Earth boring – well treating – and oil field chemistry – Earth boring – Contains organic component
Reexamination Certificate
1999-10-01
2002-03-12
Tucker, Philip (Department: 1712)
Earth boring, well treating, and oil field chemistry
Earth boring
Contains organic component
C507S145000
Reexamination Certificate
active
06355600
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to drilling, drill-in, and completion fluids, preferably water-base fluids, with good shale encapsulation properties resulting from the presence of calcium chloride and a low molecular weight, low charge cationic polyacrylamide copolymer.
BACKGROUND OF THE INVENTION
Fluids used during drilling operations include “drilling,” “drill-in,” and “completion” fluids. A “drill-in” fluid is pumped through the drill pipe while drilling through the “payzone,” or the zone believed to hold recoverable oil or gas. A “drilling fluid” is used to drill a borehole through the earth to reach the payzone. Typically a drilling mud is circulated down through the drill pipe, out the drill bit, and back up to the surface through the annulus between the drill pipe and the borehole wall. The drilling fluid has a number of purposes, including cooling and lubricating the bit, carrying the cuttings from the hole to the surface, and exerting a hydrostatic pressure against the borehole wall to prevent the flow of fluids from the surrounding formation into the borehole. A “completion fluid” is used to protect the “payzone” during the completion phase of the well.
Fluids in which water is the continuous phase provide for a fast drilling rate, and are ecologically favored over fluids in which oil is the continuous phase. Unfortunately, the walls of a wellbore frequently are composed at least in part of shale, and when exposed to water, many shales swell, slough, or spall to the extent that they may even prevent further operation of the wellbore. Shale also may slough off during gravel transport in open-hole completion, mix with the gravel, and reduce the productivity of the well by choking off the permeability of the gravel pack. The sloughing also may cause screen blinding.
Brines have long been used in the formulation of drilling fluids to take advantage of their density and their inhibitive characteristics. Clay chemistry has shown us that cationic base exchange with the negatively charged clay minerals commonly found in shale formations, limits their ability to hydrate, soften, and swell, thereby rendering them more stable in the presence of water based fluids.
Monovalent salts, such as NaCl or KCl, have long been used as make up water for drilling fluids. In the past, NaCl or KCl have provided a limited inhibitive environment for drilling hydratable shales in many areas. In offshore drilling, seawater—a complex mixture of various salts which is readily available—has frequently been used in formulating drilling mud.
Today, technological advances in the design of drilling equipment has resulted in increased penetration rates for better drilling economics. The performance of the monovalent salt systems has not been able to maintain pace with new advances in drilling technology. The need for improved drilling mud systems saw the application of oil mud systems and development of synthetic systems to meet these challenges. However, increasing environmental regulation has limited the application of these systems.
Divalent salts are known to provide an added inhibitive benefit for drilling water sensitive shales. Divalent salts are capable of developing a strong bond with and between active clay platelets in these shales, thereby further limiting the volume of hydration water that can become a part of the clay, causing it to become soft, pliable, and sticky, resulting in problems with mechanical drilling equipment and drilling fluid control. One of the most available and economical divalent salt systems is CaCl
2
. CaCl
2
systems have been around for many years, and the inhibitive characteristics of the calcium ion are well known.
Water-soluble polymers are used to thicken water-base fluids, and in part to synergistically stabilize shale. The water-soluble polymers provide the viscosity necessary to lift drilled solids from the wellbore, and tend to improve drilling rates.
Unfortunately, fluids which have shown promise in increasing the rate of penetration through shales also have tended to increase screen blinding, which can lead to huge losses of mud, with loss of rig time and high fluid costs. Water base fluids are needed which can achieve a high rate of penetration without screen blinding.
SUMMARY OF THE INVENTION
The present invention provides a drilling system fluid comprising water as a continuous phase, a first amount of a divalent metal salt, and a quantity of a polyacrylamide copolymer, wherein the amount of the divalent salt and the quantity of polyacrylamide copolymer are sufficient to produce a rate of penetration approaching that achieved using a synthetic oil-based drilling system fluid while preventing substantial screen blinding. In a preferred embodiment, the continuous phase also comprises a second amount of a monovalent salt effective to increase gas hydrate suppression and decrease density when compared to a fluid consisting essentially of only a divalent salt in the absence of the monovalent salt.
DETAILED DESCRIPTION OF THE INVENTION
The present invention involves formulating aqueous-base drilling fluids to stabilize shale encountered during drilling. The water base fluids of the present invention comprise a combination of a suitable divalent salt, a suitable monovalent salt and a relatively low charge cationic, low molecular weight polyacrylamide copolymer. In a preferred embodiment, the divalent salt is calcium chloride and the monovalent salt is sodium chloride. This fluid composition provides a rate of penetration that approaches synthetic oil-based systems, and also provides good shale encapsulating properties for control of gumbo shale without substantial screen blinding. For purposes of the present application, the term “substantial screen blinding” is defined as the formation of a mat of undissolved or dispersed polymer on the shaker screen, which blocks passage of the hole fluid through the shaker screen, causing the hole fluid to overflow the shaker screen.
The polyacrylamide copolymers of the present invention may be used in substantially any drilling, drill-in, or completion fluid. As used herein, the term “drilling fluid” or “drilling fluids” shall be interpreted to refer to any one of these kinds of fluids. Preferred drilling fluids have water as a continuous phase.
Preferred drilling fluids comprise a mixture of salts consisting of brines comprising about 5 wt % to about 20 wt %, preferably about 15 wt % of the divalent salt, most preferably calcium chloride and about 0 lb/bbl to about 70 lb/bbl, preferably about 40 lb/bbl to about 70 lb/bbl, most preferably 50 lb/bbl of the monovalent salt, preferably sodium chloride. The fluids can contain substantially any suitable salts, suitable divalent salts include, but are not necessarily limited to salts based on metals, such as calcium, magnesium, zinc, and aluminum. Suitable monovalent salts include but are not necessarily limited to those based on metals such as sodium, potassium, cesium, and lithium. The salt may contain substantially any anions, with preferred anions including, but not necessarily limited to chlorides, bromides, formates, propionates, sulfates, acetates, carbonates, and nitrates. A preferred anion is chlorine. Preferred brines comprise calcium chloride. Sodium chloride is typically added to the drilling fluid after the calcium chloride brine.
The water-base drilling fluids contain “water-soluble polymers,” defined as polymers that are capable of viscosifying a drilling fluid and/or providing filtration control for a drilling fluid. Preferred viscosifiers and filtration control agents are XAN-PLEX™ D, BIO-PAQ™ and/or BIOLOSE™, all of which are commercially available from Baker Hughes INTEQ.
The drilling fluids of the present invention also contain “polyacrylamide copolymers” to provide shale inhibition. The term “polyacrylamide copolymers” is defined herein to refer to cationic polyacrylamide copolymers having a relatively low molecular weight and a relatively low charge. The term “low molecular weight” is defined to mean copolymer units having from about 50
Evans, Jr. Frank E.
Jarrett Michael A.
Norfleet James E.
Potts Patricia A.
Xiang Tao
Baker Hughes Incorporated
Paula D. Morris & Associates P.C.
Tucker Philip
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