Early Evaluation of Uncertainties in the Incremental Condensate Recovery Through a Gas Cycling Process
- G.J. Massonnat (Elf Aquitaine Production) | C. Bachtanik (Elf Aquitaine Production) | E. Tutenuit (Elf Aquitaine Production) | P. Gouel (Elf Aquitaine Production) | N. Carles (Elf Aquitaine Production)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 1997
- Document Type
- Journal Paper
- 33 - 47
- 1997. Society of Petroleum Engineers
- 5.5.5 Evaluation of uncertainties, 1.6 Drilling Operations, 5.8.8 Gas-condensate reservoirs, 5.4.3 Gas Cycling, 5.5.8 History Matching, 4.3.4 Scale, 5.1.5 Geologic Modeling, 5.4.2 Gas Injection Methods, 5.2.1 Phase Behavior and PVT Measurements, 5.1.3 Sedimentology, 5.1.1 Exploration, Development, Structural Geology, 1.14 Casing and Cementing, 5.5 Reservoir Simulation, 5.6.4 Drillstem/Well Testing, 2.2.2 Perforating, 5.6.1 Open hole/cased hole log analysis, 1.2.3 Rock properties, 5.1 Reservoir Characterisation, 4.6 Natural Gas
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In the case of a gas condensate reservoir assessed by two wells only, the recovery mechanism (primary depletion or gas-cycling) has not been selected yet.
To succeed in the development project, the gas condensate recovery increment given by the gas-cycling option shall be completely demonstrated and above all shall not be overestimated at an early stage of the field life.
Therefore, decisions shall be made taking into consideration reliable results from the flow simulations performed on gridded models. Furthermore, these gridded models shall give evidence of the amount of information known on the reservoir heterogeneity and the associated uncertainties.
With two wells only, a good many questions concerning the sedimentary model of the reservoir remain unanswered. However, two stages of diagenesis in the shallow water sandstones have been identified : the first one leads to the formation of carbonate cementation levels, the second one is a solution stage constituting drains.
Given that the fundamental heterogeneity is unknown at this early stage, the modeling is based on the fact that the distinction between the cemented and non cemented levels can be easily made using logs.
A stochastic modeling is then performed on a grid giving greater importance to the vertical discretization Two types of methods are successively used : the indicators for facies modeling, and the multi gaussian method for petrophysical characteristics modeling.
In order to provide contrasted models clearly showing the uncertainty associated to the geological model, two "end- member hypotheses" based on the frequency and the range of the heterogeneities have been made.
Simulations carried out on a section of the reservoir only (a chronostratigraphic unit into a compartment) show that the results are sensitive to the heterogeneity range.
This is particularly illustrated in the case of the gas-cycling option where the efficiency of the gas injection is much greater in the case of low range heterogeneities than in the case of long range heterogeneities.
In order to reproduce the same results in a full-field model after upscaling, tests are carried out to optimize the clustering of fine grid cells into coarse grid cells. One clustering method is suggested whose application demonstrates that the simulation results obtained on a fine gridded model can be reproduced using a number of layers significantly reduced.
The appraisal phase is a crucial one in the life of a field given that the way the field is perceived at the end of this stage will allow making a good many major technical decisions: number of wells, recovery mechanism, type of completion, design of surface plants, ... These technical decisions, insofar as they are economically valid, will have a great influence on how the field will be developed and therefore on its final profitability.
Therefore, the huge financial investments resulting from this assessment of the field shall not be technically risky, as far as possible, and this is the reason why quantifying the uncertainties shall be kept in mind all along this stage.
In terms of reservoir, the appraisal shall be (among others) materialized by answering the following three questions:
- Which is the fundamental heterogeneity?
- Which is the best way to model this heterogeneity?
- Which will be the effects on the flows?
The fundamental heterogeneity is defined as being the heterogeneity which will have, among all the types and scales of heterogeneities found in the reservoir, the most influence on the flows, considering a given fluid and recovery mechanism.
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