Increasing the accuracy of estimated porosity and saturation for gas hydrate reservoir by integrating geostatistical inversion and lithofacies constraints

Abstract

Abstract Porosity, saturation, thickness of gas hydrate-bearing layers and their spatial distributions are key factors for production tests. The gas hydrate-bearing layers ranging from a few to tens of meters thickness are confirmed by Logging While Drilling (LWD) data and core samples at Sites W11 and SC-03, in the Shenhu area, South China Sea. Gas hydrate-bearing layers of two wells (about 200 m) show that not all layers exhibit good lateral continuity. Several thin layers (~4 m) with low gas hydrate saturations (15–20% of the pore space) or non-gas hydrate are laterally discontinuous. The interbedding of gas hydrate-bearing and non-hydrate layers with different saturations, thickness and random distribution cause the heterogeneities in the reservoir. The gas hydrate saturation and distribution inverted from deterministic and traditional geostatistical inversion have high uncertainties due to the limitations of the empirical relationship and probability density functions (PDFs). A better understanding of spatial distribution and saturation for each gas hydrate layer is useful to build a realistic net hydrate pay and resource estimation. In this study, we first divide gas hydrate saturations, porosity, acoustic impedance into low and high values based on the estimated LWD results, which is call as lithofacies. Then, we propose to create multivariate PDFs by lithofacies constraints. The acoustic impedance, saturation, thickness, porosity of gas hydrate-bearing layers and non-gas hydrate layers are inverted by displaying geostatistical inversion with lithofacies constraint. We found the spatial variations and heterogeneous distributions with a thickness ranging from 4 to 35 m and the laminated gas hydrate layers with saturations ranging from 10% to 40% are clearly shown from seismic data, which are useful for the horizontal well development method of a hydrate production test.