Temperature

To date, most thermal studies in marine hydrate reservoirs have relied on traditional heat flow surveys in which temperatures are measured to depths of 25 m below the seafloor using one of several designs of heat flow probes. Although the details of the probes vary, each type is equipped with thermistors, which are sensors whose electrical resistance varies in a regular manner with very small changes in temperature. The thermistors are arrayed along the probe, which is dropped vertically into the seafloor (Figure la). The equilibrium temperatures T (units of °C or K) measured by these thermistors yield a direct constraint on the thermal gradient as a function of depth (dT/dz). Because these data are collected in the shallowest part of the sedimentary column, the measurements typically do not directly constrain temperatures within the gas hydrate reservoir itself, which is often located at depths far greater than the depth of maximum probe penetration.

The most direct measurements of the thermal state of the gas hydrate reservoir are obtained in situ, through the use of special probes deployed in undisturbed sediments at the bottom of boreholes (Fisher and Becker, 1993; Davis et al., 1997). This technique is known as downhole temperature measurement (Figure lb). In the marine environment, such measurements have typically been acquired as part of Ocean Drilling Program (ODP) studies using one of three types of probes. These probes remain by far the best means for characterizing the thermal state of the hydrate reservoir. However, none of the probes can measure thermal conductivity, only one can determine a true thermal gradient during a single deployment, and one of the probes can only be deployed at depths at which sediments are not indurated.

Logging of fluid temperatures in open boreholes can sometimes be useful for constraining equilibrium temperatures above, below, and within the gas hydrate reservoir. Unfortunately, borehole fluid temperature logs do not usually provide an accurate characterization of equilibrium temperatures in the sediments. One difficulty is that borehole temperature logging is typically completed immediately following drilling, meaning that the fluid filling the borehole is not yet in equilibrium with the surrounding formation, where gas hydrate may or may not be present. A second problem is that fluids in open channels move more freely than pore fluids in the adjacent marine sediments and that boreholes may intersect fractures or other asperities that enhance migration of fluid into the borehole. Such processes can have a dramatic impact on fluid temperatures measured in boreholes.

By the time hydrate-bearing sediments reach the deck of a ship during marine coring operations, small pieces of hydrate disseminated throughout the sediment matrix have usually begun to break down into constituent components (gas and water). Because dissociation of gas hydrate is an endothermic process, temperature probes inserted in the core detect relatively colder regions where even very small amounts of hydrate were present in situ. Mapping of the thermal anomalies in cores immediately following recovery is therefore one of the best means for developing a qualitative constraint on the distribution of gas hydrate. Such temperature measurements, which are sometimes referred to as catwalk temperature measurements to denote their completion on the catwalk of the drilling ship, proved critical for locating gas hydrate in the cores during ODP Leg 164 (Paulletal., 1996).

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Figure 1. Common methods used to characterize the thermal state of the hydrate reservoir, (a) Traditional marine heat flow measurements are conducted in the shallowest part of the sedimentary section, often far shallower than the top of the hydrate reservoir, (b) Downhole temperature measurements in ODP boreholes determine equilibrium sediment temperature at depths as large as hundreds of meters below the seafloor. Circles schematically represent measurement depths.

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