Well Bore Washout and Under-balanced Pore Pressure Effects on the Loss of Directional Control, Seismic Energy Release, Bit-Walk, and Lead Angle

ABSTRACT: This work uses three sets of field data from three directional wells, drilled recently in south Louisiana. We refer to them as well 1, 2, and 3. In this work our objective has been to find supporting field data for our conjecture regarding bit-walk and excessive lead angle by using the actual field data. We show (1) the effect of under-balanced mud weight which results in the well bore break-out, washout, and eccentricity at the bit and stabilizers and (2) how well bore break-out, washout, and eccentricity at the bit and stabilizers and the stresses due to unbalanced pore pressure affect the bit walk tendency and the loss of directional control, specifically in relation to seismic energy release. In order to understand what happens when well breakout takes place, the authors apply the principle of rock burst to oil well drilling. Basically, we apply these principles in mining and tunneling operations where major rock breakdown and structural failures occur. In the theory part, we formulate a simple model that accounts for the stresses around the well bore, strain energy, and work done by applied stress around the washout areas. Additionally, we calculate the released seismic energy, which is an indication of stress relief phenomenon. We validate the conjecture by processing the well bore survey data to find where and how the bit deviates from the pre-determined drilling trajectories.The results of analysis show that well 1 experienced severe bit walk, which resulted in a great deal of unproductive time and financial problems for the contractor. To correct the well path, the contractor had to run additional mud motor runs, with a certain housing angle for correcting the well path. We found that severe bit-walk in this well clearly related to a long and deep well bore washout where the mud weight was less than the calculated pore pressure . In contrast to well 1, the contractor drilled well 2 with properly balanced mud weight in the nearby field, without any technical and financial problems. We found this was because no major well bore breakout had occurred in well 2. Well 3 had a low inclination angle and exhibited very low bit walk tendency. This, also, confirmed our observation that bit walk is dependent on the inclination angle, which can be as low as 5 degrees in soft rock sediments.Interestingly, we found further that when the directional control was lost in well 1, in the under-balanced pressure section of the well, the uncontrolled well path became parallel to the major in-situ principal horizontal stresses, which is almost in NE-SW direction.

INTRODUCTION