System Response During Short- and Long-Term Gas Production from a Gas Hydrate Deposit at the Site of a Planned Field Test in the Ulleung Basin of the Korean East Sea

Abstract

Abstract We investigate by means of numerical simulation the coupled flow and geomechanical system behavior during gas production from an oceanic hydrate accumulation at the site of a planned field test at the Ulleung basin in the Korean East Sea. We study the system response at two time scales: a short period (14 days) that corresponds to the duration of the planned field test, and a prolonged production period that continues to the point of hydrate exhaustion and/or reservoir abandonment. The data describing the geologic model and the system properties were obtained from earlier geophysical studies and the analysis of cores obtained from experimental wells at several locations in the Ulleung Basin. Gas hydrate accumulations at this site typically occur in deep water (2157 m) but close to the ocean floor (with their tops 140 mbsf), and are usually characterized by alternating mud (nearly hydrate-free) and sand (hydrate-rich) layers. The layered stratigraphy and the presence of mud layers preclude the use of horizontal wells and necessitate vertical wells. The analysis of the short-term production tests focuses on the expected gas production rate and the possibility of subsidence during the limited time of the planned test. The long-term study aims to address the issue of feasibility of long-term production from the hydrates at the test site in terms of production rates and the geomechanical stability of the well assembly and of the reservoir. We investigate several well operation regimes. We evaluate the sensitivity of the system to various flow, thermal, and geomechanical properties, and we account for the interdependence of flow and geomechanical properties. We estimate possible subsidence under production and we determine the conditions that have the potential to lead to wellbore and formation failure, thus defining the envelope of well operations and production conditions that maximize safe gas production.