Experimental Study on Wellbore Strengthening with Fine Particles
Authors: Hamidov Elmir , Ozbayoglu Evren , Baldino Silvio 
Published: September 2023

ABSTRACT
One of the most serious issues that arises during drilling operations is lost circulation. Significant study into wellbore strengthening has been conducted over several years in the attempt to improve the fracture gradient and reduce losses. Three ways to wellbore strengthening have been presented in general: 1) Increased fracture plug/hoop stress or "stress cage," 2) particle "screen-out," and 3) fracture propagation resistance. All these options are designed to prevent the spread of the fracture once it has begun. Filter-cake has been identified as having a significant role in wellbore strengthening. Mud cake has been shown to considerably strengthen wellbores, even to the extent of preventing fracture formation. Specifically, fine particles (e.g. ultrafine barite) have been tested to measure strengthening. A true triaxial cell was employed in this study to explore the influence of mud cake on wellbore strengthening for a variety of fluids and operating conditions. The effects of external and internal cakes, as well as the absence of mudcake, have been studied. API filter tests have also been carried out using various mud formulations.

INTRODUCTION
Drilling under partial or total lost circulation is extremely challenging. Cavernous and vugular structures, natural or induced fissures, unconsolidated sand, very porous tight sands, and tight sands with high sulfide levels are all examples of formations that can cause lost circulation. Even though it is common to use techniques like bridging, gelling, and cementing to remediate to this issue, the results vary depending on the method used. Trying to fix circulation problems can take long time and have a significant cost, especially when this results in considerable nonproductive time (NPT) (Benaissa et al, 2005). When drilling through depleted sands, there is a risk of losing circulation. This is made worse by the sands’ low fracture gradient and the high mud weight needed to avoid shear failure in nearby shales, which both add to the problem. When it comes to depleted sands, the best way to stop circulation problems is to be pro-active rather than re-active (e.g., prevent lost circulation to happen in the first place). This can be done by using techniques from both geomechanics and drilling fluid design.
Numerous procedures may be used to prevent or treat lost circulation. The first consideration should be to improve drilling operations. For instance, sophisticated geomechanical models may be used to predict the probability of a hole collapsing or losing circulation. Additionally, expandable casing, managed pressure drilling, and casing while drilling are used. (Majidi et al, 2011) Another option is to select drilling fluids with the proper rheological characteristics, which entails implementing drilling fluid with the proper rheological properties and/or additives to minimize or eliminate lost circulation. The third method of preventing fracture propagation is to use wellbore strengthening techniques, which are composed of specially formulated and sized particulate materials that are introduced into a fracture (natural or induced) and stop the fracture from propagating by isolating it from the wellbore. This can be caused by a high leaked-off fluid depositing solids inside the fracture and creating hard particles within short fractures. (Majidi et al, 2011) This can also be caused by smearing or plastering mudcake on the wellbore wall, which deposits solids within the fractures’ mouth (Marbun et al., 2014; Baldino et al., 2019).