TECHNOLOGY Soluble-silicate mud additives inhibit unstable clays

Ding Rui, Qiu Zhengsong, Li Jianying
University of Petroleum
Dongying, China

Although drilling fluids with soluble-silicate additives are not used as often as in the past, experiments in China indicate these additives can help stabilize problem clays in some well bores, and therefore warrant further attention.

Drilling fluids with soluble-silicate particles can help stabilize a well bore by blocking cracks on clay surfaces, by inhibiting the clay from hydration, and by reacting with the clay. The negative silica groups and particles block clay cracks through adsorption, sedimentation, and diffusion. In some extreme cases, the silica particles react with the clays, resulting in a firm cementation.

Silicate muds have been in use for decades and have found excellent levels of success in stabilizing clays in many oil fields.

Hole stabilization is a critical problem in drilling some clay formations. The drilling literature has many accounts of the application of soluble silicates in drilling fluids and well treating fluids, mainly for stabilizing formation clays.

The earlier silicate muds contained more than 30% sodium silicate.¹ Their rheological properties were hard to control, and their high alkalinity was a hazard.

In recent years, silicate muds have been used again, at lower concentrations, in China and in Russia to consolidate well walls.^{2 3} A number of deep wells have been drilled with dilute silicate muds in the south Xinjiang area. Potassium silicate was helpful in combating problems with differentially stuck pipe.

Experiments

Several experiments were designed to study the effectiveness of varying concentrations of potassium and sodium silicates on inhibiting clays. The goals were to determine the mechanisms of how soluble silicates inhibit clays and ultimately the optimum concentration for oil field use.

A base mud with a density of 1.03 g/cc was prepared with Anqiu bentonite, soda ash, and freshwater. A silicate was added after thorough hydration of the bentonite. The mud properties were then measured.

The particle distribution and zeta potential of the clay/silicate muds were determined with a microelectrophoresis analyzer (model DXD) and a sedimentation particle analyzer (Model SA-CP2). The concentrations of silicate were analyzed through acid/alkaline titration.

The formulas of the silicate used are Na₂O-3.02SiO₂ and K₂O-2.83SiO₂.

Cuttings from the southwest Tarim area were soaked in certain muds and solutions and rolled for 16 hr at 77 C. The particles retained on a 40-mesh screen were dried and then placed in a closed atmosphere with a relative humidity of 80%, maintained with saturated ammonium sulfate solution. The isothermal moisture adsorption was then measured.

Effect on bentonite

Table 1 [17569 bytes] lists the properties of the muds before and after a silicate was added. The silicates decreased the viscosity, yield point, and filtration of hydrated bentonite mud.

When silicate solution, bentonite, and soda ash were added simultaneously to freshwater, the solution's filtration was always much greater than that of hydrated mud, even after 2 months, suggesting that bentonite could not be hydrated in a silicate solution.

The effect of silicates on the particle distribution and zeta potential of bentonite mud are shown in Table 2 [21957 bytes]. The particle sizes of the muds are markedly increased with the addition of silicate. The zeta potential of hydrated bentonite rises significantly and that of unhydrated bentonite is less, indicating that the causes for particle enlargement are different. Perhaps the hydrated bentonite particles are enlarged by the adsorption of silicate, and the unhydrated particles are prevented from dispersion by the silicate.

Many particles settled faster than the clay particles. In silicate solutions with SiO₂:Na₂O ratios greater than 2.5, the negative silica groups predominantly exist as colloids.⁴ It is thought that silicates can deflocculate hydrated clay and inhibit unhydrated clays.

Adsorption

Specific amounts of silicate were added to 250 cc of base mud containing 1 g of bentonite. After 10 min of stirring, the liquid phase was separated through centrifugation, and the change in silicate content was determined. The procedure was repeated in which the base mud was replaced with distilled water; the silica retained in the liquid phase for Na₂O-3.02SiO₂ was 55% and for K₂O-2.83SiO₂ was 40%.

Fig. 1 [26520 bytes] shows the calculated adsorption data. Such adsorption most probably occurs at the clay edges where there are positive or fewer negative charges. The negative silica particles arrive at the entrances to the clay cracks through adsorption, sedimentation, and diffusion, preventing water from entering.

Shale hydration

The isothermal moisture adsorption data of recovered shale are listed in Table 3 [20522 bytes]. Lumps from shale cores, each about 30-40 g, were soaked in the muds and solutions shown in Table 3 [20522 bytes]. After 16 hr, the shale core lumps were taken out, washed, and soaked in freshwater for 24 hr. The samples were then weighed while wet and again after they were oven dried. The amount of water adsorbed by the cores is shown in Table 3 [20522 bytes].

It is clear that the hydration of shales treated with silicate was much lower than that of those treated in other systems containing no silicate. The polymer and FCLS may also be adsorbed by the clay but through less affinity than between the clay and silicates. The polymer and FCLS have no similarity with clay structure. Their colloidal particles are more difficult to settle than those of a silicate, leading to their lower abilities to decrease clay hydration tendency.

Silicate reaction with clays

Given some temperature increase and enough time, clays could react with orthosilicate to form new minerals which are mostly zeolites.5

Kaolin was soaked in solutions of 3% and 5% K₂O-2.83SiO₂ for 1 week at 150 C. Estimated from X-ray diffraction, the amount of kaolinite and quartz in the treated kaolin were both decreased to about half that in the untreated kaolin. The reaction products are mostly amorphous and firmly bound together, with a shear strength of 6 MPa, with the unreacted kaolin.

In another test, cuttings from 1,800-2,038 m in Well Qu 3 were pounded to less than 60 mesh. The cuttings were then soaked in 3% potassium silicate for 1-10 days at 105 C. There was no notable change, through X-ray diffraction, in clay content in the tested cuttings (Fig. 2 [25021 bytes]). It is considered that muds containing several percent of silicate may not react with most formation clays in common drilling operations.

In typical operations, formation clays are not mixed with silicate in enough extent, and the temperature is less than 150 C. in most cases. However, the clay-stabilizing effectiveness is markedly within a short silicate-to-clay contact time (several hours). Therefore, the main mechanism is not a chemical reaction, but a blocking of cracks between clay layers by silica particles, which inhibits the clays from being hydrated.

Acknowledgment

The authors thank Qian Qixiang, Gao Bo, and others at Taxinan Exploration Co. for initiating this project and for their assistance, and the China National Petroleum Corp. for financial support.

References

1. Rogers, W.F., Composition and properties of oil well drilling fluid, second edition, Houston, 1953.

2. Yuci, Zh., et al., "Dealing with the serious differential pressure pipe stuck problem in Keshen-1 well," Drilling and Completion Fluid, No. 2, 1994, pp. 34-38.

3. Khariv, I.Y., Ginkovskaya, Z.Y., and Bubna, M.G., "Composition for consolidation of well walls," SU 1,777,271.1992.

4. Iller, R.K., "Colloidal components in solution of sodium silicate," ACS Symposium Series, No. 194, 1981, pp. 95-114.

5. Mohnot, S.M., et al., "A study of mineral-alkali reaction," Society of Petroleum Engineers paper 13032, 1984.

The Authors

Ding Rui is an engineer at the University of Petroleum (East China) in Dongying, China. He is a researcher in oil field chemistry and teaches in the petroleum engineering department.

Rui graduated from the East China Engineering Institute in 1982 and received an MS from the University of Petroleum in 1994. He has published eight articles on drilling fluids in the journal of Oil Field Chemistry and the Journal of the University of Petroleum.

Qiu Zhengsong is an associate professor at the University of Petroleum (East China) in Dongying, China. He is a researcher in oil field chemistry and teaches in the petroleum engineering department. Zhengsong received an MS from the University of Petroleum in 1988. He has published numerous papers on petroleum technology.

Li Jianying is a professor at the University of Petroleum (East China) in Dongying, China. She is a researcher in oil field chemistry and teaches in the petroleum engineering department. Jianying received a postgraduate degree from the Peking Agriculture Institute in 1961. She has published numerous papers on petroleum technology.