页岩储层地应力预测模型的建立和求解

对致密页岩储层进行水平井钻井和压裂时,必须精确确定页岩气储层地应力。在分析页岩构造特征的基础上,给出考虑页岩横观各向同性的地应力计算模型,并建立声波测井资料求取页岩横观各向同性力学参数的关系式,据此研究页岩气储层地应力的分布规律。分析孔隙压力及地层升沉作用对地应力的影响。结果表明:页岩横观各向同性性质对地应力分布有较大影响,当水平与垂直方向弹性模量之比达到4时,水平地应力平均增加144%,而垂直方向与水平方向的泊松比之比达到2.2时,水平地应力平均增加16.5%；当孔隙压力变化2.4MPa时,水平地应力平均增加13.8%,而地层升沉作用对地应力的影响不大。将预测模型用于Baxter页岩的地应力预测,预测结果与其地层瞬间闭合压力(ISIP)最大误差仅为1.2%,证实模型准确可靠,可为页岩气水平井的钻井和压裂设计提供准确的基础数据。

Prediction model for in-situ formation stress in shale reservoirs

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.