Centralizer Increases Production Rates, Extends Life Of Other Tools

HDD Best Practices
By Jeff Griffin, Senior Editor | June 2009 Vol. 64 No. 6

"Stabilizing the tool is not as critical in the gumbo we encounter in Louisiana, but when we're in rock, we use a stabilizer for backreaming every time we pass through the hole."

Crumpton said centralizers generally are available in sizes ranging from 9 3/8 to 60 inches. He said that some manufacturers use oil field designed centralizers.

"Some HDD designs," he continued, "have three or four legs to support the outer centralizer ring, usually containing jet nozzles to keep the tool clean of debris that sometimes can clog the area between the ring and the body. Bladed centralizers can work in hard rock, but not soft soils. Generally, most centralizers are similar in design, some with jets, some with a larger diameter ring thickness, others using inserts or a combination of both."

Tilt-a-whirl

Crumpton compares a hole opener's performance without a centralizer to "tilt a whirl" rides once popular at fairs and carnivals.

"The hole opener will bounce from one side to the other side of the bore hole," he said. "This causes the cutters to always be skidding, making them become drag cutters.

“With the hole opener being slammed into the sides of the bore, it will always keep the cutters from being evenly and properly loaded. Due to the weight of the drill string, the hole opener will always be tilted away from the centerline of the bore causing a wobble as the tool is rotated. This wobble effect will not allow the weight to be transferred from the tooth insert to the rock efficiently. The rock will not be fractured as it should be. The effect of not centralizing includes premature and extreme body wear, shortened bearing life, broken inserts and poor bore hole geometry. The rate of penetration will not be as good as it could be. The efficiency of production will be lower."

By centralizing the reamer body, Crumpton said the reamer stays more stationary while rotating and will help align the reamer with the face it is cutting by reducing the weight of drill string which can pull down. The wobble effect also is reduced.

Another benefit of centralization is reducing the chance of "tear drop" holes, that can occur when a series of passes are made that cut lower in the formation because the tool has no centralizing support.

Crumpton equates effective use of centralizers to an art.