Pulse Testing For Reservoir Description in a High-Permeability Environment

Abstract

Summary The continuous carbon/oxygen (C/O) log, recorded by a 20-kBz pulsed neutron logging tool, locates and quantitatively evaluates hydrocarbon reserves in cased wellbores independent of the salinity of formation waters. Gamma rays emitted during neutron inelastic scattering and neutron capture provide information on relative amounts of several elements in potential reservoirs, such as C, O, silicon (Si), calcium (Ca), etc. These data can be related to oil saturation of reservoir rocks of fresh, mixed, or unknown formation water salinities and to formation lithology. Measurement principles, toot response, test pit data, and interpretive concepts are summarized. Field case examples include hydrocarbon exploration behind pipe, monitoring of production wells, secondary and tertiary recovery schemes, evaluation of heavy oil reservoirs, tar sands, and steam floods, and other case studies in clastic carbonate reservoirs. Introduction For more than a decade, pulsed neutron logs that measure the macroscopic cross section (E) for thermal neutron capture in a borehole environment have been highly successful in differentiating water from hydrocarbons behind casing in salt-water reservoirs. One such device, the Neutron Lifetime Log (NLL), has been in routine use since the early 1960's. Although successful for saline reservoirs, the NLL cannot be used to locate hydrocarbons where the macroscopic cross section of the formation water is not significantly different from that of the hydrocarbons. In addition, in a freshwater or steam flood, reliable interpretation of the NLL is very difficult. A well logging tool that can locate hydrocarbons regardless of the salinity of the formation waters has been developed and has been offered commercially for 7 years. This device, the C/O log, measures the gamma rays emitted by carbon and oxygen that are produced from inelastic scattering of fast neutrons from these elements. By combining the C/O ratio with other measurements made during a single pass with the logging tool, we can identify and measure hydrocarbons in the formation. Neutrons are produced by an accelerator source that can be pulsed with a repetition frequency of 20 kHz. Pulsing the source at this frequency enables the tool to provide good results at togging speeds of 150 to 180 ft/hr. This paper discusses the physics of the gamma ray production and the method by which the gamma rays are detected and measured by the logging instrument. The response of the tool for the matrix types and porosities normally encountered in a borehole environment is presented. In addition. interpretation techniques and several field examples are shown. Physics of the Borehole Interactions Neutron Inelastic Scattering The most practical method for finding oil behind casing is to detect and to measure gamma rays emitted from carbon in the oil. The most advantageous way to produce gamma rays from carbon is by the inelastic scattering of fast neutrons with the carbon nucleus. Inelastic scattering is the process in which the nucleus of an element becomes excited by the nuclear force interaction between a nucleus and a neutron. The neutron must possess sufficient energy to raise the energy of the nucleus to one or more of its bound states (energy levels). Once the nucleus is excited, it usually de-excites by the emission of one or more gamma rays, the energies of which are characteristic of the struck nucleus. Symbolically, the inelastic scattering of neutrons with carbon can be written as 12C(n, n ) 12C, and the carbon nucleus, after excitation, emits gamma rays of 4.43 MeV. The symbol 12C (n, n' ) 12C is a shorthand notation for the nuclear reaction: 12C+n-->12C+n'+ ray. JPT P. 2441^