GAS HYDRATES AS A DRILLING HAZARD: EXAMPLES FROM GLOBAL DEEP WATER SETTINGS

Jillian Nimblett, Craig Shipp and Floris Strijbos
Shell International Exploration and Production, Houston TX

The presence of gas hydrate in marine sediments, while possibly representing a significant drilling hazard in the petroleum industry, is rarely given adequate attention pre-drill. Without prior indication of hydrate, either by analog data at nearby offset wells or the presence of a BSR on seismic data, the existence of in situ gas hydrate is usually not considered. Failure to account for the presence of gas hydrate in drilling operations has resulted in hazardous situations ranging from minor gas bubbling to borehole instability. De-risking these scenarios is therefore of primary importance to the industry and must include robust geo-hazard analysis techniques that integrate geophysical and petrophysical modeling, and seismic interpretation as well as pore pressure evaluation. Several case histories in diverse exploration settings illustrate where gas hydrates were encountered while drilling. Based on analysis of log data as well as borehole observations, several possible mechanisms by which hydrate dissociation induced drilling hazards during well operations may be inferred. Analysis of these case studies indicate that hydrate occurrence was accompanied either by anomalous borehole temperatures, unexplained gas flows, hydrate formation on equipment, borehole instability, or any combination of the four.

Traditionally geo-hazard analysis has focused on geologic descriptions of the near-surface interval made on the basis of seismic interpretation. In particular geo-hazard analysis has mainly included identification of phenomena that fall into one of two categories: seafloor stability (e.g. expulsion features and scarps) or subsurface features (e.g. faults, shallow gas, and sand-prone intervals). Methodology used to identify these features is well established and widely accepted in industry. A key challenge now facing geo-hazard interpreters is to therefore integrate gas hydrate analysis in geo-hazard assessment. Such an effort requires first, recognition of the potential of gas hydrate dissociation as a geo-hazard and second, development of accurate methods of identifying and quantifying gas hydrate volumes prior to drilling. Ongoing detailed seismic velocity and amplitude analysis may provide further information relevant to our understanding of the in situ rock properties of hydrate-bearing intervals. These efforts will no doubt aid in the development of seismic detection tools for hydrate identification for the purposes of geo-hazard assessment.