Qualitative Analysis of Drilling Fluid Loss Through Naturally-Fractured Reservoirs

Abstract

Summary Drilling becomes extremely challenging when dealing with naturally fractured reservoirs (NFR). A comprehensive solution is developed in this study to perform qualitative analysis on drilling fluid loss rate and volume to examine how they can be affected by NFR characteristics, drilling fluid rheology, leakoff phenomenon, and wellbore condition. In this regard, the solution is applied to generate type curves to facilitate the sensitivity analysis (refer to the provided Supplementary Materials). The presented solution accounts for not only drilling fluid pseudoplasticity in the total system but also matrix medium under wellbore constant pressure assumption (by including dimensionless matrix contribution parameter). It is also able to measure mud loss advancement not only through NFR but also through homogeneous reservoirs. The developed solution is validated by reducing it to the preexisting solution (designed for Newtonian fluid case) by incorporating assumptions into it. The result demonstrates the significance of NFR properties and drilling fluid pseudoplasticity on the leakoff phenomenon and total loss volume, especially when constant pressure is established inside the wellbore. The finding reveals that three periods can be identified through generated type curves depending on NFR characteristics, drilling fluid rheology, and leakoff coefficient. Therefore, different drilling fluids with specific pseudoplasticity should be used in each period to mitigate drilling fluid loss effectively. In this regard, the study is supposed to design drilling fluid in a way to maintain its pseudoplasticity at a higher level at early and late times, while being maintained at a lower level during the transient period, a critical aspect for managed pressure drilling techniques, particularly in the context of dual-gradient drilling applications. Additionally, a procedure should be implemented to lessen the transient period while attempting to keep drilling fluid advancement occurrence at a lower rate, which shows that drilling fluid pseudoplasticity can be used as an effective tool to manage this period. The obtained result also indicates that the importance of drilling fluid rheology to control total loss volume is greater for NFR with higher leakoff than with lower leakoff. Furthermore, the greater the differential pressure inside the wellbore, the greater the importance of mud rheology to reduce drilling fluid loss. The outcome of the study not only facilitated qualitative and quantitative analyses through NFR but also enabled decision-makers to instantaneously select optimal wellbore conditions and drilling fluid pseudoplasticity.