multi - lateral wells include one or more connections between a larger diameter well casing and liner - equipped branch - wells of smaller diameter initiated with a small angle deviation from the casing axis , so as to facilitate the sequential insertion of the directional drilling string and of the liner string , used respectively for drilling and for completing each lateral branch . each such insertion requires , in the casing string , an elliptical window cutout presenting a vertical axis of more than ten feet and a horizontal axis of only a few inches , corresponding to the diameter of the branch - well .