1. Field of the Invention
The present invention relates to methods of forming a chemical casing in a well bore penetrating a weak unconsolidated zone or formation.
2. Description of the Prior Art
Rotary drilling methods are commonly utilized in the drilling of oil and gas wells. That is, the well bore which extends from the surface into one or more subterranean oil and/or gas producing formations is drilled by a rotary drilling rig on the surface which rotates a drill bit attached to a string of drill pipe. The drill bit includes rotatable cutting surfaces so that when the drill bit is rotated by the drill string against subterranean strata under pressure a bore hole is produced.
A drilling fluid is circulated downwardly through the drill string, through the drill bit and upwardly in the annulus between the walls of the well bore and the drill string. The drilling fluid functions to maintain hydrostatic pressure on formations penetrated by the well bore and to remove cuttings from the well bore. As the drilling fluid is circulated, a filter cake of solids from the drilling fluid forms on the walls of the well bore. The filter cake build-up is a result of initial fluid loss into permeable formations and zones penetrated by the well bore. The presence of the filter cake reduces additional fluid loss as the well is drilled.
In addition to removing cuttings from the well bore and forming filter cake on the well bore, the drilling fluid cools and lubricates the drill bit and exerts a hydrostatic pressure against the well bore walls to prevent blow-outs, i.e., to prevent pressurized formation fluids from flowing into the well bore when formations containing the pressurized fluids are penetrated. The hydrostatic pressure created by the drilling fluid in the well bore may fracture low mechanical strength formations penetrated by the well bore which allows drilling fluid to be lost into the formations. When this occurs, the drilling of the well bore must be stopped and remedial steps taken to seal the fractures which are time consuming and expensive.
In order to insure that fracturing of low mechanical strength formations penetrated by the well bore and other similar problems do not occur, it has heretofore been the practice to intermittently seal the well bore by cementing pipe referred to in the art as casing or liners in the well bore. The points in the well bore during its drilling at which the drilling is stopped and casing or liners are installed in the well bore are commonly referred to as “casing points”. Casing or a liner is placed in the well bore above each casing point and a sealing composition such as a hydraulic cement composition is pumped into the annular space between the walls of the well bore and the exterior surface of the casing or liner disposed therein. The hydraulic cement composition is permitted to set in the annulus thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented. This technique of cementing pipe in the well bore as the drilling progresses has a number of disadvantages including the time and expense incurred in placing and sealing the pipe as well as the reduction in the well diameter after each casing point. That is, the well diameter must be reduced below each casing point so that a smaller casing can be lowered through the previously placed casing and sealed in the well bore.
Another problem that occurs in the drilling and completion of well bores is that when the well bore is drilled into and through unconsolidated weak zones or formations formed of clays, shales, sand stone and the like, unconsolidated clay, shale and sand slough off the sides of the well bore which enlarges the well bore and often causes the drill bit and drill pipe to become stuck whereby drilling must be stopped and remedial steps taken.
Thus, there are needs for improved methods of drilling well bores whereby unconsolidated weak zones or formations are consolidated and the mechanical strength of the well bore is increased during drilling without the need to stop drilling for prolonged periods of time.