52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: With the considerable push of oil and gas exploration to the new frontiers, the greater uncertainties and narrower drilling margin need a thorough integrated assessment to different aspects of wellbore strengthening behavior. In this pursuit of addressing several wellbore issues, the behavior of formation rock is of immense importance. Before looking into the underlying behavior, one has to understand the formation properties, hydraulic communication, and formation properties of the rock samples. These critical properties effect the sealing behavior and post sealing strength of the borehole wall. After the evaluation of formation properties in the experimental setup, the near wellbore behavior can be studied.
This paper includes a comprehensive investigation towards understanding the stress redistribution and underlying smear mechanism during wellbore strengthening phenomenon. In this study, two types of rocks Bandera Brown and Michigan sandstone were investigated using experimental work and analytical models. Experimental data has been used to analyze the two competing theory based on fracturing and filter cake buildup for wellbore strengthening. An analytical model is derived to obtain the fracture length and consequently fracture width is obtained using semi-analytical approach. The fracturing and poro-elastic model is used to analyze the wellbore strengthening candidacy of the two rock samples.
In recent days, narrower mud window regime has become a routine encounter during the drilling operation, and subsequently wellbore stability problems have become ubiquitous which are dictated by in-situ stresses, fluid flow in the nearby zones, and the strength of wellbore supporting formation over time (Kiran et al., 2017). Stress alteration around the borehole wall changes the dynamics of the failure properties of existing rock. In order to avoid these failures, there are several possibilities to be explored by the researchers till date. The primary option persuaded is by the adjustment of applied mud pressure according to mud window. This mud window is constructed using the lower bound identified by formation fluid influx or borehole collapse (shear failure), and the upper bound is determined based on the tensile or breakout limit (tensile failure) (Wang et al., 2008). In addition, the secondary option exploited, such as wellbore strengthening, is critical to enhance the breakdown limit and cater with instability problems such as lost circulation.
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