The pressure and temperature regime of the shallow sedimentary section in the Deepwater Gulf of Mexico is conducive for the accumulation of gas hydrates. However, public information is limited regarding observation of in-situ hydrates during deepwater drilling operations. This paper documents drilling observations of possible gas hydrate dissociation from a well in the Deepwater Gulf of Mexico and explains how gas risk associated with potential in-situ hydrate was assessed during planning of subsequent appraisal and development activities. The paper contributes to industry knowledge of how in-situ hydrates are recognized during drilling and presents techniques for predicting hydrate-related risks during shallow hazards assessment.
An appraisal well on the Shenzi Field in Green Canyon Block 653 experienced possible hydrate-related annular flow after cementing the 22-inch casing. Although the flow did not adversely impact drilling operations at this well, indications of in-situ hydrate prompted the assessment of hydrate-related gas risk for subsequent wells. Upon revisiting the 3-D seismic interpretation of the suprasalt section and calculating the estimated base of the gas hydrate stability zone, an apparent ceiling to high-amplitude anomalies within a regional sand-bearing interval was attributed to gas trapped at the base of the hydrate stability zone. The potential for hydrate-bearing sands is being considered during site selection of future appraisal and development wells. The geotechnical implications of in-situ hydrate on soil properties will also be addressed during the engineering design of subsea production facilities.
The Shenzi Field lies approximately 104 nautical miles south of Fourchon, Louisiana (Figure 1a), on the continental slope of the northern Gulf of Mexico (GOM) approximately 5 miles north of the Sigsbee Escarpment in Green Canyon Blocks 609, 610, 653, and 654 (Figure 1b). Water depths in Shenzi Field range from approximately 4,150 feet in the northeast quadrant of Block 653 to 4,480 feet in the southeast corner of Block 654 (Figure 2a).
The seafloor in the Shenzi Field is characterized by small fault scarps related to shallow salt deformation (Figure 2b). Fault scarps are particularly abundant above a shallow salt ridge that extends from the northeast quadrant of Block 653 across the southern boundary of Block 610. The suprasalt section generally thickens away from this salt ridge. A sediment transport pathway contains seafloor furrows that trend north-to-south through Blocks 608 and 652. Possible fluid seepage sites are interpreted in the northern portions of Blocks 653 and 654.
Gas hydrate is a clathrate in which the gas molecules are encaged in an ice-like crystalline lattice of water molecules. Methane and heavier hydrocarbon gases can combine with seawater to form solid gas hydrate in high pressure and cold temperature conditions. In water depths greater than about 1,400 feet on the GOM continental slope, the pressure and temperature conditions are both high enough and cold enough for natural gas hydrates to be stable in the shallow sediments (Ginsberg, 1998; Sloan, 1998). The Gas Hydrate Stability Zone (GHSZ) is the range of depths below seafloor in which gas hydrates are thermodynamically stable.