Study On Comprehensive Interpretation Methods Of Shallow Gas Formation In Cased Hole

With the development of exploration in oil fields, shallow gas re-evaluation is getting more and more people's attention in old wells. Because the logs in open hole used for gas identification is little in old wells, and far from being able to meet the needs of shallow gas exploration.To satisfy the needs of potential evaluation of shallow gas, and make up for lack of open hole's logging information, CNL and NLL are added to run in cased hole. Therefore, it's full of necessity and meaning to establish comprehensive interpretation methods of shallow gas formation with logs in open hole and cased hole.Based on the study of characteristics of log responses to gas formation in open hole and cased hole, effective gas-indicating curves are optimized. The effect of shale on gas-indicating logs makes log responses to gas weaken. Also the methods of correction for effect of shale to gas-indicating logs are given. According to characteristics of log response to gas zone, nine overlay curves or parameters have been worked out to indicate gas fromation qualitatively. The comparison of interpretation results with gas testing shows that the overlay curves and parameters can determine shallow gas formation in cased hole effectively. After qualitative identification of gas zones, a new way to determine gas saturation in shallow formation is proposed by use of the ratio curve of far detector inelastic count rate and the product of near capture detector count rate and capture cross section in NLL. Thereby a set of semi-quantitative interpretation method is established, and the calculated saturation is good. The gas- indicating parameters are taken as input parameters of support vector machines to determine gas zones and water zones automatically. Practical application for two wells shows that the correctness of identification results is up to 80%, and good results are achieved.