Tengiz field is a super-giant carbonate reservoir located in the Western Kazakhstan. The carbonate matrix consists of almost pure calcite, which makes it very attractive for acid stimulation. Over the years matrix acid stimulation has been successfully used in Tengiz to remove near wellbore damage and enhance well productivity.
Despite successful production response from acid stimulation in the past, it was noticed from poststimulation surveillance analysis that tighter and less depleted intervals were often left untreated. This observation suggested that chemical diverting agents were not effective for long completion intervals and further opportunity for improvement was identified. In addition, existing two phase retarded acid had limitation due to high viscosity and had known issues during plant flowback.
New acid stimulation design has been proposed and successfully executed in three newly drilled wells. This acid treatment utilized staged stimulation concept, where diversion was ensured by mechanical isolation of lower intervals with inflatable packer. New single phase retarded acid system was introduced to address operational and plant processing challenges with two phase retarded acid. Core flow tests were performed prior stimulation to evaluate effectiveness of different acid systems. Results of core flow tests and wireline log data were used during simulations on new generation of matrix acidizing modeling software to determine parameters for optimal wormhole creation.
Proper planning and thorough technical assessment enabled execution of staged acid stimulation with new acid system incident free with less than 10% incremental cost in all three wells. Post-job surveillance program is in place to evaluate acid diversion and production contribution from tighter and less depleted intervals.