Oil-based critical mud weight window analyses in HTHP fractured tight formation
Oil-based critical mud weight window analyses in HTHP fractured tight formation
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DOI：
10.1016/j.petrol.2015.10.002
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发表时间：
2015-11
影响因子：
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通讯作者：
Ming Liu;Yan Jin;Yunhu Lu;Mian Chen;Xin Wen
中科院分区：
文献类型：
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作者：
Ming Liu;Yan Jin;Yunhu Lu;Mian Chen;Xin Wen
中国西部地区蕴藏着丰富的致密油气资源。目的层埋深一般为6000-7000 m，具有高温高压钻井环境、基质渗透率低、天然裂缝系统发育、地应力各向异性等特点。即使在使用强抑制性油基钻井液的情况下，钻井液漏失和井壁坍塌仍时有发生。此外，在许多威尔斯井中，当泥浆比重增加时，井眼条件变得更差。与传统的现场经验相反，坍塌压力本身存在一个窗口。研究裂缝性致密地层的特征，建立裂缝性致密地层钻井复杂性解释模型。在这种方法中，地层被划分为致密段和裂缝段。1.在致密段，采用基于孔隙弹性理论的孔压和应力分布预测模型。计算结果表明，在各向异性地应力场作用下，在钻孔附近区域，钻孔打开后的瞬间，由于孔隙-力学耦合作用，会产生异常高的孔隙水压力。诱导压力对井眼稳定性有害，需要一个泥浆比重下限来克服。在裂缝性储层中，考虑天然裂缝的走向和泥浆侵入，建立了三维弱面模型。计算结果表明，高比重泥浆会渗入裂缝的缝隙中，降低裂缝上的正应力，从而导致裂缝沿弱面沿着剪切滑移破坏。由此导出了泥浆比重的上界。3.模拟了井底温降随时间的变化规律，分别研究了其对致密段和裂缝段井壁稳定性的影响，辅助约束了钻井液密度。综合考虑上述因素，得到了一个综合的临界钻井液密度窗口，为裂缝性致密地层油基钻井液井壁稳定性分析提供了理论依据。
There are plenty of tight oil and gas reserves in western China. The depths of the target reservoirs are usually as deep as 6000–7000 m, which are featured by: HTHP drilling environments, low matrix permeability, well-developed natural fracture systems, and anisotropic in-situ stresses. Even using highly inhibitive oil-based fluids during drilling operation, mud losses and wellbore collapses still occurred frequently. Moreover, in many wells when the mud weight is added, borehole conditions become worse. Contrary to the traditional field experiences, collapse pressure itself has a window. The purpose of this work is to investigate the features of fractured tight formation and correspondingly build a model to explain the unusual drilling complexities. In this approach, the formation is divided into a tight member and a fractured member. 1. In the tight member, a model for pore pressure and stress distribution prediction around a borehole is used based on the poroelasticity theory. The calculation results show that in the near borehole region, instantaneously after the borehole is opened, an abnormal high pore pressure is induced due to the poro-mechanical coupling effect under anisotropic in-situ stress field. The induced pressure is harmful to borehole stability, and a mud weight lower bound is necessary to overcome it. 2. In fractured member, a 3D weak plane model is established considering natural fracture׳s (weak plane) orientation and mud invasion. The calculation results show that muds with hight weight will infiltrate into the aperture of fractures and decrease the normal stress on it, which may induce shear slip failure along the weak plane. Thus an upper bound of mud weight is derived. 3. The time dependent temperature drop at the bottom of a wellbore is also simulated and its impact on borehole stability of tight and fractured section is studied respectively, which helps constrain the mud weight auxilarily. By integrating the above-mentioned factors, a comprehensive critical mud weight window is obtained, which can serve as the theoretical basis for wellbore stability analyses in the fractured tight formation drilled with oil-based fluid.