Stress Field and Natural Fracture System in Polish Shale Belt Based on Borehole Images

Borehole images from six Orlen operated wells in SE Poland were investigated for structural features. Wellbore instability features were used to calculate earth stress, borehole stability and fracture stress.

Silurian shales formed in a uniformly subsiding basin albeit with abundant earthquake activity, and later tilted en-bloc to W or S. WSW dipping faults were found in most wells. They can be assigned a Hercynian origin with general tectonic transport towards ENE, but recently reactivated, as we note stress-deflection in one of the wells, at Streczyn.

Densely spaced ENE and ESE steeply dipping fractures seem to constitute a set of strike-slip fractures in response to a general N-directed horizontal stress; alternatively, they formed by hydraulic tension and later slipped tectonically. The fractures remain sensitive to strike-slip in the current stress field.

Maximum horizontal stress is relatively high, placing the area in strike-slip regime. There is high horizontal stress anisotropy but due to rock strength and low fluid pressure, the vertical wells are quite stable to drill. The regime is favourable for drilling horizontal wells along the minimum stress axis.