The Carbon/Oxygen Log Measurement

Abstract

1978 Rocky Mountain Regional Cody Wyoming May 17–19, 1978 Abstract A new pulsed neutron logging technique has been developed to unambiguously identify the presence of the element carbon. The Carbon/Oxygen Log is currently available on a limited commercial basis in North America and has been tested under a wide variety of field conditions. This paper covers the theory of the log's response and shows examples of what is available in the way of taped output. The basics of interpretation by a simplified pattern recognition technique are reviewed along with test pit measurements and verified field response. pit measurements and verified field response Introduction During the past three years, a Carbon/Oxygen logging device has been commercially available and has been used to survey several hundred wells. Despite the fact that a great many of these applications have been made as a "last resort" when definitive answers could not be derived from other more conventional logging methods, the device has scored a very high success ratio in predicting the type of fluid ultimately produced. Also complicating these initial efforts is the fact that development and improvement of the instrumentation has been continuous over this period of time and even now, a significant advance is on the verge of commercial introduction. The Dresser Atlas Carbon/Oxygen Log is made possible by an electro-mechanical device that can be pulsed in extremely short, concentrated bursts of 14 Mev neutrons and a detector system that can selectively discriminate and count the high energy gamma rays that result from the inelastic collisions of these neutrons with the atoms of carbon and oxygen. Figure 1 schematically reviews the source and time frame of neutron bombardment induced gamma radiation. Note that the inelastic or prompt gamma rays exist only while the very high energy neutron source is on and for a very few microseconds following the cessation of the pulse. From this period of time until the neutron source is reactivated, only gamma rays of thermal neutron capture, and to a lesser degree, activation gamma rays, are present. This is an extremely important point, as the 4.43 Mev inelastic carbon gamma ray is the only unambiguous nuclear indicator of that element in nature.

Continuous Carbon/Oxygen Log Interpretation Techniques

“…25 Copyright 1981 Society of Petroleum Engineers of AIME 1394 in predictable ways, which have been detailed in previous publications. [4][5][6] Basically, different elements usually can be identified from characteristic gamma ray energy levels released as a result of the neutron bombardment. The number of gamma rays at each energy level is proportional to the quantity of each element.…”

“…The distances (Aco) from the water line to the oil line in Fig. 3 can be approximated using (6) where c/> is porosity and ex and {3 are proportionality and exponential factors, respectively. The approximate values for ex and {3 for the test pits are 0.6 and 1.11, respectively.…”