Objective
The aim of the project was to demonstrate the capabilities of the MCDUFF Monte-Carlo code in predicting the downhole performance of commercial Gamma-Ray density logging tools. It can be used to help understand the reasons for observed differences between different tools in the same borehole, and to establish the accuracy of the porosities which are derived from density measurements in different environments.
For this purpose it was necessary to establish a calibration facility for Gamma-Ray density logging tools and to demonstrate its role in the improvement of log interpretation by downhole modelling.
MCDUFF incorporates the following key innovative features :
1) An advanced acceleration technique called Forced Flight.
2) The prediction of absolute counting rates in the scintillation detectors which will be an important diagnostic in downhole investigations.
3) A sensitivity option which allows the effects of small changes in the borehole environment to be accurately predicted.
The main results of the project are as follows :
1) A range of new calibration test formations for Gamma Density logging tools has been established at EUROPA in Aberdeen. To date these formations, which include a thin bed test formation, have been successfully logged by two service companies. For most of the formations tested the agreement between the log density and the density determined for each formation by core analysis was generally better than 0.02 g/cc.
2) The MCDUFF models of two commercial tools were successfully validated against good experiment data in calibration conditions.
3) The models were used in Phase 3 of the project to investigate realistic downhole conditions :
i) Evaluation of density readings from thin beds and inclined thin beds
ii) Effect of borehole shape on the response of two types of MWD tool.
Both of these problems required several Monte-Carlo calculations in order to simulate the tools motion in the borehole. In a six month period over 400 separatecalculations were carried out.
4) The calculations in Phase 3 fully demonstrated the ability of the Monte-Carlo code MCDUFF to predict the response of commercial density tools in realistic downhole conditions. These conditions are not easy to simulate in the laboratory and modelling provides a very cost effective means of generating new calibration points. In this study over 400 calculations have been carried out but these have been "shared" between three logging tools and several borehole configurations. There has been insufficient modelling on any one aspect to permit the determination of new environmental correction algorithms but the methodology has been proven.
The project has been carried out in three phases :
Phase 1 "Construction of industial calibration test formations at the EUROPA Facility".
PHASE 1 was to establish a new calibration facility for gamma density tools at the EUROPA building in Aberdeen. This would provide independent benchmark measurements that could be used to validate computer models of commercial logging tools. The gamma density test formations were designed such that both wireline and MWD tools could be accommodated.
Some of the gamma density test formations built for the first gamma density project were transferred to EUROPA and adapted for industrial use. Additional test formations were also installed to cover the operational range of interest for gamma density tools from 1 g/cc to 3 g/cc. The test formations were characterized using a reference gamma density tool, after installation.
In addition a Thin Bed Test Formations was installed at EUROPA. This was used in Phase 3 of the project to provide benchmark measurements for a commercial logging tool against which the MCDUFF modelling was compared. PHASE 2 "Validation of Monte-Carlo models of commercial logging tools using benchmark experimental data, some of which were obtained in the new EUROPA test formations".
A deliverable of the first gamma density project was the Monte-Carlo code MCDUFF for gamma density tools which was validated against experimental benchmark data including benchmark data obtained for a reference tool in well characterised test formations.
Building on the first gamma density project, the central feature of this project was the modelling of commercial logging tools using MCDUFF in downhole situations. It was anticipated that these would pose considerably more difficult problems for modelling than the reference tool. Therefore it was deemed essential to validate the models by comparing predictions with calibration measurements in the EUROPA Gamma Density Test Formations or other similar test formations, before anymodelling was carried out in more complex downhole situations.
Two Service Companies - one wireline and one MWD agreed to make the engineering drawings of their tools available for this study. Tool models were therefore set up in MCDUFF and validated against experimental benchmark data for clean hole and pertubed conditions. The validated models were then available for use in the downhole modelling study. PHASE 3 "Application of these models for forward modelling of tool responses in realistic downhole conditions". The work carried out in Phase 3 extended the earlier Monte-Carlo modelling to include the apllication of MCDUFF to a range of more realistic conditions, typical of what would be encountered downhole.
Two downhole environmental pertubations were studied in Phase 3 :
1) Thin beds including inclined thin beds
2) Borehole shapes
Both commercial logging tools were used in these studies.
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DEM - Demonstration contractsCoordinator
DT2 8DH Dorchester
United Kingdom