Effects of Porosity and Material Fluctuations on Gas Transport and Sorption Kinetics in Coalbeds
Bibliographic record
Abstract
Abstract Natural gas transport and storage in coal is important for accurate predictions of production rates from coalbeds. Recent investigations based on nondestructive imaging have shown that coals are complex materials exhibiting nonuniform pore structure. Standard approach to describe gas/coal interactions is deterministic and neglects the effects of local spatial heterogeneities in material content and porosity. In this work, adopting a weak-noise approximation, these heterogeneity effects on diffusive gas transport are investigated using a statistical approach in the presence of non-equilibrium (kinetic) gas sorption with random partition coefficient. It is found that the gas-coal system behaves distinctively in the presence of kinetics and that the coal matrix heterogeneities generate multiplicative non-trivial effects on transport. Consequently, average gas concentration field is significantly different than the one obtained using an equivalent yet purely deterministic (i.e., homogeneous) approach. Using fractional gas recovery curves, the results are shown for coals exhibiting Gaussian porosity distributions with varying correlation length scales. A new upscaled deterministic gas mass balance is proposed. The work is a unique approach for understanding coalbed environment and development of sound numerical gas production/storage models. Introduction Coal is a mixture of various minerals and organic material exhibiting an intricate pore network. Variations in its material properties (e.g., rank and maceral content) add to its structurally complex nature and influence its gas retention (sorption) capacity. Much work has been carried out in understanding the pore structure of coal. Characteristically, coalbeds are dual-porosity environments with a network of fractures imbedded within a porous matrix. The coal matrix often exhibits a multi-scale heterogeneity with pores varying in size from micrometer (macro- and mesopores) to angstrom (micro- and submicropores). The gas sorption capacity of coal tends to increase with the volume of small-scale pores displaying significantly large specific surface area. Coal matrices have traditionally been considered as a porous material with a network of interconnected macropores, (Bond, 1956, Bhatia, 1987). According to this viewpoint, natural gas migration in and production from coalbeds have similarities to production from conventional naturally fractured reservoirs. This viewpoint, however, has been disputed by Larsen and Wernett (1988) suggesting that the macropores may not necessarily be connected; therefore, the gas molecules are anticipated to reach the macropores and fractures only by diffusive transport through the microporous solid material. Walker and Mahajan (1993) and Siemons et al. (2007) provided further experimental evidence of diffusive gas transport in the coal micropores. Efforts also have been put forth to identify relationships between the matrix pore structure and its material content. Although these investigations have generally been qualitative, it is shown that the coal porosity is somewhat related to its material properties (White et al. 2005). Typically, porosity has a tendency to decrease as the coal rank, i.e., thermal maturity, increases from lignite to bituminous and anthracite. Gan et al. (1972) observed that porosity is primarily dominated by the macropores in lower rank coals. The influence of maceral, i.e., organic, composition on porosity has also been considered by several groups.
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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".