Formate-based mud has been used to drill deep gas wells in Saudi Arabia since 2004. This mud typically contains XC-polymer, starch, polyanionic cellulose, and a relatively small amount of calcium carbonate particles. This mud (typically 90 pcf, pH of 10) is used to drill a deep sandstone reservoir (310°F). Various cleaning fluids were proposed to remove drilling mud filter cake; including: solids-free formate brine, formate brine doped with organic acids (acetic, formic, citric, and lactic acids), and enzymes. The main objective of this paper is to assess the effectiveness of these cleaning fluids in removing formate drilling mud filter cake.

A dynamic HPHT cell was first used to determine characteristics of the formate mud filter cake. Drilling mud and completion fluids were obtained from the field. The experiments were conducted using ceramic disks at bottom hole conditions. Compatibility tests between formate brine and cleaning fluids were performed at 300°F and 500 psi using HPHT visual cells (see-through cells).

Circulation of solids-free formate brine did not remove the external filter cake. Citric acid at 10 wt%, formic acid, and lactic acid were found to be incompatible with formate brine at room temperature. However, these acids were compatible with formate brine at temperatures greater than 122°F. Mixing of these acids with the formate in the field may cause operational problems. Only acetic acid and enzymes were compatible with formate brine. A formula was developed that is compatible at room and reservoir temperature. This formula was effective in removing filter cake. A corrosion inhibitor was added to protect downhole tubulars. This paper will discuss the development of this formula and all tests that led to its development.

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