Drilling advances

	Vol. 228 No. 9
LES SKINNER, PE, CONTRIBUTING EDITOR, LSKINNER@SBCGLOBAL.NET

Cement-the ubiquitous drilling material. Drilling, like most other industrial operations, is actually a collection of technologies, trades and people. While it may seem somewhat distant to consider cement as an integral part of drilling, just try drilling a well without it.

In ancient times, the Assyrians, Babylonians and Canaanites used various clay mixtures to bind sun-dried mud bricks to form structures with limited cohesiveness. On a visit to the historical site of Jericho, an oasis in Israel’s West Bank, our guide explained that archaeologists had determined that the mortar that held the fables walls was actually clay mortar dug from a nearby hill. That could have contributed to the walls’ catastrophic failure.

Later, the Egyptians added lime and gypsum to the mortar mix as binding agents. That led to building larger, more substantial structures, including the pyramids. They are still held together by this early grout.

The Romans finally got it all together. They mixed slaked lime with volcanic ash (pozzolana) to form a cement mixture that would harden under water. This permitted construction of extensive jetties, docks and wharfs at seaports throughout the Roman Empire, some of which still exist today.

During the Dark Ages, the chemistry of this hydraulic cement was lost, since just about all science, especially chemistry, was considered one of the “dark” arts. Many misguided experimenters, dabbling in chemistry, eventually experienced the chemical process of combustion as an active participant. Oddly, heating is precisely what makes modern cement possible.

Archaeologists also tell us that many of antiquity’s cities are built on the ruins of previous cities. That’s not really surprising. Many cities were constructed of limestone, a soft rock that was easy to shape using bronze or iron tools. Further, it was white, which reflected sunlight, making the buildings cooler. Wooden structures were used to support roofs, provide doorframes and shutter windows.

Each time a city was conquered, the victors would kill the inhabitants and burn the city. When limestone and clay are burned, they form (you guessed it) hydraulic cement! The next time it rained, a new foundation was formed, perfect for building a new city upon-makes one wonder if the Romans were really smart or just very observant.

In the mid-1700s, British engineers re-discovered this hydraulic cement recipe. Other naturally occurring minerals, calcium, silicon, iron and alumina, were added to the mixture to provide specific properties. By the time the “oil bidness” started, Portland cement, so named because its color reminded British scientists of quarried stone from the Isle of Portland off England’s coast, was a very popular component of concrete used to build everything from bridges to tunnels to sewer systems in the US, the UK and Europe.

The petroleum industry, and especially drilling, is an offshoot of the mining industry. When shafts were dug and permanent retaining walls were needed, grout would often be placed between the shoring and the shaft face for strength. If the grout was strong enough, the shoring would be removed and reused. So it’s not much of a stretch to see how early casings were grouted in place. In fact, the grout that was pumped was actually a type of drilling mud with lots of clay, limestone dust and “poz”-the same material the Romans used, pozzolana (it’s still used today). Old timers will tell stories about “mudding in” a string of casing.

Why are we still pumping cement down wellbores? Many reasons, I believe. First, cement technology has advanced considerably over the years with the perfection of a wide variety of additives that optimize the slurry’s performance. These include fluid loss additives, extenders, defoamers, materials that accelerate or extend setting times, materials that resist channeling as the cement sheath begins to gel and support its own column hydrostatic weight, materials that can reduce slurry density, weighting materials, latex, salts, gilsonite, silica flour; and the list goes on. Clearly, there was a need for all these materials and the industry responded by providing a dazzling array of cements with properties tailored to the application.

Second, cement is relatively inexpensive. Other materials might do a better job of sealing an annulus, supporting a string of pipe and preventing external corrosion, and have the same long life as cement, but none are as inexpensive as cement. Take, for example, some of the multi-part epoxies. They can seal, support, protect and be pumped, while retaining their properties for long periods. However, their cost would break the largest bank, if pumped in the volumes that cement provides each day.

Third, the industry has come to rely on cement as the bellwether product for long-term installations. How robust is cement? Well, just try to cut and pull a string of casing that has been cemented in an old wellbore for, say, 50 years. It is not coming out of the ground, at least not easily. Every regulatory agency in the world specifies that cement plugs be set in abandoned wells, regardless of what other mechanical devices are in place, such as bridge plugs and packers. If it’s meant to be permanent, chances are it needs to be cement.

Finally, cement is a very forgiving material most of the time. Like others, I have had my own experiences with cement setting up inside a string of pipe, where I didn’t want it. Most of the time the cement got there instead of where it was supposed to be as the result of human failure-often mine. Cement will usually provide some indication of an impending problem, such as increasing pump pressure, decreased flowrate or heat of hydration in returned fluid. Steps can be taken to address the problem, if one will simply “listen to the well,” as Max Hall used to say.

What’s the newest thing coming from the cement guys? Liquid cement that stays a liquid until it’s activated and pumped. No longer must we mix dry powder with water to make a slurry. This stuff is already a slurry. It just needs to be pumped down the hole. Add enough hollow microspheres to it and the resulting hardened cement will be lighter than water. That’s right folks, it will float. It will also insulate the wellbore and prevent heat losses to the formation preventing permafrost melting or hydrate dissociation. Wow!

I wonder what the residents of Jericho would say if they could see the kinds of cement materials we have today?