While possibly representing a significant drilling hazard in the offshore petroleum industry, the presence of in situ gas hydrate in marine sediments is rarely given adequate attention before drilling of the well. Without prior indication of hydrate, either by analog data at nearby offset wells or the presence of a bottom-simulating reflector (BSR) on seismic data, the existence of hydrate is usually not considered. Failure to account for the presence of hydrate in drilling operations has resulted in well trouble, ranging from minor gas flows to borehole instability. Adressing these scenarios is therefore of primary importance to the offshore industry. These scenarios must include robust geohazard analysis techniques that integrate geophysical and petrophysical modeling, seismic interpretation, and pore pressure evaluation. Examples in diverse exploration settings are illustrated, where hydrate was 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 examples indicate that hydrate occurrence was accompanied either by unexplained gas flows, hydrate precipitation on seafloor equipment, borehole instability, or any combination of the three.
In the last two decades, global exploration and production activities have expanded further into the deepwater and ultradeepwater environments. Recently, a number of wells have been drilled, either in locations at which hydrate can theoretically exist, or in close proximity to locations where hydrate has been observed. These observations are made on the basis of the presence of a bottom-simulating reflector (BSR) or directly sampling hydrate in cores. Thus far, most operators have either avoided hydrate or drilled through 'probable' hydrate deposits without major incident. As a result, encountering hydrate during deepwater operations may have been largely overlooked as a potential drilling hazard.
Contributing factors to a lack of recognition for hydrate as a drilling hazard is complicated by the difficulty to accurately identify hydrate-laden sediment, using traditional interpretation tools. In the absence of direct seismic indicators, such as a BSR, the presence of hydrate in marine sediment cannot be confirmed reliably before drilling and measurement. A limited knowledge of the petrophysical properties of hydrate in sediment also contributes to this uncertainty, particularly due to the use of only gamma ray and resistivity logging-while-drilling (LWD) tools in the riserless section. Other evidence such as seafloor hydrate mounds (manifested as seismic amplitude anomalies at seafloor) can aid in hydrate identification pre-drill, but tend to be rare. Identification of hydrate can only be certain in cases where there is visual confirmation by remotely operated vehicles (ROV). This is because other seafloor features, such as chemosynthetic communities, may have a similar amplitude signature to hydrate mounds.
Another complication is that even where hydrate has been detected seismically, logging and drilling performance do not provide accurate hydrate characterization for subsequent wells. Deepwater wells are typically drilled riserless from the seafloor to approximately 600 m (?2000 ft) below the mudline (BML) with relatively large-hole sizes (usually greater than 20 in. diameter).