Laboratory and Field Evaluation Of a Carbon/Oxygen (C/O) Well Logging System

Abstract

A pulsed neutron log using gamma rays from inelastic neutron interactions has been developed for working over wells having low or unknown formation water salinities. C/O and Ca/Si ratios measured simultaneously reveal the amount of lime and the oil saturation for porosities above 15 percent. The measurements, in the form of a continuous log, are independent of salinity and shaliness. Introduction There has long been a need for a well logging method that directly distinguishes oil from water in formations having low or unknown water salinities. Since the 1950's several groups have been developing techniques for solving this problem using primarily stationary measurements. Texaco has also been working on a practical logging technique and has developed a carbon/oxygen (C/O) logging system capable of logging continuously at 5 ft/min. In the C/O log a pulsed 14 MeV neutron generator is used and gamma rays are detected from fast-neutron inelastic interactions with atomic nuclei. A ratio is made between the number of gamma rays from carbon and those from oxygen. The measured C/O values are related to the ratio of elemental carbon to oxygen and can be used to determine the oil saturation of a formation. In developing this C/O system it was found useful to employ a simultaneous ratio developed from inelastic calcium and silicon gamma rays (Ca/Si) so that an increase in C/O between sand and lime could be distinguished from that between water and oil. To be able to log continuously, emphasis was placed on increasing the counting rates while still maintaining adequate oil sensitivity. Apparatus A schematic diagram of the logging sonde is shown in Fig. 1. A Philips Model 18603 neutron generator was used, it accelerates deuterium ions into a tritium target to produce 14 MeV neutrons. A 125 kv power supply within the sonde supplies the accelerating potential and an ion source supply controls the width of each neutron burst and the frequency of repetition. The neutron generator operates satisfactorily at repetition rates of at least 20,000 bursts/sec. At 20 kHz, each neutron burst has a full width at half maximum of approximately 4 microsec. The average neutron yield is 1 to 2 X 10(8) neutrons/sec in either the pulsed mode or the continuous mode of operation. The gamma ray detector consists of a 2-in.-diameter X 4-in.-long NaI(Tl) crystal and an EMI Model 6097B photomultiplier tube. These, together with a preamplifier, are kept at a constant temperature of preamplifier, are kept at a constant temperature of about 0 degrees C inside a Dewar vessel that also contains an ice-filled canister. Iron is placed between the source and detector to shield the detector from some of the fast neutrons and the detector is shielded from thermal (slow) neutrons by surrounding it with boron-impregnated material. The sonde has a maximum diameter of 4 in. and is 20 ft long. The present maximum operating temperature is 125 degrees C and the pressure rating is 14,000 psi. Pulse height information (proportional to gamma ray energy) is transmitted over a coaxial logging cable 7/16 in. in diameter and 15,000 ft long. An automatic gain stabilizer, using a reference pulse from the sonde, compensates for changes in pulse from the sonde, compensates for changes in gain over the logging cable caused by temperature variations. JPT P. 1103