Wellbore instability has long been a great concern in the oil/gas well drilling industry since it may incur expensive operation cost. In this paper, we present a detailed analysis of instability of wellbore drilled in weakly consolidated rock formations through an elaborate poro-elastoplastic numerical model. The Drucker-Prager/cap plasticity model, by which both shear compressive plastic deformation and volumetric plastic deformation can be taken into account, is employed to more precisely characterize the mechanical behavior of weakly consolidated rock formations. Possible infiltration of drilling fluid into the formation has also been considered. With this model, plastic yielding and fracturing behavior of the borehole surrounding rocks under different conditions are analyzed. Results show that, during drilling with low mud pressure, the rock formation in the near-wellbore region firstly experiences some compressive plastic deformation and then shear plastic deformation. Excessive radial inward displacement of the wellbore may occur if the mud pressure is too low. If penetration of drilling fluid into the formation is prohibited, fracturing of the wellbore may not occur even with high mud pressure. Instead, high mud pressure may cause cavity expansion phenomenon at the wellbore, i.e. the wellbore will expand substantially and the deformed wellbore radius could be 2 times larger than the initial wellbore radius. In the other hand, if drilling fluid can penetrate into the formation, the pore pressure within the near-wellbore region could be raised, resulting in tensile hoop stress at the wellbore and thus fracturing of the wellbore. Some implications for drilling stable wells in weakly consolidated formations have been provided based on the numerical analysis results.
The past decades has witnessed a worldwide growth in the exploration and development of oil and gas in deepwater and ultra-deepwater due to the increasing energy demand and evolving technologies. Nowadays, more than 40% of the newly proved oil and gas resources are found in deepwater (Yan et al., 2015), thus it has been expected that the share of offshore oil and gas production from deepwater would further increase in the future. Recently, China is also launching more and more efforts to develop deepwater oil/gas resources.
Despite considerable technological advancements, deepwater drilling is still a non-trivial task and there exists various challenges during the whole process of drilling due to the complex deepwater environments (Rocha et al., 2003). Among various problems, wellbore instability is a major obstacle for achieving quick and cost-efficient drilling. Actually, wellbore instability has long been a notorious problem in the drilling industry, irrespective of whether the well is drilled onshore or offshore, resulting in remarkable economical loss and nonproductive time. For deepwater drilling, maintaining wellbore stability is an even more difficult task since the overburden stress in deepwater is relatively low due to the long water column, resulting in narrow safe mud density windows (Aadnoy, 1998; Rocha, et al., 2003). Especially in the shallow sediments, where the formation rock is generally soft and weakly-consolidated or even unconsolidated, special carefulness is needed to avoid wellbore instabilities like fracturing of the formation, lost circulation, excessive borehole closure as well as borehole collapse.
Currently, common practices of wellbore stability analysis are mostly based on linear elasticity theory (Bradley, 1979; Papanastasiou and Zervos, 2004). These traditional wellbore stability analysis procedures may overestimate the required minimum drilling mud density due to the requirement of completely no plastic deformation on the wellbore wall. Actually, for soft rocks like the shallow sediments in deepwater, it has been recognized that the wellbore can remain stable even if the surrounding rock has been loaded into plastic state (Charlez, 1997; Chen and Abousleiman, 2017; Zheng, 1998). In the other hand, soft rocks may have experienced plastic deformation before fracturing caused by high mud pressure, which is not considered in the traditional elastic model. Thus, in recent years, various elastoplastic models have been proposed and employed for analyzing the stresses and deformations around the wellbore (Chen and Abousleiman, 2017; Chen et al., 2011; Detournay and Fairhurst, 1987;McLellan and Wang, 1994;Muller et al., 2009;Wang and Dusseault, 1991), and borehole stability is realized by restricting either the area of plastic region (McLellan and Wang, 1994) or the borehole closure (Chen and Abousleiman, 2017) to be less than some designated values derived from field experiences.