Scientific Results of the Hydrate-01 Stratigraphic Test Well Program, Western Prudhoe Bay Unit, Alaska North Slope

Abstract

The United States Department of Energy, the MH21-S Research Consortium of Japan, and the United States Geological Survey are collaborating to enable gas hydrate scientific drilling and extended-duration reservoir response testing on the Alaska North Slope. To feasibly execute such a test, a location is required that is accessible from existing roads and gravel pads and that can be occupied without disrupting ongoing industry operations. A review of potential locations meeting these criteria determined the likely occurrence of gas hydrate in two fine-grained marginal-marine sands of Tertiary age in the vicinity of the inactive “Kuparuk State 7-11-12” exploration pad in the western Prudhoe Bay Unit (PBU). Existing well and seismic data for that site were insufficient to preclude the potential for free gas occurrence within the deeper (and most prospective) target sand. Therefore, with support from the PBU Working Interest Owners, Alaska Department of Natural Resources, and Petrotechnical Resources Alaska, the Hydrate-01 Stratigraphic Test Well (STW) was drilled in December 2018 to confirm the suitability of the site for future gas hydrate scientific testing. The Hydrate-01 well was successfully drilled to −3290 ft (1003 m) subsea vertical depth at a bottom hole location of approximately 900 ft (∼275 m) east of the surface location. The drilling program featured acquisition of a full suite of logging while drilling data, the collection of side-wall pressure cores, and the installation of distributed temperature and distributed acoustic sensor fiber-optic cables. The log data acquired confirmed the occurrence of gas hydrate at high saturation in two target sands. Integrated evaluation of log and sidewall core data provide petrophysical and geomechanical property information that allow for potential reservoir response to depressurization to be simulated. The deeper “B1 sand” is deemed to be most favorable for reservoir response testing as a result of confirmed gas hydrate occurrence in sediments of high intrinsic permeability, location within 100 ft (30 m) of the base of gas hydrate stability, and minimal risk for direct communication with permeable water-bearing (hydrate-free) zones. The shallower “D1 sand” provides a secondary target that is differentiated by colder in situ temperatures and the interpreted direct hydraulic communication to a lower section of non-hydrate-bearing, water-saturated sand. The Hydrate-01 log data also confirm the occurrence of at least one sub-seismic fault in close proximity to the B1 sand reservoir. To better image the distribution of the gas-hydrate-bearing reservoir sections and associated faults, a three-dimensional (3D) vertical seismic profile was conducted in early 2019 using the distributed acoustic sensors installed as part of the Hydrate-01 STW completion. Detailed two-dimensional (2D) and 3D geologic models have been constructed to enable numerical simulations to inform the planning for potential future scientific tests of reservoir response to depressurization at the site.

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)

“…A detailed description of the geology, stratigraphy, geometry, and the various units at the site is provided by Boswell et al 16 and Tamaki et al 19 and will not be repeated here. The schematic in Figure 1 provides a simplified description of the simulated system, including the dimensions of its various geologic units.…”

“…To address this issue, a long-term field test of hydrate production from a permafrost-associated reservoir on the Alaska North Slope has been pursued, with the recent drilling of the Hydrate-01 Stratigraphic Test Well confirming the occurrence of a suitable test site within the Prudhoe Bay Unit. 16 In preparation for this test, extensive geologic [17][18][19] , engineering 20 and numerical simulation studies [21][22] are being undertaken in an effort to assess the system response under a wide range of circumstances, explore the technical challenges of long-term production from hydrate reservoirs and prepare an appropriate engineering design. This study is part a larger numerical simulation investigation seeking to provide answers and inputs to the design of the proposed production test in northern Alaska.…”

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

“…To evaluate the applicability of depressurization for sustainable gas production from MH reservoirs, the MH21-S, DOE NETL, and USGS planned to conduct a long-term production test for about a year from MH reservoirs at the Kuparuk 7-11-12 site in the Prudhoe Bay Unit in Alaska North Slope. 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 .…”

Sand Migration Simulation during Gas Production from Gas Hydrate Reservoir at Kuparuk 7–11–12 site in the Prodhoe Bay Unit, Alaska

“…The production site, Kuparuk 7–11–12, in the Prodhoe Bay Unit, Alaska, has been identified as an ideal site for gas hydrate production with past extensive scientific research, and it is confirmed that the site contains two high quality gas hydrate reservoirs. , Furthermore, through logging data and side-wall coring, the detailed characterization of reservoir and nonreservoir units of the site has been established. , Based on these findings, this study simulates sand migration during the planned gas production at the 7–11–12 site. The analysis is carried out using the coupled thermal, hydrological, chemical, and mechanical (thermo-hydro-chemo-mechanical) sand migration model that is originally developed by Uchida et al To evaluate the effect of development of “skin”, two extreme cases are set to simulate conditions of sand inflow: no-sand control case in which produced solid can flow into the well freely and the perfect sand exclusion case in which the entire migrated sand with gas and water should stop and settle down in the domain near the surface of the well.…”