Resolving Mixed-Salinity Challenges With a Pulsed-Neutron Methodology

Fig. 1: (a) A set of 16 measurements in the 43.6%-dolomite formation, covering the range of 0- to 200-ppk FSAL and BSAL, with an overlay of 42.9%-limestone points. The 16 points are represented as red dots. (b) The same set of 16 dolomite points are represented as black dots and are compared with the case of oil in the borehole.

The mixed-salinity environment poses a significant challenge in petrophysics. The industry still lacks a workable solution for accurate formation measurements in the presence of variable or unknown water salinity, especially in reservoir surveillance. A pulsed-neutron (PN) spectroscopy tool was characterized by use of laboratory limestone formations and with data generated through Monte Carlo simulation. Results from a number of measurements obtained were compared with values determined from formation-tester samples, demonstrating that a solution for accurate formation-water-salinity (FSAL) determination has been established.

Introduction

FSAL is a key input parameter for water saturation Sw calculated from formation-evaluation-resistivity logs by use of an Archie type of equation (details are provided in the complete paper).

Equation 1,

 

(1)

or from reservoir surveillance pulsed-neutron-capture sigma (Σ) logs, based on the typical mixing rule,

Equation 2

. (2)

 

In Eq. 1, reservoir porosity Φ and formation true resistivity RT are typically determined from logs. Rock properties a, m, and n (lithology factor, cementation exponent, and saturation exponent, respectively) are determined from laboratory core analysis. Rw is the formation-water resistivity at reservoir conditions. In Eq. 2, the subscript “log” represents values determined from a well log, ma represents reservoir-rock matrix, hc represents hydrocarbon, and w represents water. Knowing FSAL is a must in determining Rw and Σw in Eqs. 1 and 2, respectively.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 170608, “Resolving Mixed-Salinity Challenges With a Methodology Developed From Pulsed-Neutron-Capture Gamma Ray Spectral Measurements,” by S. Mark Ma, SPE, Saudi Aramco; Harold Pfutzner, SPE, Schlumberger; A.A. Al-Hajari, SPE, and Nedhal M. Musharfi, SPE, Saudi Aramco; and Pablo Saldungaray, SPE, and Hisham Azam, SPE, Schlumberger, prepared for the 2014 SPE Annual Technical Conference and Exhibition, Amsterdam, 27–29 October. The paper has not been peer reviewed.
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Resolving Mixed-Salinity Challenges With a Pulsed-Neutron Methodology

01 August 2015

Volume: 67 | Issue: 8

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