Gas Detection by Dual-Spacing Neutron Logs in the Greater Oficina Area, Venezuela

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Published in Petroleum Transactions, AIME, Volume 210, 1957, pages 140–147. Abstract The geological and economical conditions peculiar to the Greater Oficina area are presented to demonstrate the necessity of a low-cost, well-site method of distinguishing gas-bearing formations. The method of gas detection with the neutron log is briefly discussed and its limitations are explained. A new method is described which consists in running a second neutron log with a larger source-to-detector spacing. Field data indicate that under favorable conditions the larger spacing response is proportionately more influenced by the virgin gas zone than is the shorter spacing response. One important condition is that the gas sands be invaded, with a diameter of invasion equal to about two to three times the hole diameter. The calibration of the longer spacing log is made so that the two logs read the same deflection in shale and in a known water or oil sand. Under such conditions, gas is indicated by a positive separation, i.e., the curve for the longer device is displaced to the right of the curve for the shorter device. Because of the non-linearity of the long spacing response, low porosity formations also show positive separations. In sands with large porosity variations, the gas sands can no longer be located visually. They can be detected on a plot of shorter spacing vs longer spacing deflections. Introduction In the Greater Oficina area of Eastern Venezuela, the rapid and accurate detection of gas from well logs is of primary importance because of a combination of economic, geologic, and operational conditions. Briefly, these conditions are: economic - there is no market for gas at the present time; geologic - a single well may contain as many as 30 or more oil or gas sands for which intervals to be tested must be selected; operational - first, once opened to the casing through perforations, gas sands are extremely difficult and expensive to squeeze off, and second, gas detection must be rapid because the geologist must decide immediately, at the well, which sand to perforate.