Elsevier

International Journal of Rock Mechanics and Mining Sciences

Analytical modelling of coal failure in coal seam gas reservoirs in different stress regimes

Abstract

Coal seam gas (CSG) reservoirs typically have low rock strength. During gas production, pressure depletion and matrix shrinkage may cause the differential stress around the wellbore to exceed the coal mechanical strength and result in rock failure. Coal failure has several detrimental consequences including coal fines production, permeability reduction, wellbore filling and damage to pumps and compressors. The matrix shrinkage causes a unique stress path in CSG reservoirs. However, the details of how matrix shrinkage affects coal failure still have remained uncertain. This paper presents a new workflow to evaluate stress distribution around CSG wells and predicts coal failure by coupling the effects of pressure depletion, matrix shrinkage and wellbore simultaneously. The model calculates Maximum Coal Free Drawdown Pressure ( MCFDP) by considering the effects of all contributing parameters and Mogi-Coulomb failure criterion. Data from a vertical well in the San Juan Basin in USA were used to evaluate the validity of the developed model. The developed model was applied to evaluate coal failure under three different stress regimes. The results indicate that pressure depletion and matrix shrinkage have a significant effect on coal failure in all stress regimes. In the case of a normal stress regime, it is found that vertical wellbores are the most stable during the life of a reservoir. However, in the case of strike-slip and reverse stress regimes, pressure depletion and matrix shrinkage could cause the change of stress regime and therefore, the optimum wellbore trajectory could change. Additionally, it is found that in the normal stress regime the depletion and matrix shrinkage reduces the MCFDP of horizontal wellbores more than the vertical wells. However, in the reverse stress regime, depletion and shrinkage cause more reduction of MCFDP in vertical wellbores compared to horizontal wells.

Introduction

During past decades, CSG or Coal Bed Methane (CBM) have increasingly become an important part of natural gas resource. The world CSG resource constitutes around 2980 to 9260 Tcf.1 , 2 Moreover, by the exploration of more CSG around the world and development of new technologies that enable commercial production from CSG, an increase in production from these reservoirs is expected in the future.3

CSG reservoirs are considered as unconventional gas resources because methane is not trapped by overlying seals and is normally adsorbed on the coal matrix while the coal cleat system is saturated with water. Dewatering is initially carried out to deplete the reservoir pressure and this causes the progressive desorption of gas. Gas desorption from the coal matrix causes matrix shrinkage.4 , 5 The matrix shrinkage will affect material behavior and leads to a specific stress path during production (section 2.2). This makes the CSG unique in terms of stress distribution around the well. Since the CSG reservoirs are naturally fractured and typically have low rock strength, pressure depletion, and matrix shrinkage can change the stress concentration around the wellbore so that it exceeds the coal mechanical strength and results in coal failure and coal particles production.

Coal failure may cause serious gas production issues from CSG reservoirs.6 , 7 There are wide reported problems during drilling and production, particularly in deviated and horizontal CSG wells in pressure depleted condition. It has been reported that 40% of the horizontal CSG wells in the Qinshui Basin in China have serious instability problems during drilling and production.8 Some of the recently drilled horizontal and multi-lateral wells in the Bowen Basin, Australia have experienced coal fines production during production.9 In San Juan Basin, USA, massive coal fines production has also been observed during late production, when reservoir pressure had decreased to less than 300 psi from initial pressure of 1260 psi. Matrix shrinkage, desorption with pressure depletion induced horizontal stress changes are interpreted as the cause of geomechanical failure.3 , 5 , 10 Moreover, several borehole failure and coal fines production events are experienced in depleted coal in the Arkoma Basin, Oklahoma.11

Recently, many researchers have shown that after a long-term exponential increase in permeability with depletion caused by desorption-induced shrinkage, a sudden decrease in coal reservoir permeability is observed and coincided with coal fines production.3 , 10 , 12 Keshavarz, Badalyan13 concluded that during depletion, coal permeability may decrease due to the cleat closure. Production of coal particles has adverse effects on downhole and surface facilities and artificial lift pumps. The failed coal can also plug the cleat system and cause permeability reduction.14 , 15

Palmer, Moschovidis11 investigated the coal failure mechanism and its consequences. They used a conventional sanding onset model to predict coal failure but did not consider the shrinkage effect and its induced stress path on coal failure. Liu and Harpalani16 carried out a qualitative analysis of coal failure by illustrating Mohr's circle and considered gas depletion. They noticed that methane desorption had a significant effect on in situ stress state and as a result, shear failure takes place earlier than a conventional reservoir where there is no shrinkage effect. Bai, Chen17 and Bai, Chen15 used numerical simulation to characterize coal failure and fines generation during dewatering and gas production. Recently, Espinoza, Pereira3 and Lu and Connell7 developed an analytical model to consider the effect of desorption on coal failure. However, the effect of the wellbore trajectory on stress redistribution around the wellbore was not considered. Saurabh, Harpalani18 conducted laboratory experiments to measure coal matrix volumetric strain from desorption and modeled the lateral stress changes during experiments.

This paper presents an analytical model to evaluate the stress distribution around the wellbore by coupling the effect of depletion, matrix shrinkage, and the wellbore. An analytical model is also developed to calculate the Critical Coal Free Bottom-hole Pressure (CCFBP) and Maximum Coal Free Drawdown Pressure (MCFDP). The model is validated by field data in the San Juan Basin. Moreover, the developed model is utilized to investigate the effect of different well inclinations and azimuths, pressure depletion on coal failure in different situ stress regimes.

Section snippets

Stress path in conventional reservoirs

The uniaxial strain condition is commonly used as a simplified model during depletion from a laterally extensive reservoir, which means constant vertical stress, zero lateral strain and associated changes in horizontal stresses.3 , 16 , 19 , 20 The horizontal stress variations are quantified by reservoir stress path which is defined as the changes in total horizontal stresses per unit change of pore pressure during production from a reservoir as follows: Δ S h Δ P = K = β 1 − 2 v 1 − v σ h = S h − β P Δ σ h Δ P = − β v 1 − v Where S h is

Model validation

In order to validate the developed model, field data and observed coal failure pressure in San Juan Basin in USA are utilized. The San Juan Basin spans across the northwest of New Mexico through southwestern Colorado and is one of the oldest CSG productive areas in the world. The commercial gas production was established in 1977 from Fruitland coal formation and as of 2010, there were more than 7200 active wells in the Basin.12 At the early stage of gas production, there was no coal production

Coal failure in different stress regimes

The developed methodology is applied in three different stress regimes to investigate the coal failure in CSG reservoirs. Table 3 presents the typical input data used for the analysis of coal failure in normal, strike-slip and reverse faulting stress regimes.

Conclusions

This study presented an analytical model for coal failure prediction in CSG reservoirs in different in situ stress regimes by applying the Mogi-Coulomb failure criterion. The developed model enables quantitative estimation of maximum coal free drawdown pressure in different pressure conditions. It considers the combined effects of matrix shrinkage, pressure depletion, initial in situ stress and wellbore trajectory on stress distribution around the wellbore simultaneously. The model prediction

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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