A Comprehensive Experimental Study on Wellbore Strengthening

Abstract

Abstract Experimental tests have been performed to investigate the applicability, effectiveness and engineering of various wellbore strengthening techniques which increase the mud weight window by adding lost circulation materials (LCM) to drilling fluids. The experimental techniques include fracture sealing of known fracture apertures simulated by two plates, induced or natural fractures in a sandstone or shale block with a borehole, and a transparent cylindrical LCM particles transport cell, which allows visualization of how the fracture seal is formed and broken. In the block tests, two independent stresses are applied to the block faces parallel to the wellbore to simulate horizontal stress anisotropy. Drilling fluid, with and without LCM, is pumped into the wellbore to simulate formation breakdown, mud loss, fracture sealing and wellbore strengthening. The wellbore strengthening treatments can be continuous in one step or hesitation squeeze in steps. The various types of tests performed to simulate different operational procedures include: breakdown tests under different stress anisotropy and magnitude to establish the baseline and to validate the results with rock mechanics analysis; fracture sealing and remedial treatment of an induced fracture with LCM after breakdown; preventive treatment with background LCM in the active mud system before breakdown to study the difference between preventive versus remedial treatments; and fracture sealing and strengthening of natural fractures of different apertures; among others. Several hundred tests have been performed with different stress conditions and strengthening implementation methods. Valuable lessons have been learned from this comprehensive experimental study and the results have significantly increased the success of wellbore strengthening. Due to the large volume of test results and data, this paper focuses more on the various types of tests performed using the block tester, and gives selected snapshots of test results and modeling work on wellbore strengthening mechanism and the role of filtercake on sealing quality and strengthening performance. More results will be presented elsewhere.