Wireline quality assurance and quality control (wireline QA/QC) is a set of requirements and operating procedures which take place before, during, and after the wireline logging job. The main merits of wireline QA/QC include accuracy and precision of recorded data and information. Accuracy is a measure of the correctness of the result and is generally depended on how well the systematic errors (a reproductible inaccuracy introduced by faulty design, inadequate calibration or a change in borehole) are controlled and compensated for. Precision is depended on how well random errors (errors that cannot be reproduced and are mostly related to the physics of a measurement) are analysed and overcame .
Introduction Wireline logging is a part of exploration geophysics and is mainly used to detect the presence of economically useful hydrocarbons in the Earth's sub-terrain. Products of wireline logging are wireline logs or well logs.: (Fig. 1. Ll1LOG) In the oil industry the logs are "a recording against depth of any of the characteristics of the rock formations traversed by a measuring apparatus in the well-bore". The well logs are obtained by lowering the measuring tools on a wireline (cable) into the well (borehole). The integral parts of wireline logging operations are quality assurance and quality control procedures. Quality control is the "process that defines how well the solution for a specific problem is known". Well log quality control is a "set of methods that identifies and analyses data deviations from established standards and allows the design of a remedy". Unlike measurements in well-known and controlled laboratory conditions, logging is performed in-situ and can be affected by different possible failure sources and susceptible to systematic and random errors. The objective of wireline logging job is to obtain a permanent continuous record of the rock properties penetrated by the wellbore with the result of wireline logs, fluid, and rock samples. Of all the well data sets recorded and collected, well logs are the most valuable as they are vital for reservoir and formation evaluation. Wireline (well) logs are then combined with drilling data, mud logs, and measurements while drilling (MWD) and coring information in order to choose correct testing and completion intervals to properly evaluate the production potential of the well. There are two categories of well log data: the original data (e.g. Gamma Ray log as a result of gamma rays measurements in the borehole, through an iodine crystal scintillation detector, calibrated as per normal field operational procedure) or derived data (data resulting from the processing of the original data, e. g. calculating the volume of shale). Main components of the well log data record are logs themselves (main curves for the main and repeat sections with relevant information - calibration information, parameter tables... and additional curves of the main and repeat passes including down-logs and images of quality control logs) and contextual information, such as data acquisition plans, other job reports, witness reports, and tool specification. Contextual data is of a great value for exploration but can be lost or difficult to access. With the best quality wireline logging data acquired, subsequent steps in well production and completion are more precise and effective. High quality data and conscientious data management enables the prognosis of potential problems and failures so the whole process (the whole rig or oilfield lifetime) can be adapted and configured in order to prevent possible consequences in regard to human safety, environmental preservation, and infrastructural integrity. Acquisition of geotechnical and petrophysical data is costly, but necessary. Data of dubious quality cannot yield reliably good decisions. The poorer the quality of data, the higher risk associated with decisions based upon them. The less knowledge there is about the quality of recorded and collected data, the higher is the uncertainty of said decisions and can lead to false interpretations and evaluations. A good evaluation is only possible with good quality data so it is essential to acquire the best quality data.
Factors affecting the data quality The data quality may be compromised to a greater or lesser extent by bad hole conditions, wireline equipment failures, human errors, or even extreme weather conditions. Within the bounds of the rig time costs and preservation of the personnel safety, equipment and the well, the logging engineer, operational geologists and wireline log witnesses have to ensure that the best quality data is acquired. The objective of the wireline logging services is to provide the best possible quality data in the minimum possible rig-time. Quality of the data is directly dependent on available rig-time. Factors affecting data quality and time efficiency are operational procedures and environmental conditions, but mainly problems caused by them. During wireline job planning, some factors are accountable for specific types of equipment used for specific borehole geometry and conditions (temperature, pressures and mud type used) and others are wireline responsibility on site and during the wireline logging job. Data quality and time efficiency factors
Environmental conditions Drilling mud – hole diameter affects log readings, mud type and density affect type of tools used and measurements such as conductivity and resistivity, invasion of drilling, and mud fluids into reservoir rock, forming of mud-cake, wash-outs in soft sediment, formation of stress-related break-offs. Borehole geometry – e. g. affects type of logging and tools used (wireline, pipe conveyed or LWD). Borehole environment – temperature, pressures and possible hostile (H2S ) environments affect tools used and their operating limits.
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