Drilling advances

	Vol. 229 No.12
LES SKINNER, PE, CONTRIBUTING EDITOR, LSKINNER@SBCGLOBAL.NET

New drilling muds

Recently, new drilling mud formulations have been popping like mushrooms in a cow pasture. These systems have been developed to improve hole cleaning, provide lubrication, reduce contamination, increase rheology control and reduce formation invasion-all problems of many former systems, especially water-based muds.

In the past, prior to synthetic oil-based systems, we were obliged to use water-based muds with monovalent or divalent salts such as potassium chloride to “pickle” shales. This was only partially successful in preventing sloughing, produced by shale hydration, which caused many junked hole segments. Crude and diesel oil-based muds followed, but discharging them into the environment was not only damaging, it was illegal in most places. Why create an intentional oil spill by discharging whole mud or oil-soaked cuttings?

Vegetable oils in drilling fluids came next followed by synthetic oils; both were expensive. Discharging a plant-based oil into the environment didn’t seem quite as bad as discharging a refined product. But, why have an oil sheen around a platform? Also, why would anyone want to dump such an expensive fluid?

Water-based fluids are coming back with new additives for better inhibition without the problems of oil-based fluids. Some new systems use derivatives of old molecules. For example, instead of adding glycol, one can now add glycerol to a water-based mud along with other “sister” molecules without glycol’s toxicity. The trick is getting the right combination of shale-inhibiting chemicals for the well’s depth, BHT, deviation angle, mud density and other variables, based on the shale’s nature.

Glycol derivatives are generally soluble in water-based muds below the cloud point. However, at elevated temperatures, they separate into two fluids allowing the glycol-like fluids to work on the shales independently. The glycol derivatives must be blended to the conditions of the well along with salts, stabilizers and inhibitors such as polymers and amines. Now, we have water-based mud systems that approach the shale-inhibiting qualities of some of the early oil-based muds.

Or, at least we think we do.

The problem is that they may work on one shale, but not on another. The additives aren’t cheap and most drillers are reluctant to over-treat the mud. Then, some wit decides to pump a sweep, thinking that the chemicals’ inhibiting properties can somehow be replaced by viscosity. The sweep wets the shale and all the benefits of inhibition are negated. The mud system is then labeled a failure even though a lack of mud chemistry knowledge is usually to blame. I saw this occur recently.

As a friend of mine once said, “There is a cure for ignorance called education, but stupidity is forever.”

One of the neatest things to come along is the use of film-forming agents in water-based systems to reduce invasion. In the past, reducing filtrate invasion was attempted by building a tough, impenetrable filter cake using bridging materials and water-loss chemicals. The bit would drill and a certain amount of filtrate would be pushed into the formation leaving the solids as a filter cake on the rock’s surface. Then, each time the bit, reamer, stabilizer or drillpipe connection was pulled across that hole section, more filtrate would be lost as a new filter cake formed. This process could occur numerous times. Then, depending on the depth of filtrate invasion, the logs could show nothing but residual oil saturation in some zones. Upshot: another dry hole.

Another problem involves clay destabilization in porous sandstones and, to a lesser extent carbonates, when using water-based systems. The clays, being finely divided and dispersed in the rock matrix, get wet, turn loose from their moorings and flow back along with formation fluid, plugging pore throats along the way. We call that “skin.” This can be prevented by using more expensive oil-base muds or by fracturing through the damaged zone and propping the frac open. In some cases, nothing can be done to reverse the damage.

Now with film-forming agents, filtrate invasion is limited to the first few millimeters of the sand face. Logs now see original oil saturation, and gas zones are protected from clay destabilization. Cleanup time and cost are reduced and initial production rates are high. Formations that were once considered “too tight to produce” are flowing nicely with minimal stimulation.

One application of film-forming agents involves rotary sidewall coring. These cores are bored perpendicular to the well’s centerline. In the past, getting a small-diameter rotary sidewall core out of the hole with anything but residual saturations was a miracle-all of the oil and most of the gas were displaced by filtrate. Now film forming agents in water-based muds, along with fine bridging materials, protect the core from flushing. Finally we’re beginning to see native saturations along with undamaged permeability. It’s so much easier to analyze a reservoir when the drillers can provide that kind of data to the petrophysicists and reservoir engineers!

Which brings me back to a previously stated position. I often notice that we have developed the insane notion that putting a hole in the ground is an end in itself. We don’t grasp full-cycle well life. The objective of any hydrocarbon well is putting grease or gas through a sales meter.

It’s amazing that we can get good results in both shale stabilization and film-forming additives in water-based mud systems without the expense or risk of using oil-based muds. We have come full circle: from water, to simple gel systems, to water-based muds, to oil-based muds, to synthetic oil-base muds and now back to water-based systems. Of course, as with all other parts of the drilling package, each of these has its own niche.

It makes you wonder what the mud men and chemists will come up with next. Film-forming polymers, maybe? That would mean we could just mix one additive in a pit full of water and make hole with no fear. Just the thought makes me want to get a morning tour derrickman’s job (almost).