Numerical Simulations in Support of a Long-Term Test of Gas Production from Hydrate Accumulations on the Alaska North Slope: Reservoir Response to Interruptions of Production (Shut-Ins)

Abstract

We investigate by means of numerical simulation a planned year-long field test of depressurization-induced production from a permafrost-associated hydrate reservoir on the Alaska North Slope at the site of the recently-drilled Hydrate-01 Stratigraphic Test Well. The main objective of this study is to assess quantitatively the impact of temporary interruptions (well shutins) on the expected fluid production performance from the B1 Sand of the stratigraphic Unit B during controlled depressurization over different time scales, as well as on other relevant aspects of the system response that have the potential to significantly affect the design of the field test.We consider eight different cases of depressurization, including (a) rapid depressurization over a 60-day period to a terminal bottomhole pressure PW of 2.8 MPa and (b) a slower depressurization rate to a final PW of 0.6 MPa at the end of the year-long production test, in addition to (c) a multi-step depressurization regime and (d) a quasi-linear continuous depressurization strategy. The results of the study indicate that shut-ins obviously reduce gas release and production during and immediately after their occurrence, but their longer-term effects are strongly dependent on the depressurization regime and on the time of observation, covering the entire range of potential outcomes. Shut-ins (a) have a universally strong negative effect when quasi-linear depressurization is involved regardless of the length of the production period, (b) have a strong positive effect in multi-step depressurization schemes that becomes apparent earlier for large initial pressure drops, but (c) can also appear to have practically no effect for slow step-wise depressurization at the end of the year-long production test.Shut-ins lead to a rapid reformation of hydrates, even to the point of disappearance of a free gas phase in the reservoir. Rapid depressurization regimes lead to early maximum rates of hydrate dissociation and gas production, while the maximum rates occur at the end of the production test for the cases of slower depressurization. Shut-ins do not appear to have a significant impact on water production, as the cessation of production is followed by higher rates production when depressurization resumes. Similarly, (a) the fraction of produced CH4 originating from exsolution from the water, (b) the water-to-gas ratio, and (c) the rate of replenishment of produced water by boundary inflows do not appear significantly affected by shut-ins, the effects of which seem to be temporary in the majority of the cases. The study confirmed the superiority of multi-step depressurization methods as the most effective strategies for hydrate dissociation and gas production, and showed that two observation wells (located at distances of 30 m and 50 m from the production well) are appropriately positioned and both able to capture the P, T and SG behavior during the fluid production and shut-ins in any of the eight cases we investigated. over different time scales, as well as on other rel...

Sensitivity and Uncertainty Analysis for Natural Gas Hydrate Production Tests in Alaska

“…In December 2018, as a part of the science programs toward a long-term production test, the Hydrate-01 STW was drilled at the Kuparuk 7-11-12 site, and multiple geological/geophysical data were obtained through LWD and sidewall core sampling in the target MH reservoirs (the B1 and D1 sands) and their sealing layers . On the basis of these data obtained from Hydrate-01 STW, the project partners constructed 2D/3D reservoir simulation models (the PBU Kuparuk 7-11-12 2D/3D reservoir model) and conducted several studies based on those simulation models to support the design of the production test. − Myshakin conducted a 2D simulation study for predicting the gas and water production rate for a one-year production period in Unit-B under a simplified depressurization scenario where the bottom-hole flowing pressure is 3 MPa constant to establish the design of production wells, surface production facilities, and observation wells . In addition, to support the development of the production test plan, gas and water production behaviors under more likely depressurization scenarios including multiple shut-ins based on the 2D simulation model and optimizing the simulated perforation interval to suppress water production and maintain gas production were investigated. , …”

Energy Transition and Climate Mitigation Require Increased Effort on Methane Hydrate Research

“…International methane hydrate production programs (offshore in Japan, , China, , India, , and South Korea , and onshore in U.S. Alaska North Slope ,− ) have revealed high methane hydrate concentrations in sediments with significant permeabilities required for gas production. , …”

Gas Hydrate Saturation Estimates, Gas Hydrate Occurrence, and Reservoir Characteristics Based on Well Log Data from the Hydrate-01 Stratigraphic Test Well, Alaska North Slope

“…Planning for a gas hydrate production test and accurately interpreting the resulting data are dependent on thoroughly characterizing the reservoir itself as well as the nature of the gas hydrate within the reservoir. , More broadly, quantitatively characterizing natural gas hydrate deposits is essential for understanding the scale and occurrence patterns of gas hydrate deposits locally, regionally, and globally; such characterization informs studies of gas hydrate’s role in the carbon cycle, as a subsea hazard, and as an energy resource . Subsurface characterization to support the planned Alaska production test is provided by borehole log data , and sidewall core samples from the Hydrate-01 STW along with surface seismic and 3D distributed acoustic sensor (DAS) vertical seismic profile (VSP) data. ,, …”

“…Surface and borehole geophysical data, such as seismic surveys, enable characterization over a larger spatial area; these approaches have been successfully used in conjunction with borehole data ,− and in the absence of borehole data. , The need for detailed subsurface characterization is greater in cases when production tests are planned, with reservoir characteristics and gas hydrate occurrence characteristics both needed for realistic modeling and planning. Accurate physical property estimates are critical for numerical modeling, which provides an essential tool for the planning and design of gas hydrate production tests. , As noted by Lee and Collett, many geophysical properties provide options for estimating gas hydrate saturation ( S gh ), but some of these estimates depend on the form that gas hydrate takes in the pore space (e.g., V P , V S , and electrical resistivity), whereas others relate more directly to the total amount of gas hydrate present (e.g., nuclear magnetic resonance, NMR).…”

“…Accurate physical property estimates are critical for numerical modeling, which provides an essential tool for the planning and design of gas hydrate production tests. 14,15 As noted by Lee and Collett, 26 many geophysical properties provide options for estimating gas hydrate saturation (S gh ), but some of these estimates depend on the form that gas hydrate takes in the pore space (e.g., V P , V S , and electrical resistivity), whereas others relate more directly to the total amount of gas hydrate present (e.g., nuclear magnetic resonance, NMR).…”