Study on the shape of open-pit for methane hydrate mining using 3D finite difference method and shear strength reduction method

Abstract:

Introduction

In order to recover surface type methane hydrate by open-pit mining safely and economically, it is necessary to prevent occurrences of collapses in and around the open-pit. Therefore, it is important to design the shape of open-pit so as to increase the amount of recovered methane hydrate while ensuring the stability of the ground in and around the open-pit. In this study, the stability of the open-pit for methane hydrate mining was analyzed using three-dimensional finite difference method (TDFDM) and shear strength reduction method (SSRM) and appropriate shape of open-pit was investigated.

Method

It was assumed that there was a sandy sediment containing 25% by volume of 1 cm diameter methane hydrate particles (SS25) in a cylindrical region of 300 m in diameter and 100 m in depth lying under the seabed. The stability of the ground in the process of excavating this area in the shape of an inverted truncated cone was examined by combining TDFDM and SSRM. The following two models were assumed as SS25 generated processes.

Model 1: Methane hydrate particle was formed in the pores of the sand particles in the sandy sediment on the sea floor and the SS25 was formed as a result.

Medel2: Methane hydrate was formed and grown on the surface of the sea floor, and as a result of sedimentation of sand on it, SS25 was formed.

Cohesion C and friction angle φ of SS25 were given by a weighted average of the values of C and φ of pure methane hydrate and fine sandy sediment by the respective ratios. Specifically, φ=5.7°, C=0.148MPa- (depth 0-10m), 0.155MPa (depth 10-20m), 0.178Mpa (depth 20-40m), 0.204MPa (depth 40-60m), 0.230MPa (depth 60-80m), 0.260Mpa (depth 80-100m). C and φ for outside area of the cylindrical region containing SS25 were given by different values considering SS25 generated process of Model 1 and 2.

Results and Discussion

In the case of the Model 1, the safety factor was 2.28 even when excavating to a depth of 100 m in an inverted truncated cone shape with a side slope of 63 degrees. The maximum amount of SS25 that can be recovered is approximately 5 million m³ (70% of the total existing amount). In the case of the Model 2, when excavating with the same shape as Model1, the safety factor when excavating to a depth of 90 m is 1.98, but less than 1.0 when excavating to a depth of 100 m. The maximum recoverable amount of SS25 is approximately 4.6 million m³ (66%).