FORMATION EVALUATION OF GAS HYDRATE BEARING SEDIMENTS (2008)

ABSTRACT There is a growing interest in natural gas hydrate mainly due to the possibility that it may represent a large energy resource, potentially have an impact on global climate change and play a role in submarine slope failures. It is therefore of great importance to be able to identify the occurrence of in-situ gas hydrates and accurately assess their volume within the host sediments. The evaluation of gas hydrate volumes is dependent on a number of parameters, including the areal extent of the hydrate occurrence, reservoir thickness, porosity of reservoir and degree of hydrate saturation. Normally in hydrocarbon exploration of conventional oil and gas fields, the analysis of core samples, obtained while drilling, are used to evaluate this petrophysical data. However, because hydrate disassociates and loses gas as it is brought up to normal surface temperatures and pressure, it becomes problematical in obtaining meaningful porosity and saturation values from recovered cores. Therefore, downhole well logging techniques are used in order to obtain petrophysical data. These techniques have, in the main, been adapted from the standard hydrocarbon industry methods of determining porosity and saturations using resistivity and acoustic log data, together with the deterministic relationships such as the Archie (resistivity) and Wyllie (acoustic) time average equations. Deterministic formation evaluation methods follow a step-by-step approach to arrive at one solution for each depth step. In successive steps, the lithology, porosity, and saturations are determined. Thus the analytical parameters are fixed and the solutions unique. Another approach has been to use probabilistic statistical software such as the Mineral Solver module within the Interactive Petrophysics™ software package. Probabilistic evaluation, computes a solution from an assigned mineral model and all the available logs simultaneously, using statistical methods. This probabilistic method has been applied to log data acquired from the ODP/IODP (marine) and the Mallik (permafrost) exploration wells, successfully generating gas hydrate saturation estimates, with comparable results to Archie saturation estimates at high saturations. Further investigation is required to fully determine the potential of probabilistic methods for gas hydrate evaluation. INTRODUCTION Some of the significant questions for gas hydrate research include: 1. Where do the gas hydrates occur? 2. How much gas hydrate is there? 3. Why and how do gas hydrates occur in particular geological settings? Well logging and subsequent log analysis form crucial roles in answering some of these questions, particularly in evaluating the quantity of gas hydrates present in hydrate bearing sediments. Gas hydrates are combinations of water and natural gas that, under certain conditions of high pressures and low temperatures, form solid, ice like substances, forming as cement in the pore spaces of sediment (Figure 1) as well as in layers and nodules of pure massive hydrate (Booth et al., 1998; Clennell et al., 2000; Camps et al., 2006). They naturally occur within a limited range of pressures and temperatures, found in some ocean sediments and in on-shore sediments of the permafrost regions of the Arctic (Kvenvolden et al., 2001). Hydrates form in sediments where methane is available, either from the biological breakdown of organic matter, or from thermal decomposition of an underlying petroleum reservoir. These gas hydrate deposits potentially represent a significant energy resource, with