Year of fee payment : 4. Year of fee payment : 8. Year of fee payment : There is provided a method of drilling an oil or gas well, comprising drilling a wellbore into an underground formation in which a drill bit at the lower end of a drill pipe is placed at the bottom of the wellbore; pumping a first drilling fluid into the wellbore; determining the producing zone of the underground formation; and introducing a second drilling fluid into the wellbore through the drill pipe when the borehole is within the producing zone of the underground formation, wherein the second drilling fluid is a high density drill-in fluid, having a composition different from that of the first drilling fluid and including a brine system with at least one dissolved monovalent salt and at least one dissolved divalent salt together with a suspension polymer, said drilling fluid having a density of at least about 1.
Field of Invention. The present invention relates broadly to treatment of wells and wellbores. In a preferred aspect, the invention relates to oil well drilling operations, particularly drill-in operations, and more particularly to novel drill-in fluids and methods of drilling involving the use of at least two different drilling fluids, one of which is a novel drill-in fluid. Drilling operations typically involve mounting a drill bit on the lower end of a drill pipe or "drill stem," which may then be rotated against the bottom of a hole to penetrate a formation and create a borehole.
A drilling fluid, typically a drilling mud, may be circulated down through the drill pipe, out the drill bit, and back up to the surface through the annulus between the drill pipe and the borehole wall.
The drilling fluid has a number of purposes, including cooling and lubricating the bit, carrying the cuttings from the hole to the surface, and exerting a hydrostatic pressure against the borehole wall to prevent the flow of fluids from the surrounding formation into the borehole. Another type of fluid used in oil and gas wells, distinguishable from drilling fluids, is a "completion fluid," which herein refers to any fluid that is pumped down a well after drilling operations are completed, and will refer broadly to fluids introduced during acidizing, perforating, fracturing or workover operations.
In a preferred aspect, the present invention is directed to drilling fluids. In one aspect the present invention relates to a specific type of drilling fluid, namely a "drill-in" fluid, which is pumped through the drill pipe while drilling through the "payzone," which refers to the part of the underground formation or reservoir of an oil or gas well that is believed to hold the oil or gas to be removed, i.
