Claims:1. A fluid loss control additive comprising a polymer blend of at least one cellulose or cellulose derivative and at least one polysaccharide derivative in a ratio in the range of 1:9 to 9:1, wherein said additive provides stable filtration control for an extended period of up to 16 hours at about 300 °F.

2. The fluid loss control additive as claimed in claim 1, wherein said cellulose derivative is selected from the group consisting of carboxymethyl cellulose (CMC), cationic CMC, hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), carboxymethyl hydroxyethylcellulose (CMHEC), polyanionic cellulose (PAC), and mixtures thereof.

3. The fluid loss control additive as claimed in claim 1, wherein said cellulose derivative is polyanionic cellulose (PAC).

4. The fluid loss control additive as claimed in claim 1, wherein said polysaccharide derivative is selected from the group consisting of starch and its derivatives, guar gum and its derivatives, xanthan gum, wellan gum, hydroxypropyl guar (HPG), carboxymethyl guar (CMG) carboxymethylhydroxypropyl guar (CMHPG), diutan, scleroglucan, and mixtures thereof.

5. The fluid loss control additive as claimed in claim 1, wherein the ratio of said cellulose or cellulose derivative and said polysaccharide is 1:1.

6. The fluid loss control additive as claimed in claim 1, wherein said fluid loss control additive is used in a concentration in the range of about 1 – 3% by weight of the drilling fluid.
, Description:FIELD OF THE INVENTION
The present invention relates to fluid loss control or filtration control additive for use in drilling fluids and completion fluids. More particularly, the present invention relates to fluid loss control or filtration control additive for water-based drilling fluids used in wellbores.

BACKGROUND
Natural resources such as oil and gas present in the subterranean formation are recovered by drilling a wellbore that penetrates the formation. During the drilling a fluid, called drilling fluid or mud, is injected into the well through a drill pipe and re-circulated to the surface in the annular area formed by the wellbore wall and drill string. Once back at the surface, the drilling fluid is physically and chemically treated and conditioned before it is pumped back into the well. The drilling fluid can be water-based, oil-based, pseudo-oil based or foam-based. Water-based drilling fluids are preferred these days over oil-based or pseudo oil-based for economic and environmental reasons.

The drilling operation involves driving by rotation a drilling apparatus including a column of drill pipes to the bottom of which is attached a multi-pronged/toothed drill bit. As the drill bit descends, it generates “cuttings”, or small bits of stone, clay, shale or sand. To facilitate drilling, these cuttings are to be continuously removed from the vicinity of the drill bit at the bottom of the hole. The drilling fluid is pumped down-hole through the drill pipe. The cuttings are removed from the down-hole to the surface by the drilling fluid through the annular space between the formation and the drill pipe.

In drilling operations the role of a drilling fluid is extremely important, and to perform these functions, an efficient drilling fluid must exhibit characteristics, such as desired rheological properties, capable of cooling and lubricating the drill bit as it grinds through the earth, fluid loss prevention, stability under various temperature and pressure operating conditions, stability against contaminating fluids, such as salt water, calcium sulfate, cement and potassium contaminated fluids, etc. A wide variety of additives may be added to the drilling fluid formulation to achieve the above properties.

Fluid loss prevention is a key function of the drilling fluids. For water-based drilling fluids, significant loss of water or fluid from the drilling fluid into the formation can cause irreversible change in the drilling fluid properties, such as density and rheology occasioning instability of the borehole. A wide variety of water-soluble or water-swellable polymers, such as cellulose, starch, guar gum, xanthan gum, synthetic polymers and copolymers of acrylamide, acrylic acid, acrylonitrile, and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) have been used in water-based drilling fluids. The most commonly used polymers used to build viscosity are the cellulose, guar gum, xanthan gum, and polyacrylates. Carboxymethyl cellulose (CMC), polyanionic cellulose (PAC) and carboxymethyl hydroxyethylcellulose (CM-HEC) have been known to be used traditionally.

US Pat. No. 4664816 discloses using a drilling fluid additive comprising a highly water absorbent polymer such as polyacrylate, polyacrylic acid or polyacrylamide encapsulated by a waxy substance, which dissolve or melt at borehole temperature to release the water absorbent polymer to expand by absorbing water. Another US Pat. 7256159 discloses using an oil soluble polymer prepared from monomer of styrene, alkyl acrylate, alkyl methacrylate, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, isoprene, butadiene, ethylene, vinyl acetate, etc., in the form of a gel as fluid loss reducer. The water-based drilling mud comprises an aqueous phase containing an oil soluble polymer in the form of a gel as a fluid loss reducer. US Pat. No. 8016040 discloses a fluid loss control pill comprising polyester solids that are hydrolysable and exhibit plastic deformation at formation conditions. The well completion method uses a fluid loss control pill comprising degradable polyester particles with or without a thickening polymer to control leak off to the formation.

The traditional fluid loss control additives have some disadvantages such as lack of stability at high temperatures, and uneconomical as is to be used in high concentrations to enhance the viscosity and provide effective filtration control. Further, the water-soluble polymers such as water-soluble cellulose ethers or cellulose ether derivatives, that are widely used as additives in the aqueous drilling fluid system, often pose difficulties during the formation of the aqueous drilling fluid systems due to their tendency to form lumps. In fact, on contact with water, local gel formation can occur around particles of undissolved cellulose ether, which prevent the hydration of the polymer matrix. The disaggregation/dissolution of the agglomerates or lumps when formed is very time consuming.

Another commonly used fluid loss control additive is the polyanionic cellulose which is a high-quality, water-soluble polymer that acts as a filtration control additive for fresh and salt water muds. It also acts as a viscosifier, and can resist bacterial attack. Polyanionic cellulose (PAC) is available in two forms, viz., PAC High Viscous (PAC HV) and PAC Low Viscous (PAC LV), both of which give the same degree of filtration control but different degree of viscosity. This traditionally used filtration control additive degrades significantly at temperatures of 300°F. As the bottom hole temperature of drilling deep wells is about 300°F, these traditional filtration control additives are likely to thermally degrade.

There is therefore felt need for a fluid loss control additive for water-based drilling fluids which overcomes the afore-noted drawbacks in traditionally used additives, and provides improved characteristics, such as, maintains the desired rheological properties of the drilling fluid, and obtains effective fluid loss or filtration control for extended period at bottom hole drilling temperatures above 300°F.

SUMMARY OF THE INVENTION
Accordingly, a primary object of the present invention is to provide a fluid loss control additive for water-based drilling fluids and completion fluids which overcomes the afore-noted drawbacks in traditionally used additives, and provides improved characteristics, such as, maintains the desired rheological properties of the drilling fluid, and obtains effective fluid loss or filtration control in freshwater, seawater, KCl and salt water, for extended period at bottom hole drilling temperatures of about 300°F. Also, the additive of present invention aids in tough, thin filter cake formation to minimize the risk of differential sticking. Other advantages of the additive of present invention include strong suspending capability, high ability of carrying crumbs and cleaning drilling crumbs, stabilizing the wellbore wall, preventing the mud from flowing away, protecting the drill bit and reducing the thickness of mud cake.

Other objects, aspects and advantages of the present invention will be more apparent from the following description.

Accordingly, the present invention discloses a fluid loss control additive comprising a polymer blend of at least one cellulose or cellulose derivative and at least one polysaccharide derivative in a ratio in the range of 1:9 to 9:1, wherein the additive provides stable filtration control for an extended period of up to 16 hours at about 300 °F. Preferably, the ratio of the cellulose or cellulose derivative and the polysaccharide derivative is 1:1. The fluid loss control additive is used in a concentration in the range of about 1 – 3% by weight of the drilling fluid.

The cellulose derivative is selected from the group consisting of carboxymethyl cellulose (CMC), cationic CMC, hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), carboxymethyl hydroxyethylcellulose (CMHEC), polyanionic cellulose (PAC), and mixtures thereof. Preferably, the cellulose derivative is polyanionic cellulose (PAC).

The polysaccharide derivatives are selected from the group consisting of starch and its derivatives, guar gum and its derivatives, xanthan gum, wellan gum, hydroxypropyl guar (HPG), carboxymethyl guar (CMG) carboxymethylhydroxypropyl guar (CMHPG), diutan, scleroglucan, and mixtures thereof.

DETAILED DESCRIPTION
The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting examples in the following description. The examples used herein are intended merely to facilitate an understanding of the ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

The description herein after, of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

According to a preferred embodiment the present invention discloses a fluid loss control additive for use in drilling fluids and completion fluids. In drilling fluids, the fluid loss control additive acts as a viscosifier, a flow controller and a filtration reducer. The additive has strong suspending capability, and improved ability of carrying crumbs, cleaning drilling crumbs, stabilizing the well wall, preventing the mud from flowing away, protecting drill bit and reducing the thickness of mud cake. As a completion fluid, the fluid loss control additive functions to control the fluid viscosity, suspends heavy objects, brings the filler and minimizes fluid loss. It can be used to replace guar gum in fracturing fluids.

The fluid loss control additive includes a low viscosity polymer blend to provide effective filtration control with improved thermal stability. The polymer blend comprises at least one cellulose or cellulose derivative and at least one polysaccharide derivative in a ratio in the range of 1:9 to 9:1. The ratio of the cellulose or cellulose derivative and the polysaccharide derivative may be 1:1. The fluid loss control additive provides stable filtration control for an extended period of up to 16 hours at 300 °F. The fluid loss control additive is used in a concentration in the range of about 1 – 3% by weight of the drilling fluid.

The cellulose derivative is selected from the group consisting of carboxymethyl cellulose (CMC), cationic CMC, hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), carboxymethyl hydroxyethylcellulose (CMHEC), polyanionic cellulose (PAC), and mixtures thereof. The cellulose derivative may be polyanionic cellulose (PAC).

The polysaccharide derivatives are selected from the group consisting of starch and its derivatives, guar gum and its derivatives, xanthan gum, wellan gum, hydroxypropyl guar (HPG), carboxymethyl guar (CMG) carboxymethylhydroxypropyl guar (CMHPG), diutan, scleroglucan, and mixtures thereof.

Due to the presence of carboxy group, the water dispersibility of the additive is high. Further, dissociation of Na+ creates negative site along the chain which leads to mutual repulsion between the charges, thus causing randomly coiled chains to stretch linearly, thereby increasing the viscosity. The mud containing the fluid loss additive permits the drilling machine to obtain low initial gel strength, so the mud can discharge easily.

The fluid loss control additive is designed for situations where filtration control is needed with only minimal increase in rheology. The fluid loss control additive performs well in all brine applications, especially saltwater based fluids. The fluid loss control additive may be used at all densities in either dispersed or non-dispersed systems.

The fluid loss control additive is adapted to encapsulate solids to control dispersion of active shale. The fluid loss control additive controls fluid loss in freshwater, seawater, KCl and salt water. Further, the additive aids in the formation of a tough, thin filter cake, thereby minimizing the risk of differential sticking. The additive is particularly useful in situations where filtration control is needed, but viscosity build-up is to be prevented.

The fluid loss control additive is adapted to form an envelope around exposed shales and cuttings for encapsulation, thus reducing dispersion and improving wellbore integrity. The encapsulation protects the shale from exposure to water, which otherwise tends to destabilize the shale. The fluid loss additive is a polymer blend of at least one cellulose or its derivative and at least one polysaccharide. The polymer blend provides effective filtration control with improved thermal stability and optimized rheology. The polymer blend is economical as it contains low concentration of the expensive polymers. The fluid loss control additive of present invention maintains stable filtration control for extended period of up to 16 hours at bottom hole drilling temperatures of about 300°F.

Drilling mud treated with the fluid loss control additive of present invention forms a thin filter cake with low permeability, and thus can reduce the mud loss, hole shrinkage and collapse of well bore phenomena resulting from water loss of mud. The additive inhibits/resists bacterial growth in the mud, hence there is no need to maintain high pH or use preservatives or biocides. Further, the additive remains stable over temperature variations and additions/removal of other chemical agents during the drilling process.

The fluid loss control additive may be used in water-based drilling fluids to effectively reduce the filtration rate in water-based drilling fluids. Further, it helps to enhance and stabilize viscosity, to improve hole cleaning and suspension properties in a wide variety of fluid environment. The additive effectively reduces the filtration rate of water-based drilling fluids, without any significant increase in viscosity or gel strength.

The preferred embodiment pertains to an additive composition for improving filtration control and thermal stability of a water-based drilling fluid for prolonged periods at deep well bottom hole temperature. The present invention comprises employing a novel natural polymer blend for effective filtration control with improved thermal stability. When the blended polymer is added into a drilling fluid, it substantially reduces filter loss. Considerably enhanced filtration loss is obtained by having higher molecular weight filtration additive combined in water-based drilling mud after prolonged ageing at about 300°F.

EXAMPLES
The rheological properties of the drilling fluid were determined by FANN Viscometer as described in "Standard Procedure for Testing Drilling Fluids", American Petroleum Institute, API RP 13B. The API fluid loss was measured in accordance A.P.I. Spec 13A.

EXAMPLE 1
The fresh water mud was prepared by dissolving 3.5 ppb of fluid loss control additive (polyanionic cellulose and polysaccharide derivative in a ratio of 1:1) into 350 ml of deionized water with stirring for 20 minutes. The fresh water mud system was aged under static condition for 12 hours at 25°C.

EXAMPLE 2
10 ppb of Attapulgite clay was stirred into 350 ml of saturated salt water solution for 20 minutes. 3.5 ppb of fluid loss control additive (polyanionic cellulose and polysaccharide derivative in a ratio of 1:1) was added to the saturated salt water solution. The saturated salt water mud system was aged under static condition for 12 hours at 25°C. Table I below shows the mud formulations 1 & 2.
Table I
Components Formulation 1
Fresh water mud Formulation 2
saturated salt water
Freshwater 350 ml -
Saturated salt water - 350 ml
4% NaCl brine - -
Attapulgite clay 10 ppb
Fluid loss control additive 3.5 ppb 3.5 ppb

EXAMPLE 3
350 ml of 4% NaCl brine solution was prepared. 1 ppb sodium bicarbonate, 35 ppb of API standard evaluation base clay, 3.5 ppb of fluid loss control additive (polyanionic cellulose and polysaccharide derivative in a ratio of 1:1) was added to the brine solution. The mud was aged with rolling for 16 hours at 300° F. API fluid loss was measured and thermal stability of the fluid loss control additive was checked. Table II below shows the mud formulation 3.

Table II
Components Formulation 3
4% NaCl brine 350 ml
sodium bicarbonate 1 ppb
API evaluation base clay 35 ppb
Fluid loss control additive 3.5 ppb

As seen from Tables III & IV, the fluid loss control additive of present invention gives significant results in term of filtration control, rheology and thermal stability.
Table III
FANN 35 RHEOLOY AT 25°C Formulation 1
Formulation 2

600 RPM 9 11
300 RPM 5 6
Apparent Viscosity (cP) 4.5 5.5
Plastic Viscosity (cP) 4 5
Yield Point (lbs/100 ft²) 1 1
API Fluid loss (ml) 9.2

Table IV
FANN 35 RHEOLOY AT 25°C Formulation 3
600 RPM 12
300 RPM 7
Apparent Viscosity (cP) 6
Plastic Viscosity (cP) 5
Yield Point (lbs/100 ft²) 2
API Fluid loss (ml) 10.4

EXAMPLE 4
Saturated salt water mud was prepared. 10 ppb of Attapulgite clay was added into 350 ml of saturated salt water solution for 20 minutes. 3.5 ppb of fluid loss control additive (polyanionic cellulose and polysaccharide derivative in a ratio of 1:1) was added to the salt water solution. The saturated salt water mud system was aged under static conditions for 12 hours at 25°C. For thermal stability study, 4% salt solution and 1 gm of sodium bicarbonate were mixed for 10 minute. 35 gm of API evaluation base clay was added with stirring for 20 minutes. 3.5 gm of polyanionic cellulose and polysaccharide derivative in a ratio of 1:1 was added with stirring for 10 minutes. The mixture was hot rolled for 16 hours at 300°F. Table V below shows the results in comparison with specified standards, indicating improved fluid loss control and thermal stability by use of the fluid loss control additive of present invention.
Table V
Parameter Specification Result

Fresh Water Solution
600 rpm Reading 14
300 rpm Reading 7.5
Apparent Viscosity (AV) 7
Plastic Viscosity (PV) 11cp, Max 6.5
Yield Point (YP) 3 lb/100 sq.ft Max 1
Saturated Salt Water Solution
600 rpm Reading 15
300 rpm Reading 8
Apparent Viscosity (AV) 7.5
Plastic Viscosity (PV) 13cp, Max 7
Yield Point (YP) 4 lb/100 sq.ft Max 1
Fluid loss (ml) 12.5cc, max 8
Moisture 8.0% max 7.6
Purity 90 % Min. 93.14
Thermal Stability: API Fluid loss : After Hot rolled at 300 °F for 16 hours hot rolled 12.5 ml 10.8

Embodiment of the present invention is applicable over a wide number of uses and other embodiments may be developed beyond the embodiment discussed heretofore. Only the most preferred embodiments and their uses have been described herein for purpose of example, illustrating the advantages over the prior art obtained through the present invention; the invention is not limited to these specific embodiments or their specified uses. Thus, the forms of the invention described herein are to be taken as illustrative only and other embodiments may be selected without departing from the scope of the present invention. It should also be understood that additional changes and modifications, within the scope of the invention, will be apparent to one skilled in the art and that various modifications to the composition described herein may fall within the scope of the invention.