Stability Analysis of a Horizontal Coalbed Methane Borehole in the San Juan Basin, USA

Abstract

Borehole stability analysis was conducted using STABView and FLAC geomechanical modeling for a horizontal coalbed methane well in the San Juan Basin. The objective was to determine whether the coal seam at 3,000 ft depth would yield under certain drilling and production conditions. Triaxial tests using 1-inch diameter core plugs were performed to construct the Mohr-Coulomb and Hoek-Brown failure envelopes. A reduction of the peak coal strength by approximately 30% was needed to correct the scale-dependent strength to a value that was thought to be more representative for a 4.75-inch diameter horizontal borehole. No yielding was predicted at underbalanced and overbalanced drilling conditions when the uncorrected M-C and H-B lab strength data were used. A 30% reduction in peak strength predicted no yielding during overbalanced drilling and only minor yielding during underbalanced drilling or production. The maximum extent of the yielded zone at BHP of 100 psi predicted by STABView was 37% over gauge. FLAC gave directionally similar but slightly more conservative results compared to STABView. A reasonable amount of yielding and subsequent detachment of the coal is expected along the horizontal well at the highest drawdown pressures. Stability analysis showed that drilling a horizontal well in Coal A overbalanced would be possible. Drilling the same coal seam underbalanced will produce a rim of yielded coal but no catastrophic failure, provided that the coal is not highly fractured.

Keywords:

• borehole stability
• FLAC
• horizontal coalbed well
• STABView

Notes

1. Initial failure indicated by a rapid increase in one radial direction; however, the sample recovered and maintained a similar stress-strain response up to the peak stress shown (5,860 psi).

2. Initial failure indicated by a rapid increase in both radial directions; however, the axial strain maintained unchanged up to the peak stress shown (7,400 psi).

3. Anomalous sample—silt layers contribute to the higher bulk density and Young's modulus. Results were not used for delineation of the failure envelope.

4. Initial failure indicated by increased strain rate.