Drilling large diameter intake wells with cutter bits in loose and weakly consolidated rocks

Abstract

Most of the useful aquifers in Poland occur in the Quaternary and Tertiary horizons, which are deposited at a depth of tens to hundreds of meters. Owing to the need of providing large quantities of water for municipal and industrial purposes, the aquifers are more and more frequently opened with large diameter wells of various designs. Such wells are mostly drilled with the rotary method with reverse mud circulation with the use of various bits. The Quaternary and Tertiary strata abound in loose and weakly consolidated rocks therefore cutter bits are predominantly used. They allow for high rates of drilling and shorter time of drilling of the well. This significantly influences the negative influence of drilling mud on the near screen zone of the aquifer and better hydraulic properties of the well. Drillability tests were performed with cutter bits of 0.86 and 0.67 m diameter for providing high rates of drilling of large diameter intake wells in loose and weakly consolidated rocks. The research was made for measurement sections 0.5 to 1.0 m long, and each of them was drilled at con- stant rotational velocity and axial weight on bit. Prior to the drillability tests the limitation of weight on bit and rotational velocity of bit imposed by the technical characteristic of Prakla B50 rig, strength of the string and butter bit were established. Various regression models were analyzed for the sake of finding a dependence between drilling rate and axial weight on bit and rotational velocity of bit for particular macroscopically homogeneous layers. The best results were obtained for the exponent model illustrating the influence of axial weight on bit and rotational velocity of bit on the drilling rate, which has been proved by the calculated regression coefficients and statistical parameters.