Geopressuring, secondary porosity, and low‐permeability zones occur in many of the world's sedimentary basins and are often associated with major petroleum reservoirs. We present a mechanical model which simultaneously reproduces these phenomena. Assuming pressure solution to be the dominant rock deformation mechanism, a solid‐state rock skeleton viscosity is introduced which relates the deformation of the rock skeleton to the effective stress. Numerical solutions for compaction indicate that a soft sedimentary layer preferentially compacts to form a low‐permeability zone. There is a large liquid pressure gradient through the low‐permeability zone. In the more slowly compacting region beneath the low‐permeability zone, the pressure gradient remains hydrostatic but the absolute pressure can exceed the lithostatic pressure in the absence of hydraulic fracturing. This results in the formation of secondary porosity.

In April 2005, the Chevron Joint Industry Participation Project (JIP) on Gas Hydrates organized a drilling and coring expedition to potential gas hydrate sites in Atwater Valley and Keathley Canyon in the Gulf of Mexico. In support of these activities, methods were developed to predict the mechanical and phase change stability of boreholes drilled in sediments containing gas hydrates. Models of mechanical failure and downhole temperature were constructed from seismic and log data for the wells in Atwater Valley and Keathley Canyon. Model results were compared with LWD caliper, image, and temperature logs in three boreholes. LWD logs were also used to assess drilling performance. Mechanical failure models compared favorably with deformation features observed in image logs in all three wells. An excellent match was also obtained between the modeled and measured downhole temperatures in Atwater Valley. However, for reasons that remain unknown, temperatures observed in the Keathley Canyon wellbore were generally lower than those predicted by the model. Time-lapse analysis of LWD data revealed that the equivalent circulating density (ECD) in Atwater Valley became abnormally high and coarse-grained solids were falling into the BHA annulus from uphole causing packoffs. These packoffs eventually caused the rotary to stall. Some evidence that the packoffs were caused by shallow water flows discharging large quantities of sand into the wellbore was found. Post-drill temperature simulations indicated that the LWD boreholes in Atwater Valley and Keathley Canyon were sufficiently cool to prevent hydrate from dissociating, owing in part to successful management of circulation rates in the borehole. It was also shown that loop currents at Atwater Valley helped to reduce the risk of dissociation. Introduction Gas hydrates are crystalline substances consisting of molecules of gas (e.g., methane, ethane, H2S) locked in a cage of ice1. They occur continentally in the sediments of permafrost regions such as in Alaska or Siberia, or close to the mudline in deepwater marine sediments, such as in the Gulf of Mexico or the Nankai Trough. Gas hydrates dissociate into water and gas when sufficiently heated or depressurized. Since vast amounts of gas are thought to be locked in sediments containing gas hydrates, there is growing international interest in gas hydrates as an energy resource2,3,4,5. Boreholes drilled in sediments containing gas hydrates are susceptible to a variety of instabilities. Thermal disturbances caused by drilling can lead to dissociation of gas hydrates. Instances of blowouts accompanying dissociation have been documented in the literature, particularly in permafrost regions6. It is likely that such incidents are under-reported, since operators are not always aware that they are drilling in gas hydrate zones. Since gas hydrates can enhance the strength of sediments, either by cementing the grains, or by acting as load bearing members in the pore space, the dissociation of gas hydrates during drilling can lead to a dramatic loss of mechanical competence. Furthermore, the expansion of gas accompanying dissociation may result in an abrupt increase in the pore pressure7 thereby weakening the sediment further. Thus sediments undergoing dissociation may be in an exceptionally weakened state when compared with surrounding formations.

Summary Large accumulations of methane hydrates are known to exist in the deepwater Nankai Trough off the southern coast of Japan. Because of its enormous potential as an energy source, there is growing interest in the production of methane gas from these deposits. An offshore production test was scheduled to test the technological viability to produce the methane hydrate with a depressurization method. However, methane-hydrate production may cause large amounts of compaction and subsidence, which can damage well integrity and cause loss of zonal isolation in the overburden. This condition could provide potential leakage pathways for methane gas to the surface and endanger offshore operations. The paper presents a study in which well integrity was evaluated for a methane-hydrate-production test well in the Nankai Trough. In this study, a new work flow was proposed and applied to honor both fieldwide formation heterogeneity and near-wellbore geometry. The geomechanics properties were determined through integration of seismic inversion data, log data, and core data from the Nankai Trough. Interface elements were installed between the casing and cement and the cement and formation to evaluate the impact of cement-bond quality on the well integrity. The numerical simulation was conducted through coupling of the simulation results produced by a methane-hydrate production simulator from a third party with a 3D finite-element geomechanics simulator. The study predicted the occurrence of 2 cm of subsidence on the seafloor generated by approximately 1.5% of reservoir compaction for a production test of 20 days. Over this period, casing would yield with a plastic strain of approximately 1%. Most findings indicated a low risk in the loss of zonal isolation that is consistent with the results of the production test conducted in March 2013. This simulation evaluated potential risks related to well integrity, and provided critical information for the preparation and execution of the pioneering offshore methane-hydrate production test in the Nankai Trough.

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