Necessary Roughness

This article in the Life of the Well series covers logging, testing, and cementing. Logging and Testing help us identify the potential profitability of an oil well, and cementing the well keeps the hole open and environmentally safe.

Logging

Once (or even while) a hole is drilled, there is opportunity to find out what’s in the ground and how deep it is. Sensors will be fastened to a cable and lowered into the hole. As the sensors are pulled back up, electrical signals from the sensors to a computer on the surface record a log of what was detected—hence we call this Logging.

Two major types of logging are open-hole and cased-hole. Open-hole logs are done before the well is cemented, while cased-hole logs are recorded when the casing is already cementing in the hole. As an open-hole sensor is pulled up the hole, it records the lithology (e.g., sandstone) and what might be in the rock. Cased-hole logs are primarily used to see how well the cement is holding between the pipe and the formation.

There are different kinds of open-hole sensors and logs. The most popular and least expensive kind of log is a gamma-ray log. These give indications as to whether earth at a certain depth is shale, sand, or limestone. Gamma rays are sent out into the earth, and the different types of earth bounce back a different amount of gamma rays. Oil can be found in almost any formation (I have yet to see it in granite). Most frequently we see it in sandstone, so knowing where the sands are is good.

Rock, amazingly enough, is not 100% rock, and especially in igneous rock we see a lot of air. The space between the rock particles is the rock’s porosity. This porosity can be filled with air, water, or petroleum. Halliburton’s top-of-the-line logging tools find not only rock type but also estimate how much water is embedded in the rock. If we can detect the rock type and how much water is in the rock, we can estimate how much oil and gas there is in rock.

Logging as a business doesn’t command the profit margins as some of the other services from Halliburton. Schlumberger has built their business on it.

Well Testing

In testing a well a pressure recording device is lowered into the hole, and the blow-out preventer valves I mentioned in the first post are “shut-in”, or closed. One may then wait on location for 24-48 hours. During this time, pressure builds from that oil and gas recently freed by the drilling. After time elapses, the valves are opened, the sensor is pulled, and the data collected from the device is read. If the pressure doesn’t build up satisfactorily, there may not be enough gas worth going after. If there’s a nice buildup, the company will begin cementing the well.

Well testing is the most boring type of job I’ve ever been on, but one of the most important in decision making.

Cementing

Cementing is a make-or-break step in oil well construction. A bad cement job can ruin the water table, allow a blowout, and prevent effective treatments that increase production later.

Oil may be lighter than water, but seemingly in most areas the oil and gas are found thousands of feet below the water table. We need to get the petroleum out without harming the water zone. Cement is placed between pipe and earth and solidifies to keep the oil from going into the water and to keep the water from mixing with the liquid money.

Not all wells are like the one to the right, but if I found out my well was this way, I would not only be worried about the water table but about the gas zone. Gas is much easier to produce than oil. Gas also exerts pressure on the oil, causing it to come out of the rock and up the pipe. It would be my goal to produce the second oil zone, then the first zone, then the gas zone, to get the most hydrocarbons out of the well. By cementing all of the pipe in the hole, I can be selective in which rock I want to get the petroleum out of first.

In the pre-Halliburton days, cement was mixed by hand and poured down the sides, between the casing and the earth. Clumpy cement and imperfect drilling caused cement to come down some sides of the pipe faster than other sides, leaving open spaces where hydrocarbons could come up the hole outside of the metal pipe. Erle P. Halliburton made his money by pumping cement down the middle of the pipe, then pumping water next, so that the cement pushed around the bottom of the pipe and up, like the drilling mud in the first post. The cement, under high pressure from pumps and fighting gravity, filled the space between the hole and the pipe, making for much better separation between zones. Pump enough water, and you have only water in the middle of the pipe while all the cement is outside the pipe and hardening. Brilliant, patented, and heinously profitable.

Of course, when one set of problems is eliminated, others arise. As drilling gets deeper, the earth temperature rises, about 1 degree Fahrenheit every 100 feet. Cement hardens faster as the temperature goes up, and it could harden before it has time to go around the bottom of the pipe. We can add things to the cement now to delay setting times, from simple things like sugar to our Synthetic Cement Retarder that lets us cement with earth temperatures nearing 400 degrees. There are also other design considerations, but this is the big one.

After a good cement chemical design and a good pumping job, the cement hardens, and the cased-hole logging crews arrive on location to see how well the cement held to the pipe and earth. If the results are good, the company will move on to perforating the casing at the right depth. That’s to be described in the next post in this series.

Series Navigation

«Life of the Well 1: Location and DrillingLife of the Well 3: Perforating»