A fast identification method based on the typical geophysical differences between submarine shallow carbonates and hydrate bearing sediments in the northern South China Se a

Bottom simulating reflector(BSR)has been recognized as one of the indicators of gas hydrates.However,BSR and hydrate are not one-to-one correspondence.In the Xisha area of South China Sea(SCS),carbonate rocks wildly develop,which continuously distribute parallel to the seafloor with high amplitude on seismic sections,exhibiting reflections similar to BSRs in the Shenhu area nearby.This phenomenon causes some interference to hydrates identification.In this paper,the authors discussed the typical geophysical differences between carbonate rocks and hydrates,indicating that the main difference exists in relationship between porosity and velocity,causing different amplitude versus offset(AVO)characters.Then the authors proposed a new model assuming that the carbonates form the matrix and the hydrate fill the pore as a part of the matrix.The key modeling parameters have been optimized constrained by Pvelocities and S-velocities simultaneously,and the model works well both for carbonate rock and gas hydrate bearing sediments.For quantitative identification,the authors calculated the velocities when carbonates and hydrates form the matrix together in different proportions.Then they proposed a carbonate and hydrate identification template(CHIT),in which the possible hydrate saturation(PHS)and possible carbonate content(PCC)can be both scaled out for a group of sample composed by P-velocity and S-velocity.If PHS is far larger than PCC,it is more likely to be a hydrate sample because carbonates and hydrates do not coexist normally.The real data application shows that the template can effectively distinguish between hydrates and carbonate rocks,consequently reducing the risk of hydrate exploration.

Assessment of natural gas hydrate reservoirs at Site GMGS3-W19 in the Shenhu area,South China Sea based on various well logs

To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19,extensive geophysical logging data and cores were analyzed to assess the reservoir properties.Sediment porosities were estimated from density,neutron,and nuclear magnetic resonance(NMR)logs.Both the resistivity and NMR logs were used to calculate gas hydrate saturations,the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data.The density porosity was closely in agreement with the core-derived porosity,and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates.The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity.For the gas hydrate reservoirs at GMGS3-W19,the porosity,gas hydrate saturation and free gas saturation was 52.7%,42.7%and 10%,on average,respectively.The various logs provide different methods for the comprehensive evaluation of hydrate reservoir,which supports the selection of candidate site for gas hydrate production testing.

Lateral bearing characteristics of subsea wellhead assembly in the hydrate trial production engineering

Conductor and suction anchor are the key equipment providing bearing capacity in the field of deep-water drilling or offshore engineering,which have the advantages of high operation efficiency and short construction period.In order to drill a horizontal well in the shallow hydrate reservoir in the deep water,the suction anchor wellhead assembly is employed to undertake the main vertical bearing capacity in the second round of hydrate trial production project,so as to reduce the conductor running depth and heighten the kick-off point position.However,the deformation law of the deep-water suction anchor wellhead assembly under the moving load of the riser is not clear,and it is necessary to understand the lateral bearing characteristics to guide the design of its structural scheme.Based on 3D solid finite element method,the solid finite element model of the suction anchor wellhead assembly is established.In the model,the seabed soil is divided into seven layers,the contact between the wellhead assembly and the soil is simulated,and the vertical load and bending moment are applied to the wellhead node to simulate the riser movement when working in the deep water.The lateral bearing stability of conventional wellhead assembly and suction anchor wellhead assembly under the influence of wellhead load is discussed.The analysis results show that the bending moment is the main factor affecting the lateral deformation of the wellhead string;the anti-bending performance from increasing the outer conductor diameter is better than that from increasing the conductor wall thickness;for the subsea wellhead,the suction anchor obviously improves the lateral bearing capacity and reduces the lateral deformation.The conduct of the suction anchor wellhead assembly still needs to be lowered to a certain depth that below the maximum disturbed depth to ensure the lateral bearing stability,Thus,a method for the minimum conductor running depth for the suction anchor wellhead assembly is developed.The field implementations show that compared with the first round of hydrate trial production project,the conductor running depth is increased by 9.42 m,and there is no risk of wellhead overturning during the trial production.The method for determining the minimum conductor running depth in this paper is feasible and will still play an important role in the subsequent hydrate exploration and development.