SILICON-BASED ADDITIVES IMPROVE MUD RHEOLOGY

Alexander P. Zakharov
Scientific-Industrial Enterprise of New Technologies
Moscow

Evgeny A. Konovalov
Aprelevka Division of the All-Union Research & Development Geological Oil Institute
Moscow

Environmentally safe mud additives made from water-soluble combinations of silicon, phosphorus, aluminum, and boron have replaced some of the conventional water-loss reducers and lubricating agents in certain drilling operations.

Mud additives that use these bases - combined with coal-alkaline chemicals, lignites, and acryl-type polymers - have been used as effective substitutes for hydrocarbon components in mud systems.

These additives have been found to both thin the mud and improve the mud inhibition level. On field tests in the Tyumen area of Russia, wells drilled with these inhibitors and thinners had fewer problems with bore hole stability and sloughing formations.

A distinct feature of the combined chemical additives is their ability to affect several mud properties. This cuts treatment time and often permits elimination of other costly chemicals. The additives have significant inhibitive influence on sloughing formations. Field tests have shown improved borehole stability and less clogging of the shale shakers.

Common industrial wastes, such as used sunflower oil and organic acids from the food processing industry, have found an application as a base material for lubricating additives. The large quantity of these industrial wastes in Russia makes them a good resource for future economical production of mud chemicals.

Field test results indicate that the lubricating agents reduce well bore friction up to three times less than that in conventionally drilled wells.

These mud additives have had success both onshore and offshore, including several test wells recently drilled in the Shtockmanovskoye gas field in the Barents Sea.

LABORATORY STUDIES

Sodium silicate and silicon organics were initially considered because the silicate hydrogels have extremely high adsorption activity and the silicon organics have water repelling characteristics. Acids or acid mineral salts are usually used as precipitating (or gel-forming) agents for sodium silicate solutions.

When mixed with sodium silicate, water-soluble acids follow similar reactions, resulting in the formation of silicon acid and a corresponding salt. The silicon acid forms immediately and enters a polycondensation reaction, which takes place between silicon dioxide particles.

With a pH 8, the inorganic polymer particles become amorphous with a large specific surface magnitude because of the catalyst action of the OH- ions. A final product of the reaction is some volume of amorphous substance, flake-like precipitation, or gelatin mass, depending upon the reaction conditions. Boron acid (or a composition of borax with acids and acid salts) was used as a gel-forming agent to obtain the least aggressive conditions for a silicate hydrogel filtrate.

Experimental studies with potentiometer titration and electrophorus methods proved the possibility of forming boron-silicate complexes with high acidity. The most probable scheme of interaction is between one silanol group and one OH group of boron acid. With the addition of borax, the pH range for gel-forming conditions becomes more alkaline. For drilling purposes, the gel formation time is low, usually less than 1 hr.

The specific features of boron which help in the formation of complexes are the small dimensions of a boron cation (0.2 A), the high charge (3+), and a number of stereochemical geometries. If an alkaline solution is added to a boron acid solution, the boron changes its geometry from triangular to tetrahedral.

With the pH around 8-9, the most probable boron form is the tetraborate ion B4O72-. Borate ions are precipitated easily by hydroxides and base salts of metals and are adsorbed by highly dispersed aluminum silicates.

The reaction of sodium silicate with hydrolyzing salts of three-valency metals (such as aluminum) has a number of benefits; aluminum sulfate salt is ideal from the perspective of obtaining stabilized aluminum silicate gels for use as a base material.

Aluminum salts, including aluminum sulfate, have a tendency to hydrolyze. The free ion of A13+ hydrates significantly, taking six molecules of water. With the addition of alkaline, the hydrated ion of aluminum forms soluble and insoluble hydroxide aluminum products.

The reactions from the interaction of aluminum salts with silicate and phosphate solutions are complicated. Experiments have shown that the reaction products promote interpore bridging, resulting in a sharp decrease of filtration through core samples. This function is very important for the completion of pay zones.

The process is pertinent to the application of water-soluble cellulose ethers and acrylates used widely in the drilling industry. The aggregating and complex-forming capabilities of aluminum sulfate can be used in the mixing of drilling muds.

The development of the chemical additives took into account the high reaction capability of the phosphates and the ability of the phosphates to peptize and stabilize clay suspensions. The reaction of the phosphates with aluminum salts in alkaline media results in the formation of water-insoluble aluminum combinations with hydroxide and phosphate groups. The combinations of phosphate with berates and sodium silicates are of interest because of the similarity in geometry and size of their structures.

BORON-SILICATE GEL

A silicon-based bridging agent was developed from the reaction of boron acid with sodium silicate. Sodium three-polyphosphate, a coal-alkaline material, and sodium silicate (7-15 vol %) were combined to form an efficient thinner and water-loss reducer for bentonite muds.

This mud additive has been used to prevent sloughing and caving of unconsolidated shales (siltstones). In field trials, the additive decreased the time spent circulating the hole clean and reaming the well.

The gel additive is a viscous liquid with a density of 1.12-1.13 g/cc, a funnel viscosity of 40-60 sec, and a pH of 10-11. Table 1 shows the results of the treatment of typical bentonite muds.

Most of the field testing has occurred on more than 30 wells in the Tyumen area. During the field trials, other common mud additives (carboxymethyl cellulose (CMC), polyacrylamide, etc.) were not needed except for lubricating and calcium-precipitating agents.

With the addition of 0.20.5 vol % of the thinner to the mud system, the shear strength of the treated bentonite mud decreased from 60-80 dPa down to 20-30 dPa. The water loss of the mud stayed constant or dropped slightly (1-3 cc). The gel strength dropped rather sharply along with an improvement in the well bore stability.

After the mud treatment, the well bore remained stable toy the duration of the drilling operation (up to 4 months); thus, this additive was used frequently before logging operations or the running of casing. Laboratory experiments at 120' C. and 20.0 MPa showed an increase in the duration of clay stability of 2-8 times for gel concentrations ranging from 0.35 to 0.5 vol %.

The boron-silicate gel was successfully tested during drilling in carbonate and halogenic formations in the Krasnoyarsk area of eastern Siberia. These applications, without the use of coal-alkaline and sodium three-polyphosphate, also helped prevent sloughing of the shale formations and maintained well bore integrity.

SILICON-ORGANIC THINNER

Another recently developed thinning agent is based on a silicon-organic material rather than boron acid. The addition of a polyacrylamide polymer to this thinner in a bentonite mud maintains the mud rheology and viscosity during the drilling of dispersible clays. This thinner has a high flocculating action (Table 2).

The silicon-organic thinner underwent field testing in wells drilled mainly in the European far north (Komi Republic) and in the Tyumen area. The field testing helped determine the chemical's inhibiting ability during drilling through easily dispersed clays. This additive may replace costly chemicals such as water-soluble cellulose ethers.

The best results occurred with treatment just after surface casing was cemented. On the test wells, the surface casing was set at depths of about 100-300 m. Typically, the cement plug in the surface casing was drilled out with brine treated with sodium carbonate. This brine was then treated with the silicon-organic thinner at a concentration of 1.5-2.0 vol % before drilling continued.

This treatment resulted in drilling mud with the following parameters:

Density, 1.04-1.07 g/cc
Funnel viscosity, 16-18 sec
Water loss, 7-10 cc
pH, 8-9
Plastic viscosity, 2-4 mPa sec.

These field tests in the Komi Republic showed that the silicon-organic thinner can prevent easily dispersed clays from thickening the drilling mud. The high flocculating ability allowed cuttings to be easily cleaned off the shale shakers, and the density of the circulating mud increased only slightly. The use of this thinner has increased the penetration rate 1.5-1.7 times and reduced the consumption of bits by 30-50% in this area. During tripping operations, there were no incidents of balling or drillstring sticking.

Comparative studies were performed on bentonite muds treated with this thinner, oil-based muds, and potassium muds. The silicon-organic treated muds showed the lowest amount of wetting of high-colloidal clays with no deterioration of the pay characteristics of productive formations.

LUBRICATING AGENT

An ecologically safe lubricating agent was developed from a boron-silicate vegetable oil mixture (an emulsion of used sunflower oil left over from the thermal processing of fish products).

The boron-silicate compound provided gel strength and the glycerine from the sunflower oil provided lubrication for the mud (Table 3). Sunflower oil was selected because it is plentiful at low cost and does not damage the environment.

This material was used as a substitute for crude oil and other petroleum-based lubricating agents in drilling fluids. One of the goals of the research was to find a non-polluting (not crude-oil based) additive which could help prevent failures during drilling of complicated geological structures.

Table 4 lists some of the additive's lubrication properties. The lubricity tests indicated that this mixture has several advantages over standard lubricating agents such as crude oil.

Field testing occurred on some wells drilled through terrigenous formations in the Tyumen area, in the Komi Republic, and in the Murmansk area. The initial field tests on wells drilled in the Komi Republic showed that treatment of bentonite muds with 0.30.8 vol % of the boron-silicate lubricating agent did not deteriorate gel strength, geological properties, or water loss characteristics. Standard laboratory studies conducted on samples of drilling mud taken from the test wells showed a decrease in sticking characteristics of the filter cake.

The field tests in the Arcticmorneftegasrasvedka enterprise in Murmansk are of the greatest interest. The first test was carried out on Well No. 4 at the Shtockmanovskoye gas field in the Barents Sea in the summer of 1991. The treatment of the drilling mud started at a depth of 1,544 m. The circulating drilling mud (350 cu m) was treated with 2.1 cu m of lubricating agent (0.6 vol %) during two circulation cycles.

The Theological and water-loss characteristics of the drilling mud remained essentially constant, yet friction characteristics improved significantly. Control measurements on a friction meter indicated that lubricating properties of the drilling mud improved by 24% on average. The well was successfully drilled to a final depth of 2,500 m.

This lubricity additive has been successfully tested on two other wells in the same field in late 1991. Further application of the lubricity additive occurred in the Tyumen area in 1991. The first field tests in this region showed a drop in the friction factor of 1.5-3.0 times the typical level. No complications were observed during the field tests.

EFFECT ON RESERVOIR

These additives have typically been used during drilling out and later during completion of productive formations. A number of experiments were undertaken to determine the effects on reservoir properties.

Because of the various sizes of solids and the high adsorption activity of the modified silicates, it was thought that the treated muds would create little low-permeability bridging. The bridging prevents significant penetration of fluids during drilling and cementing operations. To test this assumption, special experiments were designed to evaluate core permeability. These experiments simulated the downhole processes during drilling out of productive formations.

For the first stage of the experiment, the kinetics of drilling mud filtration through sandstone samples was studied by alternating the circulation scheme from dynamic to static conditions. These results allowed the calculation of the thickness of a bridging zone and the radius of penetration of a drilling mud filtrate. The test period corresponded to the period from the drilling out of the production formation until the casing was run.

The second stage of the simulation experiment evaluated the influence of drilling mud filtrate on the permeability of oil-bearing sandstones. The magnitude of the expected productivity was the ratio of actual formation productivity to the productivity of an ideal undamaged formation.

The measurements of instant filtration, specific filtration, filter cake thickness, and filtrate composition were used in the calculation of the expected productivity ratio and in the evaluation of the filter cake forming capabilities of the muds. The experiments showed that the penetration zone of drilling muds treated with the chemicals additives does not exceed 610 cm, an acceptable level.

BIBLIOGRAPHY

Konovalov, E.A., Zakharov, A.P., et al., "Silicate Inhibitors For Drilling Muds," Express-Information of VNIIGasProm, series on "Geology, Drilling, and Development of Gas and Gas-Condensate Fields," Moscow, No. 3, pp. 9-14, 1991.
Konovalov, E.A., Zakharov, A.P., et al., "Silicate Hydro-Gel Drilling Muds and Chemicals For Wet Drilling," information review of the Ministry of Geology of the U.S.S.R., series on "Scientific-Technical Achievements and Modern Experience in Geology and Exploration of Resources," Moscow, No. 5, pp. 53-64, 1991.