Wellbore strengthening evaluation with core fracturing apparatus: An experimental and field test study based on preventive approach

Abstract

One of the most persistent and complicated challenges during drilling a well is lost circulation, which leads to more operational cost, time, and risk. Wellbore strengthening (WBS) with the addition of special solid particles, means lost control materials (LCMs), in drilling fluid is a major solution to control the fluid loss and improve the formation pressure-bearing capacity. The preventive approach of wellbore strengthening is based on the plastering effect of mud cake to inhibit the initiation of new induced fractures or the growth of pre-existing ones. The sealing capability of the mud cake can be affected by LCM properties which has been evaluated by some plugging apparatus with slotted disk in previous researches. However, in this paper, a self-designed core fracturing laboratory setup has been developed to simulate the fracturing process of a rock sample in a more realistic situation and study the effect of LCM particles on the creation of an efficient filter cake to improve the fracture gradient of the formation. Artificial cylindrical concrete cores with a central hole have been used to eliminate the heterogeneity effect of the rock and achieve the same experimental conditions. The fracture breakdown pressure (FBP) and fracture reopening pressure (FRP) have been considered as the objective parameters for evaluation of LCM performance. The experimental results on 18 core samples with two different types of LCM and different particle size distribution show that the addition of LCM particles into drilling fluid can improve FBP and FRP up to 33 % and 72 %, respectively. This indicates that the sealing/plastering effect of mud cake can be improved by LCM properties to achieve a higher fracture pressure. In addition, different fluid systems with different LCM types and LCM sizes have various effects on the preventive approach of wellbore strengthening. Finally, one of the drilling fluid formulations with a desirable performance during laboratory tests was selected to apply for drilling the 12–1/4” hole section in one of Iranian oil fields as a preventive approach where a history of severe lost circulations had been reported. The results of field test showed that using this approach was more successful than the remedial approach used in all previous wells of this field and resulted in a significant reduction in cost and material consumption.