Differential sticking can be a significant cause of lost time and non-productive cost where formations with varying permeability are drilled. In some instances, operators have limited choice and drilling overbalanced through lower pressured formations is necessary to meet well design targets. Higher permeability sections result in high rates of fluid loss into the formation, thick filter cakes and a significant risk of the drillstring becoming differentially stuck.
A novel approach to preventing such incidents was introduced in Saudi Arabia in 2006. The treatment/system relies on a combination of two new products added to the mud system; a synthetic graphite and an organic polymer which, in combination, provide an effective seal against high permeability formations and substantially reduce occurrences of differential sticking.
The synthetic graphite and the polymer, (added in the form of an aqueous emulsion) sizes are important factors in the success of this approach. Strategies to maintain the correct level of products in the mud system during drilling are discussed in conjunction with examples of successful field applications.
Laboratory work, designed to improve on previously existing guidelines for product application in different formation types and a proposed method of determining the graphite concentration in the mud by analytical analysis are also presented. Data from all field applications to date are summarized, showing that the system described is highly effective at eliminating differential sticking, mud losses and wellbore stability issues.
Drilling trough formations with a considerable difference in pressure without being isolated, involves the high risk of lost circulation and differential stick. With a high amount of stuck incidents on those wells and an equivalent amount of sidetracks, it was obvious that a new drilling fluid design was required.
Over the years it has been found that graphite is very effective to reduce losses and improve filtration control, highly resilient synthetic sized graphite has the capacity to deform and plug a wide range of pore throats and fractures, at the same time, due to its shape and particle distribution, this graphite works as a mechanical lubricant, effectively reducing torque and drag.
Resilience is the mechanical property of materials to absorb pressure when elastically deformed. The Modulus of Resilience, Ur, can be defined as the relation of the yield stress and Young's modulus as described below.