Abstract
The importance of an accurate geomechanical model for borehole stability assessment is increasing in the petroleum industry due to the growth in the number of drilling operations in unconventional reservoirs. These reservoirs are thin layered, naturally fractured with high clay content; the presence of clay minerals in particular, make their behavior to be unpredictable and also make the rocks to become chemically active with the drilling fluid. Thus, wellbore stability analysis is crucial and challenging; In addition a good understanding of elastic and physiochemical properties of the formation would be necessary for better field development to avoid future financial losses. For this study elastic, petrophysical and physicochemical properties of the shaly Bakken Formation were tested and reported from a several number of core plugs in different wells drilled in the Williston basin, North Dakota. We measured and reported various poro-mechanical, petrophysical and physiochemical properties of the Bakken shale along with the chemical properties of the drilling mud. The direction and the magnitude of the horizontal principal stresses were measured in the field with the data acquired from advanced logging tools in several wells. All of these values are coupled and used as input parameters in a time-dependent chemo-thermo-poroelastic constitutive model to calculate in-situ stresses and pore-pressure variations around the borehole. Changes in radial and hoop stresses were also plotted in the vicinity of the well. Mohr-Columb failure criterion was applied to the model to evaluate the deformations and failure occurrences around the borehole. Finally probabilistic risk assessment was carried out to understand the sensitivity of the results to the uncertainties in the input data.
1. INTRODUCTION
Drilling in shale formations as the main target is becoming pretty popular with the recent growth in the production from shale oils throughout the world and specifically in the U.S. From many years in the petroleum industry, shales are attributed to drilling problems such as pipe sticking, washout and borehole collapse. Swelling, shrinkage and wellbore failure is a common phenomenon in chemically active shales, which is due to the high clay content [2, 3, 5, 6]. It has been reported that 90% of wellbore instability occurs in shale formations. As a result a good understanding of shale thermal, mechanical and physiochemical processes that take place during drilling operation would be inevitable to suppress the risks and future hazards.
