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Technical Paper: Solving for Reservoir Saturations Using Multiple Formation Property Measurements From a Single Pulsed Neutron Logging Tool

Society: SPWLA
Paper Number: ll
Presentation Date: 2017
 

Abstract

Pulsed neutron logging is commonly used to evaluate reservoir saturation through casing based on the ability of neutrons and gamma rays to probe the reservoir rock through casing and cement. Pulsed neutron measurements contain information, which is sensitive to a complex set of different neutron interaction properties such as capture, elastic, and inelastic cross-sections. Conventional cased-hole interpretation methods tend to use a single isolated pulsed neutron measurement complemented by multiple external inputs, often from openhole logs, and assumed formation water salinity to solve for reservoir saturation. The information that can be obtained from pulsed neutron measurements on multiple independent formation properties is often not fully utilized. In the past, some of this information has not been accessible, and conventional interpretation methods have been limited to the use of a single isolated measurement.

A recently introduced pulsed neutron logging tool provides multiple independent formation property measurements, namely sigma, neutron porosity, fast-neutron cross section, and elemental concentrations including carbon from inelastic and capture spectroscopy. All these independent measurements follow linear volumetric mixing laws, and thus are readily accessed in a linear manner by a multimineral and multifluid solver for determination of formation volumes of water, oil, or gas. We detail a new interpretation method using these multiple measurements that can be tailored to many different scenarios encountered in cased-hole logging. The key aspect of the method is to solve for the desired reservoir saturations with proper weighting applied to each measurement, based on its statistical precision and estimated accuracy level. External inputs from openhole logs or assumptions based on local knowledge can also be used coherently with the pulsed neutron measurements. In this case, the multiple independent measurements can be used as a consistency check to validate the underlying assumptions. When external inputs (openhole logs) are not available, multiple measurements from the new pulsed neutron tool make previously underdetermined problems solvable. Log examples illustrate the applications of this method for different scenarios and show that interpretation results from this new method are coherent and robust.

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