The effects of ph value of drilling fluids on mud additives

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The factors affecting the pH values of drilling fluids and the negative effects of the pH values > 12 of < 8 were studied. NaOH should be added promptly in increasing the pH values of drilling fluids. The concentration of the NaOH added in the mud caused a severe dispersion of drilled cuttings locally and the malfunction of the mud additives. At high temperatures and high salinity, NaOH was not suitable for use in increasing the pH values of drilling fluids; the more the NaOH was added, the faster the pH values were reduced, and the worse was the stability of the drilling fluid property. Some surfactants, such as AS, SP-80 and OP-10, can be used in minimizing the reduction of the pH values of the drilling fluids, but with some limitations. Drilling fluids were nor crying for a new pH buffer.

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Lye can not only enhance the hydration capacity of clay minerals, but also corrode silicoide such as clay minerals and quartz. Although adding inhibitors or using oil-based drilling fluid may effectively inhibit the hydration clay minerals, alkali corrosion of drilling fluid with high pH value remains going on, which may lead to the mechanical strength of shale weakening. Thus, it was speculated that alkali corrosion is one of the reasons leading to borehole instability of shale. Herein, an alkali corrosion experiment on quartz, clay minerals and shale with different grain size was designed and conducted, and combined with SEM analysis and rock mechanics measurements, we also explored alkali corrosion effects on shale rock structures and mechanical strength were studieel. The results showed that when the grain size of quartz and clay minerals is equivalent at micron scale, the alkali corrosion order of minerals is montmorillonite>quartz>kaolinite>illite>chlorite. In addition, the alkali corrosion rate of shale is controlled by mineral compositions and grain size, usually dominated by clay minerals. Under alkali corrosion, shale can produce a large number of corroded pores, consequently, its rock structure becomes loose and the compressive strength decreases significantly. Shale alkali corrosion occurs not only in oil-based drilling fluid during drilling, but also more evidently in water-based drilling fluid during drilling. So, weakening and effectively controlling the degree and range of alkali corrosion are one of the important ways of controlling shale borehole instability.
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