Modeling the Mechanical and Phase Change Stability of Wellbores Drilled in Gas Hydrates by the Joint Industry Participation Program (JIP) Gas Hydrates Project, Phase II

Abstract

Abstract 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.