Silicate-based drilling fluids are not new, but recent applications are proving beneficial in major drilling areas.

In the 1930s, chemists at Philadelphia Quartz, now PQ Corporation, worked with scientists at Rice University to develop a silicate-based drilling fluid that could stabilize the heaving shales found along the US Gulf Coast. Shale inhibition is still the prime reason for building a mud around soluble silicates. But lately, some new reasons have surfaced that put silicate-based drilling fluids in a new light. These include excellent environmental performance, low chemical costs and good rates of penetration (ROP). Additionally, silicate-based fluids offer benefits like anti-corrosion properties, better cement jobs and fluid loss protection.

A look at the manufacturing of soluble silicates and the resulting chemistry helps explain why silicates are low cost, environmentally friendly and versatile. Soluble silicates (sodium or potassium), are manufactured similarly to window glass, except a much higher level of alkali is fused to the sand while in the furnace. By controlling the ration of silica to alkali, the resulting glass can then be dissolved with steam to make a "waterglass" or soluble silicate. The liquid silicate is composed of a mix of different silicate species. These silicates are unique in that they can undergo four distinct chemical reactions:

• Polymerization
• Precipitation
• Surface charge modification
• Hydration or dehydration

By understanding and taking advantage of the above chemical reactions, service companies have developed numerous types of silicate muds that are suitable for a wide range of shales. The workhorse formulation for offshore drilling still remains sodium silicate with a mix of polymers and potassium and sodium salts. These types of silicate muds are presently being run in the North Sea, Mediterranean, Nigeria and Southeast Asia.

Examples of some of the newer developments in silicate muds include a low-density aerated silicate mud developed by M-I. In Pakistan, a number of wells were successfully drilled using this new system. These silicate muds were aerated using air, and thus avoided the use of expensive anticorrosion agents of gases. Another variation on running silicate muds is a "clear water" or "flocculated silicate." An inexpensive drilling fluid can be formulated using a variable concentrate of silicate, water and a flocculating PHPA. The silicate works to stabilize the well bore and prevent the dispersion of cuttings. These flocculated systems allow higher ROP and drilling to a greater depth before having to mud-up. Operators in Western Canada have drilled about 150 wells using this type of system.

The biggest trend has been the switch to potassium silicate as the base silicate for drilling on land. In 2000, North America saw five wells drilled using potassium silicate mud, but by the end of 2003 about 120 wells will have been drilled using the formulation. Potassium silicate muds have become most popular in British Columbia, where service companies typically formulate using a relatively high concentration of potassium silicate (about 10% by volume). These muds have proven very effective at controlling dispersive shales such as the Fort Simpson shale. Given that silicate stabilizes and hardens cuttings, these muds have been resistant to solid build-up, allowing the mud to be re-used three to five times. By replacing oil-based muds with potassium silicate muds, one operator in British Columbia has claimed savings on chemicals and disposal in the range of 55% over previous systems. In Colorado, Newpark Drilling Fluids has developed a drilling fluid that incorporates a low concentration of potassium silicate. This system has proven very effective at reducing washouts, lowering cementing costs and helping to increase ROP. It was estimated by one operator in the Rockies that the reduction of washout was in the range of 14% with the mud helping to contribute to a 10% reduction in drilling cost-per-foot.
The switch to potassium silicate is driven by environmental performance; in the United States, potassium silicate is approved as a fertilizer. While other drilling fluid chemicals might also claim to have fertilizer status, potassium silicate is unique in that it is a classic salt. It is not the product of an acid-base reaction, nor can it depress the freezing point of water. Assuming no toxic chemicals are added to potassium silicate muds, regulators typically allow disposal of potassium silicate drilling waste using the lowest cost methods. Disposal methods for these muds include mix-bury-cover, land spreading and land-spraying over farm land.

As silicate muds gain popularity, more work has gone into enhancing their performance using additives. Almost all the major service companies advertise an ROP enhancer for silicate systems. Lubricants have been developed to allow for longer and more highly deviated wells. PQ has developed a rheology stabilizer that will allow silicate muds to run at higher densities and temperatures. This stabilizer has also reduced the rate of depletion of silicate when drilling through calcium-based formations.