Traditional inhibiting drilling fluids show high efficiency in laboratory tests; however they are low effective in field trials. At the present time the choosing of inhibiting fluids is carried out by a multilateral engineering approach. There is a range of parameters that are used to evaluate inhibiting properties viz. swelling, clay capacity, moisture and dispersive capacity, deformation and its residual strength of the core sample, etc. These parameters and all existing methods used for evaluating inhibiting properties are incorrect, that leads to significant differences between laboratory and field test results, revealing theory and practice contradiction. It is believed that the inhibiting properties of drilling fluids help to prevent hydration and swelling of clays and ensure borehole stability. But borehole stability depends on strengthening (casing) properties, not inhibiting.
According to this situation it is recommended to specify inhibiting and strengthening properties, separating their functions. Inhibiting properties are aimed to exclude excess volume of drilling fluid and to maintain its parameters stable by reducing hydrophilicity, hydration, swelling and ability to disperse clay cuttings. Strengthening (casing) properties are aimed to maintain stability of clays and argillites on the walls of the borehole. To define inhibiting properties the authors propose to use the following indicators: concentration of colloidal fraction while drilling in clays; the drilling fluid resistance to various external aggressive factors; the drilling fluid excess volume prepared while drilling in clays. To define strengthening (casing) properties the authors of the article propose to use indicator of moisture capacity of pressed clay samples (applicable to clay rocks with coagulation structural bonds) and indicator of rock destruction (applicable to argillites and clays with phase and transitional structural bonds).
This method was applied to the drilling fluids used on the Astrakhanskoye gas-condensate field: laboratory tests gave close results to the field tests, what confirm the correctness of the method.
References
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