Earth boring – well treating – and oil field chemistry – Earth boring – Contains inorganic component other than water or clay
Reexamination Certificate
1999-11-12
2001-06-19
Tucker, Philip (Department: 1712)
Earth boring, well treating, and oil field chemistry
Earth boring
Contains inorganic component other than water or clay
C507S141000, C507S145000, C507S139000
Reexamination Certificate
active
06248698
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a relatively high density silicate drilling mud with improved rheology and filtration control properties. More particularly, the invention relates to silicate drilling fluids comprising hematite as a weighting agent and manganese oxide as a bridging agent and which have excellent rheological and filtration control properties.
BACKGROUND OF THE INVENTION
The rotary drilling of a borehole is accomplished by rotating a drill string having a drill pipe and a drill bit at its lower end. Weight is applied to the drill bit while rotating to create a borehole into the earth. The drill string is hollow and sections are added to the drill string to increase its length as the borehole is deepened. This rotary drilling process creates significant amounts of friction which produces heat along with fragments of the strata being penetrated. The fragments of the strata must be removed from the borehole and the drill bit must be cooled to extend its useful life. Both of these functions are accomplished by circulating a fluid down through the drill string and up to the surface between the drill string and the wall of the borehole. Generally, the borehole into which the casing or liner is introduced is filled with drilling mud.
Most drilling muds have either an aqueous or an oil base. Water base drilling fluids are not only popular, but often are necessary in order to meet applicable environmental regulations. The current generation of water-based drilling fluids often contains a high concentration of additives to improve borehole stability, improve lubricity, and to control seepage loss to the formation. A high concentration of additives, such as glycols, silicates, or surfactant-lubricant blends, makes it difficult to formulate some higher density fluids (esp. silicate fluids) to balance desired Theological properties with adequate shale stabilization and filtration control.
Silicate drilling fluids are capable of giving superior borehole stability. The soluble silicate in a silicate drilling fluid is believed to invade the shale matrix and either (a) the soluble silicates precipitate when they contact the Ca
2+
and Mg
2+
available in the reservoir fluid, or (b) the reduced pH encountered in the reservoir fluid causes polymerization to occur. Both mechanisms are capable of effectively plugging the pore throats of the shale and reducing filtrate invasion.
A principal requirement of any good drilling fluid, including a silicate drilling fluid, is that the fluid remain pumpable while suspending a sufficient amount of weighting additives to meet the desired density requirements, particularly with respect to preventing gasification and blow-outs. One commonly used weighting agent is barite. Unfortunately, the use of high concentrations of barite in silicate drilling fluids results in significant depletion of silicate during exposure to heat. Silicate depletion may diminish borehole stability using silicate drilling fluids.
Alternative formulations are needed in which the silicates are not depleted during exposure to heat, and which have desirable rheology, shale stabilization, and filtration control properties.
SUMMARY OF THE INVENTION
The present invention provides a silicate drilling fluid having effective rheology and filtration control properties. The silicate drilling fluid comprises water as a continuous phase, hematite as a weighting agent, and manganese oxide as a bridging agent.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based on the premise that calcium is responsible for silicate depletion from silicate drilling fluids. Calcium carbonate generally is insoluble under the conditions present during the use of most drilling fluids. In contrast, silicate drilling fluids have a relatively high pH. The high pH coupled with elevated temperatures is believed to cause reactions between calcium carbonate and sodium silicate. These reactions are believed to be responsible for the depletion of the silicates from silicate drilling fluids, and to cause an increase in fluid loss which negatively impacts borehole stability.
The present invention avoids silicate depletion by replacing barite with hematite as a weighting agent and replacing calcium carbonate with manganese oxide as a bridging agent. The result is a relatively high density (10.5 to 16 ppg), high performance water based silicate mud with adequate rheology and with adequate shale stabilizaton and filtration control properties.
The silicate drilling fluid of the present invention may be used to drill most sections of a typical well in the Gulf of Mexico (GOM). The performance parameters of major importance are: low rheology, including plastic viscosity (PV), yield point (YP), and gel strengths; minimal rheology variation between initial and heat aged properties; minimal fluid loss; and reduced silicate depletion after heat aging. In a preferred embodiment, a 16 ppg fluid has the following properties:
Plastic
Viscosity
Yield Point
Gels
API
24 hr., HTHP
30-40, cP
20-30, lb/100 ft
2
12/20, lb/100
<10 ml
<40 ml
ft
2
For a 10.5 ppg fluid, performance parameters preferably are:
Plastic
Viscosity
Yield Point
Gels
API
24 hr., HTHP
18-20, cP
10-15, lb/100
6/10, lb/100
<4 ml
<40 ml
ft
2
ft
2
In addition, initial and post heat age rheology should have minimal change and silicate depletion should be minimal.
The silicate drilling fluid preferably contains water as a continuous phase, a water-soluble viscosifying polymer, a water-soluble filtration control polymer, a glycol for filtration control and shale stabilization, a silicate, a hematite weighting agent, a manganese oxide bridging agent, salt, and a base to control pH, a preferred base being sodium hydroxide.
The silicate drilling fluid contains any suitable salt, preferably one or more monovalent salts, including but are not necessarily limited to those based on metals such as sodium, potassium, cesium, and lithium. The salt contains 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 chloride. Preferred drilling fluids comprise about 0% w/w to about 26% w/w, preferably about 20% w/w of the salt, with a preferred salt being NaCl.
The drilling fluid contains “water-soluble polymers,” defined as polymers that are capable of viscosifying a drilling fluid and/or providing filtration control for a drilling fluid. Suitable water-soluble polymers are non-toxic and include, but are not necessarily limited to water-soluble starches and derivatized versions thereof, water soluble gums and derivatized versions thereof, and water-soluble celluloses. Starches that are suitable for use in the present invention include, but are not necessarily limited to corn based and potato based starches, preferred starches being more temperature stable starches. Gums that are suitable for use in the present invention include, but are not necessarily limited to xanthan gums, wellan gums, scleroglucan gums, and guar gums. These polymers are widely available from commercial sources.
As used herein, the terms “derivatized starches” and “derivatized gums” refer to starches and gums that have been derivatized in a manner that renders them inherently non-fermentable in order to avoid the need for a preservative. Water-soluble “derivatized starches” and “derivatized gums” that should operate successfully include, but are not necessarily limited to: hydroxyalkyl starches and gums; starch and gum esters; cross-link starches and gums; hypochlorite oxidized starches and gums; starch and gum phosphate monoesters; starch and gum xanthates; and, dialdehyde starches and gums. These derivatized starches and gums can be manufactured using known means, such as those set forth in detail in Chapter X of
Starch: Chemistry and Technology
311-388 (Roy L. Whistler, et al. eds., 1984), incorporated herein by reference.
Specific examples of suitable derivatized starches and gums that fall within the foregoing cate
Gabrysch Allen
Mullen Gregory A.
Baker Hughes Incorporated
Paula D. Morris & Associates P.C.
Tucker Philip
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