Recent publications1,2,3,4,5 have described the development and utility of cased-hole neutron and density porosity measurements from a pulsed-neutron system. Porosity information from cased-hole logging can provide required data for wells with limited logging programs such as old wells or new wells where open-hole logging is not viable. In reservoir sequences typical of the Rocky Mountain region, the addition of the cased-hole density measurement has proven useful in discriminating tight porosity from gas-filled porosity and coals from shales.
Whereas a tool such as the Computalog PND-S™ pulsed-neutron system uses two detectors, analysis of pulsed-neutron density measurement physics immediately suggests that a third long-spaced detector would be helpful in measuring deeper into the formation. Thus, a prototypical system based on the standard PND-S™ was constructed with a third detector at a 95-cm spacing. This prototype provided a collection of third detector data on normal logging runs.
Log examples from the North America Rocky Mountain region demonstrate the utility of this measurement in cased-reservoir analysis.
Often cased-hole logging is used to describe reservoirs when limited or no open-hole logging data is available. This includes old wells, wells with limited logging programs that require better reservoir definition, or new wells with hole conditions that prevent open-hole logging. The PND-S™ system is commonly used in place of open-hole logs on development wells. The development and application of pulsed-neutron porosity measurements in cased wellbores are discussed in refs. 1 through 5.
PND-S™ cased-hole neutron porosity, similar to the compensated neutron porosity, is derived from its near detector (NEAR) to far detector (FAR) ratio. In the case of pulsed-neutron systems, the spatial distribution of capture gamma rays is used to measure the thermal neutron distribution. In refs. 1 and 4 the use of the Multi-Layer Perceptron neural network6 to compensate the ratio porosity for various boreholes is described. In the log examples the neutron porosity is referenced as NPHI for open-hole compensated neutron and RPHI for the cased-hole neutron porosity from the pulsed-neutron system. Open-hole density porosity is referenced as DPHI in the log examples.
The cased-hole density measurement is based on the scattering of gamma rays produced by inelastic scattering of fast neutrons. The majority of these gammas are created near the pulsed-neutron source and the die-away of this radiation is sampled at the near and far detectors. Similar to the open-hole gamma-gamma density measurement, this die-away can be parameterized as the gamma transport length which is inversely proportional to the electron density. The gamma transport length is referenced as LRHO. A simple inverse model for two detectors can be expressed as:
LRHO=(spacing) / (ln(IRA) - ln(FAI))
IRA is the NEAR inelastic count rate
FAI is the FAR inelastic count rate
The pulsed-neutron density measurement is made with a through-tubing sonde (1.69-inch diameter) so the measurement is the average value of the wellbore and formation. To remove the wellbore signal, LRHO is normalized to match offset or field data and generate density porosity. In refs. 1 and 4 the use of Multi-Layer Perceptron neural networks is described to map the inelastic measurements into density porosity.