OS1B-1:QUANTITATIVE ASSESSMENT OF GAS HYDRATE CONCENTRATION USING INTEGRATED WELL LOG MODELING

    Gas hydrate research has undergone many advances in the last decade, but there is still much to be learned about the nature and distribution of gas hydrate. Advances in this area are relevant to assessment of shallow geohazards within the gas hydrate stability zone. Typically, gas hydrate saturation is quantified using downhole resistivity and the Archie equation, which assumes sandy host sediment, constant gas hydrate morphology, and that increases in resistivity are directly related to the presence and volume of gas hydrate. We provide an improved and updated methodology that utilizes the physical properties of gas hydrate and integrates multiple well log curves to quantitatively estimate hydrate concentrations. Application of this method will help improve detection, characterization, and quantification of gas hydrate using log responses, in particular when it is not within the pore space of sandy sediments.

    We chose five legacy borehole sites that have known gas hydrate deposits within its shallow gas hydrate stability zone to test the effectiveness of this quantitative modeling technique. Two boreholes were selected from the Gumusut field offshore of Sabah, Malaysia, and three boreholes from ODP/IODP sites on the Cascadia margin: ODP Leg 204 Sites 1248 and 1250 (Hydrate Ridge) and IODP Expedition 311 Site U1328 (Bullseye Vent). Each of these borehole sites have abundant data available, and were all drilled with the intent of learning more about gas hydrate deposits. The Gumusut boreholes, in particular, underwent robust gas hydrate analysis and volumetric modeling using Archie’s equation prior to this study, making them ideal for comparing and verifying the accuracy of different gas hydrate models.

    We utilized a standard multicomponent log evaluation model, part of a commercial log evaluation software solution, for our gas hydrate model calculations. Given two of relative volume, actual log response, and expected log response given 100% composition of a given material, the software can solve for the third. This method, commonly used for petrophysical analysis and quantification of fluids and lithology, can be adopted for application to gas hydrate deposits. The strength of this technique is its ability to integrate multiple log responses and allow user definition of specific equations and models, which provides robust and reproducible results. In this case, we input actual and expected log responses of pure hydrate, background sediment, and fluids in order to obtain the relative volumes of these formation components. Using multiple log curves simultaneously (gamma ray, bulk density, compressional slowness, neutron porosity, and resistivity), and relying on the physical properties obtained from laboratory analysis and available core data, we calculated the relative abundance of gas hydrate for each of the five boreholes. These results were calibrated and checked for accuracy using field reports, core photos, wellbore images, x-ray, and infrared imagery. For the two Gumusut boreholes, modeled gas hydrate was further calibrated with core porewater freshening data, which can be used as a proxy for gas hydrate volume.

    Our results compare favorably to the Archie gas hydrate models previously calculated for Gumusut boreholes, and their accuracy (+ 4% on average from porewater freshening values) is quite good. The three IODP/ODP boreholes also resulted in reasonable models and were well-constrained and well-calibrated by documented physical properties and core data. The improved performance of the new method compared to the Archie method is likely due in part to the more robust nature of the calculation, but also because it does not assume that gas hydrate only occurs in the pore space of sandy sediment. In fact, our method is capable of distinguishing between pore-filling gas hydrate space and other morphologies. Observation of gas hydrates using the concepts described here is sure to advance our understanding of the nature and subsurface distribution of these deposits. This technique for modeling gas hydrates is a very visual and effective way to calculate and display gas hydrate distribution throughout a borehole; additionally, it is simple to implement using existing technology and gives the gas hydrate community an improved way to assess these potential shallow geohazards.