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

Abstract

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.

Simulation of axial tensile well deformation during reservoir compaction in offshore unconsolidated methane hydrate-bearing formation

“…The OCR values of the overburden layer are derived from triaxial test data on formation core samples retrieved at the Nankai Trough [34], whereas OCR = 1 is employed for the reservoir and underburden layers, in which case Equation 1 reduces to Jaky's formula. This means that the reservoir and underburden layers are normally consolidated, and this is consistent with the triaxial compression test results on reservoir and underburden sediment cores recovered at the Nankai Trough [9].…”

“…However, if gas production longer than 20 days were simulated, greater reservoir compaction would occur, and there is a risk of well failure. Yoneda et al [9] also simulated gas production at the Nankai Trough and found that tensile deformation of the well developed in the overburden layer due to reservoir compaction, which might cause the tensile failure of the gravel pack to induce sand production. This highlights the importance of analyzing the tensile deformation of the well during reservoir compaction.…”

“…Table 1 shows the list of numerical work on well integrity during reservoir compaction [16], including the ones introduced above. The main shortcomings of these studies are the omission of the well construction process prior to simulating reservoir compaction ( [9], [15], [17]- [27]) and omission of the cement sheath ( [14], [15], [20]- [24], [28]). Currently, the work by Xu [29] is the only existing work which simulates detailed well construction processes, such as cement shrinkage, and also models the integrity of the cement sheath.…”

Distributed fibre optic strain sensing of an axially deformed well model in the laboratory

“…However, the test was terminated prematurely due to sand production (Uchida, Klar and Yamamoto, 2016). One of the potential causes of sand production is considered to be well failure induced by the compaction of the unconsolidated methane hydrate reservoir (Yoneda et al, 2018).…”

Stability Analysis of Near-Wellbore Reservoirs Considering the Damage of Hydrate-Bearing Sediments

“…In addition, the differences between fully coupled and semi-coupled models were analyzed. Yoneda et al [10] developed a coupled thermo-hydro-mechanical (THM) simulator to evaluate the mechanical stability of a hydrate-bearing reservoir and well completion at the eastern Nankai Trough. The results showed that the mechanical deformation occurred in a much wider area than the range of hydrate dissociation and large shear stress occurred near the production well.…”