Sediment Instability Caused by Gas Production from Hydrate-bearing Sediment in Northern South China Sea by Horizontal Wellbore: Evolution and Mechanism

Abstract

Effective production of natural gas from hydrate-bearing sediments by using various strategies (such as depressurization) is an important way to solve the current global energy crisis. Nevertheless, hydrate dissociation during gas production can weaken sediment strength, influencing reservoir stability and subsequent gas production. Previous studies focused mainly on the analysis of production behavior of natural gas from hydrates, but few on reservoir stability. In this work, evolution of gas production, reservoir characteristics and sediment deformation were analyzed thoroughly with ABAQUS platform. Investigation on gas production revealed that the average production rate was 5.57 × 10⁴ m³/day, indicating that development strategies mentioned herein can achieve the goal of commercial development of gas hydrates. Although the changes of hydrate saturation and effective stress both affected the characteristics of hydrate reservoir throughout hydrate development operation, hydrate saturation was the main influencing factor. The contour of the distribution nephogram of reservoir characteristics basically coincided with that of the hydrate saturation distribution nephogram. Meanwhile, the yield area around wellbore appearing in the early stage of development operation corresponded to the area prone to sand production. However, the yield area near the seabed appearing in the late stage of development operation corresponded to the area prone to submarine landslide. Finally, investigation on sediment deformation indicated, except for the dissociation area, which experienced significant compaction, the sediments in other areas in the confined space experienced continuous subsidence. This study is expected to lay a theoretical foundation for proposing engineering measures to avoid uncontrollable geological disasters in the process of hydrate development.

Appendices

Appendix A: Comprehensive Model for Hydrate-bearing Sediments in Northern South China Sea

The comprehensive model mentioned herein was derived from our previous work (Li et al., 2018b). As we all know, permeability and porosity are two important parameters that describe the pore characteristics of porous media. For hydrate-bearing sediments in northern South China Sea, permeability and porosity of can be expressed as a function of hydrate saturation (S_h) and effective stress (σ). Considering the effects of both hydrate saturation and effective stress, permeability and porosity of hydrate-bearing sediments can be expressed, respectively, as:

$$ K\left( {S_{{\text{h}}} ,\;\sigma } \right) = K_{0} \cdot \left( {0.001 \cdot \sigma^{2} - 0.0572 \cdot \sigma + 1.1267} \right) \cdot \left( {1 - S_{{\text{h}}} } \right)^{7.9718} $$

(A1)

$$ \phi \left( {S_{{\text{h}}} ,\;\sigma } \right) = \phi_{0} \cdot \left( {1 - S_{{\text{h}}} } \right) \cdot \left( {1.1039 \cdot \exp ( - 0.041\sigma )} \right) $$

(A2)

where K₀ and ϕ₀ are the permeability and porosity of hydrate-free reservoir, respectively. In the comprehensive model, the elastic modulus, Poisson's ratio, cohesion and internal friction angle are written, respectively, as:

$$ E\left( {S_{h} ,\sigma } \right) = E_{0} \cdot \left( {1 + 13.25 \cdot S_{h} } \right) \cdot \left( { - 0.004\sigma^{2} + 0.0242\sigma + 0.9784} \right) $$

(A3)

$$ v\left( \sigma \right) = v_{0} \cdot \left( { - 0.0002\sigma^{2} + 0.0139\sigma + 0.9697} \right) $$

(A4)

$$ C\left( {S_{{\text{h}}} ,\;\sigma } \right) = C_{0} \cdot \left( {1 - 1.2 \cdot \left( {\phi - \phi_{0} } \right)} \right) $$

(A5)

$$ \theta \left( {S_{{\text{h}}} ,\;\sigma } \right) = 20 + 2.564 \cdot lg\left[ {\left( {59.83 - 1.785C} \right)^{2} + \sqrt {60.83 - 1.785C} } \right] $$

(A6)

where E₀ is the elastic modulus of hydrate-free sediments, and v₀ and C₀ are the initial Poisson's ratio and initial cohesion of hydrate-bearing sediments before hydrate dissociation, respectively. Graphs showing the variation of parameters mentioned above with hydrate saturation and effective stress are presented in Figure

Figure 19

Variations of parameters of hydrate-bearing sediments with hydrate saturation and effective stress

19. Based on the comprehensive model, the evolution of physical properties of hydrate-bearing sediments in drilling and production operation can be analyzed.

Appendix B: Methodology for Comprehensive Model and Hydrate Dissociation

In the ABAQUS platform, there is currently no module to solve the fluid–solid-thermal-chemical multi-physics coupling problem related to hydrate development. In this work, it was implemented by coding the subroutine of USDFLD. The methodology and algorithm for implementing the functions mentioned above through the USDFLD subroutine is presented in Figure

Figure 20

Methodology and algorithm for implementing the functions of hydrate dissociation and the comprehensive model through the USDFLD subroutine in any increment step

20. Meanwhile, some key codes involved in the subroutine are given in Figure 20. As observed in Figure 20, the implementation methodology can be summarized as the following five steps:

• (1) Definition of the comprehensive model in advance Define the relationship between the physical parameters and the two influencing factors of hydrate saturation and stress in advance in “Property” module of the ABAQUS platform.

• (2) Implementation of multi-physics coupling analysis Based on the physical parameters of sediment determined at the end of the previous increment, perform a multi-physics analysis in this increment.

• (3) Determine hydrate saturation in the present increment based on the dissociation kinetics theory Get hydrate saturation at the end of the previous increment with the state variable (SDV3) and take it as the initial hydrate saturation of the present increment. Based on the dissociation kinetics theory, determine the final hydrate saturation of the present increment.

• (4) Determine the characteristic parameters of sediment used in the next increment Effective stresses of any nodes obtained in the previous increment were got through the function GETVRM, and parameters related to mechanics, thermodynamics and seepage of sediment are obtained by the automatic traversal operation.

• (5) Obtain output at the end of the present increment Obtain the derived distribution of pore pressure (POR), hydrate saturation (SDV3), stress (S), temperature (TEMP) and other parameters to *.ODB file for subsequent analysis. Especially for hydrate saturation, this output needs the help of function GETSDV6 coded by the authors.

To facilitate readers to further carry out relevant research, complete codes of the USDFLD subroutine are depicted as follows.

C The user subroutine can realize the continuous change of physical parameters of hydrate-bearing sediments with hydrate saturation between integration points.

C This source code provides all content including code and annotation in a relatively simple way. According to this source code, simulation of related engineering geological hazards during hydrate development can be realized by other researchers.

C USDFLD subroutine will be called at each node within the investigation model to automatically determine the physical parameters.

See Figures 19 and 20