Very popular wellbore strengthening technique in shales is ‘stress caging’. In this approach, the increase of wellbore stresses is induced by use of sized-particulate additions to the drilling mud. Fractures in formation are held open with bridging material.
The bridging material must have low permeability to provide pressure isolation of the fluid in the wellbore from that in the fracture which will prevent further elongation of the fracture. To have successful stress caging, proper size, type and amount of lost prevention material (LPM) must be used. Proper drilling fluid system will also prevent problems with well stability in case of swelling shales.
Polymer mud containing soluble salts is used to inhibit shale swelling and dispersion. Another option for drilling fluid is oil based mud. 1.1 Salts One of the mentioned problems in salt formations is salt flow. The rate of flow depends on depth, temperature and tectonic factors. Prevention of sticking due to salt flow can be accomplished by proper mud system. Salt flow can also cause washouts that later cause a problem with cement jobs.
To minimize washouts oil-based mud, supersaturated salt mud or high chloride (130 000-180 000 ppm) mud is used (Sheffield et al.,1983). Earl and Nahm investigated five commercial salt inhibitors in NaCl solution in a laboratory to evaluate their impact on mud properties while drilling through salt formations. The conclusion of their research is that salt recrystallization inhibitors are effective in maintaining supersaturated conditions in salt saturated salt muds (the type of inhibitors is not revealed). That means compatible inhibitors will help in eliminating hole washouts in salt zones and reduce the risk of poor primary cementing due to hole enlargement. When it comes to casing design, the designer should plan for non-uniform loadings and use casing with higher than normal collapse rate.
Also, it will be good to use two strings of the casing through the salt zone. An overlap of 150-200 feet above and below all significant occurrences of salt is desirable to ensure coverage of the interval and protection against excessive bending and collapse stresses which may occur at the interface between flowing salt and formation that is stationary. Another thing that needs to be considered is cement job.
The important thing is to assure that a cement top is above the desired minimum, at least 500 feet above the top of salt (Sheffield et al.,1983). API cement class that should be used in salt formation is class G or H salt free. A lot of work has to be done on displacing cement in gauge holes and in washed out sections. The optimum solution is to pump cement with appropriate properties in turbulent flow in gauge holes and plug flow in washed out zones. 1.
2 Permafrost The most effective methods to control hole enlargement in permafrost are reducing fluid density, thermal conductivity, specific heat, temperature and increasing the mud viscosity. The viscosity of drilling fluid will affect flow regimes and hole cleaning while reducing thermal conductivity and specific heat is the best method for solving washout problems in permafrost. Because of the fact that air or vacuum is a poor conductor of heat, there were several researches about type of drilling mud in permafrost.
Sengupta (2011) presented microsphere based drilling fluid as a good solution for drilling problems in artic environments. Microspheres are very small spheres which are mainly empty and partially filled with water or gas (nitrogen or compressed air). In comparison with ordinary drilling fluids, microsphere based muds transfer lesser amount of heat. This will help in the prevention of melting the permafrost. The microspheres can be organic or inorganic and hollow spheres can be of glass, ceramics or plastics.
