Stability Analysis of Multilateral High Density Pad Wells in the Three Forks Formation

In order to increase the productivity from the Three Forks Formation, drilling horizontal well and multilateral ones, in particular, seems inevitable. A multilateral well is defined as a single well with one or more wellbore branches radiating from a main vertical borehole. A successful multilateral well that replaces several vertical wellbores will not only increase production, but also reduces overall drilling and completion costs, and allows more efficient hydraulic fracturing operations. Reducing surface impoacts while developing a field is a major benefit of this drilling plan. Well placement and design to have enough stability where in-situ stresses are disturbed dramatically due to short well spacing and stress shadowing is a crucial task, which should be studied prior to any drilling operation. Instability problems in multilateral wells can result from stress interplay among nearby wellbores.In this paper, instability of the borehole around the horizontal section of the high-density multilaterals, also known as pad drilling is studied. This drilling technique is growing very fast in the Williston Basin to increase production from two main hydrocarbon layers in the basin, Bakken and Three Forks Formations. Focusing on the Three Forks Formation, mechanical properties of this rock unit such as Poisson's ratio, Young's modulus and Unconfined Compressive Strength (UCS) have been extensively investigated and reported through filed data and lab experiments. These parameters, along with the calculated in-situ principal horizontal and vertical stresses, were input into a numerical simulation software, Flac 2D, to understand drilling impacts and to analyze borehole instability through a geomechanical model. For the final results, we have presented stress perturbation, rock displacements and creation of different failure zones around the borehole in the vertical and horizontal direction after the medium is penetrated.