Wells – Processes – Cementing – plugging or consolidating
Reexamination Certificate
2001-09-26
2004-11-23
Kreck, John (Department: 3673)
Wells
Processes
Cementing, plugging or consolidating
C166S288000
Reexamination Certificate
active
06820692
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to filling or plugging wells. In particular, the present invention relates to improved materials for plugging wells such as drill holes within the earth and for maintaining the plug integrity indefinitely, methods for preparing the materials and methods for using the materials.
2. Description of the Related Art
It has been well known to provide deep (on the order of several hundred feet to thousands of feet) and shallow (on the order of twenty to several hundred feet) wells such as drill holes within the earth for a variety of purposes. Relatively shallow drill holes are formed during seismic exploration, water wells, monitoring wells, cathodic protection wells and mineral exploration and extraction wells and other purposes. Deeper holes are typically formed during standard oil, gas, and/or disposal well operations. A deep drill hole is formed and then lined with a casing. The drill hole generally passes through several compositions, such as hard compacted soil, clay, loose sand, and other typical geologic materials, in addition to one or more water-bearing layers. Such water-bearing layers may represent a saline water source or a fresh water aquifer. Once the well bore is determined to be unusable, the well bore is abandoned. If left unsealed, gases and/or liquids escape from the zones
of origination and migrate through the hole. Further, the casings corrode and disintegrate causing additional migration pathways.
In particular, a fresh water aquifer may “leak” through the casing and hole into a fracture or uncharged zone, causing loss of water from the aquifer. A drill hole extending between a saline water source or petroleum and a fresh water aquifer may allow commingling of these zones, damaging both. Additionally, contamination from the surface may cause damage, such as contaminated water passing downward through the hole and casing into a fresh water aquifer. These problems can also occur with shallow holes.
To overcome these problems it has been known to plug the casings and drill holes with cement. However, cement has proven less than effective in maintaining the integrity of the seal throughout the casing over long periods of time. One problem with cement plugs is that voids can occur during placement of the cement slurry in the casing as a result of incomplete displacement of drilling fluids by the slurry. In addition, cured cement is brittle and can crack over the life of the plug due to pressure changes or due to earthquake activity. Cured cement undergoes strength retrogression at temperatures above 230° F. if the cement does not contain additional silica. All of these factors can contribute to limited success with cement plugs.
In the past, sodium bentonite has been proposed for filling drill holes. Early work focused on the use of finely ground bentonite for filling relatively shallow holes. A report entitled “Axial Shear Strength Testing of Bentonite Water Well Annulus Seals” by Fred Lee Ogden and James F. Ruff published by Colorado State University, 1989, discusses the use of bentonite as an annulus sealant. Past usage of bentonite is explained in a report entitled “Experiments in Subsurface Applications of Bentonite in Montana” by John Wheaton, Steve Regele, Bob Bohman, Dave Clark and Jon Reiten, published by Montana Bureau of Mines and Geology, 1994. Both of the foregoing reports are incorporated herein by reference.
When bentonite as ⅜″ in diameter or smaller chips is poured into a hole it begins to expand when exposed to water. This method is adequate for shallow holes since the bentonite sinks to the bottom of the hole before a significant amount of swelling occurs. However, if bentonite is poured into deep well holes, the hole may contain several hundred feet of water.
In general, high-grade and low-grade bentonite chips fall through water at an average velocity of about 1 ft/sec. Smaller bentonite granules of ⅜″ in diameter or less fall more slowly than larger particles for two reasons. First, smaller particles have more surface area per unit weight, and therefore proportionally more drag in the water. Also smaller bentonite granules are typically less dense than larger chips. A bentonite granule with a diameter of ⅜″ may have a volume of 0.5 cm
3
and weigh 1.01 grams, while a bentonite chip with a ¾″ diameter weighs 3.65 grams and has a volume of 1.50 cm
3
In this example, the smaller granule has a density of 2.02 gr/cm
3
and the larger chip has a density of 2.43 gr/cm
3
.
Once hydration begins, the density (or specific gravity) of the granule decreases as the granule swells. Similarly, the fall velocity of the granule in water decreases at a rate of about 0.009 ft/sec per minute of fall. For instance, a small granule having an initial fall velocity in water of just under 1 ft/sec, after 44 minutes of exposure to water, will fall at a rate of approximately 0.6 ft/sec. As the granule absorbs water, its density decreases approaching the density of water further slowing the fall velocity. These factors prevent small granules from effectively being used to plug deep holes with several hundred feet of water therein.
U.S. Pat. No. 5,611,400 of James et al. describes the use of coarse dry dehydrated ground chips of sodium bentonite as a well plugging material. The chips are from ¼″ to about 2″ in size. U.S. Pat. No. 5,810,085 of James et al. describes the use of large pieces of bentonite having a minimum diameter of at least ⅞″ and up to at least about 3 inches as a well hole-plugging material.
These patents describe relationships between particle size and particle performance during well plugging. They point out that each particle expands at a rate proportional to the liquid content of the particle. The rate of hydration of a given particle is related to the surface area of the particle and the volume of the particle. However, the volume and the surface area of a particle vary with respect to the particle diameter. The ratio of particle surface area to particle volume decrease as the particle diameter increases. Accordingly, the rate of hydration decreases (as does the rate of expansion) with increased particle diameter.
As noted above, it is desirable that the particles have a diameter of at least ⅞″. A particle with a diameter of less than ⅞″ hydrates and may expand too rapidly to allow the particle to reach the bottom of a deep hole before plugging the hole. By way of example, fine bentonite particles with a ⅜″ diameter may hydrate and swell to 10 times their original size and turn to a slurry state in less than 15 minutes. Often drill holes are several hundred feet deep with over a hundred feet of liquid. Each particle falls at a rate dependent upon the particle's density and the liquid's viscosity. However, generally the density of the bentonite and the viscosity of the liquids within the holes are such that a particle having ⅜″ diameter falls at a rate of one foot per second. As the particle swells, its density decreases and its surface area increases, further reducing its fall velocity. Such particles require several minutes to reach the hole's bottom. Accordingly, ⅜″ particles may swell and plug the hole before reaching the bottom or turn to a slurry state.
Another factor which plays a part in the use of bentonite as a material for plugging wells, drill holes and the like is salinity. Saline water is found in many wells. High salt contents in saline water can interfere with bentonite particles by promoting breakdown and flaking which could reduce the density of the plug when hydrated. A preferred form of bentonite would minimize these problems.
Large-particle bentonite materials have been formed heretofore as chunks and as extrudates. Chunks are accompanied by a large quantity of fines which must be removed. Chunks are irregular and often lead to bridging. Extrudates are difficult and slow to
Hejl Kieth A.
James Maurice L.
Burns Doane Swecker & Mathis L.L.P.
Chevron U.S.A. Inc.
Kreck John
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