118 / 2021-09-30 20:02:05

Wellbore Stability in Horizontal CBM wells: Practical solutions to Better Understand the Uncertainty in Rock Strength and Coal Heterogeneity in the Upper-Silesian Coal Basin

methane drainage,Coal bed methane,wellbore stability

In recent decades, the approach to Coal Bed Methane (CBM) has changed radically as concerns regarding global warming and increasing coal prices highlight the fact that there is a need for alternative energy sources which are less detrimental to the environment. Capturing methane and harnessing it into an abundant energy source could help the acceleration of the energy transition. It may also allow the mine to convert smoothly to the underground storage of compressed air, methane, or hydrogen, which will play an important role in renewable energy supply and climate change (Tao i in., 2019, Kalair i in. 2021).

Unlocking potential hidden in coal seams might be done by performing long-reach horizontal wells. However, successfully drilling horizontal coalbed methane wells depends on wellbore stability, which in turn depends on coal seam thickness, seam depth, overburden stress, wellbore length, reservoir pressure coal cohesion, and the drilling fluid used. Therefore, proper modeling of wellbore stability in both overburden and production zone can help avoid common problems such as wall collapsing, stuck pipe, or even complete loss of the borehole ( Błaż, 2020).



The problems related to the stability of horizontal/inclined degassing wells in coal rocks and the low flow rate of methane from such wells caused by low permeability of coal matrix have induced the engineers to drill the inclined/horizontal wells in permeable horizons which overly the coal. This paper presents a mathematical model to evaluate the wellbore stability of inclined wells drilled in the strata overlying coal seams and its application to seam 501 located in the Staszic-Wujek Coal Mine conditions.

In order to evaluate the wellbore stability, a series of experimental works covering petrophysical and geomechanical analysis and fluid – rock interactions were performed on rocks that cover the trajectory of wells TM1-TM5 drilled in seam 501. The analyzed rocks show very low permeability and a significant swelling pressure from 0.14 to 1.64 MPa, consistent with swelling from the range of 2.09 to 12.37%. Estimated mud densities required to assure stability of horizontal well for the coal, sandstone, and clay series were in the range of 1266 to 1549, 1064 – 1470 and 1453 - 1524 kg/m³, respectively. However, in some cases, for a given set of data, the mud density required to maintain the stability of horizontal well is greater than mud density which corresponds to fracturing pressure, what suggests the need of applying different mud composition.



Performed work provides more confidence in optimizing drilling mud weight for CBM horizontal wells by allowing a more scientifically sound and practical solution to address rock failure mechanisms associated with borehole instability.



Acknowledgement

This project is financially supported by the European Commission Research Fund for Coal and Steel (RFCS) project “Advanced methane drainage strategy employing underground directional drilling technology for major risk prevention and greenhouse gases emission mitigation” GA: 847338-DD-MET-RFCS-2018/RFCS-2018. The authors would like to express their gratitude to the research partners at PGG S.A. and Central Mining Institute for their contribution to the project.



References



BŁAŻ, S. 2020. Drilling muds for coal deposits. Nafta-Gaz, 10, 701–709, doi:10.18668/NG.2020.10.05

KALAIR, A., ABAS, N., SALEEM, M.S, KALAIR, A.R., KHAN N.  2021. Role of energy storage systems in energy transition from fossil fuels to renewables. Energy Storage, 3, 1, 1059-1074. https://doi.org/10.1002/est2.135

TAO, S., CHEN, S., PAN, Z. 2019. Current status, challenges, and policy suggestions for coalbed methane industry development in China: A review. Energy Science Engineering, 7, 1059-1074. https://doi.org/10.1002/ese3.358