During drilling or cementing, lost circulation may be encountered due to the presence of highly permeable formations, which may due to high natural permeability in sandstone, or to large natural fractures and vugs as found in limestones. This problem is critical as it increases the drilling cost due to wasted time, plus the possibility of stuck pipe and even losing the well. In most cases, viscous mud pills with lost circulation material, gel, etc …, allows, to temporarily reduce the losses but does not guarantee a durable plugging of the formation. Cement plugs, can be used but their placement and the time necessary to drill through them make their use relatively costly.
The use of an internally activated silicate solution allows to obtain a solid free Newtonian, solution with a very low initial viscosity. After a given period of time, which depends, on the fluid design and the temperature, the viscosity of the solution increases rapidly to form a gel. This gel is coherent, strong, and does not develop free water as a function of time. The very low initial viscosity of this solid free fluid allows it to be pumped directly through the bottom hole drilling assembly therefore saving time, and provides deep penetration of final gel.
A high pressure experimental set up was used to plug various cores of different permeabilities and with different saturation fluids. Results are presented on the extrusion resistance obtained after one hour and after one week. This, gel is shown to withstand differential pressures, greater than 1500 psi per foot of plugged formation. Some results on the long term stability of this gel to aggressive brines, organic compounds and oil are included.
A few case studies, of jobs performed in Europe and in the Middle East are also presented.
There have been many articles over the years referring to why lost circulation occurs, the different materials available, and how to solve it during drilling and while cementing1–4. Basically, lost circulation treatments can be divided into two categories:
Those where the lost circulation is solved by the addition of solid material the drilling mud; and
those where the problem an only be solved by the use of a non-mud system. Normally when lost circulation occurs, the first attempt to cure the problem is always by a reduction in mud weight and/or the addition of granular, fibrous, or lamellated material to the mud. The decision to use what material has been shown to be dependent on the type of lost circulation zone and the severity of the losses, but in many cases a number of different materials are used, sometimes with, sometimes without, success.
If no success is obtained by using the solid-laden mud, then it becomes necessary to use the most efficient non-mud system available. In order for these systems to be effective they have to exhibit certain properties, some of which are: