Numerical Simulation of a Class I Gas Hydrate Reservoir Depressurized by a Fishbone Well

Abstract

The results of the second trial production of the gas hydrate reservoir in the Shenhu area of the South China Sea show that the production of a gas hydrate reservoir by horizontal wells can greatly increase the daily gas production, but the current trial production is still far below the minimum production required for commercial development. Compared with a single horizontal well, a fishbone well has a larger reservoir contact area and is expected to achieve higher productivity in the depressurization development of gas hydrate reservoirs. However, there is still a lack of systematic research on the application of fishbone wells in Class I gas hydrate reservoirs. In this paper, a grid system for gas hydrate reservoirs containing fishbone wells is first established using the PEBI unstructured grid, and fine-grained simulation of reservoirs near the bottom of the wells is achieved by adaptive grid encryption while ensuring computational efficiency. On this basis, Tough + Hydrate software is adopted to simulate the productivity and physical field change of a fishbone well with different branching numbers. The results show that: the higher the number of branches in a fishbone well, the faster the free water production rate, reservoir depressurization, and free gas production rate in the initial stage of depressurization development, and the faster depressurization can effectively promote hydrate dissociation. Compared with a single horizontal well, the cumulative gas production of a six branch fishbone well can increase by 59.3%. Therefore, using multi-branch fishbone depressurization to develop Class I gas hydrate reservoirs can effectively improve productivity and the depressurization effect, but the hydrate dissociation will absorb a lot of heat and lead to a rapid decrease in reservoir temperature and hydrate dissociation rate. At the end of the simulation, the hydrate dissociation rate of all schemes was lower than 50%. In the later stage of depressurization development, the combined development method of heat injection and depressurization is expected to further provide sufficient thermal energy for hydrate dissociation and promote the dissociation of the hydrate.

A Comprehensive Review of Fishbone Well Applications in Conventional and Renewable Energy Systems in the Path towards Net Zero

“…The study conducted by He [60] primarily investigated the application of fishbone wells for the extraction of gas hydrates from reservoirs, using the Shenhu area of the South China Sea as a case study. The research indicated that whereas the use of horizontal wells significantly improved daily gas production, it still fell short of the minimum threshold required for commercial exploitation.…”

Numerical Simulation of Production Behavior with Different Complex Structure Well Types in Class 1-Type Hydrate Reservoir

“…Jin G. et al estimated that 22 sets of multilateral wells each with a completion length of 1000 m are sufficient to achieve commercial development of NGHs in China's Shenhu Sea area [22]. He J. et al simulated the productivity of a fishbone well with different lateral branch numbers and found that the production capacity of the six-branch fishbone well was about 59.3% higher than that of the single horizontal well [23].…”

Hydrate Production Behavior in China’s Second Marine Trial Production Site under High Irreducible Water Saturation and a Strong Sealing Effect

“…Yu et al used a single horizontal well system and a dual horizontal well system to simulate the long-term NGHs production behavior and found that the extremely low permeability makes it difficult to significantly increase gas production. He found through that using multibranch fishbone wells for depressurization can effectively increase the cumulative gas production in the trial production area. Zhu et al demonstrated through numerical simulation that placing a horizontal well in the three-phase layer (TPL) and approaching the bottom of the TPL can achieve higher gas production.…”