Wells – Processes – Cementing – plugging or consolidating
Reexamination Certificate
2002-07-02
2004-05-11
Bagnell, David (Department: 3672)
Wells
Processes
Cementing, plugging or consolidating
C166S285000
Reexamination Certificate
active
06732797
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention broadly relates to well cementing. The invention further relates to well plugging. The invention more particularly relates to a method of forming a cement plug in a well bore.
2. Description of the Prior Art and Problems Solved
The term, “primary cementing,” is employed by persons skilled in the art of well cementing to refer to the formation of a sheath of cement in the annular space between the wall of a bore hole drilled in the earth and the exterior wall of a casing positioned in the bore hole. The sheath is ordinarily formed as a part of the initial construction of a well, such as a well which produces hydrocarbons, for example, liquid petroleum and natural gas, from a subterranean earth formation. The purpose of the sheath is to stabilize the bore hole, support the casing in the bore hole and segregate subterranean formations which contain hydrocarbons from subterranean formations which contain water, particularly potable water. The sheath of cement can extend from the bottom of the casing to the surface of the earth. A method of forming the sheath is well known.
The term “casing” is employed in the previous paragraph to broadly refer to at least one and usually two or more tubular conduits of decreasing diameter which, together in a telescoping mode, extend from the surface of the earth to the bottom of the bore hole. In one typical example, a so-called surface casing having a large diameter continuously extends from the surface of the earth to a point below the deepest formation which contains potable water. A second casing, sometimes called a production casing whose outside diameter is less than the inside diameter of the surface casing, continuously extends from the surface of the,earth to a target formation, such as one which contains hydrocarbons. A sheath of cement is placed in the entire annular space between the surface casing and the bore hole and a second sheath of cement is placed in the annular space from the bottom of the production casing to a point above the target formation.
When the producing life of a well is complete, such as when recovery of hydrocarbons from the well is no longer economically sound, the well is abandoned. Abandoned wells pose a variety of hazards, one of which is the potential of undesirable fluids, such as hydrocarbons and/or salt water, which originate from subterranean formations penetrated by the bore hole, to migrate to and contaminate potable water in other subterranean formations which are also penetrated by the bore hole. To prevent such contamination, regulatory agencies in the several states require that abandoned wells be plugged, such as by placing a mass of hardened cementitious material in the well bore at least at points adjacent hydrocarbon producing formations and also at points adjacent potable water formations. Such plugs completely occupy the well bore volume adjacent the formations and function as a barrier to migrating fluids.
The current method of plugging a well broadly comprises forming a slurry of cement in water at the well head, introducing a continuous connected string of delivery pipe (sometimes called tubing), into the well bore until the bottom of the tubing attains a desired point of delivery of the slurry, pumping the slurry down the tubing to the bottom thereof and back up the exterior thereof, continuing pumping until a desired quantity of slurry has been deposited in the well bore to form a plug therein, and then withdrawing the tubing from the well bore. In the current method when the tubing is withdrawn from the well bore, the top of the slurry on the exterior of the tubing is preferably at the same level as the slurry in the interior of the tubing. This is referred to as a balanced plug. Before a second balanced plug can be placed, the cement in the preceding plug must first be permitted to set to a minimum hardened condition. Accordingly, if multiple plugs are required, then they cannot all be placed in a continuous operation due to the need to wait on cement to set.
Note use of the terms “bore hole” and “well bore.” For purposes of disclosure, the term “bore hole” is employed to describe the linear hole actually drilled in the earth. The wall of the bore hole is the earthen rock exposed by the drill. The term “well bore” is employed to describe the containment vessel for the conduit or the intended conduit through which fluids pass between the surface of the earth and subsurface formations penetrated by the bore hole. It is common to install a continuous string of casing in the interior of the bore hole. The volume of the interior of the casing is the well bore. The volume between the wall of the bore hole and the exterior surface of the casing is referred to as the annular space. Thus, primary cementing involves placing cement in the annular space and well plugging involves placing cement in the well bore. In the absence of a casing it is clear that there is no annular space and there is no distinction between bore hole and well bore.
Persons skilled in the art know that considerable surface equipment is required to perform the current method of well plugging. Such equipment comprises a derrick to suspend tubing in the hole, transports for delivering to and storing dry cement and water at the well head, equipment at the well head for blending and mixing the cement and water to form the slurry and a high volume/high pressure pump at the well head to pressure the slurry down the suspended tubing and back up the exterior thereof to a predetermined destination.
The current method is employed to produce cement plugs in wells regardless of depth, and is particularly useful to form plugs in wells whose internal pressures are sufficiently high to cause fluids to naturally flow to the surface of the earth. Such wells require the use of methods and equipment which function to control such pressures and to prevent the flow of fluids from the well while the cementing operation is proceeding.
A need thus exists for a method of forming cement plugs in wells whose internal pressures are not sufficiently high to cause formation fluids to flow to the surface of the earth.
THE INVENTION
Summary of the Invention
By this invention there is provided a method of well cementing which comprises forming a plug of cement in the well bore. According to the method of the invention, a suitable bore hole is first selected. Upon selection of a suitable bore hole, a liquid comprising a cementitious slurry is introduced into the well bore at the surface of the earth. The liquid is permitted to descend in the well bore by gravity, form a column of slurry to a desired point in the well bore and then permitted to harden therein to form a plug. The bulk density of the introduced liquid is selected so that it, when multiplied by the distance from the surface of the earth to a designated location in the well bore, produces a pressure which is in excess of the natural pressure at the face of any subsurface formation actually contacted by the liquid. The method of this invention thus depends upon hydrostatic pressure generated by introduced liquid and not on mechanical pressure generated by a surface pump.
A suitable bore hole is one which penetrates at least one subsurface formation which produces a well fluid other than fresh water, wherein the natural pressure of the formation is not great enough to cause the well fluid to flow from the formation through the bore hole to the surface of the earth. The formation must possess sufficient permeability and porosity to permit the well fluid to be injected into it within an acceptable period of time by pressure induced at the formation face by hydrostatic pressure in the well bore and the formation must also possess sufficient structural strength to avoid being fractured by such induced pressure.
The method of this invention features positioning all plugs required within the entire well bore in one continuous operation without stopping to wait for a preceding plug to set. Thus, the reference in the precedin
Sabins Freddie L.
Watters Larry T.
Bagnell David
Bomar S. Thomas
Weaver Thomas R.
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