シェールサンプルから廃坑ガス井の排出ポテンシャルを推定(Estimating emissions potential of decommissioned gas wells from shale samples)

2024-04-232024-04-24

2024-04-22 ペンシルベニア州立大学(PennState)

ペンシルベニア州立大学の研究チームが、稼働を停止したシェールガス井戸からのメタン排出量を推定できる新しいツールを開発しました。このツールは、井戸の閉鎖後もシェール層からメタンが拡散し続けることを明らかにし、その排出量は掘削中や運用中に見られる重要な排出源に匹敵するとされています。この発見は「Science of the Total Environment」誌に掲載されました。研究は、メタンの拡散挙動を予測する統合ガス輸送モデルに基づいており、シェールの構造とガスの動きを組み合わせています。この技術は、放棄された井戸からの長期にわたるメタン排出の監視を支援することが期待されています。

＜関連情報＞

未回収のシェールガス埋蔵量が放棄されたシェールガス井戸からのメタン排出に与える影響 Impact of unrecovered shale gas reserve on methane emissions from abandoned shale gas wells

Yun Yang, Shimin Liu, Haoming Ma
Science of the Total Environment Available online: 30 December 2023
DOI:https://doi.org/10.1016/j.scitotenv.2023.169750

Highlights

Contributions of Knudsen diffusion, slip and viscous flow to total mass transfer strongly depend on shale matrix pressure.
Knudsen diffusion is the primary mechanism for methane emissions from abandoned shale gas wells (~ 20 × 10³ m³ d⁻¹ per well).
Methane emission due to diffusive flux is comparable to the most significant operational emissions (i.e., flowback).

Abstract

Shale gas, with its abundance and lower carbon footprint compared to other fossil fuels, is an important bridge fuel in the ongoing energy transition. However, a notable concern in shale gas exploration is fugitive methane emissions during the extraction, development, and transport of natural gas. While most existing works evaluate methane emissions released by well fracking, completion and operation, the greenhouse footprint of unproductive shale gas wells (often abandoned or orphaned) has received little scrutiny. A large fraction of these emissions from abandoned shale gas wells are due to the diffusive transport of methane trapped in nanoporous shale matrix, which is poorly understood. Here, we develop a theoretical kinetic approach to predict methane diffusive flux from heterogeneous shale matrix. Our theoretical model is based on a layer sequence formulation and accurately considers multiple flow mechanisms, including viscous flow, gas slippage, and Knudsen diffusion and their mutual interactions. The model is validated against the observed methane diffusion data obtained from high-pressure and high-temperature experimental measurements on Marcellus shale. We find that methane diffusive flux increases as reservoir pressure decreases. We estimate methane emission due to diffusive transport up to 20 × 10³ m³ per well per day, which is comparable to emissions from flowback fluid. For the first time, unrecovered natural gas in the shale matrix is demonstrated to be the main source of methane emissions from abandoned shale gas wells. Given the long-lasting nature of diffusive transport to shale gas seepage, it is suggested that regulatory requirements should be implemented to provide long-term monitoring of methane emissions from abandoned shale gas wells.