EXPERIMENTAL FRICTIONAL HEATING OF COAL GOUGE AT SEISMIC SLIP RATES: EVIDENCE FOR DEVOLATILIZATION AND THERMAL PRESSURIZATION OF GOUGE FLUIDS

In order to simulate frictional heating of a hydrous fault zone we sheared bituminous coal gouge (33 wt% volatiles; 150 – 250 microns) at 1 m/s in rotary friction experiments under a normal stress of 0.6MPa. The resulting mechanical behavior is reproducible and can be divided into four stages and includes devolatilization by frictional heating, fluid pressurization, and slip weakening. Stage I is characterized by sample shortening and reduction in the coefficient of friction (f) from ~1 to 0.6. Stage II is characterized by high frequency (~5 Hz) oscillations in stress and strain records and by hydrocarbon gas emissions. Stage III is marked by rapid weakening (f ~ 0.1 to 0.35) and sample shortening, together with continued gas emissions. Stage IV produces stable stress records and continued weakness (f ~ 0.2), but without gas emission.

Stage I shortening is due to compaction of the gouge and the weakening is attributed to mechanical or thermal effects. Stage II behavior is interpreted as due to coal gasification and rapid fluctuations in fluid pressure, resulting in high frequency stick-slip type behavior. Fluid pressure briefly exceeded the normal stress (0.6 MPa), resulting in sample elongation. The dramatic reduction in shear stress in stage III (f ~0.2) is attributed to gas pressurization by pore collapse and corresponds to an instability. Microstructural observations indicate the deformation was brittle during stages I and II but ductile during stages III and IV. Cooling occurred during stage IV, due to cutoff of frictional heat by thermal pressurization. Finite element models indicate the center of the fault zone became hot (~900^oC) during stage II, whereas the edges remained relatively cold (< 300^oC). Vitrinite reflectance of coal samples show an increase in reflectance from ~0.5% to ~0.8% over the displacement interval 20–40 meters (20-40 seconds), indicating vitrinite responds to frictional heating on a short time scale. The energy expended per unit area in these low stress, large displacement experiments (7 to 87 m), is similar to that of higher stress (60 MPa), short displacement (1 m) mid-crustal earthquakes.