Formation damage due to invasion by drilling fluids is a well known problem. Many zones contain formation clays which hydrate when in contact with water such as the filtrate from drilling fluids. These hydrated clays tend to block the producing zones, primarily sands so that oil and gas cannot move to the borehole and be produced.

These zones are also damaged by solids which are carried into the openings with the fluid. The movement of drilling fluids and filtrate through these openings also causes dislodging and migration of solids in place in the formation. These solids can lodge and block movement of produced hydrocarbons.

Invasion is caused by the differential pressure of the hydrostatic column which is generally greater than the formation pressure, especially in low pressure or depleted zones. Invasion is also due to the openings in the rock and the ability of fluids to move through the rock, the porosity and permeability of the zone.

Because of this differential pressure, drillers have long used filtrate control mechanisms to control the movement of drilling fluids and filtrate into and through the formation openings. This mechanism involves adding particles to the drilling fluid which are then deposited onto the borehole wall while circulating and drilling. These particles are generally some combination of bentonite, starch, lignins, polymers, barite, and drilled solids. They are used to plug and seal the borehole due to particle size and shape, and some control is also due to the viscosity of the filtrate when water soluble polymers are used. Although this wall cake forms a semipermeable barrier, some filtrate moves through and into the zone.

Wall cake control, then is not complete and some filtrate water is allowed to contact the producing zone. Another disadvantage of wall cake mud is that when filtrate moves through, the solids are screened out and left on the cake. This causes the cake to become thicker and can lead to differential sticking of the drillstring.

More recent technology has seen the development of Low Shear Rate Viscosity (LSRV) fluids. LSRV is created by the addition of specialized polymers to water or brines to form a drilling fluid. These polymers have a unique ability to create extremely high viscosity at very low shear rates. These LSRV fluids have been widely used because of of their carrying capacity and solids suspension ability. They have been accepted as a way to minimize cuttings bed formation in high angle and horizontal wells, and as a way to reduce barite sag in high weight muds.

Recent studies and field experience indicate that his LSRV is helpful in controlling the invasion of drilling fluids and filtrate by creating a high resistance to movement into the formation openings. Since the fluid moves at a very slow rate, viscosity becomes very high, and the resulting resistance to flow allows the drilling fluid to be contained within the borehole with a very slight penetration. This has been beneficial in protecting the zones from damage as well as reducing differential sticking in these fluids.

Lost circulation is also a severe problem in rotary drilling. Lost circulation occurs when the differential pressure of the hydrostatic column is much greater than formation pressure, the openings in the rock are able to accept and store drilling fluid so that none is returned to the surface for recirculation. The fluid is lost downhole and becomes an expensive and dangerous problem. Lost circulation can lead to hole instability, stuck drill pipe, and loss of well control. At the least, it halts drilling operations and requires expensive replacement volume to be used.

In addition to the fluid volume being lost, expensive lost circulation materials (LCM) are required. These are usually fibrous, granular, or flake materials such as cane fibers, wood fibers, cottonseed hulls, nut hulls, mica, cellophane, and many other materials. These LCM materials are added to the fluid system so that they may begin to build and seal. These LCM materials themselves are damaging to the zones, and because they many times must be carried in the drilling fluid to maintain circulation, solids removal is halted and high solids mud results.

A new technique of aiding in severe lost circulation control involves introducing a blend of surfactants to a fluid system which has the ability to form bubbles. These bubbles provide a means of reduced density, thereby lowering the differential pressure at the loss zone. With sufficient bridging materials, the density reduction is able to restore circulation even in extremely severe loss situations.

Recent experience with the combination of the LSRV polymers and the density reduction surfactants has led to the development of a fluid and application technique which controls formation invasion and serves as a lost circulation preventative system. The LSRV polymers have a beneficial effect on the surfactant so that the bubble wall is made tougher, it is more effective in reducing density, and actually provides strength to a "bubble bridge" which forms downhole as the bubbles expand to fill the openings exposed when drilling. The LSRV polymers also provide a resistance to movement into the zone so that losses are prevented when the combination of polymers and surfactants are in use as the zone is being drilled. In this way, lost circulation is prevented when used while drilling a loss zone.

The invasion control concept works the same way. Differential pressure is reduced, a downhole bridge to the openings is created, and resistance to movement into the opening is reinforced. Any fluid which moves into the zone is clean, and essentially solids-free so that the damage is significantly less than another fluid which might be used. No solids or particles are involved in this method of treating so solids removal systems can remain in use and the fluid can be kept as clean as possible with any low solids system. Since there are no clays involved, the system is maintained as a flocculated system, and flocculants can be used to enhance solids removal even when drilling through potential lost circulation zones.