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.

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Accurate fracture characterization has a huge impact on production forecasts and evaluation of projects in Naturally Fractured Carbonate fields. Pulse/Interference Tests can give valuable information about fracture network by providing additional constraint for spatial fracture distribution between wells, which can be honored with application of geologically consistent trends (Levchenko et.al, 2017). However, operational execution of the Pulse/Interference tests is very challenging. Therefore, usually limited data are available from the planned surveillance jobs leaving large sections of the reservoir without information from Pulse/Interference tests, which could be used for calibration of the Fracture Model. Lack of Interference data can be resolved by examining already available measurements, which are generally used for other purposes. For example, the primary purpose of Pressure Transient Tests (PTTs) is obtaining kh and skin values, while Permanent Downhole Gauges (PDHG) are mostly utilized for recording reservoir pressure, which is used for conventional History Matching process. As it was found from this study, historical data from both PTTs and PDHGs could be a source of additional high value information of Interference tests occurring in the field, which nobody was aware of. Properly designed Pulse/Interference Tests are very difficult to execute in the field with high production deliverability requirements. However, examining historical data can reveal a lot of good quality Interference Tests, which were recorded and stored for decades, but not used for Fracture Model characterization. Additional information from Interference tests were applied to calibrate a full-field simulation model, significantly improving quality of the history match in comparison with previous models, and improved confidence in the production forecasts.

Drilling in a mature reservoir can involve multiple added risks over time including reservoir depletion that may cause serious drilling problems and significant cost increase. Effective management of these problems during well planning stage and during execution is crucial to prevent issues with drill pipe, casing and logging tools that are at a higher risk for differential sticking, Differential sticking has the potential to become a significant issue for drilling operations because of the wide variation in pressure in different parts of the Tengiz reservoir. The degree of depletion depends on the variability of the reservoir, which itself is dependent on the properties of the different geological facies. Reservoir hole section can have different pressure regimes, which require higher mud weight and thus high potential for differential sticking problems. Consequently, as the Tengiz reservoir pressures depletes, the risk of differentially stuck pipe, casing or logging tool increases. During current drilling campaign there were three wells that experienced major issues caused by differential sticking and lead to significant cost associated with sidetrack operations and lost in hole charges. Pore pressure predictions and proposed well trajectory are some of the most important factors for prediction of differential sticking issues. For predictions during well planning "differential factor" was defined, which incorporates well angle and reservoir pressure. As a result, it helps to understand if differential sticking risk is high and use preventative measures if needed to reduce it. Guidelines were developed to help to mitigate potential problems while drilling across differentially pressured zones and include identification of signs of sticking, bottom hole assembly design considerations, preventative mud treatments and reactive measures to optimize ability to drill wells and reduce non-productive time associated with differential sticking. After implementation of above mentioned measures there were only two minor differential sticking events observed.