Natural Fracture Compressibility and Permeability Hysteresis: Liquid vs. Gas
Bibliographic record
Abstract
Abstract Natural fracture compressibility and permeability estimation is important for evaluating well performance for wells completed in unconventional hydrocarbon and enhanced geothermal energy systems exhibiting a complex fracture geometry where unpropped and natural fractures contribute to flow. This experimental study compares liquid and gas natural fracture compressibility and permeability hysteresis in low-permeability rocks, with examples from the Montney and Duvernay formations. A diverse suite of core plugs (horizontal), differing in lithology (siltstones/sandstones, organic/clay-rich shales), mineralogy (quartz/clay-rich), helium porosity (2-9%), and permeability (~0.0001-0.001 md) were analyzed. Core plugs were fractured under differential stress inside a biaxial core holder. Gas and liquid fracture permeability measurements were then performed at varying stress (500-4000 psi) under loading and unloading conditions representative of fluid depletion and injection, respectively. Assuming a planar fracture geometry and that the cubic law applies, fracture width and compressibility were then calculated using fracture permeability, stress data, and core plug dimensions. Water and liquid hydrocarbons were used for liquid permeability measurements. Natural fracture compressibility (gas: 5·10−5-5·10−4 psi−1; liquid: 1·10−5-7·10−4 psi−1), permeability (gas: >30 darcy; liquid: <30 darcy), and porosity (gas >8.5%; liquid: >8.5%) were consistently larger for gas than liquid. Interestingly, however, the hysteresis in fracture attributes (compressibility, permeability, and porosity) caused by loading/unloading was consistently larger for liquid than gas. Larger hysteresis for liquids is presumably due to the ‘softening’ effect on fracture asperities under stress, and elevated inelastic reduction in (fracture) roughness for liquids compared to gases. Notably, the average empirical (natural) fracture compressibility value commonly assumed in fracture modeling (~1·10−4 psi−1) falls within the range of measured fracture compressibility values (1·10−5-7·10−4 psi−1). However, experimental fracture compressibility values covered a broad range, as opposed to the widely accepted assumption of ‘average’ fracture compressibility adopted for modeling. Interestingly, for loading and unloading, fracture compressibility followed two distinct paths, regardless of fluid type. Fracture compressibility was consistently larger for loading than unloading. The latter observations suggest that larger degrees of hysteresis in complex fracture regions may occur for liquids than gases, and for injection versus production. Natural (and induced) fracture compressibility and permeability, while important controls on well performance for complex fracture cases, are challenging and time-consuming to measure for low-permeability rocks in the laboratory, particularly over multiple stress cycles. As a result, natural fracture compressibility and permeability hysteresis data are sparse in the literature. For low-permeability rocks, these data have been primarily measured with gas. The developed workflows and provided examples are beneficial for verifying empirical and analytical correlations used for evaluation of fracture compressibility and permeability, and their hysteresis in low-permeability sedimentary rocks.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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 source (direct Gemma or distilled Codex), 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".