There are three main categories of well Logging: Electrical, Nuclear and Acoustic/Sonic.[2] Resistivity logging is an electrical well logging method and as such should be conducted in an open/uncased hole. Usually resistivity logs are displayed on track 4 of a well log and are displayed in ohm meter (Ωm) units[1] Resistivity logging was the first rock property that was logged and began the development of well logging methods.[2]
Well logs can also be used as control points for seismic reflection data.[1] The well logs and borehole seismic data can be used to predict the seismic response in the area. This provides a well tie that allows the seismic time data to be accurately tied to depth data for improved accuracy in data interpretation.[5]
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Core analysis and Well logging are two essential procedures undertaken as part of the formation evaluation process which, when used alongside each other can help to determine fundamental properties of a reservoir and how these properties can affect hydrocarbon production. This report aims to provide an overview of these two procedures by considering the numerous properties which can be measured using data provided as well as considering the various types and methods of coring and well logging. The relationship between core and well log data has also been analysed using a case study (STEZYCA OIL AND GAS FIELD, located in Poland). By studying core and well log data acquired from this field, analysing and correlating both sets of data, it is possible to determine important properties such as porosity, permeability, lithlogy and saturation while also reducing uncertainty within the formation. The analysis was performed was on the basis of well log interpretation and core sample description (micro-photographs and cross-sectional view made available). The well logs analysed consisted of data from a Gamma Ray log, Neutron log, Density log, Sonic log and Resistivity log. Three samples have been analysed from the same well. Using the core samples, performing the required laboratory tests to obtain key parameters such as permeability, porosity, saturation, capillary pressure etc., are essential to fully evaluate the formation, but are not always sufficient. Correlation of core and well log data, is an essential process as is analysing and comparing data for calibration to characterise lithology. This is important in order to calibrate the data, reduce uncertainty and as a result, gain a more complete understanding of what to expect within the formation.
With answers to these questions amongst others, it is possible to determine the productivity of a reservoir In terms of its ability to produce hydrocarbons (Halliburton). Core analysis and Well logging are two essential procedures undertaken as part of the formation evaluation process which can help to provide the above information. This report focuses on providing an overview of Core analysis and Well logging by considering the various parameters which can be measured using data provided as well as looking at the different types and methods of coring and well logging. The relationship between core and well log data has also been analysed using a case study (Stezyca Oil and Gas Field), by studying core data acquired, tests performed and well logs made available in order to correlate both sets of data to define the lithlogy, porosity, permeability and saturation
A process of obtaining core samples in which a sidewall coring tool containing explosives is lowered into the well. The explosives are used to propel a number of small, hollow, cylindrical tubes into the sides of the well to retrieve samples of the rock. As the coring tool is raised to the surface, the tubes, which are connected to the tool by wires, are pulled up as well. The samples recovered by this method are often shattered due to impact fracturing caused by the explosives, resulting in erroneous measurements of rock properties, however they are usually excellent for indication of lithology and interpretation of potential productivity.
Well logging is the process of creating written record data which are gathered during the drilling of a borehole. The importance of well logging is that the records enable information about any characteristics of a reservoir in subsurface layers to be determined. Therefore, properties of a formation such as porosity, permeability, thickness and fluid saturation distribution can be evaluated. Additionally, physical properties including temperature, electrical current, radioactivity and sonic reflection can be detected by running tools into the wellbore. Although, local information about properties of a reservoir can be determined by collected cores from the well, electrical logging is the only source that gives continuous information or data about subsurface layers. The problem with the analysis data is that although the cores obtained during drilling can give very important information about the subsurface layers, when they are exposed to expansion and compression, the physical property of a rock sample can be changed significantly. Moreover, vibration during drilling can change their properties too. The saturation of the core can be changed due to contact with mud resulting in measurements of parameters such as porosity, permeability and saturation to be unreliable. As a result well logging plays a significant role in obtaining more reliable data..
In this type, the tools are lowered into the well by a cable. The obtained data are transmitted to the surface recording system via the cable. In this method Well logging is performed after interruption or termination of drilling. Therefore, drilling must be stopped and therefore the well must be empty. This is a main disadvantage of this method.
In this type, the logging device is attached on the drill and collected data are memorised in the tool. The data are downloaded to the surface record system when drilling is terminated or interrupted. LWD is efficient in horizontal drilling where wire line method cannot be applied.
Cased hole logging: This type is used after well completion. The main cased hole logs are Cement bound logging (CBL) and Variable density logs (VDL). Both logs are used to consider the process of well completion.
Generally, density logs are the key indicator of porosity in most wells by providing a value of formation bulk density (ρb). This is obtained using a density logging tool shown in Figure 5. The dual spacing formation logging device carries a gamma ray source and two detectors. Dense formations absorb many gamma rays, while lowdensity formations absorb fewer. Consequently, high-count rates at the detectors illustrate low-density formations, while low count rates identify high-density formations.
Well logs can be used to determine important petro physical properties in a drilled borehole such as porosity, permeability and saturation. These parameters are not measured directly by well logging tools, they are derived by interpreting data provided by these tools.
Being able to correctly identify the lithology can assist in well to well correlation. For example, the use of porosity logs for lithology identification is one of the steps. Considering the most simple case, say if the formation consists mainly of two minerals, reading from porosity logs used in correlation with cross-plots, can be useful when describing the lithology of a formation. Using the data acquired above, one can find both porosity and lithology of the rock (Figures 10 and 11). Core analysis was performed under laboratory conditions. The sample of core used was taken and examined to recover direct physical measurements to provide geological characteristics and engineering information, enabling better decision making in relation to other cored wells nearby. Furthermore, the data acquired from the core plugs to characterize the reservoir as a whole, on its own is never enough to validate and evaluate the formation. Here the use of well logs becomes extremely useful and a good engineer is able to use, capture and recognize this and use it to correlate both sets of data to better validate and evaluate the formation. In order to be able to analyse what was done on for the Stezyca oil and gas field in terms of analysis of cored samples and well logging data to better understand and evaluate the formation, we broke the analyses into two sections:
The fundamental of well-log interpretation analysis and formation analysis is a training course that gives participants comprehensive training on good logging and interpretation. This Zoe course provides a broad understanding of the physical principles of the tools used in the logging process and will cover the whole interpretation workflow for the formation evaluation.
By completing the Fundamentals of Well Log Interpretation Analysis and Formation Evaluation course, all the people involved will know the fluid and rock characteristics, read and understand well-logs, explore, measure and run the surrounding corrections for the logging equipment. The course also covers the log analysis beginning with the lithology identification, share volume, water saturation, and porosity.
The people involved will identify the free water level and the reservoir fluid contacts. The delegates will understand the methods of formation evaluation. In this Zoe course, the core analysis learned will help lower the unreliability in the log interpretation, including fluid saturation, permeability, and porosity. This Zoe training strives to have a complete and in-depth basis for increasing knowledge in other specialized interpretation methods comprising well log information not trained here.
As the team lead on this task, KGS prepared well-based stratigraphic interpretations within an IHS Petra software database system. The Petra project database contains location and header information on 10,416 wells across the five-state Study area and the bordering regions of Indiana, Michigan and southern Ontario, Canada (Figure 4-1), more than 8000 of which are Lexington/Trenton or deeper penetrations. Geophysical well logs have been loaded into the Petra project for 1978 wells (Figure 4-2). Of these, the vast majority are from digital Log ASCII Standard (LAS) files, which have facilitated log-based correlations and calculations. 2ff7e9595c
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