Oil production from many major producing reservoirs is now maintained by water injection programs. The resulting mixed and often very low- salinity formation water presents a serious problem when monitoring water saturation with traditional pulsed neutron capture (PNC) tools. A slim carbon/oxygen (C/O) pulsed neutron tool tested recently in many regions around the world addresses the problems encountered in low- and mixed- salinity environments. The tool's dual -ray detectors and flexible neutron burst timing allow it to be used for carbon/oxygen logging and, in a separate pass, for PNC logging. New detector and accelerator technologies as well as advanced data processing allow logging speeds that meet or exceed those of traditional, large, single-detector C/O tools.
More and more reservoirs around the world are produced by water injection today. This leads to low and/or unknown formation water salinities which are unsuitable for PNC logging. As a result, C/O logging, in use for over two decades in reservoirs with low formation salinities, is becoming increasingly important for determining the formation water saturation Sw.
Unfortunately, until now, C/O logging has been hampered by several shortcomings such as large tool size, slow logging speed and sensitivity to the borehole fluid. In many cases, the large tool size did not allow the tool to pass through tubing, making it necessary to remove the tubing string. The new slim tool not only overcomes this disadvantage, but also logs at speeds equal to or faster than those of larger C/O tools.
The Reservoir Saturation Tool (RST*) tool has two high efficiency detectors and flexible neutron pulsing electronics and can be used for C/O and PNC logging without any hardware change in the same trip into a well. The tool diameter is 1-11/16 in. and 2-1/2 in. This paper concentrates on the 1- 11/16 in. RST tool, since results from the larger tool have been presented elsewhere. The tool is combinable with other production logging tools which can be run either during C/O logging or during separate passes depending on the logging speed requirements.
The following paragraphs present the basic tool capabilities, data analysis and interpretation, and also give examples of logs from several geographical regions.
The basic RST tool string is shown in Fig. 1. From the bottom up it consists of the accelerator controller cartridge, the sonde, the acquisition cartridge, the telemetry cartridge including a casing collar locator (CCL) and on top an optional -ray sonde. The tool is run either on coax- or hepta-cable. Other production-logging sondes, like temperature and pressure sensors, or a gradiomanometer can be combined with the RST tool. The information from the additional sensors can be used to enhance the C/O or Sigma interpretation.
The accelerator controller cartridge contains the power supplies and the control electronics for the pulsed neutron generator. The sonde houses the neutron generator with its high-voltage supply, and the two -ray detectors with preamplifiers and high-voltage supplies. The acquisition cartridge contains the main amplifiers, ADCs and digital signal processors (DSP) for acquisition of the pulse height spectra, as well as electronics to acquire the time decay spectra (multichannel scaler, or MCS). The total tool length, including bottom nose, gamma-ray and head, is 38 ft.
The tool has two basic operating modes: A single neutron burst mode is used for C/O logging (as in the previous generation GST* tool) and a dual neutron burst mode for capture-sigma logging. The pulse timing for the inelastic C/Omode is shown in Fig. 2 together with the spectra associated with a particular time gate. The neutron burst is 20 sec long, and is repeated at 100 sec intervals. Burst rise and fall times are less than 1 sec.
The use of dense, fast GSO scintillators coupled to specially designed high-throughput electronics allows very high counting rates.