Drilling and completion of long, highly-deviated or horizontal wells have become normal in most of the mature petroleum reservoirs, especially for unconventional plays. Field observations indicate that infill wells could pass through depleted zones where geo-mechanical property changes are induced by stimulation and production of the offset wells. This often leads to wellbore instability during infill well drilling. To better understand and tackle this problem, this paper presents a comprehensive modeling study and sensitivity analysis to provide recommendations for operators on infill well planning.

In this paper, an integrated workflow coupling fracture, reservoir and geo-mechanic models is introduced, to systematically study the depletion effects and the key parameters controlling the process. A base case study is carried over a well completed in the Permian Basin with geo-mechanical earth model created from well logs and field data. The integrated workflow with finite element computation was applied to predict the induced stress change after stimulation and production. Infill well wellbore stability and performance were analyzed for different scenarios: 1. Different well spacing 2. Different drilling time. Sensitivity studies were made by altering geo-mechanical and reservoir parameters: 1. Stress/Formation anisotropy 2. Rock strength 3. Pore pressure.

Based on the numerical results from this comprehensive modeling and sensitivity study, dominant factors for wellbore stability are estimated. In a normal stress environment, due to the change in the stress state from production and the increase in differential stress around the wellbore, the knowledge of equivalent mud weight required to reduce the wellbore stability issues on the lateral can be critical in order to drill and complete the well successfully. The need to change mud weight and possibility of wellbore breakouts and mud losses while drilling the infill well can be imminent. In addition, the capability of the workflow to model the pressure depletion and associated stress conditions with respect to time provides us insights on stress magnitude, stress anisotropy, stress reorientation, and impact of timing and spacing of infill wells on the safe mud weight window to optimize the field-scale development strategies.

The study presents a reference for strategic planning for the operators drilling their infill wells on an existing well pad. Preventative methods such as higher mud weight could be applied in advance to avoid wellbore failure. Better hole cleaning and mud weight design could prevent mud loss especially for the closely spaced infill wells. The study provides insights to understand the controlling factors over stress changes and their effects over depletion magnitude during infill drilling. Therefore, guiding us to understand the key triggers for wellbore stability issue in unconventional formations. This, in general, will help improve the overall economical returns from reservoir development stand point.

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