基于页岩岩石物理等效模型的地应力预测方法研究

Prediction for in-situ formation stress of shale based on rock physics equivalent model

地应力的精确预测是对页岩地层进行水平井钻井轨迹设计和压裂的基础.本文在分析页岩构造特征的基础上,提出了适用于页岩地层的岩石物理等效模型的建立流程,并以此为基础实现了最小水平地应力的有效预测.首先,通过分析页岩地层的矿物、孔隙、流体及各向异性特征,将其等效为具有垂直对称轴的横向各向同性介质,进行了页岩岩石物理等效模型的构建;然后建立了页岩地层纵横波速度经验公式,并将该经验公式与岩石物理等效模型均应用于实际页岩工区的横波速度预测中,二者对比表明,本文中建立的页岩气岩石物理等效模型具有更高的横波预测精度,验证了该模型的适用性;最后,利用该模型计算各弹性刚度张量,进而实现了页岩地层最小水平地应力的预测,与各向同性模型估测结果对比表明,该模型预测的最小水平地应力与地层瞬间闭合压力一致性更高,且储层位置更为明显,具有较高的实用性.

The effective prediction of in-situ formation stress is the basis of trajectory design and fracturing of horizontal wells in shale formation.The forecasting method of in-situ formation stress based on assumption of anisotropy has high prediction precision.But it is difficult to obtain elastic parameters this method required.Starting from the establishment of shale rock physics equivalent model,this paper calculates the elastic parameters of different directions and achieves accurate prediction of minimum horizontal stress.According to the structural characteristics analysis of shale,this paper puts forward the building process of rock physics equivalent model suitable for shale,and on this basis,achieves accurate prediction of minimum horizontal stress.First of all,through the analysis of mineral,porosity,fluid and anisotropic characteristics of shale formation,we equate it to VTI medium and construct the shale rock physics equivalent model.Then we establish the empirical formulas of Pwave and S-wave velocity applicable to shale formation.The rock physics equivalent model andempirical formulas are all applied and compared in the S-wave velocity prediction of actual shale work area to verify the applicability of our model.Finally,the model is used to calculate the elastic stiffness tensors,thus predict the minimum horizontal stress of shale formation.The forecasting method of in-situ formation stress is applied to actual shale work area,and following results are obtained：（1）The comparisons of S-wave velocity predictions using the shale rock physics equivalent model and empirical formulas show that shale rock physics equivalent model has higher prediction accuracy,and it is reliable and applicable in this work area.（2）The P-wave and S-wave velocities in horizontal direction and vertical direction have some differences.They show that there are characteristics of VTI medium in this shale formation and the assumption of VTI medium in our equivalent model is reasonable.（3）Comparing with the estimated result of isotropic model shows that the minimum horizontal stress estimated based on the rock physics equivalent model has higher consistency with instantaneous shut-in pressure（ISIP）,and the reservoir position is more apparent.This illustrates the practicability of our model.The shale rock physics equivalent model established in this paper is applicable to the studied shale work area.It can be used to estimate S-wave velocity,analyze the anisotropic characteristics and calculate elastic parameters in different directions in this work area.In addition,the model also works for the shale formation with similar characteristics of anisotropy,mineral,porosity and fluid.But because of the assumption of a few mineral species and randomly distributed pores,this model cannot be applied to shale formation with complex mineral species and many vertical cracks.For these shale formations,we still need further research.Based on the established rock physics equivalent model,we calculated the elastic stiffness tensor in different directions and according to the computational formula of in-situ formation stress,we predicted the minimum horizontal stress of well A accurately.It can provide some guidance for shale formation fracturing.But in the optimal selection process of fracturing area,we should fully consider a variety of factors such as in-situ stress,rock brittleness,and hydrocarbon distribution et al,improve the effectiveness of fracturing and increase the production of shale gas.