OVERVIEW

Sandstone acidizing in horizontal wells is difficult because of placement limitations. Horizontal zones are typically longer than zones in vertical wells. Horizontal intervals can be many thousands of feet. Mechanical isolation with packers, which is the most effective method of placing acid in vertical wells, is not often practical in horizontal zones. Too many packer settings are required, extending beyond the limits to which current commercial packer systems can be effectively seated and reseated.

Diverter choices are also limited. Ball sealers are usually not effective in horizontal wells, because they tend to the bottom side of the pipe. Foam diversion can be effective, to some extent. In any case, it is known that any attempt to divert acid fluids with a nondamaging diverter is better than no diversion at all.

Economides and colleagues have presented a clever method for the diversion of acid in horizontal completions. ^[1] Their method uses coiled tubing to pump the acid treatment. Nonreactive fluid is pumped through the annulus between the coiled tubing and production tubing to create backpressure. This dual action forces the acid treatment fluids into the formation at or below the end of the coiled-tubing string.

This technique has also given rise to the pumping of viscous diverter stages down the annulus, while acid is pumped through coiled tubing, in both sandstones and carbonates. During treatment in this manner, coiled tubing can be reciprocated, or drawn along the treatment interval at rates corresponding to desired treatment volumes to be pumped...