A gas reservoir in east Ordos basin, is characterized by lithology complexity, low porosity, and pore structure heterogeneity, which complicates the logging response and creates an especially low contrast in the resistivity response. Therefore, it is hard to accurately evaluate the reservoir using only conventional logs. In addition, engineering issues that are always encountered makes it difficult to log in an open hole environment.
To solve the challenges mentioned above, a multi-function pulsed neutron measurement is introduced to solve for minerals, porosity, and gas identification. This technology is suitable for open hole and cased hole environment because it has a small diameter of 1.72 in. With the measurement from inelastic and capture spectroscopy together with total organic carbon (TOC) measurements, solutions for precise mineral volumes and lithology are provided. In cased hole environment, minerals are acquired with minimized effect from casing and cement by applying correct offset for the measurements from the near and far detector. Time domain measurement provides neutron porosity, sigma, and fast neutron cross section, which together act as a solver for porosity and a gas indicator. Fast neutron cross section is derived from total gamma-ray counts originating from inelastic interactions and is sensitive to the formation’s ability to attenuate high energy neutrons. This measurement is effective for differentiating gas from rock matrix and fluids, such as oil and water. Its response is different from the neutron porosity response in that it does not correlate to hydrogen index. Neutron porosity and fast neutron cross section combination analysis is thus a good indicator for gas reservoir. For cased hole environment, self-compensated neutron porosity is a complement if no open hole porosity logging data is acquired. In combination with other technology, including advanced magnetic resonance data etc. a special reservoir evaluation and fluid identification method was established.
The methodology has been applied by multiple wells drilled for tight gas reservoir in both open hole and cased hole environments and achieve a good success. The reliable measurement provides an integrated solution for tight gas reservoir evaluation in this area.
