Laboratory insights into the effects of methane hydrate on the anisotropic joint elastic-electrical properties in fractured sandstones

Fractured hydrate-bearing reservoirs show significantly anisotropic geophysical properties. The joint application of seismic and electromagnetic explorations is expected to accurately assess hydrate resources in the fractured reservoirs. However, the anisotropic joint elastic-electrical properties in such reservoirs that are the key to the successful application of the joint explorations, remain poorly understood. To obtain such knowledge, we designed and implemented dedicated laboratory experiments to study the anisotropic joint elastic-electrical properties in fractured artificial silica sandstones (with fracture density of about 6.2%, porosity of approximately 25.7%, and mean grainsize of 0.089 mm) with evolving methane hydrate. The experimental results showed that the anisotropic compressional wave velocities respectively increased and decreased with the forming and dissociating hydrate, and the variation in the increasing trend and the decreasing extent of the velocity perpendicular to the fractures were more significant than that parallel to the fractures, respectively. The experimental results also showed that the overall decreasing trend of the electrical conductivity parallel to the fractures was steeper than that perpendicular to the fractures during hydrate formation, and the general variations of the two conductivities with complex trend were similar during hydrate dissociation. The variations in the elastic and electrical anisotropic parameters with forming and dissociating hydrate were also found to be distinct. Interpretation of the experimental results suggested that the hydrate binding to the grains evolved to bridge the surfaces of fractures when saturation exceeded 10% during hydrate formation, and the bridging hydrate gradually evolved to floating in fractures during dissociation. The experimental results further showed that the anisotropic velocities and electrical conductivities were correlated with approximately consistent trends of different slopes during hydrate formation, and the joint elastic-electrical anisotropic parameters exhibited a sharp peak at the hydrate saturation of about 10%. The results suggested that the anisotropic joint properties can be employed not only to accurately estimate hydrate saturation but also possibly to identify hydrate distribution in the fractures.

Effects of Temperature Gradient and Cooling Rate on the Formation of Methane Hydrates in Coarse Sand

Temperature gradient and cooling rate have an obvious effect on formation of methane hydrate. The process for formation of methane hydrate in coarse sand is monitored to tmderstand the relationship between temperature gradient and cooling rate and nucleation, growth and distribution of methane hydrate by using the electrical resistivity method. The results show that the change of resistivity can better reflect the nucleation and growth and distribution of methane hydrate. Temperature gradient promotes the nucleation, formation, and formation rate of methane hydrate. At a temperature gradient of 0.11℃/cm, the rate of methane hydrate formation and saturation reaches a maximum. Cooling rate has little effect on the methane hydrate formation process. Judging from the outcome of final spatial distribution of methane hydrate, the cooling rate has an obvious but irregular effect in coarse sand. The effect of tempera^re gradient on distribution of methane hydrate in coarse sand is less than that of cooling rate. At a temperature gradient of 0.07℃/cm, methane hydrate is distributed uniformly in the sample. If the temperature gradient is higher or lower than this value, the hydrate is enriched in the upper layer of sample.

Radioactive Element Distribution Characteristics of Red Mud based Field Road Cement before and after Hydration

Red mud was firstly used as a raw material in sintering field road cement. Then, the radioactive element distribution characteristics of red mud based field road cement（RFC） before and after hydration were comparatively investigated. The experimental results indicated that the specific activity of ^（226）Ra and ^（232）Th increased after sintering process from raw material to clinker, as a result of concentrating effect on ^（226）Ra and ^（232）Th during sintering process, but the specific activity of ^（40）K decreased after sintering process as a result of volatilization effect. Radionuclide ^（226）Ra mainly distributed in RFC silicate phases（C_xS）, ^（232）Th distributed more in RFC interstitial phases than RFC silicate phases（C_xS）, ^（40）K mainly distributed in RFC interstitial phases. With increasing hydration ages of RFC pastes, the specific activity of ^（226）Ra kept increasing, ^（232）Th remained consistency all the same and ^（40）K declined. The radioactivity of RFC was in the recommended safe limit of Chinese National Standards GB6566-2010 during its preparation and application process.

Distribution of gas hydrate in fractured reservoirs:Insights from anisotropic seismic measurements

Seismic properties of hydrate-bearing reservoirs that are affected significantly by the hydrate distribution are key for quantitative assessment of the reservoir.The knowledge of hydrate distribution in fractured reservoirs remains poorly understood.To obtain such knowledge,we measured and analyzed five anisotropic velocities needed to fully characterize the seismic anisotropy in an artificial sandstone with aligned fractures during hydrate formation associated with varying distribution.We showed that while the formation of hydrate improved the velocities,the improvement was more significant for hydrate saturation above 10%.We also showed that the increasing trends varied among the anisotropic velocities when hydrate saturation was above 10%.Specifically,the compressional wave velocity travelling vertical to the bedding plane and the shear wave velocity with polarization perpendicular to the bedding plane increased more rapidly than the other compressional and shear wave velocities,respectively.Interpretation of the anisotropic seismic results suggested that the hydrate tends to bind to the grains in the fractures at low hydrate saturation,and becomes to bridge the fracture surfaces when the hydrate saturation exceeds 10%.The results have provided new insights into the hydrate distribution and its resulting anisotropic seismic properties in fractured reservoirs.This will pave the way for the successful assessment of hydrate in fractured reservoirs.

DURABILITY OF HARDENED FLY ASH PASTE

The mechanical properties and durability (mainly frost - resistance and carbonation resistance) of fly ash - CaO - CaSO//4 center dot 2H//2O hardened paste are studied. The relationship among durability of hardened fly ash paste, the quantity and distribution of hydrates and the initial paste texture of hardened fly ash paste is presented. (Author abstract) 3 Refs.