Abstract
We develop a neural network workflow, which provides a systematic approach for tackling various problems in petroleum engineering. The workflow covers several design issues for constructing neural network models, especially in terms of developing the network structure. We apply the model to predict water saturation in an oilfield in Oman. Water saturation can be accurately obtained from data measured from cores removed from the oil field, but this information is limited to a few wells. Wireline log data are more abundantly available in most wells, and they provide valuable, but indirect, information about rock properties. A three-layered neural network model with five hidden neurons and a resilient back-propagation algorithm is found to be the best design for the saturation prediction. The input variables to the model are density, neutron, resistivity, and photo-electric wireline logs, and the model is trained using core water saturation. The model is able to predict the saturation directly from wireline logs with a correlation coefficient (r) of 0.91 and an error of 2.5 saturation units on the testing data.
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Appendix [52, 53]
Appendix [52, 53]
Wireline logs are continuous measurements of downhole formation through electrical instruments. The logging is performed during and after drilling a well where the tool is lowered to the formation through electrical cables. The interpretation of wireline logs provide indirect valuable information of the formation that has been drilled, such as lithology, porosity, saturation, and permeability. There are many tools used for wireline logging, such as gamma ray detector, density and neutron, resistivity measurement, and sonic travel time.
Wireline logs are continuous electrical measurements of downhole formation through electrical instruments. The measurements are performed at each depth of required formation, typically at every 150 cm provided through a long band of paper. The logging is performed during and after drilling a well where the tool is lowered to the formation through electrical cables. The interpretation of wireline logs provide indirect valuable information of the formation which has been drilled, such as lithology, porosity, saturation, and permeability. There are many tools used for wireline logging, such as gamma ray detector, density and neutron, resistivity measurement, and sonic travel time.
The gamma ray tool is a passive logging tool that detects the natural radiation of gamma rays from the formation, which are result of high-energy electromagnetic radiation [53, 54]. The gamma ray tool gives information about the lithology of the formation where high gamma rays are related to shaly environment, whereas low readings are interpreted as clean sands. The density log tool emits gamma ray into the formation that collides with the electrons in the formation [53, 54]. In the process, the gamma rays are attenuated. The counts rates of the scatted gamma ray at a fixed distance from the source are inversely related to the electron density of the formation; consequently, the bulk density of the formation can be calculated. The photo-electric absorption index gives information about the lithology of the formation where the photo-electric measurement primarily response to the rock matrix [53, 54]. The neutron logging tool bombards the formation with high-energy neutron. The high-energy neutron interact with the nucleus of the atoms in the formation where each interaction causes lose of neutron energy [53, 54]. The hydrogen atoms has the same mass of the neutron, hence lowers the speed of the neutron significantly. The slowing down rate is determined by the hydrogen index of all components of the formation and formation fluids that contact a significant fraction of hydrogen. The distance over which the neutrons have traveled before they reach a lower-energy level is related to the amount of the hydrogen atoms present in the formation. A combination of density and neutron tool gives indication of the lithology of the formation besides the gamma ray. The resistivity logging tool basically measures the resistivity of the formation. By measuring the resistivity, the water saturation can be calculated.
The coring and core analysis provide direct measurements of petrophysical properties in the laboratory. The core basically represents a whole section of rock extracted from the drilled formation. In the laboratory, samples from the core are taken for physical measurements such as porosity, saturation, and permeability. The physical properties from the core analysis represent the ground truth, and they are compared to the wireline logs calculated petrophysical properties. However, the core is an expensive method and limited only to few wells in the formation, whereas the wireline logs are abundantly available in most of the wells in the formation. Water saturation can be determined directly from cores taken from a well in the field. The widely used laboratory method to determine the water saturation is the Dean-Stark method. In this method, the fluid saturation is determined by distillation of the water fraction and extraction of the oil fraction from a sample. The process starts by vaporizing the water in the sample by boiling the solvent. This vaporized water is then condensed and collected in a calibrated trap. At this stage, the volume of the water in the sample can be determined. The solvent is also condensed and then flows back over the sample to extract the oil. After the water and oil have been removed, the sample is dried. The weight of oil is calculated by the difference between the total loss in the sample weight and water weight removed from it. The loss in the sample weight is calculated by measuring the weight of the sample before and after extraction.
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Al-Bulushi, N.I., King, P.R., Blunt, M.J. et al. Artificial neural networks workflow and its application in the petroleum industry. Neural Comput & Applic 21, 409–421 (2012). https://doi.org/10.1007/s00521-010-0501-6
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DOI: https://doi.org/10.1007/s00521-010-0501-6