Earth boring – well treating – and oil field chemistry – Well treating – Contains organic component
Reexamination Certificate
2002-06-17
2004-11-09
Tucker, Philip C. (Department: 1712)
Earth boring, well treating, and oil field chemistry
Well treating
Contains organic component
C507S269000, C166S293000, C166S295000, C106S638000, C106S713000, C106S789000
Reexamination Certificate
active
06815399
ABSTRACT:
The present invention relates to a reactive plugging fluid designed to gel rapidly when subjected to high shear stress. The invention also relates to a method for plugging a subterranean formation zone, especially for curing massive mud losses when drilling a well.
Lost circulation while drilling is a major problem. The well cost dramatically increases due to the lost time from delayed well production and also due to associated drilled problems such as pipe sticking and safety issues. The most common technique to combat lost circulation is to add into the drilling fluid a lost circulation material (LCM). Granular flakes and fibrous particles, essentially based on cellulosic materials, are used for sealing off fractures, vugs and porous zones. Minerals such as mica are also commonly used. If even high concentrations of lost circulation materials fail to restore the drilling fluid circulation, a cement plug is placed. The cement plug consolidates the voids but also fills the open wellbore and needs to be drilled before continuing the well drilling. Quite often, the procedure must be repeated several times before achieving a correct seal.
Other techniques involve the use of reactive fluids. Two reactive fluids are either mixed near the formation where lost occurs with a first fluid pumped through the drill-string and a second fluid displaced down the annulus. At the interface of the two fluids, the turbulent flow allows the rapid formation of a rubbery solid mass commonly known as a gunk. Another practice uses crosslinked polymer gels whose reaction is initiated on surface. In both case, the technology is highly risky since slight changes in the composition, temperature or fluid contamination may lead to premature gellation in and around the bottom hole assembly, leading to major operation failure.
It is also known to use as plugging fluids so-called rheotropic liquids that thicken when subjected to high stress. U.S. Pat. No. 4,663,366 discloses such a polycarboxylic acid containing water-in-oil emulsion where the oil phase contains hydratable water-swelling hydrophilic clay such as bentonite and the aqueous phase contains a dissolved polyacrylamide and a polycarboxylic acid. The setting of this plugging fluid takes place as a result of a swelling of the bentonite when bentonite contacts water. Each dispersed droplet of the aqueous phase is coated with a polymeric material so that the contact only occurs when the emulsion is subjected to high shear forces that break this coating.
Another rheotropic plugging fluid is known from WO94/28085. Like the emulsion of the U.S. Pat. No. 4,663,366, the fluid is based upon a ‘loose’ invert emulsion. The continuous phase provides an encapsulation medium for a crosslinker and the internal phase consists of a high concentration of a polymer while the interfacial tension between the two phases is maintained by a concentration of a lipophilic surfactant
A preferred plugging fluid of the WO94/28085 patent application consists of about 25% by volume of a continuous phase containing an hydrophobic liquid selected from mineral oils, vegetable oils, esters and ethers, an emulsifier on a triglyceride basis, bentonite and calcium hydroxide and of about 75% by volume of a dispersed aqueous phase containing water, xanthane and optionally, a weighting material such as barite. When this type of fluid experiences a significant pressure drop, an inversion of the emulsion occurs and the crosslinker is released into the aqueous phase resulting in the formation of a gel.
This latter type of plugging fluid can be stored for several weeks without reacting and pumped with a centrifugal pump for several hours. Gellation is fast and triggered only by subjecting the plugging fluid to high shear forces, for instance when forced through the drill bit. However, the use of this type of plugging fluid is limited by lack of robustness and shrinkage over time. Moreover, above a temperature threshold of about 90° C., the gel becomes less rigid and turns into a viscous fluid due to the breaking of the crosslinked bonds.
It would therefore be desirable to provide a new plugging fluid, which provides a more robust gel, both in the initial gel phase and in the days following. It would also be desirable to provide a plugging fluid with better thermal stability. There is also a need in well control for better procedures, including placement strategies to help in making jobs successful.
Thus, the invention provides a plugging fluid for plugging a subterranean formation zone surrounding a drill hole consisting of an emulsion comprising:
a. an oil phase containing
an oil
an emulsifier
2.4-4 kg of cement per liter of oil
b. an aqueous phase containing
water
12-16 g of a polysaccharide per liter of water
wherein the oil to water volume ratio ranging from 20:80 to 25:75.
The emulsion is believed to be invert (water-in-oil) but is actually suspected to be direct (oil-in-water) with further water droplets within the large oil droplet, i.e. an invert emulsion in a direct emulsion.
The principle of the setting of the plugging fluid of the present invention is essentially the same as for the plugging fluid of WO94/28085 discussed above. It is the crosslinking of the polysaccharide that causes the gel formation. Polysaccharides are known as state-of-the-art polymeric viscosifiers used extensively in the oil industry. A crosslink bond is created between a metal ion and the hydroxyl groups along the polymer chain of the polysaccharides. Upon exposing the plugging fluid to a pressure drop greater than 2 MPa over a small dimension, it is believed that the emulsion inverts or flips from its invert state into a more stable direct state. The rupture of the emulsion droplets releases the cement into the water phase thus providing Ca2+ as metallic divalent ion to crosslink with the polysaccharides and forming the gel structure.
With time, it is believed that the rigid gel structure loses oil, probably through migration through the porous and high permeable gel structure. As the oil is removed and more cement particles become water-wet, the cement begins to hydrate and generate a low compressive strength over time. The strength developed by this material is orders of magnitude higher than the strength developed with cement-free emulsions and provides further strength to consolidate weak formations.
Advantageously, with an increase of the temperature, the removal of oil and thus the hydration of the water-wet particles seem to be accelerated. This translate into accelerating the gel into a more cementitious material which is more thermally stable at temperatures as high as 140° C.
The crosslink bond created between the metal ion and the hydroxyl groups concurs if the pH is ranged between 11 and 13. When the crosslinking agent is calcium hydroxide as preferred in the emulsion of WO-94/28085, such a high pH may be difficult to achieve with some problematic oils such as winter grade diesels that contain surfactants detrimental to gellation. Advantageously, the use of cement as crosslinking agent totally overcomes this difficulty since the pH is instantly increases as cement is added to the emulsion. Thus the addition of cement increases the protection from acidic effects and moreover, as the cement sets, it rends the system immune to pH changes and increases the resistance to mechanical effects by a factor of seven hundred when compared to a soft gel alone.
Any clean liquid hydrocarbon can be used for the oil phase. The oil may advantageously be selected from any base oil suitable for drilling fluids such as mineral oils, vegetable oils, esters and ether oils, diesel, alpha-olefins, polyolefines, n-alkanes, and mixtures thereof. Selected oils must be of compatible with the used drilling fluids and the environmental regulations that for instance prohibit use of aromatic containing oils on offshore rigs.
Conventional commercially available emulsifiers can be used, selected on the basis of their compatibility with alkali environment and the intended temperature of use. Lipophilic surfactants, used to pre
Arsanious Kamal
Johnson Les
Murphy Patrick
Quinn David
Toney Allen R.
Echols Brigitte L.
Mitchell Thomas O.
Nava Robin
Schlumberger Technology Corporation
Tucker Philip C.
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