The Photoelectric Factor (Pe) is a key formation property that helps to identify and distinguish between formations of different lithologies. Pe is only a good measure of the rock matrix properties, as long as standoff and borehole fluids do not overly influence the measurements. For Wireline applications, in which instruments are pressed against the formation, invasion and mud cake (Allioli, 1997) are the main factors that influence the measurement. In contrast, Logging-While-Drilling (LWD) applications are more challenging due to the additional dynamic standoff behavior of sensors mounted in a rotating drill string and potentially changing drilling-fluid environments. Radiation-transport simulations were performed for a new-generation 4.75-in neutron-porosity and gamma-gamma density logging-while-drilling tool to establish the main factors influencing the Pe response. On this basis, an algorithm was developed to map the spectral gamma-ray response, caliper, mud Pe, and mud weight to formation Pe. This resulted in the proposed environmental corrections algorithm providing an improved accuracy compared to hitherto employed configurations. The paper presents a method to calculate drilling fluid Pe from information available in mud reports and closes with a laboratory and field-test benchmark of the developed corrections.
The photoelectric factor (Pe) is a material parameter describing the absorption rate of photons propagating through the material. The concept of using photoelectric absorption parameters in well logging was pioneered in the late 1950s, targeting mainly mining applications (Voskoboinikov, 1958) (Blyumentsev, 1962) or coal exploration (Utkin, 1965). Early tools and services either investigated the position of the discontinuity in scatter gamma-ray spectra due to the К photoelectric absorption edge scale (Voskoboinikov, 1961) or to use a ratio of two energy windows or channels to derive photo-absorption indices (Berzin, 1966).
The photoelectric factor is closely related to the photoelectric absorption cross section, mainly depending on the average atomic number of the chemical composition of the material and, therefore, the lithology and mineralogy of the rock formation. The energy dependence of absorption cross sections as well as incoherent and coherent gamma-ray scattering cross sections for the example of calcite (CaCO3) are depicted in Fig. 1. The photoelectric absorption cross section of gamma rays in matter (σph) for most atoms in downhole formations (up to approx. Z = 30) is approximately proportional to Z4.6:
(equation)