Forming a Seal Independent of Formation Permeability to Prevent Mud Losses—Theory, Lab Tests, and Case Histories
Authors: Wang Hong Max , Savari Sharath , Whitfill Donald L. , Yao Ziheng 
Published: January 2016

A Roman arch effect of a round shape wellbore naturally creates a stress concentration that is called the hoop stress for the wellbore. Assisted by the hoop stress, a round wellbore typically has a great potential to hold much more pressure than a fracture in the same far-field stress environment. However, if the wellbore has a flaw leading to an induced fracture, this hoop stress, although it may be very large, is bypassed and cannot help to hold the wellbore pressure anymore.

Induced fractures are hydraulic fractures. The fluid volume invading a fracture widens the fracture progressively. To stop an induced fracture by forming a seal to the fracture, it is important that the seal forms after only a small volume of fluid flows into the fracture. This fluid volume is actually the spurt loss of the lost circulation materials (LCM) fluid during the formation of the seal; therefore, controlling the spurt loss to low values so as to not open the fracture too wide is critical.

New Fast-Sealing LCM (FS-LCM) products were created and tested to quantify the spurt for good and reliable performance. Each of these products has a critical sealing width that is defined by its largest particle size. The products are designed to seal a wide range of fractures effectively. A slot disk used for its spurt loss at various concentrations characterizes the FS-LCM.

Furthermore, for a specific rock with specified mechanical properties, the maximum allowed invasion volume for an induced fracture, before the fracture opens to the maximum sealing width of a FS-LCM, can be calculated using a textbook hydraulic fracturing theory. Adding enough of the FS-LCM to control the fluid spurt volume so that it is smaller than this maximum invasion volume is the necessary criterion to help ensure that the seal forms before the induced fracture opens too wide to seal. In this Wellbore-Shielding (WS) preventive wellbore strengthening method, rock mechanical properties are directly linked to the concentration of a standardized FS-LCM.

Conventional LCM may have to depend on formation permeability to form a seal to an incipient induced fracture and prevent mud losses. However, weak formations that are drilled through daily are not always permeable. Furthermore, permeability is reduced in permeable formations because drilling fluid, especially nonaqueous drilling fluid, tends to have excellent control over fluid loss by instantaneously forming a tough mud cake on a newly fractured surface. A permeable formation, therefore, may behave as if it were impermeable.

With the Wellbore-Shielding (WS) method, a seal can form even when there is no fluid loss leaking into the matrix formation. In other words, this engineering allows seal formation that is independent of formation permeability. Therefore, the method may also work in impermeable formations because of its unique approach.

The case histories presented demonstrate how a wellbore strengthening job can be designed and implemented on land and offshore. The paper also demonstrates why the reliability of the application might not be compromised even when rock data are not accurately known.