Because the integrity of the wellbore plays an important role in many well operations, this work focused on studying the complex problem of the stability of multilateral junctions at different orientations in a three-dimensional anisotropic in-situ stress field, based on finite element three-dimensional modeling using commercial software. Stress-displacement analysis in steady-state was coupled with transient phenomena to compute stress behaviors and changes in pore pressure due to disturbances created by drilling. This coupled approach allows for the inclusion of some time dependent processes and the non-linear processes that influence the behavior of the system compounded by rock, fluids contained in the rock, and in-situ stresses. This paper shows the results of stress-displacement analysis of multilateral junctions. For the first time, using the "stress cloud" concept, critical areas regarding failure in the junction area were identified. In order to propose strategies to optimize drilling and completion design of multilateral wells, this analysis included variations of geometry (angles between the lateral and the mainbore), placement, and orientation of the junction. The results demonstrated that the most stable junction, independently of the depth of its placement, is with the lateral wellbore axis oriented parallel to the maximum principal in-situ stress (sH). Similarly, based on the mechanical response of rock, the results also demonstrated that junctions of multilateral wells should be placed as close as possible to the hydrocarbons zones. The significance of this work rests on the reliability that this kind of approach gives for simulating the phenomena encountered in wellbore stability analysis of multilateral junctions in most of the possible cases in oilfield applications, regarding stability.
1. INTRODUCTION
Wellbore stability analysis has been the subject of study and discussion for a long time. The integrity of the wellbore plays an important role in many well operations during drilling, completion, and production. Problems involving wellbore stability occur principally through changes in the original stress state due to removal of rock, interactions between rock and drilling or completion fluids, temperature changes, or changes of differential pressures as draw down occurs. For the particular drilling case, support provided originally by the rock is replaced by hydraulic drilling fluid pressure; this creates perturbation and redistribution of stresses around the wellbore that can lead to mechanical instabilities. These instabilities cancause lost circulation or hole closure in the case of tensile or compressive failure respectively. In severe situations, hole closure can cause stuck pipe and loss of the wellbore. These events lead to an increase of drilling costs.
Although there exists a significant amount of information related to wellbore stability, most of the information addresses the study of stability in the vicinity of the wellbore for a single hole. When two holes interact, the interference that a lateral hole causes on the stresses around the mainbore is particularly interesting. However, information about research conducted in a multilateral scenario where two holes interact is limited. Aadnoy and Edland [1] and Aadnoy and Froitland [2] investigated the effect of wellbore geometry on the stability of multilateral junctions using the elasticity theory.
