Technical Paper: Accidental Geomechanics - Capturing In-situ Stress from Mud Losses Encountered while Drilling

Society: SPE
Paper Number: 78205
Presentation Date: 2002
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The accurate determination of minimum horizontal stress magnitudes (Sh) can be critical to the economic success of oilfield operations from exploration through abandonment. Sh magnitudes are needed to optimally design drilling, completion and workover operations. While Sh magnitudes could be determined from a well-designed stress test, they are typically estimated from leak-off tests. These estimates are often inaccurate due to the different mechanisms affecting the leak-off pressure (LOP). These factors include among others the possible existence of natural or drilling induced fractures, the unknown stress concentration surrounding the wellbore, and the variability in test procedures used. These factors lead to significant differences between reported LOPs and Sh.

However, high quality stress measurements are often "accidentally" made while drilling. Most mud loss events characterized by wellbore ballooning, simulate an extended leak-off test, a high-quality stress test yielding an accurate value of Sh1. Wellbore ballooning is a term used to describe mud losses when the mud pumps are turned on followed by mud gains when the mud pumps are turned off. Wellbore ballooning is often caused by the opening and closing of fractures 2,3. When circulating, the higher wellbore pressure opens and propagates the fractures and when circulation stops, the lower wellbore pressure closes the fracture returning mud to the wellbore. Fracture opening and closing pressures are better estimates of Sh, and can be quantified using downhole annular pressure while drilling measurements (APWD). Because circulation often starts and stops multiple times, multiple opening and closing pressures are recorded. The addition of time-lapse resistivity to locate the fractures completes this elegant and high quality stress measurement technique.

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