Article pubs.acs.org/EF
Crystal Growth Inhibition Studies for the Qualification of a Kinetic Hydrate Inhibitor under Flowing and Shut-In Conditions Eduardo Luna-Ortiz,*,† Matt Healey,† Ross Anderson,‡ and Eyvind Sørhaug§ †
Xodus Group, Production Assurance Division, Cheapside House, 138 Cheapside, London, EC2V 6BJ, U.K. Hydrafact Ltd, Quantum Court, Research Avenue South, Heriot-Watt University Research Park, Edinburgh, EH 14 4AP, U.K. § Talisman Norge AS, Verven 4, 4014 Stavanger, Norway ‡
S Supporting Information *
ABSTRACT: One of the methods to control the formation of hydrates in oil and gas pipelines is the injection of kinetic hydrate inhibitors (KHIs). The accepted understanding is that KHIs slow down or interfere with hydrate nucleation, forcing an extended “induction time” (time to emergence of viable hydrate crystals) at a given subcooling. As a result, KHIs are commonly evaluated by measuring induction times in the laboratory. However, this experimental approach has some limitations, notably in that data can be stochastic due to the nucleation element, raising questions over reliability/transferability, with multiple repeats often required to establish clear trends. As KHIs also exhibit powerful growth inhibition properties, a new crystal growth inhibition (CGI) method for the evaluation of KHIs has been previously developed with the aim of providing a means to more rapidly evaluate KHIs in a robust manner. This method shows that KHIs induce a number of well-defined hydrate CGI regions with different growth rates as a function of subcooling, and these can be used to reliably evaluate inhibition performance on quite short time scales. In this work, we present the results of an experimental program for the qualification of a commercial KHI to be used in a greenfield development using this CGI method. The aim of the laboratory work was to determine required inhibitor dosage, investigate the effects of a corrosion inhibitor (CI) on KHI performance, and evaluate the potential for KHI inhibition during shut-in/restart, in addition to flowing conditions. The program focused on CGI methods for evaluation in addition to standard induction time measurements. A methodology to recreate pipeline flowing, shut-in, and restart conditions was also developed and used. The CGI approach was found to offer advantages in the speed of KHI assessment and provides a useful decision-making tool with respect to KHI field deployment. Data also correlate with and compliment traditional induction time results which still provide valuable information on the degree of “nucleation” inhibition offered on top of crystal growth inhibition. In addition to offering excellent hydrate inhibition under flowing conditions, results suggested the KHI could readily offer good protection for long periods of shut-in (e.g., >168 h at up to 15 °C subcooling) followed by restart, reducing or negating the need for depressurization procedures in the event of shut-in.
1. INTRODUCTION Gas hydratesor clathrate hydratesare ice-like nonstoichiometric crystalline compounds formed when hydrogen bonds of water form a lattice of polyhedral cages (host) enclosing small gas molecules (guests) typically at high pressure and relatively low temperature conditions. Common guest molecules are light hydrocarbons (C1 to C4) and gases such as N2, CO2, and H2S. The guest gas molecule provides thermodynamic stability to the hydrate structure through van der Waals interactions. Hydrates can be encountered in a number of different crystal structures. The fit of the guest molecule in the water cages determines the crystal structure that a particular gas or gas mixture will form. In the oil and gas industry, the most common hydrates structures encountered are s-I and s-II, and in rare occasions, s-H. A detailed description of the structure and physicochemical properties of gas hydrates can be found elsewhere, e.g., ref 1. From a flow assurance perspective, gas hydrate formation represents a major concern as they can block production pipelines causing considerable lost production, safety problems, potential irreparable damage and, in extreme cases, abandonment and replacement. The most common method to avoid hydrate formation is the injection of thermodynamic inhibitors © 2014 American Chemical Society
(THIs), such as methanol (MeOH) or ethylene glycol (MEG), which depress the activity of water, shifting hydrate stability conditions to lower temperatures/higher pressures. However, large volumes of inhibitor may be required, resulting in high OPEX and CAPEX (due to storage, transport, and regeneration), health and safety risks, and environmental concerns. As a result, there is growing interest in the application of kinetic hydrate inhibitors (KHIs) as a more cost-effective and environmentally friendly method to prevent hydrate formation. KHIs are typically based on water-soluble polymers which do not thermodynamically inhibit hydrate like THIs, but traditionally have been considered as nucleation delayers, inhibiting hydrate nucleation/growth for a certain amount of time at a particular subcooling. They have the benefit that they require low dosages (typically doses are 1−3 vol %) and are less volatile (due to high molecular weight of the active polymers) and less flammable than MEG or MeOH. KHIs have been used in many fields worldwide with successful results.2−5 A review of different chemistries of KHIs can be found elsewhere.6 Received: December 17, 2013 Revised: April 3, 2014 Published: April 14, 2014 2902
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hydrate crystals.18 As KHI inhibition of crystal growth is primarily governed by thermodynamics (crystal surface adsorption is generally assumed) rather than kinetics, quantifying this form of inhibition can be considered a more rapid means to reliably evaluate KHIs in the laboratory when compared with nucleation/induction time measurements, in addition to offering greater confidence in KHI performance if thermodynamically stable crystals actually form. In summary, the CGI method consists of a number of hydrate growth−dissociation cycles, leaving a small but measurable hydrate seed (typically, 24 and >168 h shut-in used for blank and KHI (with and without CI) tests, respectively. For restart, the warm-up rate was ∼2.0 °C/h with the impeller speed increasing incrementally by 100 rpm every 10 min upon restart until the 500 rpm standard mixing rated was reached. Cool-down and warm-up rates are related to operational scenarios that are expected in the real field and estimated with multiphase pipeline transient simulations. Experimental Program. The experimental program was divided into four work packages (WPs): Work Package I − Hydrate Stability Zone/Dissociation Conditions. This WP involved experimentally determining equilibrium hydrate dissociation conditions (points on the hydrate stability zone curve) for the synthetic gas (KHI and CI free) to firmly establish the hydrate risk region and to check thermodynamic model predictions. Work Package II − Baseline System Evaluation (Blank Case). In this WP, the aim was to generate hydrate growth rate/pattern data with no KHI/CI present (flowing/mixed conditions). In addition, the SCHR methodology was used with mixing following a 24 h shut-in/ static period to determine whether hydrate had nucleated/or not upon restart and subsequent growth rates. Work Package III − KHI Evaluation. In this WP, CGI region data was generated for a pressure range with KHI present in the system (no CI present). Performance of KHI was determined for three different dosages at the highest pressure condition (largest subcooling in absolute value) to select a final suitable target dose. Following
Figure 1. Schematic illustration of the type of autoclave cell used in the experimental studies. cell jackets from programmable cryostats. Pressure and temperature data are continuously monitored and recorded by a personal computer (PC). A temperature probe and pressure transducer have been calibrated with an accuracy of ±0.2 °C and ±0.3 bar, respectively. Methods. Hydrate dissociation point measurements for the synthetic gas were made using constant volume isochoric equilibrium step-heating.29 This method involves cooling the system rapidly to a high subcooling to induce hydrate formation and then warming in steps until dissociation is complete. At each step, sufficient time is given for equilibrium to be achieved (stable pressure), with interpolation of equilibrium points yielding the final hydrate dissociation point. The crystal growth inhibition (CGI) evaluation technique20 was used to determine KHI performance regions as a function of subcooling at a range of pressures and KHI doses. Complete inhibition (indefinite/infinite induction time)/green zone, partial inhibition/ amber zone, and rapid failure regions/red zone are reliably (highly repeatable) determined. Region nomenclature and descriptions/ definitions are provided in Table 2.
Table 2. KHI Induced CGI Region Nomenclature and Typical Hydrate Growth Ratesa region CIR SGR (VS) SGR (S) SGR (M) RGR SDR
full region name complete inhibition region slow growth region (very slow) slow growth region (slow) slow growth region (moderate) rapid growth region slow dissociation region
typical hydrate growth rates 0.00%/h 0.01 ( symbol and time in italics indicate rapid KHI failure did not occur by at least this time when the experiment was terminated. b Based on one single experiment. cBased on three repetitions.
under well mixed conditions, and operating in this region could cause problems unless for very short durations. SCHR Tests and Induction Time Measurements. Shut-in, cool down, hold, restart (SCHR) experiments were performed for the inhibited gas with 3.0% KHI + 200 ppm of CI at maximum absolute subcooling conditions and a range of pressure conditions. Results of these tests are reported in Table 9. Following SCHR tests as noted, standard induction time (ti) and time to rapid hydrate growth (tf) measurements were then made for flowing conditions at various subcoolings/pressures. Results for these tests are reported in Table 10 with data for the lowest and highest pressure conditions plotted in Figures 18 and 19 respectively. As can been seen from Table 9, all SCHR tests resulted in successful restarts after extended periods of shut-in (192−275 h) at maximum absolute subcoolings, in agreement with findings for tests with no CI present. As expected, during shutin, the absence of mixing (of gas and water) combined with the KHI severely restricted the formation of hydrate, with only 0.3−0.6% conversion of water calculated. It is concluded that this low fraction of hydrate meant that upon restart, sufficient “free” KHI polymer was still available (as opposed to being adsorbed on hydrate crystal surfaces) to inhibit hydrate growth during the restart period until the rising temperature took the system conditions outside the hydrate region. This is consistent with CGI data, e.g., for the highest and lowest pressure tests, for the SGR(S) conditions of initial restart, KHI inhibition strength would be expected to be sufficient to prevent significant hydrate growth for the few hours prior to the temperature rising back
Figure 18. Plot of measured induction time and time to KHI failure (rapid, uncontrolled hydrate growth) as a function of subcooling for 3.0% KHI at the lowest test pressure (∼28 bar). CGI regions are shown for comparison. Dashed lines indicate apparent minimum ti and tf trends.
into the CIR (complete inhibition) then to outside the hydrate stability region. Induction time and time to failure measurements for flowing conditions (Figures 18 and 19, Table 10) are consistent with CGI and SCHR test results, with ti and tf rising rapidly from close to zero at the RGR boundary to very high values within the SGR(S) region as the CIR is approached with reducing absolute subcooling. While for the highest specified absolute subcooling conditions of ∼15 °C, nucleation and growth to
Table 9. Results for Simulated Shut-In, Cool down, Hold, Restart (SCHR) Tests at Maximum Subcoolings at Low, Medium, and High Pressure Test Conditions for 3.0 vol% KHI with 200 ppm of CIa % KHI
T (°C)
P (barg)
ΔTs‑II,si (°C)
tcd (h)
tsi (h)
% hydrate, shut-in (max)b
tr (h)
% hydrate, restartb
succ restart?c
3.0 3.0 3.0
3.0 3.3 3.3
28.4 36.4 49.4
−11.7 −13.0 −14.8
19 20 23
192 192 275
0.3 0.3 0.6
6.0 7.0 7.5
N.D. N.D. N.D.
Y Y Y
Cells were cooled at ∼0.75 °C/h to set subcooling (for conditions inside the hydrate region) following shut-in and warmed at ∼2.0 °C/h upon restart to outside the hydrate stability region. ΔTs‑II,si = total subcooling at shut-in, tcd = cool down time (to set subcooling) inside hydrate region, tsi = shut-in time at set subcooling. tr = time in hydrate region following restart and warm up. b% = vol % of total aqueous phase converted to hydrate. c Denotes whether restart was successful, i.e., whether flowing conditions were re-established with no obvious problematic hydrate formation. N.D. = none detectable. a
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors kindly appreciate permission of Talisman and Xodus Group to publish this paper. E.L.O. and M.H. would like to acknowledge the assistance of other members of the Production Assurance Division, in particular, Kamila Szklarczyk, Jude Esangbedo, and Mark Bracewell. The authors would like to thank the anonymous reviewers for their valuable comments and corrections, which we found very helpful to improve our paper.
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Figure 19. Plot of measured induction time and time to KHI failure (rapid, uncontrolled hydrate growth) as a function of subcooling for 3.0% KHI at the highest test pressure (∼52 bar). CGI regions are shown for comparison. Dashed lines indicate apparent minimum ti and tf trends.
failure occurs on the scale of a few hours under well-mixed conditions, by absolute subcoolings of ∼13 °C, nucleation times were >24 h, with subsequent growth inhibition extending to ∼100 h or more across the pressure range investigated.
4. CONCLUSIONS In this work, an experimental crystal growth inhibition (CGI) method was used for the performance evaluation of a commercial KHI (in the absence and presence of CI) for a real field multicomponent gas system. The CGI method was previously developed with the aim of circumventing some of the problems that can be associated with traditional induction time studies (namely stochasticity) while reducing the time needed to obtain robust KHI performance data. The CGI method exploits the fact that, in addition to being nucleation inhibitors, KHI polymers are also powerful crystal growth inhibitors and provide reliable data on KHI long-term hydrate protection for a range of conditions, including flowing, shut-in, and restart, offering a useful decisionmaking tool with respect to KHI field deployment. In addition to CGI measurements, complementary induction time (specifically, time to when hydrate was first detected) studies showed that even though results were poorly repeatable due to inherent stochasticity, they still provide valuable information on the degree of nucleation inhibition offered on top of crystal growth inhibition and correlate well with the latter. Finally, a methodology to recreate real pipeline flowing, shutin, and restart conditions was developed, with results agreeing closely with CGI and induction time data in demonstrating that the KHI could offer good protection for long periods of shut-in (e.g., >168 h at up to 15 °C absolute subcooling) followed by restart.
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NOMENCLATURE CAPEX = capital expenditure CGI = crystal growth inhibition CI = corrosion inhibitor CIR = complete inhibition region HSR = hydrate stability region KHI = kinetic hydrate inhibitor OPEX = operational expenditure RGR = rapid growth region SCHR = shutdown, cooldown, hold and restart SDR = slow dissociation region SGR = slow growth region SGR(M) = slow growth region (moderate) SGR(S) = slow growth region (slow) SGR(VS) = slow growth region (very slow) THI = thermodynamic hydrate inhibitor WP = work package
Symbols
G = gas H = hydrate Ih = ice hexagonal structure MEG = monoethylene glycol MeOH = methanol P = pressure s-I = hydrate cubic structure I s-II = hydrate cubic structure II s-H = hydrate hexagonal structure H tcd = cool down time (to set subcooling) inside hydrate region ti = induction time tf = time to KHI failure/rapid hydrate growth tg,si = time to hydrate growth following start-up tsi = shut-in time tr = time in hydrate region following restart and warm up T = temperature W = water Greek Letters
ASSOCIATED CONTENT
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S Supporting Information *
CGI method heating cooling curves for the 3.0% vol KHI and 200 ppm of CI for different test pressures. This material is available free of charge via the Internet at http://pubs.acs.org/.
ΔT = subcooling ΔTs‑II = subcooling from the hydrate s-II stability conditions (hydrate equilibrium temperature minus test temperature) ΔTs‑II,si = total subcooling at shut-in (hydrate equilibrium temperature minus test temperature at shut-in)
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