shutdown/restart experiments hydrate testing protocol. All of the testing runs at water cut volume fraction w = 0.25 and two pressures, 5 and 12.5 MPa were observed to be pass runs. The hydrate testing runs with water cut volume fraction w = 0.4 were observed to be pass runs at 5 and 10 MPa. The water cut volume fraction w = 0.4 run at 12.5 MPa with chemical b was observed to be a fail run. One run was performed with water cut volume fraction w = 0.5 and 10 MPa. This run was observed to be a “pass” run. Rheological Measurements with Chemical b. For all pass hydrate runs with chemical b, the viscosity measurements were performed with the HP rheometer. The hydrate slurry

Figure 4. Schematic of HP rheometer system.

Figure 5. Rheological measurement of hydrate slurry created at water cut volume fraction w = 0.25 and 12.5 MPa with chemical a with flowing experiment protocol: apparent viscosity μapp and shear rate γ̇ as a function of time. Blue ◇, apparent viscosity; red ×, shear rate.

reached a constant value at each shear rate after the shear thinning effect was completed. After reaching the shear rate of 1000 s−1, the shear rate was reduced to 300 s−1 in various steps to determine if the slurry viscosity returned to its original value. The slurry viscosity reverted back to its original stable value at 1000 s−1 demonstrating that the shear thinning behavior was reversible at these conditions. The hydrate slurry rheological measurements were further performed for the water cut volume fraction w = 0.25 at 12.5 MPa with shutdown/restart experiment protocol and a water cut volume fraction w = 0.4 at 5 MPa under the flowing experiments protocol. The results of stable and constant viscosity are plotted versus the shear rate for these runs and are shown in Figure 6. The viscosity of the hydrate slurry created using the shutdown/restart protocol was found to be higher than the viscosity of hydrate slurry created using the flowing D

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viscosity measurements were completed at the shear rates of 300, 500, 750, and 1000 s−1 and at 278.15 K. The average values of hydrate slurry viscosity with chemical b are plotted in Figure 7 for the various hydrate runs. The hydrate slurry

Figure 8. Hydrate slurry density ρ at various water cuts and pressure with chemical b for pass runs only. Blue ◆, water cut volume fraction w = 0.25; red □, water cut volume fraction w = 0.4; green ▲, water cut volume fraction w = 0.5. Figure 7. Hydrate slurry apparent viscosity μapp as a function of shear rate γ̇ at various water cuts and pressure with chemical b for pass runs only. Blue ◆, water cut volume fraction w = 0.25, pressure = 12.5 MPa; green ▲, water cut volume fraction w = 0.25, pressure = 5 MPa; black ×, water cut volume fraction w = 0.4, pressure = 5 MPa; red ○, water cut volume fraction w = 0.5, pressure = 10 MPa.

viscosities with chemical b were observed to be comparatively lower than the slurry viscosities with chemical a under similar test conditions. Figure 7 shows that for the constant water cut volume fraction w = 0.25, the hydrate slurry viscosity increased with a decrease in pressure for this work. At high pressures, more solution gas was dissolved in the continuous liquid/condensate phase, which resulted in a decrease in the slurry viscosity. Hence, the high system pressure would be useful in transporting the hydrate slurry due to the positive effect of solution gas on hydrate slurry viscosity. For a constant pressure (5 MPa), the hydrate slurry viscosity decreased with an increase in water cut volume fraction from w = 0.25 to w = 0.4 in this work. The hydrate viscosity for water cut volume fraction w = 0.5 and 10 MPa was similar to the hydrate viscosity for water cut volume fraction w = 0.4 and 5 MPa. Hydrate slurry densities were measured gravimetrically for all pass hydrate runs with chemical b and are shown in Figure 8. As expected, the hydrate slurry density decreased with an increase in pressure at constant water cut due to the effect of solution gas. For constant pressures, the hydrate slurry density increased with an increase in water cut due to a higher relative density of brine sample than the hydrocarbon condensate.

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CONCLUSIONS The following conclusions can be drawn from these hydrate AA-LDHI effectiveness tests and its rheological behavior study for hydrate/gas condensate/water system: 1. Hydrate AA-LDHI effectiveness tests were successfully performed with a sapphire PVT cell equipped with an up/down motion mixer with two types of AA-LDHI chemicals. The hydrate AA-LDHI runs were deemed to be fail ones for the runs where the hydrate slurry could not be mixed with the mixer that can mix the hydrate slurry with the viscosity up to 75 mPa·s.

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2. For all pass AA-LDHI runs, the hydrate slurry could be transferred to HP rheometer system for viscosity measurements as a function of shear rate. 3. The hydrate slurry viscosity decreased with an increase in shear rate from 300 to 1000 s−1 at water cut volume fraction w = 0.25 and 12.5 MPa with chemical a, demonstrating shear thinning behavior. The slurry viscosity recovered back to its original stable value when the shear rates were decreased from 1000 to 300 s−1 implying that the shear thinning behavior was reversible under these conditions. 4. The shutdown/restart experiments protocol yielded more viscous hydrate slurry than the hydrate slurry that was created using flowing experiments protocol due to the absence of mixing/shearing effect with mixer off conditions at 5 °C for 8 h during stabilization period in shutdown/restart experiments protocol. 5. Chemical b was more effective than chemical a for all pass hydrate runs because the hydrate slurry viscosities with chemical b were lower than the slurry viscosities with chemical a at similar test conditions. 6. At a constant water cut, the hydrate slurry viscosity increased with decreasing pressure due to the effect of solution gas dissolved in the continuous liquid/ condensate phase. High-pipeline operating pressure would be useful in transporting the hydrate slurry due to the positive effect solution gas has on hydrate slurry viscosity reduction. 7. As expected, the hydrate slurry density decreased with an increase in pressure at constant water cut due to the effect of solution gas. The hydrate slurry density increased with an increase in water cut at constant pressure due to a higher relative density of brine.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +1-780-450-1668. Notes

The authors declare no competing financial interest. E

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ACKNOWLEDGMENTS Authors would like to thank Donavon Robinson for meticulously performing the hydrate AA-LDHI tests and its rheological behavior experiments at Schlumberger Reservoir Laboratory in Edmonton. Thanks are also due to the operator/ client to give permission to anonymously publish this work.

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