FORMATION EVALUATION OF GAS HYDRATE BEARINGSEDIMENTS

摘要

There is a growing interest in natural gas hydratemainly due to the possibility that it may represent alarge energy resource, potentially have an impact onglobal climate change and play a role in submarineslope failures. It is therefore of great importance to beable to identify the occurrence of in-situ gas hydratesand accurately assess their volume within the hostsediments. The evaluation of gas hydrate volumes isdependent on a number of parameters, including theareal extent of the hydrate occurrence, reservoirthickness, porosity of reservoir and degree of hydratesaturation. Normally in hydrocarbon exploration ofconventional oil and gas fields, the analysis of coresamples, obtained while drilling, are used to evaluatethis petrophysical data. However, because hydratedisassociates and loses gas as it is brought up to normalsurface temperatures and pressure, it becomesproblematical in obtaining meaningful porosity andsaturation values from recovered cores. Therefore,downhole well logging techniques are used in order toobtain petrophysical data. These techniques have, inthe main, been adapted from the standard hydrocarbonindustry methods of determining porosity andsaturations using resistivity and acoustic log data,together with the deterministic relationships such as theArchie (resistivity) and Wyllie (acoustic) time averageequations. Deterministic formation evaluation methodsfollow a step-by-step approach to arrive at one solutionfor each depth step. In successive steps, the lithology,porosity, and saturations are determined. Thus theanalytical parameters are fixed and the solutionsunique. Another approach has been to use probabilisticstatistical software such as the Mineral Solver modulewithin the Interactive Petrophysics? software package.Probabilistic evaluation, computes a solution from anassigned mineral model and all the available logssimultaneously, using statistical methods. Thisprobabilistic method has been applied to log dataacquired from the ODP/IODP (marine) and the Mallik(permafrost) exploration wells, successfully generatinggas hydrate saturation estimates, with comparableresults to Archie saturation estimates at highsaturations. Further investigation is required to fullydetermine the potential of probabilistic methods for gashydrate evaluation.