I have stated that drilling is 80 percent knowledge and 20 percent luck. Bad luck happens when a driller encounters a loss zone, thus preventing the driller from finishing the hole. In these situations, it is up to the driller to create luck. Loss zones need to be dealt with quickly and efficiently. The following steps help prevent losing the borehole when drilling into a loss zone.
1. Sample Logs, Geology and Experience
As a young mud engineer, I was invited by Ortman Drilling to help drill a geothermal test hole in downtown Indianapolis, at a site that would become a nature center. After evaluating the sample logs for the area, we expected to lose circulation at a depth of 88 feet and again at 105 feet. We removed the “luck factor” by having basic geologic knowledge of the area. I worked with this very same talented drill team at Ball State, and we encountered many lost circulation zones together. These guys were lost circulation experts for central Indiana. The team prepared to lose circulation once they drilled to 80 feet in the ground. At 88 feet, the drill bit encountered fractured limestone that immediately caused partial fluid loss. The experienced drill team agreed that understanding the loss zone, along with preparation, would be the key to drilling past the zone and completing the borehole.
2. Hole Volume and Required Drilling Volume
First, the drill team prepared 2.5 times the total borehole fluid volume. The hole design:
- 6.25-inch diameter hole to a total depth of 400 feet
- 6.25-inch diameter bit uses a borehole fluid volume of 1,593 gallons per foot
- 400 feet x 1.593 gallons per foot = 637.23 total gallons
- 637.23 x 2.5 = 1,593 total gallons
Note that you can take the drill rod diameter out and reduce the fluid volume by 300 gallons, but it is a good practice to build in a safety factor.
Using 2.5 times the fluid volume allows the operator to explore into the loss zone before losing all of the fluid on the surface. When drilling into a fractured zone for the first time, it is important to note how much fluid is lost and at what depth it is lost. The idea is to drill beyond the loss zone back into competent formation. The team could have started with a 300-gallon mud pan or a 600-gallon small mud managing system, but we expected the loss zone to take more fluid than either of the smaller options could provide. There is a fine line between stopping a partial loss and creating a huge loss circulation area. The goal is to shut the partial loss down as quickly as possible. That is hard to do with only 300 to 600 gallons of drilling fluid. There has to be enough volume to create a patch and have enough volume to put pressure against the new plug.
3. Visualize the Loss Zone
The good thing about having enough fluid volume to drill into the loss is that you have the chance of drilling beyond the zone back into a competent formation. You have time to learn about the loss or fracture zone. The bad thing about having enough fluid volume to drill into the loss formation is that you can do more damage if you do not stop the loss. If you drill into a fractured loss zone and immediately lose 50 percent of your fluid, it is time to stop drilling, raise the rods off the bottom, shut off the mud pump and prepare for WAR. Yes, there is a possibility that you could open the valve to the water truck or hydrant, mix some mud while drilling and get through the zone. However, in reality, every gallon of fluid that goes into a loss zone opens up a bigger loss zone. The drilling fluid is washing out any matrix that Mother Nature had deposited over the last 10,000 years. To get past the partial loss zone, we need that delicate matrix to stay intact.
We plan our attack by selecting the right drilling fluid and plugging media. Drillers’ notes can help recreate the encountered loss zone; go back and review how much fluid was lost and at what depth. Consider how the bit reacted to the loss zone. Did the bit have lots of rattling or chatter? Did the rods fall several feet? The goal is to build a mental picture of the loss zone. Fractured rock drills differently than glacial till and radically different than drilling into a void. At 88 feet, if the bit encounters a zone that lost 5 to 10 percent of your fluid then it is possible the loss zone is small fractures. If that 88 feet formation takes 80 percent of your fluid, and the rate of penetration met no resistance for 5 feet, then you have a huge void. Start to create a picture of the loss zone. What does it look like? Downhole cameras and logging tools are great ways to see what is going on downhole. However, realize that it is very hard to analyze downhole video, and logging tools are very expensive to lose in the hole. Your imagination works great at visualizing the downhole conditions.
4. Selecting the Right Lost Circulation Media
In Indianapolis, we hit the loss zone, pulled up off the bottom and discussed what the drill had encountered. The situation seemed simple at first. The bit started to chatter as the team penetrated the fractured limestone. The mud pit lost 10 percent of the fluid volume between 88 and 91 feet. As the team drilled to 95 feet, another 25 percent of the fluid was lost. The team had a partial loss zone but not a total loss zone. They mixed up 600 gallons of drilling fluid, ensuring that the viscosity and filtrate were within recommended parameters. If the base fluid transporting the Lost Circulation Material (LCM) to the fractured zone is of poor quality or has bad properties, then the chance of the LCM working is less.
Next, the team discussed which LCM to select. At 88 feet, we understood that the zone took little fluid, but as the team drilled into the fractured limestone, the fluid loss increased. We decided to use a combination of Baroid IDP’s N-Seal, Diamond Seal and Benseal. The team visualized a loss zone with small fractures on top and larger cracks toward the middle and bottom of the zone. To shut down the variety of loss zones encountered, the team selected products with a variety of size. Each LCM is designed to patch fracture zones differently. N-Seal is intended to build stability in the drilling fluid, creating a webbinglike matrix with the fluid over the small fractures. Diamond Seal slowly absorbs water, increasing in size over time. It can be pumped into a larger void space and swell, closing the loss zone over time. Benseal is a product used for grouting water wells. It hydrates rapidly and can fill fracture spaces. It is a great product because it can be easily pumped downhole and create a strong patch. Granular bentonite as an LCM is always a smart option. The combination of three different sizes of materials working together shut down the loss zone encountered from 88 feet to 100 feet. Beyond 100 feet, the fractures became total loss zones, and the team had to evaluate the entire project. The job could be drilled, but estimating cost and time was impossible.
Other Factors to Combating Loss Circulation
Sometimes the best way to combat loss zones is to assess the project; question the drilling method and techniques used to drill the hole. Many loss zones are drilling induced or, better yet, driller induced. Poor drilling fluid properties, such as excessive mud weights or high viscosity, can cause loss zones. Couple poor drilling fluids with bad drilling techniques, and loss zones are inevitable. Solids control can help maintain proper drilling fluid parameters. Changing drilling methods can help. For example, drilling with foam can be a better option in fragile zones. Think of the downhole conditions like your first woodworking project. Your instructor told you to predrill the hinge holes for the cherry music box you were making. It seemed like one extra step that was not necessary, so you drove the screw into the wood and split it. You had broken the fracture gradient of your Mother’s Day gift. You could patch it with putty, but it would never have been as strong as it was before the wood split. Downhole conditions are the same way; sometimes moving is a better option than continuing the fight.
It is always important to be environmentally responsible and have the groundwater’s best interests in mind when drilling. The project in Indianapolis was downtown, and we did not have to worry about water wells in the area. Always ask, where did my drilling fluid go? The fluid took the path of least resistance. That path could be impacting a water source or water well. When drilling water wells use NSFapproved loss circulation material and products that can be produced back out of the formation. Your loss zone could be another person’s water source.