Application of Integrated Reservoir Studies and Techniques To Estimate Oil Volumes and Recovery—Tengiz Field, Republic of Kazakhstan
- Kaveh Dehghani (Chevron ETC) | Dennis J. Fischer (Chevron Intl. Ltd.) | Mark Skalinski (Tengizchevroil Kazakhstan)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2008
- Document Type
- Journal Paper
- 362 - 378
- 2008. Society of Petroleum Engineers
- 5.8.7 Carbonate Reservoir, 1.6 Drilling Operations, 5.6.2 Core Analysis, 5.1.5 Geologic Modeling, 5.6.4 Drillstem/Well Testing, 5.5 Reservoir Simulation, 3.3.1 Production Logging, 5.5.11 Formation Testing (e.g., Wireline, LWD), 5.8.6 Naturally Fractured Reservoir, 2 Well Completion, 5.5.2 Core Analysis, 5.2 Reservoir Fluid Dynamics, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.3.4 Scale, 5.1 Reservoir Characterisation, 5.6.1 Open hole/cased hole log analysis, 5.2.1 Phase Behavior and PVT Measurements, 4.1.5 Processing Equipment, 5.5.5 Evaluation of uncertainties, 5.4.2 Gas Injection Methods, 1.6.9 Coring, Fishing, 5.6.3 Deterministic Methods, 5.5.8 History Matching
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Recent technical studies and probabilistic techniques have been integrated to build reservoir models for Tengiz field. Tengiz is one of the deepest supergiant oil fields in the world and is on the shore of the Caspian Sea within the Republic of Kazakhstan. Knowledge of the uncertainties inherent in oil recovery is a key to proper reservoir management of this important oil field.
Probabilistic techniques gave insights into the importance and ranking of key reservoir uncertainties. This ranking is used to reduce reservoir uncertainties and to guide reservoir management activities.
A structured Monte Carlo Technique has been used to construct several thousand static models of Tengiz field. Construction of Monte-Carlo models is fast and allows the impact of key uncertainties to be explored. Static reservoir uncertainties investigated include wireline log calibration, structural elevation, geostatistical parameters (such as variogram length), water saturation, and location of the boundary between facies intervals.
Investigating the uncertainty in the dynamic performance of the model is computationally intensive. Experimental design techniques were used to minimize the number of computationally expensive simulation runs. Key dynamic variables investigated include: oil in place (OIP), gas/oil relative permeability, permeability enhancements, vertical-to-horizontal permeability ratio, well productivity, the water/oil contact (WOC) depth, and production capacity.
The methodology for generating the range of possible outcomes for some development scenarios in Tengiz is discussed here. The application of Monte Carlo and experimental design techniques to identify the high impact static and dynamic parameters is reviewed.
The construction of a probabilistic distribution of OIP and recoveries is demonstrated for one development alternative for Tengiz field. This study found that for this development alternative, the OIP, gas/oil relative permeability, and ratio of vertical to horizontal permeability have the greatest impact on the estimated oil recoveries.
Tengiz field is one of the world's deepest supergiant oilfields under production. It is currently operated by Tengizchevroil (TCO), a joint-venture company led by Chevron since 1993 (50% Chevron, 25% ExxonMobil, 20% KazMunaiGas, and 5% LukArco). A common pressure gradient and similar oil composition across the reservoir indicate effective communication among different units of the reservoir. The oil is highly undersaturated and is currently being developed by primary depletion through fluid expansion.
A comprehensive plan to develop the Tengiz resource and supporting infrastructure requires a thorough appraisal of all the feasible alternatives. Although more than 1 billion STB have been produced, the field still is not fully appraised. To bracket the ranges of ultimate recovery and the economic outcome for each development alternative, the static and dynamic parameters with the most impact on the recovery are identified. Diligent data gathering and screening exercises reduce uncertainties in the reservoir and fluid properties, interconnectivity of various reservoir units, and distribution of rock facies.
Field Background Information
Tengiz Geologic Setting. Tengiz field is in western Kazakhstan, at the northeastern edge of the Caspian Sea (Fig. 1) (Collins et al. 2006). This isolated carbonate buildup was discovered in 1979 and produces oil from a Devonian to Carboniferous carbonate sequence. Since mid-2006, 120 wells have penetrated the reservoir and have gathered more than 6 km of continuous core and key modern wireline log data. The field is also covered by a 3D seismic survey.
The isolated carbonate buildup at Tengiz field formed in Middle Devonian time on a regional high and grew essentially uninterrupted until the early Bashkirian. It is positioned among a number of other significant carbonate buildups that occur in the Pricaspian basin (Fig. 2) (Collins et al. 2006). When growth of the buildup ended in early Bashkirian time, the carbonate sequence was blanketed by a sequence of sediments associated with closure of the Pricaspian basin. The carbonate buildup at Tengiz field consists of a series of backstepping platforms. The area of the platforms decreased dramatically over time from approximately 210 km2 in the late Famennian to approximately 90 km2 in late Visean time. A significant depositional wedge, which has a maximum thickness of more than 800 m, was created during the late Visean on an older platform sequence. This wedge is commonly referred to as the rim and flank (Fig. 3) (Kenter et al. 2006). The final stage of Tengiz deposition in Bashkirian time accumulated platform carbonates over the Serpukhovian sequence.
In 2001, a collaborative study by joint venture partners Chevron, ExxonMobil, and TCO established a stratigraphic framework for Tengiz field based upon an integration of all data collected to date (seismic, biostratigraphic, core, and well logs). This study forms the definitive stratigraphic reference for Tengiz field. This framework includes a hierarchy of second-, third-, and fourth-order sequences (Fig. 4) (Weber et al. 2003).
Within the main productive interval at Tengiz, seven bounding discontinuity surfaces (sequence boundaries and maximum flooding surfaces) are recognized on seismic data (Weber et al. 2003). Four supersequences (from oldest to youngest, Tournaisian-Lower Visean, Lower Visean-Upper Visean, Upper Visean-Serpukhovian, and Bashkirian) extend from the Famennian supersequence boundary to the Bashkirian (Collins et al. 2006). Each supersequence is divided into a transgressive sequence set overlain by a high stand sequence set. In older nomenclature, the producing intervals were designated as Unit 1 (Bashkirian, Serpukhovian, and upper Visean), Unit 2 (lower Visean and Tournaisian), and Unit 3 (Famennian).
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