Wellbore Stabilization Increases Fracture Gradients and Controls Losses/Flows During Drilling
Authors: Sweatman Ron , Wang Hong , Xenakis Harry 
Published: December 2007

This paper presents formation treatment technology that helps (1) reduce well-construction costs, (2) improve and maintain wellbore integrity during drilling, and (3) enable economical production from reservoirs with challenging drilling and cementing conditions. The technology presented helps stabilize and improve wellbore conditions by several different downhole mechanisms and types of sealing materials. This paper emphasizes one of the most promising of these mechanisms: increased near-wellbore fracture gradients (NWFG), or wellbore pressure containment (WPC) caused by stress alterations in the rock surrounding cracks induced by sealants forced into the cracks to slightly widen them near the wellbore. Industry studies1,2 sometimes refer to these stress alterations as "stress cages." The alteration process3 has been described as a WPC failure mechanism in which tensile force switches to compressive force as the fracture is sealed near the wellbore. Each side of the wellbore connecting the two fracture wings is moved by the compressive force in directions perpendicular to the fracture plane and proportionally to the increase in fracture width. As a result of this WPC improvement, engineers may plan wells with fewer casing/liner strings and with upper hole sections of smaller diameter. Under certain conditions, it is possible to construct the "one-trip" well, wherein only one bit-run is needed to drill to total depth (TD) from the surface-casing shoe. The new technology can also help prevent or mitigate unstable hole conditions that interfere with the drilling process.

Wellbore-stabilization technologies presented are designed for wells with narrow mud-weight windows (MWW) that can lead to well-construction incidents such as (1) loss of circulation, (2) failed shoe tests, (3) poor annular zonal isolation, (4) the need for extra strings of casing to enable drilling through depleted formations, and (5) lack of sufficient mud weight to prevent hole collapse and gas kicks. The key process in the system is placement of sealant in wellbore cracks to strengthen the wellbore to enable the use of a greater range of mud weights, i.e., a wider MWW.

Results from a numerical simulation analysis model are reported. The model is an aid to well planners in their efforts to design wells with (1) fewer casing strings, and (2) small-diameter upper-hole casings.

Case histories from around the world indicate that this process can result in substantial cost savings and that it represents a real breakthrough in drilling and completion technology. Improved primary cementing and sustained annular zonal isolation during production are also described. The fracture gradient (FG)-increasing treatments were applied during drilling to prevent lost circulation while cement slurry was being placed in setting casing and liner strings. Production cost savings included shorter periods to start of production, improved well integrity, prevention of annular wellhead pressures, no skin damage in reservoirs, and easy seal removal from producing natural fractures.