Permeability Variation Models for Unsaturated Coalbed Methane Reservoirs

Abstract

-A large number of models have been established to describe permeability variation with the depletion of reservoir pressure to date. However, no attempt has been made to draw enough attention to the difference in the effect of various factors on permeability variation in different production stages of unsaturated CoalBed Methane (CBM) reservoirs. This paper summarizes the existing and common permeability models, determines the relationship between various effects (effective stress effect, matrix shrinkage effect and Klinkenberg effect) and desorption characteristics of the recovery of unsaturated CBM reservoirs, then establishes two improved models to quantificationally describe permeability variation, and finally discusses the effects of various factors (gas saturation, cleat porosity, Poisson's ratio and shrinkage coefficient) on permeability variation. The results show that permeability variation during the recovery of unsaturated CBM reservoirs can be divided into two stages: the first one is that permeability variation is only affected by the effective stress effect, and the second is that permeability variation is affected by the combination of the effective stress effect, matrix shrinkage effect and Klinkenberg effect. In the second stage, matrix shrinkage effect and Klinkenberg effect play much more significant role than the effective stress effect, which leads to an increase in permeability with depletion of reservoir pressure. Sensitivity analysis of parameters in the improved models reveals that those parameters associated with gas saturation, such as gas content, reservoir pressure, Langmuir volume and Langmuir pressure, have a significant impact on permeability variation in the first stage, and the important parameters in the second stage are the gas content, reservoir pressure, Langmuir volume, Langmuir pressure, Poisson's ratio, Young's modulus and shrinkage coefficient during the depletion of reservoir pressure. A comparative study of the improved models indicates that the improved SD model has a greater sensitivity to various parameters than the improved PM model and the improved models describe permeability dynamic variation more exactly than the original ones.Résumé -Modèles de variation de perméabilité pour des réservoirs de gaz de houille insaturé -Un grand nombre de modèles ont été établis à ce jour pour décrire la variation de perméabilité en fonction de l'épuisement de la pression du réservoir. Toutefois, aucune tentative n'a été faite pour attirer suffisamment l'attention sur la différence d'effet des différents facteurs sur la variation de perméabilité dans différentes phases de production des réservoirs de gaz de houille insaturés (CoalBed Methane, CBM). Le présent article résume les modèles de perméabilité existants et usuels, détermine la relation entre les différents effets (effet de contrainte effective, effet du rétrécissement

Meshless method simulation and experimental investigation of crack propagation of CBM hydraulic fracturing

“…Although coal rock belongs to fragile material, the tip of existing crack inside the wellbore under hydraulic pressure will not be in the form of simple single brittle fracture, but rather in the form of the combination of brittle-elastic-plastic due to the crustal stress from coal reservoirs, especially buried deeply [20,21]. This process can be considered as three procedures: (1) the fracture tip produces brittle fracture while it is subjected to the maximum fracture strength resulting from hydraulic pressure, (2) due to the constraint of in-situ stress, the fracture will propagate under the continuous action of hydraulic pressure till the pressure is unloaded or fracture propagation is complete, and (3) the fracture occurred will be partially closed after pressure unloading, thus forming the plastic fracture.…”

Role of Pore-Size Distribution in Coals to Govern the Klinkenberg Coefficient and Intrinsic Permeability

“…χ is determined via experiments with certain assumptions ,,, and also with numerical models. , Characterization of gas permeability evolution as a function of pressure in the laboratory using eq is a common practice . However, experimental or real field gas permeability in tight reservoirs is better estimated if the slippage or slip effect is considered along with the effective stress changes. ,− The slippage effect describes the permeability enhancement associated with gas flow in microporous conduits due to the acceleration of the nonstationary layer of gas molecules in contact with the pore capillary walls (see Figure of Letham and Bustin). The governing equation of slippage effect, applicable for the gas flow in tight capillaries, combines Poiseuille’s and Darcy’s laws and is expressed as where k a and k ∞ denote apparent and Klinkenberg corrected/intrinsic permeabilities, respectively, and b is the Klinkenberg coefficient, also known as the slippage factor or the slip factor.…”

“…The Klinkenberg corrected permeability in tight reservoir rocks have been studied through state-of-the-art novel experiments ,,,− and also by numerical and analytical models. ,− , The studies were mostly focused on the variation of the Klinkenberg coefficient as a function of stress and demonstrate that permeability evaluation of tight reservoirs, such as coal seams at depth, should consider the role of effective stress and the Klinkenberg coefficient. They showed that the Klinkenberg coefficient increases with pore pressure at constant effective stress for an adsorbing gas. , Initially, the Klinkenberg coefficient increases with increasing pore pressure at constant confining stress; however, at even higher pore pressures, the Klinkenberg coefficient may continue to rise or decrease .…”

“…The behavior is similar for adsorbing and inert gases because, at constant pore pressures, the effect of adsorption-induced swelling is insignificant. At higher pore pressures, the role of the Klinkenberg coefficient is negligible, and permeability modifies in response to stress changes, only. ,,,,,, However, the pressure at which the slippage effect can be neglected depends on the type of tight rocks, the reservoir stress state, and the nature of flowing gas. Laboratory experiments and numerical models are suitable to separate the pressure-related effects from the slippage effect.…”