Wells – Processes – Cyclic injection then production of a single well
Reexamination Certificate
1999-11-17
2001-10-16
Suchfield, George (Department: 3672)
Wells
Processes
Cyclic injection then production of a single well
C166S053000, C166S371000
Reexamination Certificate
active
06302206
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates to a treatment for a shut-in gas well and in one aspect relates to a method for treating a gas well just after shutting in the well to prevent accumulated fresh water in the wellbore from damaging the gas producing formation.
2. Background
In producing hydrocarbon gas (i.e. natural gas) from subterranean formations, it may become necessary to temporarily “shut-in” a gas producing well from time to time during its operational life. For example, it may become necessary to shut-in a well when the demand for gas is low and the produced gas can not be sold or handled economically. In such instances, the well is shut-in for an indefinite period after which it is then reopened and production is resumed. Unfortunately, in many wells, it has been found that the production rate of gas from the reopened well is substantially less than it was before the well was shut-in.
One reason for the decrease in the resumed production from a previously shut-in well is believed to lie in the fact that natural gas (hereinafter referred to as “gas”), especially that which is produced from high temperature, subterranean formations, is normally saturated with water vapor at reservoir conditions. As the gas is produced through tubing to the surface, the temperature of the gas cools roughly in relation to the geothermal gradient which inherently exists in the wellbore. As the gas cools, the water vapor in the gas begins to condense out of the gas stream and onto the wall of the production tubing. This condensed water will be essentially free of any mineral ions, hence, it is effectively “fresh” water. Accordingly, after a sustained period of production, a substantial portion of the inner wall of the production tubing will be coated with a film of condensed, fresh water.
While the well is flowing, the dynamics of the high-velocity production stream usually cause the condensed, fresh water to adhere to the tubing wall or move upward toward the wellhead. However, when the well is shut-in and the flow of gas ceases, this film of liquid, fresh water loses the upward shear force which has been holding the water on the tubing wall. Gravity now causes the condensed water to flow downward within the tubing where it collects as a small column of fresh water in the bottom of the wellbore which, in turn, is in communication with the gas producing formation through the perforations in the well casing.
It is well documented that fresh water can be highly detrimental when placed in contact with a hydrocarbon producing formation (e.g. gas producing formation). For example, fresh water can cause severe swelling of the clays commonly found in most gas producing formations. This swelling results in closing flow paths through the formation thereby severely reducing the permeability (i.e. flow capacity) of the formation. Also, fresh water can cause other damage to the formation; i.e. it may adversely affect the relative permeabilies of the formation fluids; it may cause undesirable migration of “fines” within the formation; it may cause decementation or unconsolidation of the formation; etc. Any or all of these factors can severely reduce the flow of gas from the formation into the wellbore (hence the production rate of gas) when the well is reopened for production.
SUMMARY OF THE INVENTION
The present invention provides a method for treating a gas well which is producing a gas stream from a subterranean formation through a wellbore after the well has been shut-in in order to prevent any fresh water which may have condensed out of the gas stream from damaging the gas producing, subterranean formation during the shut-in period. This treatment allows the well to produce at substantially the same rate when it is put back on production. Basically, the present method comprises shutting-in the gas well and then injecting an additive into the well to convert any condensed, fresh water into an aqueous solution which is non-damaging to said subterranean formation.
More specifically, the present invention provides a method for treating a gas well wherein the production is stopped and an additive is injected into the well to convert any accumulated fresh water into a solution which will not damage the formation during the shut-in period. The additive may be any chemical or compound which will dissolve in or react with fresh water to alter its composition to a solution which will not damage the gas producing formation when it comes in contact therewith. For example, the additive can be selected from halide salts of alkali or alkaline earth metals; e.g. sodium chloride, potassium chloride, calcium chloride, etc., or it can be an alcohol or like solution.
The additive, e.g. salts, can be injected in solid form or in solution, e.g. brines or saline solutions. The additive can be injected through the production tubing or it can be pumped through a separate injection tubing placed within the well annulus. The additive can be injected by manually manipulating valves at the wellhead or it can be injected automatically upon shutting-in the well. In one embodiment, the well annulus is filled a solution of an additive and after the well is shut-in, is forced into the production tubing through a gas-lift valve by increasing the pressure in the well annulus.
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Miller Marvin Eugene
Wilson Scott James
Sloat Robert E.
Suchfield George
Vastar Resources Inc.
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