Standard analysis of NMR logs, based on the use of a unique T2 cut-off to discriminate free and bound fluid and estimate irreducible water saturation and permeability, gives erroneous results in complex lithologies. In the example of a mixed platform presented, the main difficulty came from the strong variations between clastics, dolomite and limestones due to the heterogeneity of the porous network. In the proposed method, illustrated by a field case, a single T2 cut-off (derived from lab NMR) for both sandstone and dolomite was found and gave good results whereas a more detailed porous network description was necessary to handle limestones. A complete set of lab measurements was used to characterize the porous network and classify carbonates following their relaxation behavior: The results obtained with this method based on core data integration are presented and validated by irreducible water saturation from centrifugation and conventional permeability measurements. Full understanding of the porous system based an extensive core analysis is necessary to link the continuous log response and the dynamic parameters of the reservoir. Small changes in mineralogy affect significantly production performance. Introduction A complex geology requires an extensive coring program to understand better the rock properties and link their petrophysical behaviors (irreducible water saturation, permeability) to laboratory measurements. A simple description of core is not sufficient to integrate crucial information like mineralogy and pore geometry. Nuclear Magnetic Resonance (NMR) logs can provide the main petrophysical properties such as porosity, irreducible water and permeability but needs a careful calibration on core NMR. In this work, it is shown that applying directly a model from core is not enough. A further understanding of rocks is necessary to enforce a model applicability. A key to this understanding is an objective confrontation between mineralogy, porous network characterization (qualitatively and/or quantitatively) and dynamic data such as capillary pressure. After presenting the geological and petrophysical context, a complete review of lab measurements is done and the NMR log intrepretation model is explained, illustrated by various figures that compare core and log data. Finally some conclusions are drawn and some recommendations are expressed concerning the benefits and the dangers of using NMR in complex lithology.
In the future natural gas will make a growing contribution to energy supply. Since gas production is constant during the year and its consumption almost seasonal, the importance of underground gas storage (UGS) is increasing because it acts as a buffer between production and consumption. This paper presents a workflow for reservoir and cap rock petrophysical and geomechanic characterization based on an integrated log interpretation, critically reviewing the value and reliability of information that can be gathered from the different log types. Geomechanical characterization is an essential trait in the UGS workflow, especially when the initial formation pressure is exceeded to increase the storage capacity and enhance gas deliverability or in the case of aquifer storages. The workflow consists in the following steps: 1) Conventional reservoir petrophysical characterization. This is achieved by means of gamma ray, neutron, density and resistivity logs, and nuclear magnetic resonance logs for verification of the nature and quantity of the pore fluids. 2) Cap rock petrophysical characterization. An innovative approach with nuclear magnetic resonance log is described.3) Reservoir and caprock geomechanical properties characterization. Information is provided by advanced sonic tools, allowing estimation of the stress profile magnitude, calibrated with micro-hydraulic fracturing tests (stress tests).The direction of the stresses is determined through image logs. 4) Well integrity assessment. Sonic and ultrasonic cement evaluation tools are recommended. 5) Reservoir monitoring. A pulsed neutron tool can be used in order to identify the position of the gas-water contact during gas injection and withdrawal cycles. Case studies are provided in order to complement the theoretical workflow description.
In the future natural gas will make a growing contribution to energy supply. Since gas production is constant during the year and its consumption almost seasonal, the importance of underground gas storage (UGS) is increasing because it acts as a buffer between production and consumption. This paper presents a workflow for reservoir and cap rock petrophysical and geomechanic characterization based on an integrated log interpretation, critically reviewing the value and reliability of information that can be gathered from the different log types. Geomechanical characterization is an essential trait in the UGS workflow, especially when the initial formation pressure is exceeded to increase the storage capacity and enhance gas deliverability or in the case of aquifer storages. The workflow consists in the following steps: 1) Conventional reservoir petrophysical characterization. This is achieved by means of gamma ray, neutron, density and resistivity logs, and nuclear magnetic resonance logs for verification of the nature and quantity of the pore fluids. 2) Cap rock petrophysical characterization. An innovative approach with nuclear magnetic resonance log is described.3) Reservoir and caprock geomechanical properties characterization. Information is provided by advanced sonic tools, allowing estimation of the stress profile magnitude, calibrated with micro-hydraulic fracturing tests (stress tests).The direction of the stresses is determined through image logs. 4) Well integrity assessment. Sonic and ultrasonic cement evaluation tools are recommended. 5) Reservoir monitoring. A pulsed neutron tool can be used in order to identify the position of the gas-water contact during gas injection and withdrawal cycles. Case studies are provided in order to complement the theoretical workflow description.
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