Gas hydrates located within the foundation sediments of deepwater offshore structures represent a hazard to foundation integrity. A positive method of detecting gas hydrates is needed. Studies indicate that the cone penetration test and electrical resistivity measurements show promise for in situ detection of hydrates.
Gas hydrates are solid ice-like materials formed by gas molecules included in a crystalline water lattice. Considered a laboratory curiosity since their discovery by Sir Humphrey Davy in 1810, hydrates received renewed study in 1934 when it was discovered they were the cause for blockages in natural gas pipelines. Later, natural gas hydrates were discovered beneath the permafrost in Northern latitudes, and subsequently they were suspected, based on seismic signatures, of existing in oceanic sediments. Hydrates have now been recovered at several locations in the worlds oceans. Ocean-related hydrates are thermodynamically stable in deep waters and can be encountered to depths of several hundred meters below the mudline, depending on the geothermal gradient.
In the case of offshore platforms, the reservoir oil is at a typical temperature of 100C. Oil rising through the conductor pipes over a period of time may cause the temperature in the soil around the conductors to exceed the upper stable temperature of hydrates causing hydrates to melt if present. The decrease in sediment strength and stiffness due to hydrate melting could lead to reduction of the bearing capacity of the foundation and to excessive settlements.
The problem is complicated by the lack of positive methods to detect hydrates in the sediments and by the different morphologies that hydrates may take in nature. The approach which has been taken by researchers in the Offshore Technology Research Center involves the development of in situ tests coupled with geophysical methods to detect hydrates in sediments. It was determined that a probe which can be pushed into sediments would be useful for detecting hydrates. A review of various methods which could be used concluded that cone penetrometer resistance, thermal conductivity, electrical resistivity, pore pressure, temperature, and perhaps density, would all be helpful in detecting hydrates. This paper discusses the development of several of these measurements.
BACKGROUND OF GAS HYDRATES Molecular Structure
The hydrogen-bonded water molecules in hydrates surround gas molecules in a cage-like structure. Gas molecules less than 0.6 nm in diameter form a crystal structure name structure I (1.2nm cell) whereas molecules with diameters between 0.6 and 0.69 nm form hydrates with structure II (1.73nm cell). The physical properties of the two structures do not differ much, although the chemical properties are vastly different.
Mechanisms of Generation and Occurrence
Hydrates may be formed from either biogenic or thermogenic gas in the hydrate stability zone. This zone is a function of temperature and pressure, as well as the gas composition. In the Gulf of Mexico, hydrates have frequently been sampled in surface (0-5m) sediments in water depths of greater than about 500 meters.