A sloping depositional surface, known as clinoform, is commonly associated with prograding strata deep into water and this surface can be imaged with seismic (J.L.Rich, 1951). During a sea level drop carbonate sediment factories tend to shut down and results in periods of non-deposition. These clinoform surfaces can be cemented, resulting in a partially sealing effect. Therefore characterizing clinoforms is crucial to better understand the reservoir dynamics and hydraulic communication throughout the field. In the Karachaganak field, Wireline Formation Pressure acquisition along the sub-horizontal well section has played a major role in the identification of semi-transmissible flow restrictions in the field. In particular, plots of depth corrected pressure measurements against distance from wellhead showed clear discontinuities confirming the existence of pressure baffles. Once these clinoforms are identified, their properties have to be calibrated against the measured pressure data, process that historically has been implemented by manual trial-and-error approach which is time-consuming and often frustrating. This paper proposes a methodology based on assisted history matching solutions to constrain the properties and sealing degree of clinoform's regions to both Wireline Formation and Static Bottom-Hole Pressure data. The proposed approach allows engineers to integrate geological and reservoir engineering workflows into a single model driven by state-of-the-art history matching optimization techniques. This makes possible to systematically sensitize the properties of the clinoforms assuring consistency between static and dynamic models. As a result, observed pressure has been matched, and hence the characterization of the clinoforms properties was considerably improved in a short time compared with the timeframe required by the traditional manual approach. The presented workflow is a valuable tool to set methods and gain experience using assisted history matching techniques, and furthermore, it contributes to a change in history matching philosophy by semi-automating laborious tasks achieving faster and more physically coherent solutions.
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.
Sour gas injection (SGI) pilot was successfully implemented in Tengizchevroil (TCO) in 2008, successfully achieving pressure maintenance in Tengiz platform, re-injecting gas with high H2S concentration and improving oil recovery within pilot area. Over the years, gas breakthrough and GOR increase has been observed in majority of producers in SGI pattern. Due to limited surface gas handling capacity, producing SGI pattern wells with elevated GOR and H2S has been a challenge. At the same time, it is critical to manage voidage replacement ratio in the pilot area to maintain sufficient miscible gas sweep and ensure enough pressure release. Inability to inject all the sour gas to the reservoir leads to equivalent oil production cut at the Plant. Specialized SGI Operating Strategy has been in place since the commencement of the project with the aim of developing broader understanding of injected gas front evolution and operating the SGI Pilot in a way to maximize value for TCO by balancing between maximizing lower GOR/H2S front-end plant production vs. maintaining sufficient well injectivity. The production and injection well targets and surveillance activities were tailored towards fulfilling these strategic objectives. The operating strategy has been revisited several times to ensure appropriateness to business needs at any given time. Specialized reservoir simulation model is utilized for short and long-term injection forecasts to develop plans to provide sufficient well injection capacity at any given time. This paper will discuss challenges and best practices associated with (1) operating elevated GOR/H2S wells in a gas constrained environment while performing reservoir data acquisition program in highly congested area to fulfill SGI pilot objectives and (2) finding the trade-off between front-end plant production and injection goals.
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