Sensitivity analysis of seismic reflectivity to partial gas saturation

Geophysics ◽  
2007 ◽  
Vol 72 (3) ◽  
pp. C45-C57 ◽  
Author(s):  
Carmen T. Gomez ◽  
Robert H. Tatham
Keyword(s):  
Sensitivity Analysis ◽  
Shale Gas ◽  
Poisson’S Ratio ◽  
Essential Element ◽  
Elastic Parameter ◽  
Poisson's Ratio ◽  
Density Contrast ◽  
Water Contact ◽  
Gas Saturation ◽  
Seismic Reflectivity

We analyze the sensitivity of seismic reflectivity to contrasts in density, seismic propagation velocities, Poisson’s ratio, and gas saturation using the complete Zoeppritz equations. Sensitivities of reflection coefficients to each bulk elastic parameter are computed as the partial derivatives of the seismic reflectivities, relative to each parameter. The sensitivity of reflectivity to gas saturation is then calculated as the full derivative of the reflectivities with respect to gas saturation, assuming both a homogeneous and a patchy distribution of gas in the pore fluids. We compute sensitivities for a sealing shale/gas-sand interface and a gas-sand/wet-sand (gas-water contact, GWC) interface. For the SH-SH reflectivity, the effect of density contrast is strongest in the 30°–50° range of incidence angles for the fluid-fluid interface and at nearer offsets for the shale/gas-sand interface. P-SV reflectivity forthe fluid-fluid interfaces is more sensitive to density contrast in the range of angles of incidence from 30° to 60°. The overall response of P-SV reflectivity to gas saturation throughout all offsets is dominated by the Poisson’s ratio of the gas sand. In the case of P-P reflectivity, the sensitivity to gas saturation increases with increasing incidence angles. The sensitivity of P-SV reflectivity to gas saturation tends to be greatest in the 20°–40° range of incidence angles. For SH-SH reflectivity, the sensitivity to gas saturation for most offsets is controlled mainly by the density contrast, and the sensitivity to density decreases with increasing offset. There is still not a generally accepted seismic reflection method to discriminate commercial gas concentrations from low gas saturation. From the sensitivity analysis, we conclude that the use of P-SV or SH-SH amplitude variation with offset (AVO), integrated with the P-P AVO, will be an essential element in understanding this problem fully.

Topology Optimisation of Composites Containing Base Materials of Distinct Poisson’s Ratios

2014 ◽  
Vol 553 ◽  
pp. 813-817
Author(s):  
Xu Ran Du ◽  
Mike Xie ◽  
Xiao Ying Yang ◽  
Zhi Hao Zuo
Keyword(s):  
Sensitivity Analysis ◽  
Poisson’S Ratio ◽  
Poisson's Ratio ◽  
Topology Optimisation ◽  
Base Materials ◽  
Maximum Stiffness ◽  
Natural Composites ◽  
Constituent Phase ◽  
Poisson's Ratios ◽  
Poisson’S Ratios

From recent studies on natural composites such as nacre and bone, it has shown that the mechanical properties of the composite are significantly affected by the Poisson’s ratio of each constituent phase. In some cases it is found that when the Poisson’s ratio approaches the incompressibility limit, the stiffness of the composite in one or more directions can increase dramatically, in some cases by two or more orders of magnitude than the softer phase. In this paper we investigate designing the composite of maximum stiffness by a topology optimisation approach. The method used is based on the bi-directional evolutionary structural optimisation (BESO). The Optimisation problem is formulated and it is solved by a searching algorithm based on the sensitivity analysis. The effect of interpolation function in the sensitivity analysis is studied. Examples of different combinations of Poisson’s ratios are presented. The stiffness is found to increase from its base value. In the case of one phase having negative Poisson’s ratio, the increase is very significant. It is concluded that the proposed method is effective in optimising the stiffness of this class of composite.

Analytical Method to Evaluate Casing Stress during Multi-Fracturing for Shale Gas Wells

2019 ◽  
Vol 944 ◽  
pp. 1050-1060 ◽  
Author(s):  
Xue Li Guo ◽  
Jun Li ◽  
Yi Jin Zeng ◽  
Shi Dong Ding ◽  
Xu Zhang
Keyword(s):  
Analytical Model ◽  
Shale Gas ◽  
Poisson’S Ratio ◽  
Poisson's Ratio ◽  
Sensitivity Analyses ◽  
Stress Deviator ◽  
Fluid Temperature ◽  
Analysis Model ◽  
Gas Wells ◽  
Cement Sheath

An analysis model of casing stress distribution and its variation regularities presents several challenges during hydraulic fracturing of shale gas wells. In this paper, an analytical mechanical - thermal coupling method was provided to evaluate casing stress. In the model, the casing, cement sheath, and formation (CCF) system was divided into four stress field induced by uniform stress, deviator stress, shear stress, and thermal stress,. Based on this analytical model, the parametric sensitivity analyses of casing stress such as mechanical properties, operation parameters, and geo-stress were conducted during multi-fracturing. The results indicated the casing stress increased first, then decreased with the increase of cement sheath modulus. However, it always decreased with the increase of cement sheath Poisson's ratio and the injection fluid temperature. The casing stress increased dramatically with the increase of δ. However, it decreased first, then increased with the increase of fracturing pressure. Higher fluid temperature, cement with small modulus and large Poisson’s ratio were effective to decrease the casing stress. In conclusion, the analytical model can accurately predict casing stress and become an alternative method of casing integrity evaluation for shale gas wells. It is a useful and efficient method for a preliminary design, being capable of simulation the actual situations in order to assess the casing stresses and integrity.

Some effects of velocity variation on AVO and its interpretation

Geophysics ◽  
1993 ◽  
Vol 58 (9) ◽  
pp. 1297-1300 ◽  
Author(s):  
Yu Xu ◽  
G. H. F. Gardner ◽  
J. A. McDonald
Keyword(s):  
Velocity Gradient ◽  
Poisson’S Ratio ◽  
Incident Angle ◽  
Elastic Parameter ◽  
Poisson's Ratio ◽  
Velocity Variation ◽  
Elastic Parameters ◽  
Amplitude Variation ◽  
Amplitude Variation With Offset ◽  
Meaningful Relationship

In recent years interest has increased in the interpretation of the amplitude variation of reflected signals as a function of offset (AVO). A more meaningful relationship for interpreting reflection coefficients at the target horizon is amplitude variation with incident angle (AVA). The challenge is to convert from AVO to AVA. The effects of velocity variation in the overburden on amplitude variation with offset (AVO) and on the final inversion of AVO data into velocity, density, and Poisson’s ratio can be significant. Examples are given here for subsurface medium with a vertical velocity gradient range of [Formula: see text] to [Formula: see text]. When the medium is treated as homogeneous in the conversion from AVO to AVA, this velocity variation causes significant errors (about 10 percent) in both the gradient of AVA and in the normal incident reflection coefficient. Such errors produce errors of similar magnitude in the inversion of AVA data into the elastic parameters of velocity, Poisson’s ratio, and density. The errors depend on the velocity gradient, the offset range, the elastic parameter contrast across the interface, and the interface depth.

scholarly journals New Analytical Method to Evaluate Casing Integrity during Hydraulic Fracturing of Shale Gas Wells

2019 ◽  
Vol 2019 ◽  
pp. 1-19 ◽  
Author(s):  
Jun Li ◽  
Xueli Guo ◽  
Gonghui Liu ◽  
Shuoqiong Liu ◽  
Yan Xi
Keyword(s):  
Stress Field ◽  
Hydraulic Fracturing ◽  
Shale Gas ◽  
Poisson’S Ratio ◽  
New Method ◽  
Poisson's Ratio ◽  
Sensitivity Analyses ◽  
Fluid Temperature ◽  
Gas Wells ◽  
Cement Sheath

An accurate analysis of casing stress distribution and its variation regularities present several challenges during hydraulic fracturing of shale gas wells. In this paper, a new analytical mechanical-thermal coupling method was provided to evaluate casing stress. For this new method, the casing, cement sheath, and formation (CCF) system was divided into three parts such as initial stress field, wellbore disturbance field, and thermal stress field to simulate the processes of drilling, casing, cementing, and fracturing. The analytical results reached a good agreement with a numerical approach and were in-line with the actual boundary condition of shale gas wells. Based on this new model, the parametric sensitivity analyses of casing stress such as mechanical and geometry properties, operation parameters, and geostress were conducted during multifracturing. Conclusions were drawn from the comparison between new and existing models. The results indicated that the existing model underestimated casing stress under the conditions of the geostress heterogeneity index at the range of 0.5–2.25, the fracturing pressure larger than 25 MPa, and a formation with large elastic modulus or small Poisson’s ratio. The casing stress increased dramatically with the increase of in situ stress nonuniformity degree. The stress decreased first and then increased with the increase of fracturing pressure. Thicker casing, higher fluid temperature, and cement sheath with small modulus, large Poisson’s ratio, and thinner wall were effective to decrease the casing stress. This new method was able to accurately predict casing stress, which can become an alternative approach of casing integrity evaluation for shale gas wells.

The Model for Calculating Elastic Modulus and Poisson's Ratio of Coal Body

2013 ◽  
Vol 6 (1) ◽  
pp. 36-43 ◽  
Author(s):  
Ai Chi ◽  
Li Yuwei
Keyword(s):  
Elastic Modulus ◽  
Poisson’S Ratio ◽  
Fractured Rock ◽  
Rock Block ◽  
Poisson's Ratio ◽  
Linear Elastic ◽  
Study Results ◽  
The Face ◽  
Block Face ◽  
Coal Rock

Coal body is a type of fractured rock mass in which lots of cleat fractures developed. Its mechanical properties vary with the parametric variation of coal rock block, face cleat and butt cleat. Based on the linear elastic theory and displacement equivalent principle and simplifying the face cleat and butt cleat as multi-bank penetrating and intermittent cracks, the model was established to calculate the elastic modulus and Poisson's ratio of coal body combined with cleat. By analyzing the model, it also obtained the influence of the parameter variation of coal rock block, face cleat and butt cleat on the elastic modulus and Poisson's ratio of the coal body. Study results showed that the connectivity rate of butt cleat and the distance between face cleats had a weak influence on elastic modulus of coal body. When the inclination of face cleat was 90°, the elastic modulus of coal body reached the maximal value and it equaled to the elastic modulus of coal rock block. When the inclination of face cleat was 0°, the elastic modulus of coal body was exclusively dependent on the elastic modulus of coal rock block, the normal stiffness of face cleat and the distance between them. When the distance between butt cleats or the connectivity rate of butt cleat was fixed, the Poisson's ratio of the coal body initially increased and then decreased with increasing of the face cleat inclination.

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