Estimation of the Stress Magnitudes in Basel Enhanced Geothermal System
Estimation of the Stress Magnitudes in Basel Enhanced Geothermal System
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发表时间：
2015
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通讯作者：
B. Valley;A. Evans
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作者：
B. Valley;A. Evans
原地应力状态在决定低渗透EGS油藏中用于开发热交换器的水力刺激注入的岩体响应中起着主要作用。因此，必须在任何刺激过程的地质力学模型中指定应力及其非均质性。本文给出了瑞士巴塞尔花岗岩EGS储集层应力值的评价结果。最小水平主应力SHmin的分布受水压试验的约束，而最大水平主应力SHmax的大小是不确定的。在这里，我们通过分析来自5公里深的BS-1井的声波电视观测仪测井的爆裂宽度数据，得出SHmax的估计值。花岗岩顶以下2.42公里处有81%的钻孔受到崩落的影响，这有利于检验估计的深度趋势。分析的一个主要目标是评估四种不同的失效标准对SHmax震级估计的影响。标准包括Rankine、Mohr-Coulomb、Mogi-Coulomb和Hoek-Brown 3D。所有这些都是使用从井底附近取下的岩心塞上的单个多阶段三轴压缩试验的强度数据进行参数化的。考虑了井壁的所有应力分量，包括钻井冷却井壁产生的残余热应力，采用了一种数值方法从估计的漏斗宽度推导出SHmax震级。以前对漏斗宽度的研究表明，大的、小范围的波动与裂缝有关，裂缝反映了强度或应力的变化，或者两者兼而有之。在更大的尺度上，漏斗宽度随着深度的增加而减小。假设岩石的强度特性沿钻孔长度没有明显的系统性变化，这是没有证据的，大范围趋势的结果是SHmax剖面的小梯度。这一结果与失效准则无关，也与分析中使用的Shmin曲线无关。SHmax的绝对值取决于所使用的失效准则。考虑中间应力(Mogi-Coulomb和Hoek-Brown 3D)强化效应的准则的屈服剖面违反了4公里以上地壳强度的摩擦极限，而Mohr-Coulomb和Rankine准则的剖面则不同(后两者在孔隙压力和井筒压力相等的情况下基本相同，并且在与我们的分析相关的Shmin和SHmax范围内)。Mohr-Coulomb/Rankine准则剖面表明，SHmax的趋势是从4200m以上的走滑断层向下方的走滑/正断层转变。这与油藏刺激期间记录的震源机制相当一致，在所考虑的深度范围内，显示了走滑和正断层的混合。
The in-situ state of stress plays a major role in determining the response of the rock mass to hydraulic stimulation injections used to develop heat-exchangers in low-permeability EGS reservoirs. As such, stress and its heterogeneity must be specified in any geomechanical model of the stimulation process. This paper presents the results of an evaluation of stress magnitudes in the granitic EGS reservoir in Basel, Switzerland. The profile of minimum principal horizontal stress, Shmin, is constrained by hydraulic tests, but the magnitude of the maximum horizontal principal stress, SHmax is uncertain. Here we derive estimates for SHmax by analysing breakout width data from an acoustic televiewer log run the 5 km deep borehole BS-1. Some 81% of the borehole below the granite top at 2.42 km is affected by breakouts, which is favourable for examining the depth trends of the estimates. A primary objective of the analysis was to evaluate the impact of four different failure criteria on the SHmax magnitude estimates. The criteria where Rankine, Mohr-Coulomb, Mogi-Coulomb, and Hoek-Brown 3D. All were parametrized using strength data from a single multi-stage triaxial compressive test on a core plug taken from near the well bottom. A numerical approach was employed to derive SHmax magnitude from the estimated breakout widths, taking into account all stress components at the borehole wall including the remnant thermal stress arising from the cooling of the borehole wall by the drilling. Previous studies of breakout width have shown that large, small-scale fluctuations are associated with fractures, which reflect variations in strength or stress, or both. At larger scales, breakout width tends to decrease with depth. Assuming there is no significant systematic change in the strength characteristics of the rock along the length of the hole, for which there is no evidence, the large-scale trend has the consequence of implying a small gradient of the SHmax profile. This result is independent of the failure criterion, and also of the profile of Shmin used in the analysis. The absolute values of SHmax depend upon the failure criterion used. Criteria that consider the strengthening effect of the intermediate stress (Mogi-Coulomb and Hoek-Brown 3D) yield profiles that violate frictional limits on the strength of the crust above 4 km, whereas the profiles of the Mohr-Coulomb and Rankine criteria do not (the latter two are essentially identical for the case where pore pressure and wellbore pressure are equal and in the range of Shmin and SHmax relevant for our analyses). The Mohr-Coulomb/Rankine criteria profiles indicate a trend in SHmax from favoring strike-slip faulting above 4200 m to strikeslip/normal faulting below. This is reasonably consistent with focal mechanisms recorded during the reservoir stimulation which show a mix of strike-slip and normal faulting throughout the depth range considered.