The main objective from carbonate matrix acidizing design is to enhance the wormhole network and to avoid any induced damage caused by treatment fluid and pre flush/post flush fluids. Optimization of treatment fluid recipe aims to avoid potential source of damage such as sludge, emulsion and wettability alteration to maintain the new acquired permeability in the acid injection phase. The skin factor trend is modeled during injection and clean up phases to assess matrix acidizing interaction with formation.
This paper will be focusing on real-time skin trend monitoring model which takes wellhead pressure and flowrate values as a two main parameters for formation response interpretation during both Acid injection and flow back clean up phases. Injection multi-rate drawdown well test model was used to monitor the skin trend for injection phase and used forward multi-rate drawdown test model for clean-up flow back phase in post soaking period. The permeability contrast was considered to monitor the diversion effect.
During post flush phase, the treatment fluid (15% Acid including additives) was squeezed into the formation using post flush #1 (mainly solvent system) followed by post flush # 2(mainly 2% formic acid). The wells left shut-in for prolonged soaking period (24 hours). The results show that Skin trend at the end of flow back phase is more than skin value at the end of injection phase by considerable difference which indicate induced and added skin value. The suspect for the source of this increased skin value could be either prolonged soaking period and/or non-optimum treatment fluid design. During flow back period, excessive fluctuating in BS&W and pH was observed which can imply water block and rock wettability alteration.
The ratio of Productivity index at the end of clean-up to the index at the end of injection phase is defined in this paper as "Stimulation Efficiency" and the difference in the skin value at the end of acid injection and the skin at the end of clean up phase defined as "Acidizing Induced Skin" value. The average of this added Skin value ranges from +3 up to +10 with 30% to 40% average stimulation efficiency which lead to increase in skin value that was lowered during acid injection. Production test after each acidizing job validated the skin value ranges at the end of clean up phase.
This efficiency value combined with BS&W trend and content behavior during clean up phase, will give the stimulation engineer an overview to optimize the acid additives and post flush recipe to mitigate above mentioned damages.