Study of the formation mechanisms of CO₂ hydrates from matching the experimental data with a porous media setting by multiphase flow-geochemical-thermal reservoir simulator

Highlights

• Reliable reaction module of CO₂ hydrate is built by a reservoir simulator.

• Sensitivity analysis is used to know the effects of flow property on CO₂ hydrate.

• CO₂ hydrate reaction module is established based on a good history matching.

• CO₂ hydrate formation begins at the interface between the water and gas.

Abstract

In this study, a multiphase flow-geochemical-thermal reservoir simulator, CMG STARS, was used to build a reliable reaction module of the CO₂ hydrate formation based on laboratory test results in order to investigate the behaviors of CO₂ hydrate formation. This research was based on experiments conducted in previous studies by the team from the National Taiwan University of Science and Technology (NTUST). For the experimental data matching, a sensitivity analysis was conducted to understand the effects of flow property, reaction, and thermal property parameters on the behaviors of CO₂ hydrate formation. This study successfully established a CO₂ hydrate reaction module based on a good history matching. Both the experiment and simulation demonstrated that the CO₂ hydrate is formed at the interface between the water and gas. The concentration, accumulation, and distribution of the CO₂ hydrate affected by the free gas transport was observed in the simulation.

Introduction

As a result of the global warming, the climate of the Earth has undergone a significant change. Climate change has had widespread impacts on both humans and the environment [1]. Global warming is majorly caused by the emission of large amounts of CO₂, a greenhouse gas that is produced by the excess use of fossil fuels [2]. Governments world-wide are eager to find effective ways to decrease the rate of global warming. A possible solution exists in reducing the CO₂ concentration in the atmosphere via carbon capture and storage (CCS), which involves injecting CO₂ into deep underground depleted petroleum reservoirs or aquifers.

The injected CO₂ might remain mobile in the porous media in the deep formation, this makes the stored CO₂ might migrate upward. The leakage of CO₂ might happen if the stored CO₂ flow into a permeable fault or a weak-integrated wellbore. To make sure of the containment of CO₂ geological storage, the function of geo-chemical mechanism is critical. The formation of CO₂ hydrates underground has been demonstrated as a more secure form of CO₂ storage.

Gas hydrates are solid compounds that are formed by small size gas molecules, such as CH₄ or CO₂, and a certain amount of water molecules. Under high pressure and low temperature conditions, the water molecules tend to form crystal-like structures that act as micro-sized cages, trapping the gas molecules inside [3]. Because of the solid crystal structures, the gas molecules are packed closely and tightly inside of the structures. It is known that a unit volume of CO₂ hydrates can release 162 volume units of CO₂ gas under standard conditions [4]. CO₂ storage via solid CO₂ hydrates zeros the mobility of the stored CO₂; and consequently, there is no risk of leakage.

The formation of CO₂ hydrate in a deep rock formation is desirable not only because of the high storage capacity and zero mobility it provides in the porous media, but also because of the important role it can play during the methane (CH₄) hydrate recovery process. The minimum pressure of CO₂ hydrates formation is substantially lower than that of CH₄ hydrates formation at the same temperature [5]. This allows for CO₂ hydrates to form more easily in the methane hydrate deposits, resulting in a CO₂/CH₄ exchange reaction that releases CH₄ gases to be produced by production wells. CH₄ hydrate recovery via CO₂ injection also benefits from the formation of CO₂ hydrates, which allow for CO₂ storage and maintains the stability of the reservoir.

Numerical simulations are a useful tool that can be used to study the hydrate reactions that occur during the CO₂ injection [6], [7], [8]. The reliable reaction module for CO₂ hydrate formation is crucial for the performance of the reservoir simulation relating to the gas production from a gas hydrate deposit or the CO₂ geological storage. A reliable CO₂ hydrate reaction module can be developed based on a conducted laboratory experiments [9], [10], [11] with history matching techniques. By matching the simulation calculations to the experimental data, the behaviors of CO₂ hydrate formation in an experiment chamber can be revealed. Another contributions from the simulation model are to know the CO₂ hydrate distribution inside of the experimental chamber, which is not easy to be observed visually during the course of experiments with limited windows [12,13].

In the literature, there are a lot of simulation jobs conducted to be compared with laboratory data of CH₄ or CO₂ hydrate reaction. However, most of them are done by the process simulation method which focuses on the chemical reaction itself without discussing the fluid flow and heat flow within the reactor [10,11]. Under the consideration of in-situ reservoir condition, the chemical reaction properties aren't the only controls of the gas hydrate formation behavior. Once the characteristics of porous media is considered in the laboratory experiment, the reaction properties obtained from process simulation matching cannot solve all of the parameters to build a reliable reservoir simulator module for hydrate simulation. The simulation study using a multi-phase flow simulator (or reservoir simulator) is very rare [9]. To our knowledge, this study is the first paper inspecting the authenticity of the multiphase flow-geochemical-thermal reservoir simulator doing CO₂ hydrate formation simulation with a laboratory scale grid model under the consideration of the porous media.

The best way to have the thermal dynamic parameters of CO₂ hydrate reactions used for the simulations of hydrate production and CO₂ geological storage is from the duplication of an experiment by the numerical model. The purpose of this study is to build a numerical simulation model in accordance with a real laboratory experiment. A reliable reaction module of CO₂ hydrate formation is obtained by matching the numerical calculations to experimental data. The CO₂ hydrate distribution inside the experiment chamber and the characteristics of CO₂ hydrate formation thus can be investigated numerically. Those well-defined thermal dynamic parameters of CO₂ hydrate reactions can also be further used for the future simulation studies.