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

Abstract

Abstract 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 fractures2,3. 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. Examples are presented from 2 GOM wells where fractures in both sands and shales have been responsible for ballooning. Annular pressures measurements are then analyzed to determine Sh. Introduction Geomechanical solutions are being used in an increasing number of drilling and completion operations across the industry because geomechanical problems represent a large portion (in some cases by far the largest) of an operator's non-productive time (NPT). One reason for this increase is the trend toward the drilling of more challenging wells. However, the quantity and quality of applicable input data required to solve the geomechanical problem is often inadequate. A key property required to solve many geomechanical problems is Sh, the minimum horizontal stress. The magnitude of Sh is the most important input in determining the upper limit to the mud weight window. Uncertainties in this calculation lead to the loss of expensive drilling fluids and productive time spent on-bottom drilling. This is experienced more in deepwater wells where the pore pressure-fracture gradient (PPFG) window is narrow and when drilling in producing fields where the state of stress is changing with time. A better understanding of Sh is vital for the efficient drilling of deepwater wells and the planning of a drilling strategy that accounts for the adverse stress effects caused by depletion.