ULTRASONIC VELOCITY MEASUREMENTS DURING EXPERIMENTAL CH 4 HYDRATE FORMATION AND CO 2 EXCHANGE

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1 Proceedings of the 7th International Conference on Gas Hydrates (ICGH 211), Edinburgh, Scotland, United Kingdom, July 17-21, 211. ULTRASONIC VELOCITY MEASUREMENTS DURING EXPERIMENTAL CH 4 HYDRATE FORMATION AND CO 2 EXCHANGE James J. Howard, Keith C. Hester, Jim C. Stevens Reservoir Laboratory Group, ConocoPhillips Bartlesville, OK 744 USA Marisa B. Rydzy Centre for Rock Abuse, Department of Geophysics Colorado School of Mines Golden, CO 841 USA ABSTRACT Laboratory measurements of compressional and shear wave velocities at ultrasonic frequencies were acquired in a newly designed sample holder that allowed for continuous data collection during hydrate formation while simultaneously monitoring hydrate saturation and distribution with Magnetic Resonance Imaging. The ultrasonic transducers were made of piezoelectric crystals embedded in PEEK end-pieces that included openings for fluid flow. High-quality waveforms were recorded with a high signal-to-noise ratio. The measured arrival times were converted to velocities, first by hand and then with a first-motion detection module prepared for a conventional petrophysical software package. The software approach allowed the analysis of all the data that were collected every 6 seconds over a several day process, rather than selecting individual waveforms. Significant changes in the waveforms, including shifts in velocity and amplitude, were observed during hydrate formation. Velocities measured before and after hydrate formation agreed with results of previous studies. The final velocities compared favorably with model data. The injection of liquid carbon dioxide converted some of the remaining free water into hydrate with concomitant small increases in velocities.. Keywords: wave velocity, hydrate saturation, hydrate morphology NOMENCLATURE G Shear modulus (GPa) K Bulk Modulus (GPa) L Sample length (m) S hy Hydrate saturation (fr) S wi Initial water saturation (fr) t Measured arrival time (μs) t o Baseline arrival time of waveform through end pieces (μs) V p Compressional (P) wave velocity (m/s) V s Shear (S) wave velocity (m/s) ρ Mass density (kg/m 3 ) INTRODUCTION Acoustic measurements in the field, such as seismic or sonic logging, are used to identify and characterize natural gas hydrate deposits. Quantification of the amount of gas hydrate present requires calibration of the geophysical data that can only be obtained with the help of laboratory measurements. A number of studies measured wave velocity in hydrate-bearing sand packs at various experimental conditions, including hydrate saturation [1-3]. In these tests Corresponding author: Phone: , fax: , james.j.howard@conocophillips.com

2 hydrate saturation was determined either by the amounts of water or methane gas consumed during formation. A critical aspect of those studies was the uncertainty associated with actual amounts of hydrate formation and final saturation within the pore space. This study used MRI images of the whole core to quantify the transformation of free water and gas to hydrate and to determine hydrate saturation and its distribution along the length and radius of the sand pack. METHODS The experiments were run in a sample holder designed to fit inside a Magnetic Resonance Imager (MRI) so that images of water saturation and distribution could be used to assist the interpretation of the ultrasonic measurements. The cell consisted of a low-inductance fiberglass outer sleeve with titanium end pieces. The sand pack was sandwiched between two PEEK platens and surrounded by heat-shrink tubing. A set of 5 khz piezoelectric P- and S-wave crystals were embedded in the PEEK platens, which provided shielding from the magnetic field associated with the MRI and improved the impedance matching between sample and transducers. The end pieces contained several ports for wiring pass-throughs and for fluid circulation through the sample. The sample holder was cooled by circulating chilled Fluorinert through the confining pressure line, thereby achieving two goals with one solution. Computer-controlled high-pressure Quizix pumps managed the flow rates of the various liquids and gases, along with confining and pore pressure. Sand packs were created by compacting wet Ottawa F11 sand into a mold created by using pre-shrunk Teflon shrink-tubing attached to one of the PEEK end pieces (Figure 1). Porosity and initial water saturation were determined by the amount of sand and water used in the mixture. A porosity of approximately.4 was obtained by uniform compaction of the sand-water mixture, with a minimum water saturation of.2 It was necessary to add more water into the wet-sand mixture in order to reach higher initial water saturations used in several tests. The distribution of the water within the sand pack was confirmed by MRI images, with the intent of preparing uniform saturation distributions along the length of the sand pack. The sand and water mixtures were compacted until the porosity was approximately 4%. The assembly of the sample holder was then completed and installed in the MRI at room temperature. The sample was initially confined at 2.1 MPa before methane was injected into to cell. After a stabilization period, the pore and confining pressures were increased to 8.3 and 1.3 MPa respectively. Figure 1. Sample holder with shrinkwrap Teflon around PEEK end piece with uncompacted sands shows first step in sample preparation (left). Once the shape was defined by the tubing over the end piece, the sand was compacted to a constant volume, (i.e. height of sample)(right). The core holder system was then cooled to 4 o C. Under these conditions the water and methane gas converted to hydrate as determined by monitoring the MRI total intensity. Previous studies demonstrated the relationship between hydrate formation, gas consumption and the loss of MRI signal as the two fluid phases were converted into a solid phase [4]. These tests were run under conditions of excess methane available for conversion to hydrate with the consumption of methane being carefully monitored. Precooling the circulation system resulted in very fast hydrate formation rates with it stabilizing within a day for low initial water saturation conditions and several days for sand packs with higher initial water saturation. MRI images of the sand pack were collected with standard 3-D imaging pulse sequences that required acquisition times of 1. to 2.5 hours per step. The data were evaluated with an IDL-based module that calculated average intensity values

3 for different orientations and volumes of the data corrected for background noise. The display images were processed with ImageJ. tp P-Wave The wave speeds were determined by a standard pulse-transmission technique. P- and S- waveforms were sampled at a digitization rate of 1. MHz (.1 μs) for up to 248 points. Waveforms were collected approximately every minute. Initial calculations of wave velocity were done manually with visual inspection of selected waveforms. First arrival peaks were picked through a series of subjective choices based on the shape of the waveform. The choice of using the initial deflection of a peak versus the centroid of the same can result in differences in arrival times of 1. to 1.5 μs. Amplitude tp S-Wave Time (microseconds) Figure 2. P- and S- waveforms illustrate where first arrivals were selected at the center of the peak rather than an earlier break point. ts 1 The initial sample length was determined on the sample outside the sample holder, while changes in length during the experiment were monitored with 1-D MRI profiles. Slice thickness along the length of the core was 25 microns (2.5 *1-4 m). Since there were not any observable changes in MRI profile length, the calculation of transit times was based on a constant sample length. Velocity for p- and s-waves was calculated from the first-arrival time as L V p, s = [1] t t o where L was the sample length (m), t was the travel time (s) of the identified waveform feature of interest, and t o was the travel time of the comparable feature when the PEEK end pieces holding the transducers were matched face-toface. The P-wave transducer had a baseline travel time of 12.1 *1-6 s, while the S-wave transducer baseline value was 19.7*1-6 s. The peak center was chosen as the first arrival event since it was more easily recognized and more favorable for any automated peak finding algorithm (Figure 2). Subsequently, a peak-picking module was developed for a commercial log analysis software package (LogIC, Logicom E&P) that was adapted for easy data input for the laboratory formatted results. This allowed for the evaluation of all the waveforms collected over the five to fifteen day interval associated with each test. First arrival was determined from the centroid of the first deflected peak (either positive or negative) that exceeded a relative or absolute intensity threshold. RESULTS The first experiment focused on hydrate formation under high initial water saturation conditions (S wi =.76). Hydrate formation was followed by depressurization that led to hydrate dissociation. The sample was then repressurized to reform gas hydrate (Figure 3). There was a significant loss of water during the depressurization step such that the second stage of hydrate formation took place with low water saturation (S wi =.21). Initial V p and V s in the partially water-saturated sand was 88 m/s and 4 m/s, with a V p /V s of Hydrate formation began on June 3 rd with a rapid decrease in MRI intensity that indicated the increase in hydrate saturation. The increase in hydrate saturation in the sand corresponded to rapid increases in V p and V s to 35 m/s and 136 m/s, with the V p /V s remaining stable at 2.2. Hydrate formation slowed significantly after the initial rate and it required almost two weeks to convert most of the

4 remaining water to hydrate. The oscillating nature of the V p results during hydrate formation was due to the slow digitization rate of the waveform used in this early experiment. Each interval around 3 m/s corresponded to a.1 μs difference in arrival time, which was the resolution of this test. A two day power outage affected data acquisition in the middle of the test, but not the stability of the hydrate-bearing core. system was repressurized to 8.3 MPa and hydrate reformed. Despite the lower hydrate saturation the V p and V s values recovered to average values of 3485 m/s and 165 m/s, which exceeded the values observed at initial high hydrate saturation (Figure 4) Low Water Content ~2% vp vs MRI Velocity (m/sec) High Water Content ~9% Velocity [m/s] MRI Hydrate Saturation Hydrate Saturation 5 6/1 6/3 6/5 6/7 6/9 6/11 6/13 6/15 6/17 6/19 6/21 Figure 3. P-wave (blue) and S-wave (red) velocity and hydrate saturation (green) changes during hydrate formation, dissociation due to depressurization and reformation for high initial water saturation test. Hydrate saturation was determined from MRI intensity averaged over the entire sand pack. Once hydrate formation stabilized at approximately S hy =.75 the system pressure was dropped from 8.3 MPa to 3.86 MPa, below the stability for methane hydrate. There was a rapid loss of hydrate saturation to zero and concomitant decrease in V p and V s to values of 71 m/s and 44 m/s, which were close to the initial sand pack measurements at the beginning of the test. The V p /V s ratio, however, was 1.61 rather than the 2.21 value observed for the initial high water saturation measurement. The depressurization also resulted in a large loss of water from the system into the outlet line as observed in the MRI intensity that was only one-fourth the original value. The estimate water saturation then was only.2 and presumably redistributed in the pores from the original water saturation. The.1 Figure 4. P-wave (blue) and S-wave (red) velocities acquired over a range of hydrate saturations for low and high initial water saturation conditions. Hydrate saturation was determined from changes in MRI intensity averaged over the length of sample. The second test consisted of a hydrate formation stage in the sand at low initial water saturation, followed by the injection of liquid carbon dioxide. The low water saturation (S wi =.21) had initial V p and V s values of 8 m/s and 48 m/s with a V p /V s ratio of Cooling the sample resulted in rapid hydrate formation as monitored by the MRI; within a day much of the available water pore was converted to hydrate (S hy =.19). The V p and V s velocity increased to 29 m/s and 145 m/s following the conversion of water to hydrate (Figure 5). The V p /V s ratio increased to 2. and remained stable. These velocities when compared to hydrate saturation followed the low initial water saturation trend of high velocity at low hydrate saturations observed in the first experiment though the actual values were slower than observed in the first test (Figure 4).

5 Vp 25.2 Velocity (m/sec) 2 15 S hy Vs Hydrate Saturation /22 7/23 7/24 7/25 7/26 7/27 7/28 7/29 7/3 7/31 Figure 5. P-wave (blue) and S-wave (red) velocity and hydrate saturation (green) changes during hydrate formation followed by the injection of liquid carbon dioxide. Hydrate saturation was determined from MRI intensity averaged over the entire sand pack.. Hydrate formation was uniform along the length of the core as determined from 2-D longitudinal slices of the MRI images (Figure 6). Transverse slices of the core showed an even radial distribution of hydrate with no preferred growth at either the center or edges of the sand pack. The embayment at either end resulted from nonuniform initial water distribution around the inlet/outlet lines. Once hydrate began to form the loss of MRI signal intensity was uniform. Figure 6. A series of longitudinal MRI images of water saturated sand pack show the transformation of water (bright colors) to hydrate (black) over a 2 day period (top to bottom). The absence of MRI signal indicated that water and gas had converted to solid-phase hydrate. Hydrate formation during this test was uniform along the length of core. Injection of liquid carbon dioxide began July 26 and caused a slight increase in the hydrate saturation. Soon afterwards during continued injection, the hydrate saturation decreased slightly, along with a 1 m/s decrease in V p and 5 m/s decrease in V s (Figure 5). The V p /V s ratio remained constant through this stage. The carbon dioxide displaced free methane gas from the pore volume and likely converted any remaining reactive water into hydrate. The waveforms showed shifts in amplitude and frequency, particularly with the S-wave, upon the introduction of liquid carbon dioxide (Figure 7).

6 Figure 7. VDL display of P- and S- waveforms in second experiment around the time of liquid carbon dioxide injection (~21:) showed a shift in velocity. The positive (red) and negative (blue) portions of the waveforms illustrated a change towards lower frequency illustrated by the broadening of the stripes on the arrival of the S- wave. DISCUSSION The measured P- and S-wave velocities were similar to values obtained by others in comparable laboratory tests on sand packs [1-3] and with measurements recorded in the field at Mallik and Mt. Elbert [5,6]. The velocities at initial water saturation prior to hydrate formation were strongly dependent upon the actual amount of water in the pores. The first test with an initial water saturation of S wi =.76 was characterized by a V p /V s of 2.21, which was comparable to values measured during hydrate formation. In contrast, the lower initial water saturation of the second test, S wi =.21, and the first test after the depressurization step that resulted in loss of water, both were characterized by V p /V s of These lower values were more representative of gas-filled porosity in unconsolidated sands while the larger V p /V s values indicate mostly liquidfilled pores [7]. The distribution of velocity as a function of hydrate saturation in the first test illustrated the importance of initial conditions, particularly the distribution of the water in the pores. These results were compared to velocity values calculated using effective medium-based models of ultrasonic responses at different hydrate distributions (Figure 8). The model calculations were based on the work of Ecker et al [8] and used input parameters of.43 for porosity, effective stress of 2.1 MPa, 1 point contacts per grain and standard values for bulk and shear moduli of the components (Table 1). The velocities associated with hydrate formation in the high initial water saturation sample in the first test fell near the load-bearing model over the entire range of hydrate saturation. The velocity increase over this broad range of hydrate saturation was small. Velocities for hydratebearing sediments formed at low initial water saturations fell between the trends for the graincementing and enveloping-cement models. The velocity change with hydrate saturation was significantly greater. Hydrate formation at low water saturation following depressurization and loss of water in the first test (open circles) showed a rapid increase in velocity. The second test designed for low initial water saturation showed a similar trend as the first test, but offset to slightly higher saturations (pluses). The maximum velocity at final hydrate saturation was significantly less than that recorded in the first test. Variable distribution of water under these conditions was the likely source of different stiffness in these tests. Vp (m/s) Enveloping Grain-Cement Pore-Filling Hydrate Saturation Load-Bearing Figure 8. Measured P-wave velocity as function of hydrate saturation for tests that started with high (filled symbols) and low (open symbols) initial water saturations compared to effective medium model predictions for different habits of hydrate distribution in pores.

7 K G [Gpa] ρ [kg/m3] V p [m/s] V s [m/s] ν [GPa] Quartz Hydrate Water Methane Table 1: Elastic moduli and densities used in model calculations [9]. A possible scenario was that at high initial water saturations the hydrate formed in the pore centers and away from contact with the grains. Growth was slow because there were few gas-water interfaces for nucleation. Under these conditions the hydrate formed as a continuous network throughout the pore space [9,1]. In contrast, under low initial water saturations the hydrate formed near the grains, either as grain-to-grain contacts or surrounding the grains as cement. These distributions were a reflection of the change in the water/gas interface in a water-wet porous medium at low saturation. In both conditions the formation of hydrate generated stiffness in the sand pack, but requiring significantly different amounts of hydrate [11]. The displacement of free methane remaining in the pores after initial hydrate formation with liquid carbon dioxide in the second test resulted in a small decrease in measured velocities, but no change in the V p /V s ratio. The velocity of liquid carbon dioxide at these conditions was calculated as 5 m/s, while literature values of methane gas are 48 m/s, so the change of fluid in the remaining pore volume did not affect the bulk properties very much. This was contrasted to the effect of fluid displacement when water, V p =15 m/s, replaced gas in numerous studies [12]. An additional explanation was that the growth of hydrate added sufficient rigidity to the frame such that changes in pore fluid had minimal impact on how acoustic waves were transmitted through the sample. The process of hydrate formation created changes in the pore volume from initial conditions of water and methane gas in the pores to a state of just methane gas in the hydrate-saturated pores. The addition of a small volume of hydrate in a grain cement habit overwhelmed any changes in pore fluid on the total compressibility and acoustic velocity of the sand pack. The importance of developing a consistent selection procedure for arrival times was magnified with the use of short samples. The P- wave arrival varied by as much as 1.5 μs depending on whether the point was chosen as the first inflection or the centroid of the first discernible peak. While the former was often preferred in other studies, the latter was easier to discern and pick consistently. This difference in arrival time translated to as much as 5 m/s for the fastest intervals when hydrate formation had reached a stable state. In addition, the sampling or digitization rate of the ultrasonics data acquisition system also impacted the final velocity as a difference in.1 μs in arrival time translated into 2 m/s difference in velocity for these short samples. A Monte-Carlo-based sensitivity analysis showed that roughly 67% of the error associated with these velocity calculations was due to the uncertainty associated with 5.*1-4 m resolution on the original length measurement. The uncertainty associated with the selection of arrival times with.1 μs resolution was responsible for the rest of the error. Since sample length was not monitored during these experiments as done in other studies, considerable care is warranted when comparing these results with those from different experimental setups. The collection of waveforms every minute during a multiple-day experiment seemed at times to be excessive, especially when the standard practice was to analyze them by hand. The use of standard petrophysical logging software created useful displays of these large amounts of data and offered suggestions for future study of velocity changes and dispersion during these hydrate exchange experiments.

8 CONCLUSIONS The formation of methane hydrates in a sand pack under in the laboratory was successful under a number of initial water saturation conditions. The procedure used in these experiments reliably produced uniform distributions of initial water and the subsequent hydrate along the length of the sand pack. The measured ultrasonic compressional and shear velocities for water-saturated sand packs increased significantly when the water was converted to methane hydrate. The MRI and pump volume data indicated that not all of the water was transformed into hydrate in these tests. The increase in compressional velocity from 1 m/s for water saturated sand to ~3 m/s for the hydratebearing sand were comparable with previously reported values. A key contribution of these experiments was the inclusion of MRI images that provided not only information on gas hydrate saturation and distribution, but also showed that not all of the available water was transformed into hydrate even under optimum conditions. The MRI images indicated that a fraction of the water remained in a liquid state. Whether it was reactive water or bound non-reactive water was not explored in this study. Nonetheless, this was one of the few studies where actual hydrate saturation was determined accurately rather than inferred. The injection of liquid carbon dioxide into the methane-hydrate filled pores resulted in small changes in the ultrasonic velocity. The accompanying MRI images revealed that some of the remaining free water in the methane hydrate system was converted into a hydrate phase, thus increasing the solid pore-filling phase. The small velocity decrease may reflect changes in hydrate distribution in the pores. There were distinct trends of V p and V s relative to hydrate saturation that reflect the distribution of hydrate in the sand packs. Low initial water saturations generated velocities that fall between the effective medium model trends for graincoating and cement-enveloping hydrate. In contrast, higher initial water saturations produced velocity trends that were closer to the load-bearing model. ACKNOWLEDGEMENTS The authors thank ConocoPhillips for permission to present this paper and to Joe Pumphrey and Kenny Tilley, Logicom E&P, for development of the module for analysis of laboratory ultrasonics data within LogIC. REFERENCES [1] Waite W, Winters W, and Mason D. Methane hydrate in partially water-saturated sand. American Mineralogist 24; 89: [2] Kwon T, and Cho G. Evolution of compressional wave velocity during CO 2 hydrate formation in sediment. Energy Fuels 29; 23: [3] Waite W, Santamarina J, Cortes D, Dugan B, Espinoza D, Germaine J, Jang J, Jung J, Kneafsey T, Shin H, Soga K, Winters W, and Yun T. Physical properties of hydrate-bearing sediment. Reviews of Geophysics 29; 47: [4] Stevens J, Baldwin B, Graue A, Ersland G, Husebø J, and Howard J., Measurements of hydrate formation in sandstone. Petrophysics 28: 49(1): [5] Collett T, Lewis R, Winter W, Lee M, Rose K, and Boswell R. Downhole well log and core montages from the Mount Elbert gas hydrate stratigraphic test well, Alaska North Slope. J. Marine Petrol. Geol, 211; 28(2): [6] Lee M, and Collett T, Elastic properties of gas hydrate-bearing sediments. Geophysics 21; 66(3): [7] Schön J. Physical properties of rocks: fundamentals and principles of petrophysics. Amsterdam: Elsevier Inc., 24. [8] Ecker C, Lumlet D, Dvorkin J, and Nur A. Sediments within gas hydrates: Internal structure from seismic AVO. Geophysics 1998; 63: [9] Helgerud M, Dvorkin J, Nur A, Sakai A, and Collett T. Elastic wave velocity in marine sediments with gas hydrates: Effective medium modeling. Geophysical Research Letters 1999; 26:

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