Earth boring – well treating – and oil field chemistry – Earth boring – Contains enzyme or living microorganism
Reexamination Certificate
2000-11-13
2004-11-16
Tucker, Philip C. (Department: 1712)
Earth boring, well treating, and oil field chemistry
Earth boring
Contains enzyme or living microorganism
C507S201000, C507S100000, C507S200000, C507S902000, C507S921000, C507S922000, C166S300000, C166S301000, C166S305100, C166S308200, C166S308300, C166S308400
Reexamination Certificate
active
06818594
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to compositions and methods used for hydrocarbon exploitation such as in the drilling of and production from wells, especially oil and gas wells. More particularly, the invention relates to such compositions and methods which alter the physical or chemical properties of a polymeric component of an oil field fluid or residue, such as decomposing a polymeric viscosifier or fluid loss control agent contained in such fluid or residue in response to a defined chemical or physical signal.
2. Description of Related Art
The selection of materials for well construction is essential to the successful completion of an oil or gas well. Among the most important is the selection of a drilling fluid. A drilling fluid having the desired properties is passed down through the drill pipe, out a nozzle at the drill bit, and returned to the surface through an annular portion of the well bore. The drilling fluid primarily functions to remove cuttings from the bore hole; lubricate, cool and clean the drill bit; reduce friction between the drilling string and the sides of the bore hole; maintain stability of the bore hole; prevent the inflow of fluids from permeable rock formations; and provide information on downhole conditions. The composition of a drilling fluid is carefully selected to optimize production within the vast diversity of geological formations and environmental conditions encountered in oil and gas recovery. At the same time, the fluid should not present a risk to personnel, drilling equipment, or the environment.
Drilling fluids may be water, oil, synthetic, or gas based. The composition is typically tailor-made to specific drilling conditions, varying in size and distribution of suspended particles, density, temperature, pH, pressure, salt concentration, alkalinity, electrical conductivity, lubricity, and corrosivity, all of which may be influenced by the surrounding geological formations. Further explanation of the properties of fluids useful in the recovery of oil and gas may be obtained from a review of the publication, H. C. H. D
ARLEY
& G
EORGE
R. G
RAY
, C
OMPOSITION
AND P
ROPERTIES
OF D
RILLING
AND C
OMPLETION
F
LUIDS
1-37 (5
th
ed. 1988); and C
HILINGARIAN, ET AL
., D
RILLING
AND D
RILLING
F
LUIDS
, D
EVELOPMENTS IN
P
ETROLEUM S
CIENCE
11 (1981).
Water-based drilling fluids, or muds, may consist of polymers, biopolymers, clays and organic colloids added to an aqueous based fluid to obtain the required viscous and filtration properties. Heavy minerals, such as barite or calcium carbonate, may be added to increase density. Solids from the formation are incorporated into the mud and often become dispersed in the mud as a consequence of drilling. Further, drilling muds may contain one or more natural and/or synthetic polymeric additives, including polymeric additives that increase the rheological properties (e.g., plastic viscosity, yield point value, gel strength) of the drilling mud, and polymeric thinners and flocculents.
Polymeric additives included in the drilling fluid may act as fluid loss control agents. Fluid loss control agents, such as starch, prevent the loss of fluid to the surrounding formation by reducing the permeability of filter cakes formed on the newly exposed rock surface. In addition, polymeric additives are employed to impart sufficient carrying capacity and thixotropy to the mud to enable the mud to transport the cuttings up to the surface and to prevent the cuttings from settling out of the mud when circulation is interrupted.
Most of the polymeric additives employed in drilling mud are resistant to biodegration, extending the utility of the additives for the useful life of the mud. Specific examples of biodegradation resistant polymeric additives employed include biopolymers, such as xanthans (xanthan gum) and scleroglucan; various acrylic based polymers, such as polyacrylamides and other acrylamide based polymers; and cellulose derivatives, such as dialkylcarboxymethylcellulose, hydroxyethylcellulose and the sodium salt of carboxy-methylcellulose, chemically modified starches, guar gum, phosphomannans, scleroglucans, glucans, and dextrane. See U.S. Pat. No. 5,165,477, which is incorporated herein by reference.
Most drilling fluids are designed to form a thin, low-permeability filter cake to seal permeable formations penetrated by the bit. This is essential to prevent both the loss of fluids to the formation and the influx of fluids that may be present in the formation. Filter cakes often comprise bridging particles, cuttings created by the drilling process, polymeric additives, and precipitates.
For a filter cake to form, it is important that the mud contain bridging particles, particles of a size selected to seal the pore openings in the formation. While finer particles may be carried deeper into a formation, bridging particles are trapped in the surface pores, and form the foundation for the filter cake. The bridged zone in the surface pores begins to trap successively smaller particles, and fluids interchange until an essentially impenetrable barrier is formed.
The formation of a filter cake seal is fostered by an imbalance of pressure of the mud column over the pressure exerted by fluids within the formation. It is recommended that drilling fluid pressure exceed the pressure exerted by fluids in the pores of the formation by about 200 psi. Pore pressure depends on the depth of the formation, the density of the pore fluids, and geological conditions. Similarly, the outward pressure exerted by the drilling fluid is a function of the density of the drilling fluid and the depth of the formation in question.
Since the outward pressure of the mud column is usually greater than the pressure exerted by the pore formation, it is also a primary function of the filter cake to prevent drilling fluid from continuously permeating into formations surrounding the well bore. The permeability of the filter cake is dependent upon particle distribution and size, in addition to electrochemical conditions of the mud. The composition of the drilling fluid can be adjusted to increase or decrease permeability, for example, by adding soluble salts, or increasing the number of particles in the colloidal size range. Fluid from the mud which permeates the barrier is known as filtrate. The probability of successful completion of a well may depend, in large part, upon the filtration properties of the mud being matched to the geological formations, and the composition of the filtrate. For further explanation of the properties and formation of filter cakes, see H. C. H. Darley and George R. Gray, C
OMPOSITION AND
P
ROPERTIES OF
D
RILLING AND
C
OMPLETION
F
LUIDS
, (5
th
ed., 1988).
Although filter cake formation is essential to drilling operations, the filter cake can be a significant impediment to the production of hydrocarbon or other fluids from the well. Damage to producing formations can occur by directly plugging the surface of the rock, M. J. Economides, et al., P
ETROLEUM
W
ELL
C
ONSTRUCTION
, John Wiley and Sons, N.Y., 1988, p.121, or indirectly by plugging the hardware placed in the well. Ladva, H. K. J., et al., “Mechanisms of Sand Control Screen Plugging From Drill-In Fluids and its Cleanup Using Acid, Oxidizers and Enzyme Breakers,” SPE 39439 (Feb. 18, 1998). Removal of the blockage presented by the filter cake may be essential to the commercial viability of the well. Many methods are used to remove filter cake damage, including concentrated acids, strong oxidizers, chelating agents and enzymes. Because enzymes are highly specific, they do not react or degrade the materials commonly found within a subterranean formation or used in well bore operations, such as limestone, iron, resin coated proppants, tubings and the like. This makes enzymes an excellent candidate to destroy the filter cake without harming the completion hardware or personnel.
As disclosed by U.S. Pat. No. 5,247,995 (“the '995 patent”), incorporated herein by reference, the permeability
Ballard David A.
Freeman Michael A.
Jiang Ping
Mistry Kishor Kumar
Norman Monica
Conely Rose LLP
M-I L.L.C.
Tucker Philip C.
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