Summary Overpressure is a common feature among productive unconventional shale reservoirs, such as the Bone Spring (BSPG) and Wolfcamp (WFMP) Formations of the Delaware Basin (DB) of west Texas and southeastern New Mexico, and is thought to be a strong driver of well productivity. Compared with conventional reservoirs and shales in normal pressured conditions, the effects of overpressure on the mechanical properties of shales is not well understood. Here we present an analysis of overpressure in clay-bearing siliciclastic facies of the BSPG and WFMP Formations of the DB and implications for mechanical properties of the reservoir. Estimation of the effects of overpressure on mechanical properties of unconventional shale reservoirs is determined through use of the sonic overpressure indicator (SOPI). The method requires log model results that accurately characterize variations in lithology and porosity for the formations of interest. The SOPI (ΔT/ΔTN)2, where ΔT is the measured compressional sonic transit time, and ΔTN is the forward-modeled result for normally pressured conditions, can be used with elastic moduli and their interrelationships to compare estimates of mechanical properties including Poisson’s ratio ν, the Biot or effective stress coefficient α, and Young’s modulus E, in normal and overpressured conditions. Results presented here are broadly applicable to overpressured unconventional reservoirs that contain significant clay volume (>0.1 v/v) and exhibit low porosity (<0.08 v/v), comparable to that of siliciclastic-rich facies of the WFMP Formation. To account for increased VP/VS ratio, we regard overpressurization of shaly facies as an irreversible thermodynamic process that transforms a normally pressured siliciclastic system. At stress below the yield point, which is taken as the limit of normal pressure, the system responds elastically to stress; beyond this point, during overpressurization, the system responds as an elastic/plastic medium with strain hardening. We regard elastic moduli as descriptive of mechanical energy stored in this system. This perspective enables Poisson’s ratio for the overpressured system νOP to be computed from an estimate of the normally pressured system νN using (ΔT/ΔTN)2. Overpressure also results in a limited increase of the Biot or effective stress coefficient α. Moreover, recognition that overpressure results in a decrease of Young’s modulus, that is, EOP/EN < 1, provides a means of estimating the amount of strain energy stored by the formation due to overpressurization. We believe that when exposed to lower pressures by wellbore construction, this strain energy stored in overpressured unconventional reservoirs drives creep, which affects interpretations made using geomechanical models. We have developed and tested computational models based on biaxial or plane strain for vertical wells and uniaxial strain for horizontal wells that describe how creep likely affects estimation of minimum horizontal stress Shmin and pore pressure from instantaneous shut-in-pressure (ISIP) measurements. Thus, for overpressured unconventional reservoirs, ISIP determinations differ from tectonic Shmin by an amount related to ν and EOP/EN.

中文翻译：

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超压对非常规页岩储层力学性能的影响，通过新使用声波超压指示器

总结 超压是生产性非常规页岩储层的共同特征，例如德克萨斯州西部和新墨西哥州东南部特拉华盆地 (DB) 的 Bone Spring (BSPG) 和 Wolfcamp (WFMP) 组，被认为是良好的生产力。与常压条件下的常规储层和页岩相比，超压对页岩力学性质的影响尚不清楚。在这里，我们分析了 DB 的 BSPG 和 WFMP 组的含粘土硅质碎屑相中的超压以及对储层力学性质的影响。超压对非常规页岩储层力学性质影响的估计是通过使用声波超压指示器 (SOPI) 确定的。该方法需要测井模型结果，以准确表征感兴趣地层的岩性和孔隙度变化。SOPI (ΔT/ΔTN)2，其中 ΔT 是测量的压缩声波传播时间，ΔTN 是常压条件下的正向模拟结果，可与弹性模量及其相互关系一起使用，以比较包括泊松比在内的机械性能估计值ν，Biot 或有效应力系数 α，以及杨氏模量 E，在正常和过压条件下。这里提出的结果广泛适用于超压非常规储层，这些储层含有大量粘土（＞0.1 v/v），孔隙度低（＜0.08 v/v），与WFMP组富含硅质碎屑的相相当。考虑到增加的 VP/VS 比率，我们将页岩相的超压视为转变常压硅质碎屑系统的不可逆热力学过程。在以正常压力极限为屈服点以下的应力下，系统对应力作出弹性响应；超过这一点，在超压过程中，系统会以弹性/塑性介质的形式响应应变硬化。我们将弹性模量视为存储在该系统中的机械能的描述。这种观点使得超压系统的泊松比 νOP 可以从使用 (ΔT/ΔTN)2 的常压系统 νN 的估计值中计算出来。过压还会导致 Biot 或有效应力系数 α 的有限增加。此外，认识到超压会导致杨氏模量降低，即 EOP/EN ＜1、提供了一种估计由于过压而由地层存储的应变能量的方法。我们认为，当井筒施工暴露于较低压力时，存储在超压非常规油藏中的这种应变能会驱动蠕变，这会影响使用地质力学模型进行的解释。我们已经开发和测试了基于垂直井的双轴或平面应变和水平井的单轴应变的计算模型，这些模型描述了蠕变可能如何影响从瞬时关井压力 (ISIP) 测量中估计的最小水平应力 Shmin 和孔隙压力。因此，对于超压非常规油藏，ISIP 测定与构造 Shmin 的差异与 ν 和 EOP/EN 有关。