Resolving the Mixed Salinity Challenges with a Methodology Developed from Pulsed Neutron Capture Gamma Ray Spectral Measurements

Abstract

The mixed-salinity environment poses a challenge in petrophysics. Many efforts have been made, but the industry still lacks a workable solution for accurate formation measurements in the presence of variable or unknown water salinity, especially in reservoir surveillance when well completion restrictions require running slim logging tools. A pulsed-neutron (PN) spectroscopy tool was characterized using laboratory limestone formations and with data generated through Monte Carlo simulation. Seventy-six measurements were conducted with conditions of openhole (OH) and cased-hole (CH) completion, porosities of 15.4 and 42.9%, borehole fluids of water and oil, and borehole and formation water salinities of 0, 20, 50, 100, and 200 ppk. These were supplemented with 88 modeled points to better define the porosity (extending down to 0%) and salinity dependence of the tool response. Twenty additional measurements were taken in a dolomite formation to investigate lithology dependence and ten additional measurements were taken with oil in a 43.9% porosity formation to investigate fluid saturation dependence. Salinity is derived from data obtained with a PN tool operated in the carbon/oxygen (C/O) mode. Ratios of the elemental yield of chlorine (Cl) to that of hydrogen (H) are computed from the near- and far-detector capture gamma-ray spectra. Using a scheme similar to C/O log interpretation for oil saturation, crossplots of the near- and far-detector Cl to H ratios were generated. A quadrilateral is formed on this crossplot by separately varying the borehole and formation water salinities. The established quadrilaterals, a newly developed algorithm to handle intermediate values of near and far ratios and of porosity, and independently obtained borehole water holdup (Yw) and reservoir water saturation (Sw) are then used to compute the borehole and formation water salinities that produced the measured Cl to H ratios. Results obtained were compared with values determined from formation tester samples, demonstrating that a solution for accurate formation water salinity determination is established, thus resolving the challenge of mixed salinity in formation evaluation and reservoir surveillance.