Abstract
Accurate evaluation of formation fracture pressure is essential to prevent induced fracturing and mitigate drilling fluid losses, representing one of the major challenges in oil and gas well drilling operations. Conventionally, formation breakdown pressure (FBP) is regarded as an intrinsic rock property and is assumed to be independent of drilling fluid properties. However, the results of this study demonstrate that improving the sealing performance of the drilling fluid can significantly influence both the formation breakdown pressure and the fracture reopening pressure (FRP). Specifically, the purposeful application of drilling fluid additives, particularly lost circulation materials (LCMs), enhances the formation's pressure-bearing capacity and effectively prevents lost circulation, consistent with the preventive wellbore strengthening approach. To investigate this effect, twenty-five drilling fluid formulations containing various combinations of LCM were prepared, and their fluid loss behavior and mud cake quality were evaluated using an API filter press apparatus. Based on these initial results, five drilling fluids exhibiting the most favorable performance were selected for further evaluation using a wellbore strengthening evaluation device. The results show that the drilling fluid formulated with the best LCM combination forms a thin, low-permeability mud cake and exhibits better sealing properties. As a result, increases of up to 42% in FBP and 95% in FRP were observed. These improvements highlight the important role of well-designed drilling fluids in preventing lost circulation. Furthermore, a qualitative agreement in experimental trends between API filter press data and wellbore strengthening test results indicates that API filter press measurements can serve as a rapid and cost‑effective preliminary screening method, helping to optimizing the experimental workflow.