Mechanism of Liquid-Phase Adsorption and Desorption in Coalbed Methane Systems - A New Insight into an Old Problem
Bibliographic record
Abstract
Abstract Coalbed methane (CBM) recovery is a process of desorption rather than adsorption. Traditional experimental studies of methane adsorption/desorption on moist coal samples illustrate that, there is little hysteresis between adsorption and desorption processes, and these two behavior could be described by Langmuir equation. However, it is widely believed that coal porosity is normally occupied by water, and few researches focus on the gas sorption mechanism in condition of pore systems saturated with water. In this condition, there is an equilibrium between methane molecular dissolution into pore water and adsorption on (or desorption from) pore surface. And it is a typical liquid-solid internal interaction rather than gas-solid internal interaction. This paper presents experiments with methane adsorption/desorption on coal samples saturated with water and illustrates the significant difference between liquid-solid adsorption and gas-solid adsorption. In this work, we carry out three sets of experiments with coal samples immersed in water, and the equilibrium pressure is P=5.445 MPa, 5.220 MPa and 2.965 MPa respectively. Subsequently, we conduct desorption experiments based on the former adsorption experiments to simulate the production process in aqueous environment with decreasing pressure. During desorption process, we are surprised to find that desorption amount changes slightly with different pressure drops, and desorption ratio for four sets of experiments is only 3.36%, 4.98% and 4.21% respectively. This dramatic results indicate that the desorption amount from saturated aqueous solution is not sensitive to pressure, which differs considerably from gas-phase desorption case. Furthermore, we analyze the Gibbs free energy change during adsorption/desortion processes in these two different systems: gas-solid and liquid-solid interface systems. It should be noted that, in an ideal gas-solid interface system (such as adsorption/desortion on flat surface case without the effect of pore structure), the Gibbs free energy change in adsorption are approximately equal to that in desortion processes, thus, the phase transition between absorbed layer and bulk is quasi-reversible, and no hysteresis exist in these two processes. However, in the liquid-solid interface system, due to the additional energy consumed by nucleation, it requires more energy for unit molar of methane during desorption process than adsorption process, which indictes that adsoption/desorption of liquid-solid interface are unreversible. Based on this theory, we established a hysteretic model to describe the hysteresis phenomenon during adsoption/desorption from aqueous environment, and this model can be perfectly verified by our experiment results. Our result demonstrates that the process of the methane desorption from micropores under water saturated condition is hysteretic seriously. This meaningful finding indicates that when almost no free gas exists in matrix pores, i.e., only absorbed gas and dissolved gas exist, and adsorbed gas almost cannot desorb and gas production of CBM wells will be very low. In other words, the larger amount of adsorbed gas doesn't always yield the larger gas productivity, and free gas in matrix pores could induce adsorbed gas to desorb.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".