A new approach for understanding the mechanism of wellbore strengthening theory

Abstract

To achieve fast, safe and economical drilling operations through formations containing natural fractures or vugs, all fractures and vugs must be perfectly sealed using proper fractures seal materials in a process called wellbore strengthening. Wellbore strengthening may elevate the overall tensile strength of the treated formation to a value approaches the intrinsic tensile strength of unfractured rock.Numerous studies have been conducted for testing fractures seal materials suitability for wellbore strengthening application in various formations. However, there is no solid and simple theoretical explanation on how wellbore strengthening process can recover the tensile strength and hence the fracturing pressure of the treated naturally fractured formation.In this study, test specimens cored from an artificial (building) sandstone bricks were used to measure the unconfined compressive strength (UCS), Brazilian indirect tensile strength (BTS), and establishing the multi-stage triaxial compression (MS-TCS) test. Core samples from the same sandstone were previously used for wellbore strengthening studies using crushed dates palm seeds. Crushed dates palm seeds succeeded to completely stop mud losses at a pressure equal to 6.9 MPa.Data obtained from unconfined compression, multi-stage triaxial compression, Brazilian indirect tensile, and wellbore strengthening tests were collectively used to develop a clear theoretical understanding of the mechanism of tensile strength recovery caused by wellbore strengthening effect.It was found that wellbore strengthening effect in sealing natural fractures and vugs is similar to the confining pressure effect in closing the induced shear fractures in the test specimen during a multi-stage triaxial compression test.For the tested sandstone, using crushed dates palm seeds as a fracture seal material has improved wellbore strengthening from zero tensile strength (complete loss of mud circulation) to 2.56 MPa tensile strength at which there was a complete fracture seal (zero mud loss). The recovered tensile strength (2.56 MPa) is approaching the intrinsic tensile strength (BTS = 2.7 MPa) of the same sandstone in its initial unfractured state.