Oil based drilling fluids and mixture for use in oil based drilling fluids

Technical field

The present invention relates to non-aqueous drilling fluids (NDF) and to a mixture for use in non-aqueous drilling fields.

Background Art

In the exploration for oil and gas drilling fluids are generally used to serve certain functions such as lifting the cuttings to the earth's surface, lubricating and cooling the drill bit, maintaining the downhole pressure, etc... There are two major classes of drilling fluids by means of aqueous and non-aqueous (oleaginous) based drilling fluid. Normally non-aqueous based drilling fluid which is known as well as invert emulsion- is based on diesel, or mineral oil as continuous phase, and contain viscosity modifier such as organophilic clay or synthetic polymers, aqueous phase normally CaC^ brine, emulsifiers and wetting agents, fluid loss control agents such as synthetic hydrophobic polymers or gilsonite or asphaltene and weighting agent such any heavy weight particulates with a specific gravity normally >2g/cm ³ .

The commonly weighting agents used to control the drilling fluids density are barite (BaSO ₄ , specific gravity (SG) minimum 4.2), manganese tetraoxide (Mn ₃ O ₄ , SG 4.7-4.9), calcium carbonate (CaCO ₃ , SG 2.7-2.8), ilmenite (FeTiO ₃ 4.5-4.7), hematite (Fe ₃ O ₄ , SG 4.9-5.2), siderite (FeCO ₃ , SG 3.96), galena (PbS, SG 7.2-7.6), celestite (SrSO ₄ , SG 3.96) and others.

In oil based fluids the weighting agent with the help of emulsifier and wetting agent is dispersed in the oil phase. Weighting agents, particularly weighting agents having an average particle size of <10pm have a tendency to agglomerate or flocculate forming larger agglomerates particularly at high temperature -above >150 °C and high density >1.6SG. Such agglomeration is undesirable because it alters the fluid properties such as the rheology, the filtration and settling. For example formation of agglomerates tends to increase the plastic viscosity, yield stress and the gel strength of the fluids and increase the fluid loss into the subterranean formation. Severe flocculation can cause many operational challenges such as drillpipe sticking, increase the drillpipe torque, reduce the rate of penetration, borehole instability and loss circulation. Such challenges have a direct impact on the total drilling cost. To overcome such formation of agglomerates a chemical agent normally called deflocculant, dispersant or thinner is added to the drilling fluids to minimize the particles-particles interaction.

The currently existing deflocculants such as Omin_cote® from Baker Hughes, Versathin™ HF from Ml Swaco, or OMC™ from Baroid have the disadvantage being environmentally harmful or not performing well under various drilling circumstances. In terms of exotoxicity some of these products have been identified as having potential environmental concerns and these deflocculants are not or hardly biodegradable.

US patent 7,638,466 B2 discloses a method of drilling borehole in subterranean formation at a temperature in the range of about 40°F to about 250°F. The patent describes the use of non-ionic surfactant as a thinner that is produced by the reaction of carboxylic acid with ethylene oxide, propylene or butylenes oxides.

US patent 7,871 ,962 B2 describes a flat rheology drilling fluid using polycarboxylic fatty acids as rheology modifier.

In addition of being biodegradable the deflocculating agent should comply with a number of requirements. It must be compatible with mineral oil, it must be stable at a temperature up to at least 400 °F, and provide satisfactory rheology for the drilling fluids at different temperatures.

There is thus a need for oil based drilling fluids containing deflocculants which are biodegradable, are compatible with mineral oil, weighting agents and other additives normally used in oil based drilling fluids and are thermally stable up to at least 400 °F.

Description of Invention The present invention thus relates to oil based drilling fluid comprising a weighting agent, which drilling fluid contains alcohol ethoxylates with a molecular weight between 200 and 2000 Dalton as a deflocculant.

The alcohol ethoxylate preferably has a molecular weight between 300 and 600 Dalton.

The drilling fluid preferably contains 1 to 15 g/l fluid of alcohol ethoxylate, and more preferably between 1.5 and 9 g/l fluid of alcohol ethoxylate.

The invention further comprises a dry mixture for use in oil-based drilling fluids comprising a weighting agent and a deflocculating agent wherein the deflocculating agent is alcohol ethoxylate with a molecular weight between 200 and 2000 Dalton as a deflocculant.

The alcohol ethoxylate preferably has a molecular weight between 300 and 600 Dalton.

The alcohol ethoxylate can have non-ionic, anionic or cationic character and can be used either in one of the above forms or as a mixture of different forms, and can be used in oil based drilling fluids containing different weighting agents such as a manganese tetraoxide, barite, hematite, ilmenite and others.

The dry mixture preferably comprises 0.02-2.0 % by weight of alcohol ethoxylate based on the weight of weighting agent, and more preferably 0.02- 1.0 % by weight of weighting agent of alcohol ethoxylate. An amount between 0.06-0.3 % by weight based on the weight of weighting agent is particularly prefered.

By the dry mixture of the present invention it is easy to add weighting agent and deflocculating agent to oil-based drilling fluids making sure that flocculation and formation of agglomerates of weighting agent in oil-based drilling fluids are avoided.

The alcohol ethoxylate is biodegradable, is stable at high temperatures up to and above 400 °F and provides satisfactory rheology and shows good filtration and low settling properties for the oil based drilling fluid. When alcohol ethoxylate is added separately during the preparation of drilling fluid there is no special sequence for the addition of alcohol ethoxylate. It can be added after the addition of emulsifiers, water, viscosifier and fluid loss additive and prior to the weighting agent. It can also be added after having made the full drilling fluid.

When making the dry mixture of weighting agent and alcohol ethoxylate, the alcohol ethoxylate is preferably sprayed on the surface of the weighting agent particles.

The preparation of some alcohol ethoxylates used in the present invention is described in WO 2005/085321.

The non-ionic alcohol ethoxylates can be described with the following general formula;

RO [(CH ₂ CHR ¹ O)x (CH ₂ CHR ² O) _y ] _z -H

RO is derivable from a mixture of alcohols ROH and the alkyl group (R) can be linear or branched.

R ¹ and R ² can be H or aliphatic hydrocarbon chain (linear or branched)

X, Y and Z are independent and can have the values from 1 to 20.

A general formula for anionic alcohol ethoxylates can be illustrated as follows;

R-O-(CH ₂ -CHR ¹ -O) _x -CH ₂ -COOH

R=linear alkyl, branched alkyl or alkylaryl, x can have a values from 1 to 20 and R ¹ = H or CH ₃

The feedstock alcohols used in the preparation of both alcohol ethoxylate and carboxy methylated alcohol ethoxylate can be; 1) fatty alcohols gained from fatty acid triglycerides or wax ester; 2) alcohols prepared by hydroformylation of olifins obtained from the Fischer-Tropsch process from carbon monoxide (CO) and hydrogen (H ₂ ) gases; 3) alcohols prepared from petrochemical sources using the Ziegler process or the Shell higher olefin process etc...

More details about non-ionic and anionic alcohol ethoxylates can be found in the US patent application 2008/0207494. The biodegradability of the alcohol ethoxylates at the end of 10-day window according to OECD 301 B standard Biodegradation Test Protocol should be at least >20 %, preferably more than 40 % and most preferably above 60 %.

Example 1

Four oil based drilling fluids for use at high pressure, high temperature, (HPHT), were made containing different deflocculating agents. The compositions of the four drilling fluid A, B, C and D are shown in Table 1. All four drilling fluids had a specific gravity of 2.1.

The preparation and testing of the drilling fluids were conducted according to API 13B Standard.

Table 1 : Recipe and mixing procedure of HPHT oil based fluid with a density of 2.1SG.

The composition of the drilling fluids is identical except for the deflocculant used. In drilling fluid A, a conventional non-biodegradable polyolefin/organic sulfonate blend deflocculant from Baker Hughes sold under the trademark Omin-Cote ^® was used. In drilling fluid B a biodegradable alcohol ethoxylate composed of an alcohol with alkyl chain of C12-C13 and an ethylene oxide (EO) block having an average content of EO unit of 6.5, an average molecular weight of 479 Dalton and hydrophilic/hydrophobic balance (HLB) of 12 was used. This deflocculant is from Sasol North America Inc. sold under the trademark Novel ^® 23E6.5.

In drilling fluid C a biodegradable C13-alcohol polyethylene glycol ether carboxilic acid from Sasol Germany GmbH sold under trademark Marlowet ^® 4538 was used. This is an anionic deflocculant and has a molecular weight of 566 Dalton. In drilling fluid D a biodegradable alcohol ethoxylate (isononanol, ethoxylated, propoxylated, carboxymethylated) from Sasol Germany GmbH sold under trademark Marlowet ^® 1072 was used. This is an anionic deflocculant and has a molecular weight of 333 Dalton. The four drilling fluids A, B, C, D were tested for rheology, filtration, electrical stability and static SAG factor before static heat aging (BHA) and after static heat aging (AHA) at 150 °C.

The following equipment was used to test the drilling fluids:

Fann rheometer with thermo-cup, high temperature and pressure filtration system for measuring fluid loss at HPHT, pH-meter, oven 250 and 500 ml hot aging cell and precision balance.

Static sag test was performed according to the following procedure;

A) Pour the drilling fluid to be aged into a stainless steel ageing cell.

b) Close the cell properly and pressurize the cell with 300 psi N ₂ gas.

c) Static age cell upright at desired temperature for 16 hours

d) When ageing time is completed, take ageing cell out of oven and cool down to room temperature. The cell is to be kept upright.

e) Carefully bleed off the pressure prior to disassembling the cell. f) Suck up the clear supernatant liquid from the top of the fluid. Transfer to a measuring cylinder and record the volume in ml. g) Suck up 20ml mud from the top of the ageing cell with a syringe and transfer it to a pycnometer.

Determine the weight of the mud in the pycnometer.

Determine the density of the mud, by dividing the weight of mud in the pycnometer by 20.

h) Suck up 20ml mud from the bottom of the ageing cell with a syringe and transfer it to a pycnometer.

Determine the weight of the mud in the pycnometer.

Determine the density of the mud, by dividing the weight of mud in the pycnometer by 20.

Sag factor = density bottom/ (density top + density bottom)

The results of the tests of oil-based drilling fluid A, B, C, D are shown in Table 2.

Table 2: Properties of 2.1SG oil based drilling fluids containing different deflocculant agents.

The results in Table 2 show that the non-ionic deflocculant according to the present invention is effective in keeping the rheological properties quite stable after heat aging for 16h at 150 °C. Also the anionic deflocculant according to the present invention such as Marlowet ^® 1072 and Marlowet ^® 4538 are also reducing the viscosity, but the filtration was quite high with the anionic deflocculant. That might be related to an interference with the fluid loss agents. This example shows that biodegradable alcohol ethoxylates can replace the currently used products which might pose risk to the environment.

Example 2

To simulate the effect on oil-based drilling fluids according to the invention of collolidal fine particles that might be created from the formation during drilling, grinded montmorillonite particles were added to oil-based drilling fluids. Four drilling fluids E, F, G, and H having the compositions shown in Table 3 were mixed.

Drilling fluid E did not contain deflocculant, and drilling fluids F, G and H contained 8 g/l alcohol ethoxylate deflocculants according to the invention. The deflocculants used were as follows:

Drilling fluid F: The flocculant used in drilling fluid F (Novel ^® 23E6.5) is the same as the deflocculant used in drilling fluid B in Example 1.

Drilling fluid G: Ethoxlylated alcohol (Alcohol C10-C16) ethoxylated having a molecular weight of 589 Dalton produced by Sasol North America Inc. and sold under trademark Novel ^® 23E9.

Drilling fluid H: The flocculant used in drilling fluid H ( arlowet ^® 1072) is the same as the deflocculant used in drilling fluid D in Example 1.

Table 3: Composition and mixing procedure for oil based drilling fluids with a specfic gravity of 1.9 contaminated with grinded montmorillonite.

Mixing Time

Materials Unit Qty.

(min.)

Mineral Oil* ml 220,0

Viscosifier - Organically modified hectorite** grams 3,0 5

Viscosifier - Organically modified clay grams 2,0 5

Lime grams 20,0 2

HT Primary Emulsifier grams 20,0 2

HT Secondary Emulsifier grams 16,5 2

Dl Water ml 22,8 2 Calcium Chloride grams 7,5 5

HT Filtration Control - organophilic lignite grams 12,0 2

HT Secondary fluid loss control grams 1 ,5 5

Montmorillonite grams 20,0 5

508,0

Weighting Agent (Mn ₃ 0 ₄ ) grams 10

^* Mineral oil used has a specific gravity of 0.79-0.81 (at 15.6 °C) and a viscosity of 1.9-2.2 cp (at 40 °C)

**A clay mineral belonging to smectite group, with a chemical formula of Na ₀₄ Mg ₂₇ Li ₀ 3Si ₄ Oio(OH) ₂ .

The drilling fluids E, F, G, H, were tested for rheology, filtration, dynamic sag, and dynamic filtration.

Dynamic sag was determined as follows:

Sag tendency of the drilling fluids was determined by Viscometer Sag Shoe Test (VSST), which measures the sag tendency under dynamic conditions. This test uses the Fann 35 viscometer and a sag shoe, which collects sagged weight material into the collection well of the sag shoe through which samples are drawn at the end of the test to determine the sag tendency of the fluid. VSST is calculated using Eq. 1 :

VSST (Ibm / gal) = 0.833 * (WT2 - WT1 ) (1 )

Where WT2 = weight of 10 ml of the drilling fluid after shearing for 30 minutes And WT1 = weight of 10 ml of the drilling fluid before shearing for 30 minutes

VSST is an improvement over VST in which no sag shoe is inserted in the viscometer thermocup, although the other procedures are the same. Aldea et al. (2001) stated that a fluid would have moderate-to-low sag tendency when the density change measured on the VST is less than 1 Ibm/gal.

The results of the tests are shown in Table 4. Fluid (1.9SG) properties before and after heat aging at 200 °C and

*) The dynamic filtration was conducted on HPHT dynamic filter press at 150°F, stirring speed of 200rpm and with differential pressure of 500psi. For the tests, permeable ceramic disk with mean pore diameter of 50 pm was used. The results depicted in Table 4 show that the 10min gels of the drilling fluid E, (without deflocculant) is excessively increased after heat aging while gels of drilling fluids F, G and H containing deflocculants according to the invention were significantly less increased than the blank sample E. The above example shows as well that alcohol ethoxylate with 6.5 EO units is better than one with 9 EO units since the sag of the fluid and the total filtrate volume with novel 23E6.5 were less that the fluid with novel 23E9. That means there is an optimal chain length for the ethylene oxide block that gives the best adsorption of the deflocclant on the Μη ₃ θ ₄ surface. When the chain reach a certain length steric hindrance might occur which lower the amount of the adsorbed molecules. Example 3

In order to simulate oil based drilling fluids contamined with G-cement, drilling fluids I, J, K, L were mixed. Drilling fluids J, K and L are according to the invention. Drilling fluid I does not contain deflocculant.

The fiocculant used in drilling fluid J is the same as the deflocculant used in drilling fluid F in Example 2. The deflocculant used in drilling fluid K is the same as the deflocculating agent used in drilling fluid G in Example 2 and the deflocculating agent used in drilling fluid L is the same as the deflocculating agent used in drilling fluid D in Example 1. The drilling fluids had a specific gravity of 1.9.

The composition of the drilling fluids is shown in table 5.

Table 5: Receipe and mixing procedure of HPHT oil based drilling fluid with a density of .9SG contaminated with G-cement

The four drilling fluids I, J, K, L were tested for rheology, gel formation and dynamic sag before heat aging and after heat aging at 200 °C and 500 PSI pressure.

The results are shown in Table 6. 11

Table 6

The properties of the drilling fluids J, K and L according to the invention contaminated with cement are in the desired range and are stable after heat aging, indicating that alcohol ethoxylates can be used to stabilize oil based drilling fluids contaminated with cement.

Example 4

In this example a combination of non-ionic and anionic alcohol ethoxylates was used to control the rheology of oil based drilling fluids having a density of 2.1SG.

Three oil based drilling fluids, M, N, O were made with compositions as shown in Table 7.

Table 7

As shown in Table 7, drilling fluid M contained the non-ionic deflocculant Novel ^® 23E6.5, drilling fluid N contained a combination of non-ionic (Novel ^® 23E6.5) and anionic deflocculating agents (Marlowet ^® 4561) in weight ratio of 1 :1 and the drilling fluid O contained the anionic deflocculating agent Marlowet ^® 4561.

The properties of the three drilling fluids were tested and the results are shown in Table 8.

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Table 8

As can be seen from Table 8 the combination of non-ionic and anionic alcohol ethoxylate as deflocculating agents not only improves the stability of the drilling fluid, but also improve the filtration properties of the fluid. This can be related to the good dispersability of the Mn ₃ 0 ₄ particles that led to formation of a thinner filter cake with low porosity and permeability.

Example 5

In this example different weighting agents are tested with alcohol ethoxylate as deflocculating agent. The deflocculating agent used in this example is Novel ^® 23E6.5. The weighting agents are API barite (BaS0 ₄ ), manganese tetraoxide (Mn ₃ 0 ₄ ) and micronized ilmenite (Fe ₃ Ti0 ₃ ) and the specific gravity of all three compositions was 2.1. The compositions of the oil based drilling fluids is stated in Table 9. 14

Table 9.

The rheology, filtration, electrical stability and static sag before and after heat aging at 150 °C were measured and the results are shown in Table 10.

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Table 10

As can be seen from Table 10 the use of alcohol ethoxylate gives very good results for drilling fluids containing the different weighting agents.