Acid stimulation of limestone and chalk reservoirs is a popular method to increase production. By flowing acid through hollow cylinder chalk specimens in the laboratory, we have created specimens with wormhole networks that have allowed us to study the impact of acidizing on rock strength in dedicated solids production tests. Comparing acidized samples with virgin reference samples reveal a reduction in the rock strength and earlier onset of borehole failure and solids production. The same effect is seen for both increasing depletion and increasing drawdown, but it is much more pronounced for the increasing drawdown case. Even though the rock strength is clearly reduced by the acidizing, there are no signs of wormhole collapse away from the near-borehole region.
Injection of acid into limestone or chalk reservoirs may be a cost effective way to stimulate oil production. The acidizing process results in the formation of wormholes that may act as flow channels and therefore increase the near wellbore permeability of the formation [1,2]. However, these wormholes may have unwanted side effects on formation integrity, especially for high depletion or drawdown, and these effects are not fully understood .
We have previously studied the mechanical stability of wormholes created by acidizing of 1.5" chalk plugs by performing hydrostatic compaction tests, revealing that the wormholes collapse a few MPa below the onset of pore collapse, with a significantly larger pore-collapse pressure for oil-saturated samples as compared to saturation with brine . In addition, the acidized samples showed a reduction in the pore-collapse pressure compared to intact plugs.