Open Hole Well Logs

The most recent well log summaries for hydrates is that by Anderson et al. (2005) and the log summary in the Mallik 2002 case study by Collett et al. (2005). While a brief overview is presented below and exemplified in the Mallik 2002 case study, the reader interested in details is encouraged to review the above two documents for a thorough exposition of hydrate open hole well logging. 

Collett (1983) and Collett and Ehlig-Economides (1983) studied the logs of 125 wells in the Prudhoe Bay region, to find 102 hydrate occurrences in 32 wells. The analyses of other permafrost wells were based upon the logs of a known hydrate well, the Northwest Eileen State Well Number Two, in which ARCO/Exxon recovered hydrate cores in 1972. Other studies on the effects of hydrates in wells and logs in the permafrost are by Weaver and Stewart (1982), Collett et al. (1984), Kamath and Godbole (1987), Mathews (1986), Collett (1992), Prensky (1995), with the most recent review of hydrate well logs in by Anderson et al. (2005). 

Nuclear magnetic resonance logging presents a new and efficient means of hydrate detection. Dallimore and Collett (2005, p. 21) summarize theNMR method as follows  

Kleinberg et al. (2005) and Takayama et al. (2005) show that NMR-log measurement of sediment porosity, combined with density-log measurement of porosity, is the simplest and possibly the most reliable means of obtaining accurate gas hydrate saturations. Because of the short NMR relaxation times of the water molecules in gas hydrate, they are not discriminated by the NMR logging tool, and the in situ gas hydrates would be assumed to be part of the solid matrix. Thus the NMR-calculated porosity in a gas-hydrate-bearing sediment is apparently lower than the actual porosity. With an independent source of accurate in situ porosities, such as the density-log measurements, it is possible to accurately estimate gas hydrates saturations by comparing the apparent NMR-derived porosities with the actual reservoir porosities. Collett and Lee (2005) conclude that at relatively low gas 

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A large investment is made by oil and gas companies in acquiring open hole log data. Logging activities can represent between 5% and 15% of total well cost. It is important therefore to ensure that the cost of acquisition can be justified by the value of information generated and that thereafter the information is effectively managed. 

Hydrocarbon-water contact movement in the reservoir may be determined from the open hole logs of new wells drilled after the beginning of production, or from a thermal decay time (TDT) log run in an existing cased production well. The TDT is able to differentiate between hydrocarbons and saline water by measuring the thermal decay time of neutrons pulsed into the formation from a source in the tool. By running the TDT tool in the same well at intervals of say one or two years (time lapse TDTs), the rate of movement of the hydrocarbon-water contact can be tracked. This is useful in determining the displacement in the reservoir, as well as the encroachment of an aquifer. 

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