Differentiation of Gas in Sand/Shale Environments Using Pulsed Neutron Capture Measurements
Authors: Morris C.W. , Vaeth J.F. , Verret D.J. 
Published: December 2007

Pulsed neutron capture (PNC) tools have been used successfully to evaluate hydrocarbon zones in Gulf of Mexico wells for many years. These tools measure both capture and inelastic gamma ray counts resulting from a high energy neutron burst. This information can be used to determine the capture cross-section and porosity characteristics of the formation. Modern tools, with both near and far detectors, provide additional data concerning total count rates, elemental yields, and the borehole environment.

Hydrocarbons are usually identified by the characteristic sigma response. However, gas bearing zones are sometimes difficult to identify in a variable sand/shale environment using standard cased-hole pulsed neutron logging techniques. Problems differentiating the hydrocarbon type in low porosity, shaly sand or thin laminar intervals result primarily from the smaller neutron capture cross-section of gas. Both sigma and neutron porosity values decrease in the presence of gas. Hydrocarbon composition, reservoir pressure, and mineralogy of the formation also influence the tool measurement. Presented is a general and computationally efficient algorithm which provides an enhanced qualitative gas identification flag. This new algorithm is based on the inelastic gamma ray nuclear response to formation gas in typical sand/shale lithology. The characteristic gas response technique can be utilized with both existing and new generation PNC tool measurements and can be used to reevaluate old data sets. Field examples are presented which illustrate the accuracy and resolution of the algorithm in low porosity and shaly sand intervals. Production results from these zones have confirmed the reliability of the new weighted inelastic ratio interpretation and the technical superiority of this technique when used to refine and calibrate petrophysical evaluations.

Introduction

Pulsed neutron capture tools first became available to the petroleum industry in 1968 with introduction of the new cased-hole technology by two service companies. These tool systems measured the thermal decay time of a neutron bombarded into a formation which manifested itself as a gamma ray of capture as the neutron was 'thermalized' to a lower energy state. This 'decay time' value was originally presented on the log presentation as 'Tau' (The time needed for the thermal neutron to loose it's 14 MeV potential, from consecutive collisions with elements more dense than the mass of a neutron, to less than the 3 MeV level and finally being absorbed in a collision with a more dense element. A gamma ray is released from the host element which is then detected at the tool). With initial work at qualitative interpretation of this data, the term 'Macroscopic Capture Cross-Section' was devised and became what is now termed 'Sigma' of the formation. The relationship between Tau and Sigma is a fixed relationship: Sigma = 4550/Tau, where Tau is in microseconds (of decay time). In a sense, this was the first attempt at qualitative interpretation as the PNC technology was born.

In the 29 years that have transpired since, service companies have endeavored to enhance and specialize this technology to assist their customers in differentiating between gas, oil, and water. In Schlumberger, the technology evolved through five different tool designs to arrive at the Thermal Neutron Decay Time (TDT-P*) tool of 1986.

The TDT-P tool emits regular pulses of fast neutrons (the TDT-P generates 12 times more neutrons/second in yield than previous tool systems) and measures the rate at which thermal energy level neutrons are captured. This rate is approximately exponential in character.

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