Offshore Methane Hydrate Production Test Research Articles

Simulations of hydrate reformation in the water production line of the second offshore methane hydrate production test in Japan’s Nankai trough

Natural gas hydrates are a potential new energy resource, however, challenges for stable, long-term gas hydrate production remain. This study examines the flow assurance risk of hydrate re-formation in production lines during gas hydrate production, which is a key issue in designing reliable production systems and operations. The focus of this study is the second offshore methane hydrate production test conducted in the Nankai trough area of Japan in 2017, where the depressurization method with a pump, wellbore gas separation, and two production lines for gas and water was applied. In this field test, separation inefficiencies caused significant quantities of gas to be produced from the water production line, posing a risk of hydrate re-formation. To understand and aid in management of this risk, we have developed models of hydrate growth and dissociation in water-dominant flow and conducted flow simulations that were compared to field data from the water production line in this field case. The simulation predictions suggest that the hydrate re-formation was not significant in this production test due to use of flexible hoses with a low overall heat transfer coefficient. The thermal insulation of the flexible hoses contained the heat generated by hydrate formation inside the production lines, resulting in a heat transfer limited system with low hydrate volume fractions (<5.2 vol%). Further simulations were conducted to estimate the sensitivity of such systems to insulation and materials choice, where a system with the flexible hoses replaced with carbon steel pipes was predicted to experience volume fractions four times greater (<19.8 vol%). Given hydrate production test systems typically take the form of short vertical lines, this indicates that pipe insulation may be critical in managing the risk of plugging due to re-formation. This insight is particularly important given the widespread use of carbon steels in subsea production facilities. Simulations enabled by the algorithm presented in this work will inform design choices for future methane hydrate production facilities. The use of thermal insulators will likely limit the degree of hydrate re-formation in such systems, and enable the deployment of management strategies that minimize or eliminate chemical injection.

The void fraction and frictional pressure drop of upward two-phase flow under high pressure brine condition

The offshore methane hydrate production test conducted in Japan has demonstrated a successful production of methane gas from marine gas hydrates using depressurization methods. The method produces methane gas by dissociating methane hydrate, which is stable at low temperature and high pressure conditions, by lowering the bottom-hole pressure in the wellbore. Under this method, stable control of bottom-hole pressure is necessary to ensure continuous methane gas production. Therefore, understanding the behavior of upward gas–liquid two-phase flow, involving gas-brine mixture, in a wellbore is essential for the proper pressure drop evaluation in the production line. In the present study, void fraction and frictional pressure drop behaviors under nitrogen-brine two-phase flow conditions is investigated using the high-pressure test facility of 52.7 mm inner diameter and 28 m height. The measured void fraction was assessed using drift-flux model, and the results showed that the decrease in bubble diameter under bubbly flow in gas-brine two-phase flow conditions resulted in decrease in bubble rise velocity and uniformity of bubble distribution. It was also found that, under bubbly flow in gas-brine mixture, the frictional pressure drop tends to further decrease with elevation. Based on these findings and obtained experimental dataset, the new frictional pressure model for gas-brine two-phase flow under high-pressure conditions is developed in the present study.

Numerical investigation on the long-term gas production behavior at the 2017 Shenhu methane hydrate production site

In 2017, an offshore methane hydrate production test was successfully conducted at well SHSC-4 in the Shenhu Area of the South China Sea, but the long-term gas production behavior is still unknown and requires further investigation. In this study, a multi-layered methane hydrate reservoir model with three sublayers of the hydrate-bearing layer (HBL), three-phase layer (TPL), and free gas layer (FGL) was built based on the actual geological conditions at this site, and a short-term simulation was initially conducted to verify the validity of the reservoir model. Afterwards, the long-term simulations were conducted to predict the hydrate dissociation and gas production behaviors in the reservoir and investigate the contributions of each sublayer to the total gas production, and the effects of the intrinsic permeability of each sublayer on the gas production were fully examined. The simulation results indicated that the average gas production rate (1.83 × 103 ST m3/d) was less than half of that confirmed during the 2017 Shenhu production test (5.15 × 103 ST m3/d). The majority of the total gas production originated from the free gas in the FGL (56.5%), followed by the methane gas released from hydrate dissociation in the HBL (24.1%), and the TPL contributed the least to the gas recovery (19.4%). In addition, if the method of permeability enhancement was applied to the methane hydrate reservoir at well SHSC-4, the gas production could be greatly promoted, but the mechanisms were different. Finally, the following application priority was recommended: HBL > FGL > TPL.

Production performance and numerical investigation of the 2017 offshore methane hydrate production test in the Nankai Trough of Japan

Since the first offshore methane hydrate production test was conducted in the Nankai Trough of Japan in 2013, the second production test was also conducted at the same test site in 2017, but information about this field test is limited. Thus, in this study, we aimed to report detailed information about the 2017 production test for the first time, and conducted short-term simulations of the oceanic methane hydrate production from the multilayered reservoir models for model validation. Then, we predicted the long-term gas production behavior for a production period of 5 y, and investigated the effect of the production intervals on the processes of hydrate dissociation and gas production. The simulation results indicated that most of the released gas from hydrate dissociation was actually produced indirectly in the aqueous phase rather than directly in the gas phase, and the proper utilization of the packers on the wellbore could effectively promote gas production, but also caused excess water production. Therefore, high-performance gas-water separation devices and water production management are necessary for future production tests. Furthermore, the 5-y average gas production rates for each case designed in this study were estimated as 1.01 × 104–1.21 × 104 ST m3/d, which were far less than the commercial exploration level of 3.0 × 105 ST m3/d, so improvements in the production strategies should be considered for future production tests.

Mechanical Response of Reservoir and Well Completion of the First Offshore Methane-Hydrate Production Test at the Eastern Nankai Trough: A Coupled Thermo-Hydromechanical Analysis

Summary During gas production from offshore gas-HBS, there are concerns regarding the settlement of the seabed and the possibility that frictional stress will develop along the production casing. This frictional stress is caused by a change in the effective stress induced by water movement caused by depressurization and dissociation of hydrate as well as gas generation and thermal changes, all of which are interconnected. The authors have developed a multiphase-coupled simulator by use of a finite-element method named COTHMA. Stresses and deformation caused by gas-hydrate production near the production well and deep seabed were predicted using a multiphase simulator coupled with geomechanics for the offshore gas-hydrate-production test in the eastern Nankai Trough. Distributions of hydrate saturation, gas saturation, water pressure, gas pressure, temperature, and stresses were predicted by the simulator. As a result, the dissociation of gas hydrate was predicted within a range of approximately 10 m, but mechanical deformation occurred in a much wider area. The stress localization initially occurred in a sand layer with low hydrate saturation, and compression behavior appeared. Tensile stress was generated in and around the casing shoe as it was pulled vertically downward caused by compaction of the formation. As a result, the possibility of extensive failure of the gravel pack of the well completion was demonstrated. In addition, in a specific layer, where a pressure reduction progressed in the production interval, the compressive force related to frictional stress from the formation increased, and the gravel layer became thin. Settlement of the seafloor caused by depressurization for 6 days was within a few centimeters and an approximate 30 cm for 1 year of continued production.

Technology Focus: New-Frontier Reservoirs I (July 2017)

Technology Focus Congratulations! You are one of a select group to open your JPT and find this page! For your effort, you deserve something special, and I know what you want. In 5 minutes or less, you want to know how to immediately make a step change in your business and improve your own promotion prospects in the process. You want to know the key technologies you should be implementing now. To develop your longer-term strategy, you want an awareness of future trends now. You don’t want just anyone to tell you these answers; you want it to come from the very best people in the industry. Consider your wish granted. According to a poll of leaders with more than 220 years of collective experience (see JPT online for names), the top theme to have affected our industry in the past 2 years is Fracture-Stimulation Optimization—Given the high cost, fracture design has gained its fair share of scrutiny in the downturn. The results of statistical production analysis have varied by play. Fluid types and chemical additives have been removed or modified. Alternative proppants have been evaluated and, in some cases, trialed successfully outside their previously specified safe-performance ranges. Nothing is sacred. The top theme to affect our industry in the next 2 years is predicted to be Intelligent Operations—The industry is moving toward increasing the level of automation in all activities. All areas from exploration to shipping will be touched by these technologies. This includes drones for safer, demanned operations as well as robotics on drilling rigs. The other key area of automation is predictive analytics, which allows us to analyze multivariant problems like never before. Fracture-design optimization, drilling analytics to improve rate of penetration, and optimized operator vehicle routing for field-staff and plant optimization are just a few areas that will be transformed. Now, wasn’t it worth the effort to read this? Recommended additional reading at OnePetro: www.onepetro.org. SPE 182230 Issues and Challenges With Controlling Large Drawdown in the First Offshore Methane-Hydrate Production Test by S. Sakurai, Japan Oil, Gas, and Metals National Corporation, et al. SPE 180399 Petrophysical Evaluation of Natural-Gas Hydrates—North Slope, Alaska by Douglas Hupp, Schlumberger, et al. SPE 180818 Proppant Management: A New Challenge To Develop Unconventional Reservoirs in Argentina by J.C. Bonapace, Halliburton

Issues and Challenges With Controlling Large Drawdown in the First Offshore Methane-Hydrate Production Test

Summary The first offshore methane-hydrate production test was conducted in the Eastern Nankai Trough area of Japan in 2013, subjecting a gas-hydrates reservoir to large drawdowns by reducing bottomhole pressure (BHP) for in-situ dissociation of gas hydrates. This pioneering test has proved the feasibility of the depressurization method through demonstration of gas production from a deepwater gas-hydrates reservoir. Approximately 119 500 std m3 of gas was produced during a continuous flow period of 6 days. However, reservoir response to a range of drawdown conditions was not attainable, which is important for reservoir evaluation, because drawdown became uncontrollable after unintended water production through the gas line occurred. Gas and water released from the dissociation of gas hydrates were separated by use of a downhole gas-separation system. The separated gas and water were produced to surface by means of two dedicated gas and water lines. Drawdown was executed by pumping out water into the water line by use of an electrical submersible pump (ESP). Drawdown control was designed to regulate the liquid level (or hydrostatic pressure) in the gas line by controlling the ESP frequency and the water-line surface backpressure. Analysis of production data supported by flow simulations indicated that continuous water production through the gas line was the main reason for the loss of drawdown control. The trigger of the water production was that the water column in the gas line reached surface because of the rising water level resulting from the produced gas, which also lightened the water column and lowered the BHP. Consequently, the continuous water production made it difficult to regulate the drawdown as intended. The analysis concluded that the risk of water production through the gas line could be significantly lowered if a choke valve was installed at the surface gas line and/or the ESP had high tolerance to the presence of free gas. This first field trial has provided valuable information in understanding the methane-hydrate production system to further improve/develop strategies in controlling large drawdown in the system.

Key Findings of the World’s First Offshore Methane Hydrate Production Test off the Coast of Japan: Toward Future Commercial Production

Marine methane hydrate in sands has huge potential as an unconventional gas resource; however, no field test of their production potential had been conducted. Here, we report the world’s first offshore methane hydrate production test conducted at the eastern Nankai Trough and show key findings toward future commercial production. Geological analysis indicates that hydrate saturation reaches 80% and permeability in the presence of hydrate ranges from 0.01 to 10 mdarcies. Permeable (1–10 mdarcies) highly hydrate-saturated layers enable depressurization-induced gas production of approximately 20,000 Sm3/D with water of 200 m3/D. Numerical analysis reveals that the dissociation zone expands laterally 25 m at the front after 6 days. Gas rate is expected to increase with time, owing to the expansion of the dissociation zone. It is found that permeable highly hydrate-saturated layers increase the gas–water ratio of the production fluid. The identification of such layers is critically important to increase the en...

Evaluation of Gas Production from Marine Hydrate Deposits at the GMGS2-Site 8, Pearl River Mouth Basin, South China Sea

Natural gas hydrate accumulations were confirmed in the Dongsha Area of the South China Sea by the Guangzhou Marine Geological Survey 2 (GMGS2) scientific drilling expedition in 2013. The drilling sites of GMGS2-01, -04, -05, -07, -08, -09, -11, -12, and -16 verified the existence of a hydrate-bearing layer. In this work gas production behavior was evaluated at GMGS2-8 by numerical simulation. The hydrate reservoir in the GMGS2-8 was characterized by dual hydrate layers and a massive hydrate layer. A single vertical well was considered as the well configuration, and depressurization was employed as the dissociation method. Analyses of gas production sensitivity to the production pressure, the thermal conductivity, and the intrinsic permeability were investigated as well. Simulation results indicated that the total gas production from the reference case is approximately 7.3 × 107 ST m3 in 30 years. The average gas production rate in 30 years is 6.7 × 103 ST m3/day, which is much higher than the previous study in the Shenhu Area of the South China Sea performed by the GMGS-1. Moreover, the maximum gas production rate (9.5 × 103 ST m3/day) has the same order of magnitude of the first offshore methane hydrate production test in the Nankai Trough. When production pressure decreases from 4.5 to 3.4 MPa, the volume of gas production increases by 20.5%, and when production pressure decreases from 3.4 to 2.3 MPa, the volume of gas production increases by 13.6%. Production behaviors are not sensitive to the thermal conductivity. In the initial 10 years, the higher permeability leads to a larger rate of gas production, however, the final volume of gas production in the case with the lowest permeability is the highest.

Sedimentary facies and paleoenvironments of a gas-hydrate-bearing sediment core in the eastern Nankai Trough, Japan

From June to July 2012, the Ministry of Economy, Trade and Industry completed a pressure core-sampling well (AT1-C) for determining reservoir properties at the first offshore methane hydrate production test site on the north slope of the Daini–Atsumi Knoll off the Atsumi and Shima peninsulas. Coring through the targeted gas-hydrate-bearing reservoir was completed using the hybrid pressure coring system and the extended shoe coring system. The coring program achieved 61% recovery through 60 m of the hole. The recovered core consists of alternating clayey to sandy siltstones and very fine to fine sandstones. The stratigraphic pattern indicates upward thinning of the sand layer. Grain-size distributions indicate that the transition to mud facies is marked by an increase in fine particles and more poorly sorted sediments. Sedimentary structures and bioturbation characteristic of conventional cores are absent in the pressure cores due to the effects of fluidization attributed to the release of water from dissociation of gas hydrates. The core sediments were subdivided into four facies: Facies 1 (hemipelagic setting or slope) structureless silt; Facies 2 (abandonment surface mud drape) silt-dominated alternating beds of sand and silt; Facies 3 (non-amalgamated channel deposit) proportionally interbedded sand and silt; and Facies 4 (semi-amalgamated channel deposit) sand-dominated, with alternating beds of sand and silt. The results aid the interpretation of subsurface data and to quantitatively constrain geological models, thereby reducing uncertainties in the development of reservoirs.

System Monitors Sandface for Deepwater Offshore Gas-Hydrate Production

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper OTC 25328, ’A Deepwater Sandface-Monitoring System for Offshore Gas- Hydrate Production,’ by S. Chee, SPE, T. Leokprasirtkul, T. Kanno, O. Osawa, Y. Sudo, M. Takekoshi, and H. Yu, SPE, Schlumberger, and K. Yamamoto, Japan Oil, Gas, and Metals National Corporation, prepared for the 2014 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed. The world’s first offshore gas-hydrate production was carried out successfully in deepwater Japan at Nankai trough in 2013. In this project, one production well and two sandface monitoring wells were drilled and installed with a combination of distributed-temperature-sensing (DTS) and array-type resistance-temperature- detector (RTD) sensors. The objective of the sandface-monitoring system was to capture the hydrate-dissociation front dynamically changing during the production test and to monitor long-term reservoir stability with the selected temperature sensors. Introduction Interest is growing worldwide in methane hydrate as a unique and challenging natural-gas resource. In order to produce and use these resources, it is necessary to establish the production technology and understand how methane-hydrate dissociation behaves. Japan has studied methane-hydrate exploration during the past decades and planned to conduct the world’s first offshore methane-hydrate production test in 2013. In such a production test, the dissociation progress and the stability of the methane-hydrate reservoir can be captured by long-term continuous temperature monitoring, which is conducted in real time and autonomously before, during, and after the production test. One of the more challenging requirements is that the deepwater sandface-monitoring system must be self-operated by subsea battery without cable connection from the surface for a long-term monitoring period of 18 months. Thus, the reliability and redundancy of the monitoring system were key design challenges above data measurement quality and offshore deployment tasks. Outline of the Sandface-Monitoring Wells One production well and two monitoring wells were drilled. The methane-hydrate reservoir in the Nankai-trough area has methane-hydrate-bearing sand-rich intervals in turbidite fan deposits. For the production test, approximately 50 m of methane-hydrate zone was selected that is located in shallow unconsolidated reservoir approximately 300 m below the seafloor in a water depth of 1000 m. Deepwater sandface-monitoring systems were deployed in the two monitoring wells in the first quarter of 2012, approximately a year before the methane-hydrate production test started in the first quarter of 2013. The production test continued for a week, and then the recovery of the sandface-monitoring systems was performed in the third quarter of 2013. Therefore, the in-situ temperature monitoring started 1 year before the production of methane hydrate and continued for approximately half a year after the production test in order to monitor the long-term stability of the methane-hydrate- reservoir behavior. After the monitoring system was recovered from the seafloor, temperature data were retrieved from data storage for analysis.