Gas hydrate accumulation and saturations estimated from effective medium theory in the eastern Pearl River Mouth Basin, South China Sea

2017 ◽  
Vol 5 (3) ◽  
pp. SM33-SM48 ◽  
Author(s):  
Jin Qian ◽  
Xiujuan Wang ◽  
Timothy S. Collett ◽  
Dongdong Dong ◽  
Yiqun Guo ◽  
...  
Keyword(s):  
Gas Hydrate ◽  
Effective Medium Theory ◽  
Effective Medium ◽  
P Wave ◽  
Well Logs ◽  
High Resistivity ◽  
Pearl River Mouth Basin ◽  
Medium Theory ◽  
Anisotropic Models ◽  
Hydrate Layer

Pore- and fracture-filling gas hydrates were identified from the core samples at several sites during the second Guangzhou Marine Geological Survey (GMGS2) expedition. Well logs indicated that gas hydrate occurred in three distinct layers at site GMGS2-08. The gas hydrate saturations calculated from well-log data and the seismic responses for the three gas hydrate-bearing layers, especially within the middle carbonate layer, were poorly known. We estimated gas hydrate saturations using isotropic and anisotropic models based on the mineral composition of the sediments and the effective medium theory. In the upper and lower gas hydrate-bearing layers, saturations estimated from anisotropic models are close to those estimated from pressures cores and chlorinity data. The average saturation using an anisotropic model in the upper (fracture-dominated) hydrate layer is approximately 10% with a maximum value of 25%. In the lower (fracture-dominated) layer, the horizontal and vertical gas hydrate-filled fractures and visible gas hydrate were formed with a maximum saturation of approximately 85%. For the middle layer, well logs show high P-wave velocity, density, high resistivity as well as low gamma ray, porosity, and drilling rate, together indicating a carbonate layer containing gas hydrate. The hydrate saturations calculated from isotropic models assuming hydrate formed at grain contacts are less than 20%, which fit well with two values calculated from chlorinity data for this layer. The upper gas hydrate layer shows no clear seismic response and probably consisted of small fractures filled with gas hydrate. The middle carbonate and lower fracture-filled gas hydrate-bearing layers show pull-up reflections, with the carbonate layer exhibiting relatively higher amplitudes. Pore-filling gas hydrate was also identified just above the depth of the bottom-simulating-reflector (BSR) from the GMGS2-05 drill site. Below the BSR, the push-down reflections, polarity reversal, and enhanced reflections indicate the occurrence of free gas in the study area.

Modifying the generalized effective-medium theory of induced polarization model in compacted rocks

Geophysics ◽  
2020 ◽  
Vol 85 (4) ◽  
pp. MR245-MR255
Author(s):  
Tong Xiaolong ◽  
Yan Liangjun ◽  
Xiang Kui
Keyword(s):  
Maxwell Equations ◽  
Effective Medium Theory ◽  
Induced Polarization ◽  
Effective Medium ◽  
High Resistivity ◽  
Hydraulic Permeability ◽  
Mineral Particle ◽  
Polarization Model ◽  
Rock Composition ◽  
Medium Theory

The generalized effective-medium theory of the induced polarization model (GEMTIP) is a mathematical-physical model derived from the Maxwell equations based on the effective-medium approach. Compared to the Cole-Cole model, the GEMTIP parameters are better related to the structural parameters of reservoir rocks, such as rock composition, mineral particle size, porosity, and specific surface; therefore, it can better describe the induced polarization (IP) characteristics of tight oil and gas reservoirs. However, GEMTIP is not suitable for high-resistivity perturbed media, and it does not account for interfacial polarization, which occurs between two media that share the same resistivity. Starting from the theoretical assumptions of the GEMTIP model, we derived an extended GEMTIP model (MGEMTIP) by adding an equivalent surface current term into the Maxwell equations for a heterogeneous medium. The complex resistivity parameters predicted by two models are compared through numerical simulation, and the results demonstrate that MGEMTIP can more accurately predict the DC resistivity and the chargeability of heterogeneous media. MGEMTIP is suitable for characterizing the polarization phenomena of rock with high salinity, low porosity, low hydraulic permeability, and a disseminated perturbed medium. Furthermore, the testing of rock samples for the inversion of IP parameters with MGEMTIP revealed that the predicted chargeability is higher than the inverted chargeability from the experimental data. This difference is strongly correlated with rock hydraulic permeability. MGEMTIP provides a petrophysical basis for the forward modeling and inversion of IP parameters of compacted rocks. The quantitative relationships between model IP parameters and reservoir parameters also provide a theoretical foundation for predicting reservoir permeability using electromagnetic methods.

Biot–Gassmann theory for velocities of gas hydrate‐bearing sediments

Geophysics ◽  
2002 ◽  
Vol 67 (6) ◽  
pp. 1711-1719 ◽  
Author(s):  
Myung W. Lee
Keyword(s):  
Elastic Properties ◽  
Gas Hydrate ◽  
Effective Medium Theory ◽  
Velocity Ratio ◽  
Effective Medium ◽  
Unconsolidated Sediments ◽  
Medium Theory ◽  
Velocity Models ◽  
Time Average ◽  
Hydrate Bearing Sediments

Elevated elastic velocities are a distinct physical property of gas hydrate‐bearing sediments. A number of velocity models and equations (e.g., pore‐filling model, cementation model, effective medium theories, weighted equations, and time‐average equations) have been used to describe this effect. In particular, the weighted equation and effective medium theory predict reasonably well the elastic properties of unconsolidated gas hydrate‐bearing sediments. A weakness of the weighted equation is its use of the empirical relationship of the time‐average equation as one element of the equation. One drawback of the effective medium theory is its prediction of unreasonably higher shear‐wave velocity at high porosities, so that the predicted velocity ratio does not agree well with the observed velocity ratio. To overcome these weaknesses, a method is proposed, based on Biot–Gassmann theories and assuming the formation velocity ratio (shear to compressional velocity) of an unconsolidated sediment is related to the velocity ratio of the matrix material of the formation and its porosity. Using the Biot coefficient calculated from either the weighted equation or from the effective medium theory, the proposed method accurately predicts the elastic properties of unconsolidated sediments with or without gas hydrate concentration. This method was applied to the observed velocities at the Mallik 2L‐39 well, Mackenzie Delta, Canada.

Characterization of elastic properties of near-surface and subsurface deepwater hydrate-bearing sediments

Geophysics ◽  
2013 ◽  
Vol 78 (3) ◽  
pp. D169-D179 ◽  
Author(s):  
Zijian Zhang ◽  
De-hua Han ◽  
Daniel R. McConnell
Keyword(s):  
Wave Velocity ◽  
Elastic Constants ◽  
Gas Hydrate ◽  
Effective Medium Theory ◽  
Pore Space ◽  
Seismic Interpretation ◽  
P Wave ◽  
Near Surface ◽  
Free Gas ◽  
Hydrate Bearing Sediments

Hydrate-bearing sands and shallow nodular hydrate are potential energy resources and geohazards, and they both need to be better understood and identified. Therefore, it is useful to develop methodologies for modeling and simulating elastic constants of these hydrate-bearing sediments. A gas-hydrate rock-physics model based on the effective medium theory was successfully applied to dry rock, water-saturated rock, and hydrate-bearing rock. The model was used to investigate the seismic interpretation capability of hydrate-bearing sediments in the Gulf of Mexico by computing elastic constants, also known as seismic attributes, in terms of seismic interpretation, including the normal incident reflectivity (NI), Poisson’s ratio (PR), P-wave velocity ([Formula: see text]), S-wave velocity ([Formula: see text]), and density. The study of the model was concerned with the formation of gas hydrate, and, therefore, hydrate-bearing sediments were divided into hydrate-bearing sands, hydrate-bearing sands with free gas in the pore space, and shallow nodular hydrate. Although relations of hydrate saturation versus [Formula: see text] and [Formula: see text] are different between structures I and II gas hydrates, highly concentrated hydrate-bearing sands may be interpreted on poststack seismic amplitude sections because of the high NI present. The computations of elastic constant implied that hydrate-bearing sands with free gas could be detected with the crossplot of NI and PR from prestack amplitude analysis, and density may be a good hydrate indicator for shallow nodular hydrate, if it can be accurately estimated by seismic methods.

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