Achieving effective stimulation across zones not readily accessible is a common challenge in the oilfield. Several authors1-3 have presented case histories in which horizontal wells completed with slotted liner, gravel pack, internal gravel pack, or plain sand screens were difficult to stimulate because there was no easy way to effectively remove drilling fluid filter cake from the wellbore walls. Similar inaccessibility is encountered when dealing with straddled completions, where out of necessity, the producing zone is placed behind straddle packers that are extremely challenging to reach. Another accessibility problem is encountered when a "fish" becomes caught up across the perforated interval and accessibility is again denied.
Should such wells produce trouble-free and meet expectations, there is not much to ponder; but if not, the options for executing an enhanced stimulation treatment have been limited. Generally, treatment has consisted of pumping and squeezing away the treatment into the perforations and hoping for the best. The preferred technique is to improve the injectivity into the perforations by "spotting" reactive fluids at the perforations or using some type of mechanical action such as "perforation jetting" to break up blockages in the perforation tunnels. This is only possible when perforations or pays are accessible through the wellbore.
These problems become more complicated if the zones of interest are sandstone reservoirs. A study4 in the late 1990s revealed a success ratio in the 30–40% range when treating such formations.
This paper primarily deals with a case history in which a sandstone reservoir, not previously matrix-acidized, was producing at sub-par rates, eventually declining to a negligible production status. The zones were completed between straddle packers and were only accessible through a sliding side door (SSD), 100 ft above the perforations.
The prudent decision was to stimulate, but make all efforts to maximize the chances of success. Apart from applying the best practices in sandstone acidizing technology, a true fluidic oscillator (TFO) was included as a stimulation tool. This tool provides a continuous pressure pulse in the fluid system that allows solids buildup within the perforations to fatigue and break up while the acid system works on the rock matrix.
The results from the treatment of this well were exceptional and will be detailed in this paper. The production improvement was 10-fold over its prestimulation performance.