Investigating geological storage of carbon dioxide in a gas hydrate reservoir of the Shenhu area, South China Sea

Abstract

Deep-water shallow-buried sand layers above the base of methane hydrate stability zone (BMHSZ) have been recognized as favorable reservoirs for gas hydrate accumulation. Carbon dioxide (CO2) injection into the sand layers provides significant advantages over onshore geological storage options because of CO2 hydrate formation and high CO2 density under conditions of pressure and temperature. However, reservoir characterization and geological conditions for storing CO2 in gas-charged sand layers below the BMHSZ have not been well studied.We investigate deep-water shallow-buried strata in the Shenhu area by analyzing and interpreting 3D seismic and well logs data. We present the W07–W23 gas hydrate reservoir that consists of two Lower Pleistocene sand zones: gas hydrate and free gas accumulation within levee facies in the upper zone and free gas accumulation within channel facies in the lower zone. Analysis of well data shows a 51 m thick total gas section interbedded with gas hydrate and inferred gas hydrate saturation as high as 20% of sediment pore space. Average porosity in the reservoir is estimated from log data at approximately 50%. Permeability in the hydrate-free sediments, as inferred from empirical equations retrieved from the offset wells, ranges 3–5 mD. We estimate that a total of 13–40 × 109 m3 of gas could be contained within the two zones and they may store roughly 5.7–11.4 × 107 ton of CO2. These results provide an improved understanding of gas hydrate reservoirs for occurrence of gas hydrate, free gas accumulations and CO2 storage capability below the traditional BMHSZ.