Summary
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.
Introduction
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).
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
27 June 2006
- Meeting paper published:
31 August 2006
- Revised manuscript received:
10 April 2007
- Manuscript approved:
22 July 2007
- Version of record:
25 April 2008
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