Drilling water wells on Kharyaga oilfield, Timan-Pechora region of Russia, often resulted into poor water production due to aquifer formation plugging with weighting materials and drilled solids and insufficient filter cake removal.
Bentonite-based drilling fluids have been traditionally used for water wells drilling in this area. Bentonite particles invasion into the unconsolidated sand water-bearing horizons is almost impossible to prevent and cure, so in most of the cases actual water production rates achieved proved to be lower than estimated ones. Also, when the wells were put on production considerable sand control related issues often occured. During the drilling phase, when passing through unconsolidated sands, borehole instability problems due to sands washout and hole sloughing have been frequently experienced.
In order to avoid considerable formation damage, borehole instability and increase water production, saturated sized salt drilling fluid system has been proposed for two water wells drilling.
Saturated sized salt drilling fluid has been formulated by water saturation with sodium chloride salt following by treatments with modified sized salt of appropriate grade and additions of alkalinity, filtration, viscosity and foaming control agents. The system showed very stable properties throughout operations with tight LPLT filtration, good thin and smooth filter cake and perfect rheological characteristics. Drilling and coring operations in sand and clay formations have been performed with no problems, neither several days of stand-by had any influence on wellbore stability. Electric logs have shown perfect in-gauge hole.
After the section has been drilled and liner with filter screens has been run to bottom, the filter cake was easily removed by displacing and circulating hole with fresh water to dissolve sized salt particles.
When the water wells were put on production, both of them have shown very good production rates in comparison with other water wells drilled in the field and in comparison with estimated production rates for these two wells also. No signs of sand/fines control related issues have been either experienced during more than two years of water production.
To minimize formation damage due to drill-in and completion fluids solids invasion into the production zone as a whole and aquifer formations in particular, drilling fluids bridging particles should satisfy two important criteria:
they should be of appropriate size to rapidly develop a thin and tough external filter cake in order to filter out drill solids and polymers from the filtrate entering the formation; and
this filter cake should be easily and fully removable without much additional force applied onto the formation.
Rapidly developing external filter cake prevents solid particles suspended in the drilling fluid from invading deeply into the formation, plugging the pores and hence reducing the rock permeability. A good filter cake also helps to support walls of the unstable wellbore and prevent wash out, sloughing and pack off.
If this filter cake can be easily removed (dissolved) during completion operations, this gives an additional advantage of cleaning out and opening the pores plugged by the bridging material particles without additional operations required, such as scraping or washing the filter cake away.
Formation protection and consolidation by a good, easily removable filter cake during the drilling phase will not only increase the production rates and decrease skins, but will also help to mitigate any sand and fines control related issues in future, when the well is put on production.
Among the factors which determine formation damage due to pay-zone particles invasion the following major ones can be pointed out:1
Particle size distribution in the mud.
Formation permeability / pore size distribution.
Concentration of drilling fluid solids.
Mud circulation rate and rheology.