GB2178785A - Well drilling fluids, fluid loss additives therefor and preparation of such additives - Google Patents

Abstract

A fluid loss additive for well drilling fluids consists of air blown tall oil pitch, preferably having a softening point (ring and ball) of 100 DEG to 165 DEG C.

Description

SPECIFICATION Well drilling fluids, fluid loss additives therefor and preparation of such additives The present invention is concerned with well drilling fluids, with fluid loss additives for such fluids, and with the preparation of such fluid loss additives. The term "drilling fluid" is used herein in its conventional sense, that is to include completion, packer and workover fluids, as well as drilling fluids as such.

During the rotary drilling of oil or gas wells a "drilling fluid" or "drilling mud" is continuously pumped to the bottom of the bore hole where drilling is taking place and recirculated to the surface. The circulating drilling fluid serves to lubricate the drill as it cuts, removes cuttings from the bottom of the bore up to the surface, and exerts a hydrostatic pressure on the drilled formation which prevents the escape of oil, gas or water up the bore hole.

Aqueous drilling fluids comprising a suspension of clay in water are well known in the art, but for applications where it is necessary to stabilse the well bore, for example when drilling water-sensitive formations or when the bottom hole temperature is high, fluids in which the liquid phase consists of oil or of an oil/water emulsion are more usually used.

Emulsion drilling fluids are both cheaper and easier to handle than fluids in which the liquid phase consists entirely of oil. Such emulsion fluids are available as water-in-oil formulations, i.e. oil as the continuous phase and water as the disperse phase, or as oil-in-water formulations. However, the latter tend to damage oil-bearing formations as a result of the filtration of water from the fluid into the formation, which causes a reduction in the permeability of the formation and a decreased rate of oil production.

Water-in-oil emulsions are, therefore, generally preferred for applications requiring an emulsion-based drilling fluid. Such a drilling fluid typically comprises the following constituents: (1) oil - this is generally diesel oil or a low aromatics oil; (2) emulsified water droplets - these may be water as such or a sodium chloride or calcium chloride brine; (3) finely divided solids - these are present to increase the density of the fluid - a wide variety of solids may be used, such as barite; (4) viscosifier - to suspend the solids - organophilic clays are commonly used for this purpose; (5) emulsifier - to stabilise the emulsion of water droplets in the continuous oil phase; (6) wetting agent - this constituent coats the solid particles and stabilises the dispersion of the solids in the oil, thus avoiding coalescence and water wetting of the solids;; (7) lime - to saponify fatty acids and increase the pH of the aqueous phase; (8) fluid loss reducing additive.

A single material may act as ingredients (5) and (6); thus blown tall oil has been used to fulfill both functions, though it is more usually used simply as the emulsifier (5) with another material being used as the wetting agent (6).

The main difficulty found with water-in-oil emulsion drilling fluids is the tendency of the liquid phase to leach into surrounding permeable formations. This phenomenon is reduced by the solid, but porous, filter cake formed on the walls of the borehole by the solids present in the drilling fluid, but it is not completely prevented and it is usual, therefore, for such drilling fluids to contain a fluid loss reducing additive, (8) above, to minimise such loss. Among the materials which have been used as fluid loss additives are, for example, asphalt, gilsonite and amine lignite.

We have now found that tall oil pitch which has been air blown so that its softening point is raised, is an excellent fluid loss additive for emulsion-based, and water-based, drilling fluids. Preferred blown tall oil pitches for this purpose are those having a softening point (determined by the ring and ball test, as are all tall oil pitch softening points given herein) of from 100" to 165"C.

According to one aspect of the present invention, therefore, there is provided a well drilling fluid which contains, as a fluid loss additive, an air blown tall oil pitch.

Blown tall oil pitches having softening points below 100"C are known and are used, for example, in printing inks; we are not aware that blown tall oil pitches having softening points of 100"C or above have previously been described or commercially available.

According to another aspect of the present invention, therefore, there is provided a fluid loss additive for drilling fluids, which consists of air blown tall oil pitch having a softening point of from 100"C to 165"C.

Tall oil pitch is cheap and readily available as the residue from the distillation of tall oil; it is low in fatty acids and high in fatty esters, higher alcohols, and sterols. Blowing air through tall oil pitch at an elevated temperature partially oxidises and polymerises the material and drives off volatiles. Blowing reduces the volume of the pitch by about 30% and increases the viscosity/softening point; the softening point of the resultant blown pitch is therefore indicative of the degree of oxidation-polymerisation which has occurred.

We have found that optimum properties as a fluid loss additive are given by blown tall oil pitches which have a softening point of from 125" to 130"C.

The blown tall oil pitches used according to the invention are obtained by blowing air through tall oil pitch at a temperature of from 260" to 275"C and continuing blowing until the blown pitch has the re quired softening point. The reactions which take place during blowing are overall exothermic so that the temperature may rise above 275"C; an increase in temperature above 275"C is acceptable. The temperature range 260" - 275"C represents a threshold temperature which must be achieved in order for the desired reactions to take place.

The pitch may be blown with other oxygen containing gases, but there is no advantage in using any gas other than air. Depending on the rate at which the air or other oxygen-containing gas is passed, the process typically takes from 8 to 16 hours under optimum conditions.

The drilling fluids in which the fluid loss additive according to the invention is used may in all other respects be conventional and suitable constituents and their proportions for such drilling fluids will be known to those skilled in the art. It is generally preferred to use the fluid loss additive at the rate of 5.0 to 8.0 pounds per barrel (14.3 to 22.8 kg/m3), although for certain special applications the rate may be as low as 1.0 or as high as 15.0 pounds per barrel (2.85 or 42.8 kg/m3).

In order that the invention may be more fully understood, the following examples are given by way of illustration only.

Example 1 A base mud of the following composition was prepared: Pounds per barrel (kglm3) low aromatic oil 0.543 ( 1.55) calcium chloride 31.9 ( 90.9) water 0.219 ( 0.62) barite 273.0 (778.1) organophilic clay 6.0 ( 17.1) emulsifier 3.0 ( 8.55) wetting agent 1.0 ( 2.85) To this composition was added 5.0 pounds per barrel (14.3 kg/m3) of blown tall oil pitch having a softening point of 125"C.

Samples of the composition with and without the blown pitch additive were aged at 1210C in a rotating oven for 16 hours.

Physical characteristics, that is plastic viscosity (in centipoises), yield point (in lbs/100 sq. ft.and Pascals), gel strength 10 second (in Ibs/100 sq. ft. and Pascals), gel strength 10 minutes (in Ibs/100 sq. ft. and Pascals), and electrical stability (in volts), of the untreated and treated drilling fluids were then measured and their respective fluid losses were determined in a high pressure high temperature filtration unit which simulated the conditions found in field use. The results obtained are shown in Table 1.

TABLE 1 Untreated Treated with 5.0 lbslbbl blown pitch Plastic viscosity, cps 25 31 Yield point, Ibs/ 100 sq. ft. (Pa) 6 (2.9) 10 (4.8) Gel strength 10 second, Ibs/100 sq. ft. (Pa) 4 (1.9) 4 (1.9) Gel strength 10 mien., lbs/100 sq. ft. (Pa) 6 (2.9) 10 (4.8) Electrical stability volts 320 360 HTHP fluid loss at 500 psi (3.4 Pa) and 121"C oil (ml) 7.2 4.0 water (ml) 1.6 0.0 It will be seen that the presence of the blown pitch in the treated fluid significantly reduced fluid loss and, in particular, led to no loss of water at all.

Example 2 Example 2 A base mud of the following composition was prepared: Pounds per barrel (kgim3) low aromatic oil 0.657 ( 1.87).

calcium chloride (77%) 42.6 (121.4) water 0.254 ( 0.72) rev dust (inert clay) 15.0 ( 42.8) organophilic clay 6.0 ( 17.1) emulsifier 4.0 (11.4) wetting agent 2.0 ( 5.7) To this composition was added 8.0 pounds per barrel (22.8 kg/m3) of blown tall oil pitch having a softening point of 125"C.

Samples of the composition with and without the blown pitch additive were aged at room temperature and at 94"C in a rotating oven for 16 hours, respectively.

Physical characteristics and fluid losses were measured as described above with the addition of an API fluid loss test which measures filtrate loss across a similar filtration unit having a pressure differential of 100 psi (0.68 Pa) and at ambient temperature. The results obtained are shown in Table 2.

TABLE 2 Aged at room temperature Aged at 94"C Air blown pitch, ppb (kg/m) 0 8 (22.8) 0 8 (22.8) Plastic viscos ity, cps 20 25 35 49 Yield point, lbs/100 sq.

ft. (Pa) 9 (4.3) 9 ( 4.3) 25 (12) 26 (12.5) Gel strength 10 second, lbs/100 sq. ft. (Pa) 4 (1.9) 3( 1.4) 14 ( 6.7) 17 ( 8.2) Gel strength 10 min., Ibs/100 sq. ft. (Pa) 7 (3.4) 5 ( 2.4) 22 (10.6) 40 (19.2) API fluid loss 100 psi.

(0.68 Pa), ml 2.6 2.2 3.0 1.4 HTHP fluid loss at 500 psi (3.4 Pa) and 149"C Oil (ml) 7.0 4.0 9.0 2.0 Water (ml) 4.0 Trace 3.0 0.0 In both cases, the presence of blown pitch significantly reduced fluid loss and effectively removed water from the filtrate.

Claims (10)

1. A well drilling fluid which contains, as a fluid loss additive, an air blown tall oil pitch.

2. A drilling fluid according to claim 1, in which the air blown tall oil pitch has a softening point of from 100" to 165"C.

3. A drilling fluid according to claim 1 or 2, in which the air blown tall oil pitch has a softening point of from 125" to 130"C.

4. A drilling fluid according to any of claims 1 to 3, in which the drilling fluid is of the water-in-oil emulsion type.

5. A drilling fluid according to any of claims 1 to 4, which contains the additive in an amount of from 1.0 to 15.0 pounds per barrel.

6. A drilling fluid according to any of claims 1 to 5, which contains the additive in an amount of from 5.0 to 8.0 pounds per barrel.

7. A fluid loss additive for drilling fluids, which consists of air blown tall oil pitch having a softening point of from 100" to 165"C.

8. An additive according to claim 7, which consists of blown pitch having a softening point of 125" to 130 C.

9. A process for making an additive according to claim 7 or 8, which comprises blowing air through tall oil pitch at a temperature of from 260" to 275do and continuing blowing until the blown pitch has the desired softening point.

10. A drilling fluid according to claim 1, substantially as herein described in Example 1 or 2.