The requirement for appropriate placement in matrix acidizing to achieve efficient, effective stimulation or removal of formation damage has long been recognized. Despite this, assessing the effectiveness of an acid treatment in terms of placement and treatment efficiency prior to field deployment remains a challenge.
This paper presents a scenario where carefully designed core flood tests have been deployed alongside the use of a near-wellbore simulator to model and predict acid stimulation using viscosified fluids for treatments in a range of scenarios. Dual linear core flood tests were conducted to assess placement and clean-up efficacy of a viscosified acid treatment.
The efficiency of the treatment was then modelled using a state-of-the-art computer simulator. The computer simulator used is one which has been used extensively to model and optimize scale-inhibitor squeeze treatments in long-reach/complex wells. Its capability was extended to evaluate effects of fluid viscosification (staged or otherwise) on stimulation treatments.
Cases where partial/localised clean-up of a damaged zone was achieved, resulting in uneven stimulation were also examined. Results obtained from the model were validated with dual-linear core flood tests using simulated damaged and non-damaged zones. Resulting laboratory injection/diversion was compared to model predictions.
Core flood tests highlighted the importance of laboratory simulation of viscosified treatments. These tests showed how improved placement can be achieved by careful fluid design using viscosity alterations. The model was used to demonstrate the benefit (or otherwise) of different degrees of viscosification on evenness of treatment placement for systems with permeability contrasts, pressure differentials, presence of a water-producing zone, and various degrees and depths of skin per zone (including consideration of the effects on placement of acid reactivity on this damage).
Results showed that often even modest viscosification of the treatment fluid (~20 cP) improved placement. The greater the fraction that was viscosified (for staged treatments), the more even the placement. Viscosification invariably improved placement where a difference in native permeability existed (as opposed to a difference due to damage) and in most cases reduced the fraction of treatment fluid entering a water-producing zone.