The only wells that are straight or follow a smooth curve are in the pictures in well plans. Real wellbores are shaped by the mechanics of directional drilling tools, the skills and attention of drillers, the force of gravity, and the path followed by hydrocarbon-rich seams of rock.
“Well path modeling commonly generates smooth curves, whereas an actual well contains severe doglegs and other irregularities,” said Robello Samuel, technology fellow, drilling, at Halliburton. A major interest for Samuel is creating measures to address a problem he raised in a 2009 paper (SPE 124710): “There are not clear criterion for defining the quality of the wellbore.”
There are several companies with methods for estimating the frictional force added by irregular wellbores, which grew out of the long-standing effort to more accurately measure the path of narrow holes drilled miles underground.
They are seeking more realistic measures of wellbore quality than those offered by established formulas, such as the minimum curvature method, which assumes the well follows a smooth curve based on the shortest possible radius from the previous measurement point.
It is an elegant formula, but years of experience and testing have shown that it poorly describes the work of rotary steerable tools or directional drilling tools using bent mud motors to build curves. In the process, the data sent back to the surface can lead to errors in critical measures, particularly in the total vertical depth. The level of uncertainty is greater when using a bent housing, but neither tool is immune to error.
Crooked wellbores may undulate with doglegs, spiraling, and washouts. In a word, it is tortuous. Rocks can test a driller’s ability to keep drilling on course. Gravity also can also alter the course of the drill bit to below the planned course....
The Search for Measures of Drilling Imperfection
Stephen Rassenfoss, JPT Emerging Technology Senior Editor
12 November 2015