Staged Acid Stimulation Initiative in Giant Carbonate Reservoir
Abstract: 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 interva… Show more
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Cited by 5 publications
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It is becoming more common for operators around the world to use alleged conformance control completions as a means of managing inflow zones and controlling production. When this type of completion is introduced in a field, it is extremely important to analyze its effectiveness at very early stages of the project to achieve maximized zonal contribution together with proper compartmentalization in current and subsequent completions, since this will have a significant impact on the future life of the entire field. A thorough analysis should include understanding zonal isolation before and after acid stimulation, fluid distribution inside the compartments during the treatment, and confirmation of completion integrity. Analyzing completion performance by introducing additional downhole monitoring systems or devices is costly and is more appropriate for the long term. Another option, surveillance with wireline technology, may not provide definite conclusions due to limited acquisition extent. Alternatively, coiled tubing (CT) can provide a fit-for-purpose integrated solution to data acquisition and analysis challenge. The proposed approach uses distributed temperature sensing technology along with real-time data streaming capabilities to provide an instantaneous insight on wellbore dynamics, thus enabling informed decisions on treatment optimization, as well as yielding reliable information on interzonal communication. This study is based on a success story of intervening with CT on 10 wells, with a total of 40 compartments in a carbonate reservoir in the Caspian region. Distributed temperature evolution models are used to build a signature library characteristic of specific flow events in the wellbore. The study consists of distributed temperature surveys lasting from 30 minutes to 6 hours that were acquired before and during the acid stimulation of each conformance compartment. Unique temperature features are identified in specific flow events, such as communication between compartments, loss of completion integrity, and effective stimulated area determination, to name a few. Those events are hypothesized and corroborated using downhole point measurements. A significant finding is that communication between zones occurs through several possible paths (i.e., through the formation/matrix or via the completion). The stimulation strategy can be modified accordingly, leveraging downhole data to maximize completion efficiency. This combination of transient distributed temperature and point measurement data provides an insight into wellbore and reservoir flow dynamics and facilitates an optimized stimulation strategy.
It is becoming more common for operators around the world to use alleged conformance control completions as a means of managing inflow zones and controlling production. When this type of completion is introduced in a field, it is extremely important to analyze its effectiveness at very early stages of the project to achieve maximized zonal contribution together with proper compartmentalization in current and subsequent completions, since this will have a significant impact on the future life of the entire field. A thorough analysis should include understanding zonal isolation before and after acid stimulation, fluid distribution inside the compartments during the treatment, and confirmation of completion integrity. Analyzing completion performance by introducing additional downhole monitoring systems or devices is costly and is more appropriate for the long term. Another option, surveillance with wireline technology, may not provide definite conclusions due to limited acquisition extent. Alternatively, coiled tubing (CT) can provide a fit-for-purpose integrated solution to data acquisition and analysis challenge. The proposed approach uses distributed temperature sensing technology along with real-time data streaming capabilities to provide an instantaneous insight on wellbore dynamics, thus enabling informed decisions on treatment optimization, as well as yielding reliable information on interzonal communication. This study is based on a success story of intervening with CT on 10 wells, with a total of 40 compartments in a carbonate reservoir in the Caspian region. Distributed temperature evolution models are used to build a signature library characteristic of specific flow events in the wellbore. The study consists of distributed temperature surveys lasting from 30 minutes to 6 hours that were acquired before and during the acid stimulation of each conformance compartment. Unique temperature features are identified in specific flow events, such as communication between compartments, loss of completion integrity, and effective stimulated area determination, to name a few. Those events are hypothesized and corroborated using downhole point measurements. A significant finding is that communication between zones occurs through several possible paths (i.e., through the formation/matrix or via the completion). The stimulation strategy can be modified accordingly, leveraging downhole data to maximize completion efficiency. This combination of transient distributed temperature and point measurement data provides an insight into wellbore and reservoir flow dynamics and facilitates an optimized stimulation strategy.
Acid treatment is a common well stimulation technique widely used for both oil and gas wells. However, there is a challenge, that for any given acid solution, rock lithology and permeability, and reservoir conditions, there are optimum values of acid injection volume and rate. Deviation from these optimal values during stimulation treatments reduces the acid job efficiency. We developed a robust and efficient method of identification of the optimum parameters based on digital core approach. The developed workflow includes: a) construction of a digital avatar of a core sample using 3D microCT tomography; b) pore-scale direct reactive flow modeling using a combination of the chemical kinetics/thermodynamics (in assumption of partial local equilibrium) with the method of density functional theory in hydrodynamics (an efficient tool for pore-scale modeling of multiphase flow able to handle different complex physical phenomena); c) core scale simulations in the framework of the Darcy based approach using upscaling from the results of the direct pore-scale simulations; d) input of the obtained parameters into the acidizing simulator to determine optimum acid type, rates, and volumes. We illustrate the developed workflow on example of an optimum injection rate determination in the case of Silurian dolomite dissolution by hydrochloric acid. The pore-scale simulations were performed using 3D microCT models with 2.5 μm/voxel resolution. These simulations allowed to determine the dependence of dolomite dissolution rate on the fluid injection rate and predict the transport properties of damaged rock. The correlations obtained from high resolution simulations were then applied in core-scale modeling of dissolution process using continuous Darcy based model (with 100 μm/voxel resolution). The transport properties of a core were populated using the results of pore scale simulations. Then several core scale simulations of dolomite dissolution with different acid injection rates were performed to obtain numerically the dependence of its influence on the number of pore volumes injected until the breakthrough (PVBT). PVBT dependence on the injection rate in a form of characteristic curve was incorporated into the advanced acidizing simulator. Being calibrated this way, the simulator was then used to model acidizing treatment in a dolomite reservoir with the similar properties as digitally acidized core. Modeling showed that post stimulation skin values are lower and expected wormhole length is bigger when digitally calibrated pore volume to breakthrough (PVBT) curve is used, if compared with modelling of the same treatment with non calibrated acid-rock interaction curves. Consequently, outcomes of this well scale modeling suggest that the use of digitally calibrated PVBT curves results, for this case, in optimization of required acid volume and associated operational footprint. The suggested approach improves the process of obtaining PVBT characteristic curve by application of digital core analysis technique. It allows to test numerous "what if’ scenarios and to evaluate the effect of different factors on mineral dissolution rate at pore scale. This paves the way for improvements in acidizing job design by increasing the consistency between the models used for reactive flow modelling and pore scale heterogeneity of real rocks.
Oil and gas companies operating carbonate oil and gas condensate fields in Kazakhstan have been carrying out acid stimulation activities leading to a substantial increase in hydrocarbon production. Nearly all treatments were considered a success. Nevertheless, a certain level of optimization in the production enhancement methods that could, potentially, have brought additional technical and financial benefits, were overlooked due to various reasons. A comprehensive review of historical treatments on several fields located in West-Kazakhstan region was performed to identify areas to improve post-stimulation well performance. This review identified improvements including "cleaner" fluid selection, optimised design and treatment schedules. Historical treatments in the oil field typically used straight hydrochloric acid as the main acid, polymer-gelled (self-diverting) acid as the chemical diverter, and linear guar gel for displacement, and diagnostic tests. The application of a modern single-phase retarded acid to replace the straight hydrochloric acid was identified as a key improvement that would yield more efficient wormhole generation and an improved stimulation ratio. Another opportunity for improvement was to upgrade the chemical diversion system from polymer-based self-diverting acid to a viscoelastic surfactant-based (polymer-free) diverting acid system. The use of an oil-based displacement fluid with high retained permeability instead of linear gel and to reduce the hydrostatic pressure post-acidizing, thereby improving flowback, was also employed. Extended core flow testing for regained permeability and solubility were carried out with several acid systems to compare their capabilities and efficiency to create conductive wormholes, and their dissolution capacities. Additionally, emulsion, and sludging tendency upon contact with wellbore tubulars and formation crude was checked to verify the acids’ compatibility with hydrocarbons produced from the target reservoir. After the prerequisite laboratory testing, field trials commenced applying various combinations of fluid technologies in high-rate matrix stimulation treatments. The optimizations resulted in higher (normalized) post-stimulation productivity index (PI), facilitated formation cleanup, and enabled more efficient operations. A similar approach is, currently, being implemented in other stimulation projects in the region, and the results are being replicated. As has been mentioned above, one of the main enhancements implemented as part of this work is the employment of the single-phase retarded acid. Most of the published literature discussing application of the acid covers the cases of stimulation of relatively hot reservoirs (BHST>100°C) as acidizing of high-temperature carbonate rock using traditional hydrochloric acid is a great challenge. The current paper provides details of the case studies, where the acid system was successfully implemented in combination with several other stimulation technologies for mid-temperature ranges. One of the objectives was also to assess whether application of reduced volumes of the retarded and diverting acids would still lead to improved wells’ productivity. Positive results of the laboratory studies, treatment modeling, and field trials were validated by the increasing normalized post-stimulation PI with each optimization step.
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