Prediction model for in-situ formation stress in shale reservoirs

Key words:  shale  in-situ stress  transversely isotropy  mechanical parameters

Abstract:

It is important to predict the in-situ rock stress in drilling or fracturing of horizontal wells in tight shale formation. Based on the rock mechanic characteristics of shale, an in-situ formation stress model was developed, in which the transversely isotropy of shale formation was considered, and its mechanical parameters were derived through acoustic logging data. The model was used to study the distribution of in-situ stress in shale formation, and the effect of pore pressure and subsidence or uplift on the in-situ stress was analyzed. The results show that the transversely isotropy of shale rock has a significant effect on the in-situ stress. When the ratio of horizontal to vertical elastic modulus is equal to 4, the horizontal in-situ stress can increase 144% on average. When the ratio of vertical to horizontal Poisson 's ratio is equal to 2.2, the horizontal in-situ stress can increase 16.5% on average. When the pore pressure increases by 2.4MPa, the horizontal in-situ stress can increase 13.8%. However the subsidence or uplift is not an important factor to the in-situ stress. Applying the model to the Baxter shale formation, it is found that the maximum error is 1.2% between the prediction results and that measured via instantaneous shut-in pressures(ISIP). It demonstrates that the transversely isotropy model is correct and can provide accurate data for drilling and fracturing of horizontal wells in tight shale formation.