An Improved Method for Calculating Water Resistivities From Chemical Analyses
Authors: Talash A.W. 
Published: December 2007

An equation has been developed for calculating water resistivities from chemical analyses. This equation provides a means of obtaining resistivities of formation waters directly from chemical analyses without the aid of tables, conversion factors or resistivity vs concentration plots. The study included 885 water analyses covering a variety of oil producing formations for which the water resistivity and chemical analysis were known. A majority of the water analyses were obtained from fields in East Texas, while the remaining data comes from fields in South Arkansas and North Louisiana. The accuracy obtained by applying this method was found to be better than the accuracies of other methods reported in the literature. The average deviation for the South Arkansas and North Louisiana data was 3.77 per cent and 4.68 per cent for East Texas fields.

Introduction

Several methods of calculating water resistivities from chemical analyses have been reported in the literature. Both Tixier and Jones proposed methods based on the Palmer system. These methods require that the chemical analyses be given in terms of primary and secondary alkalinity and salinity. In the Tixier method, one adds the reciprocals of the resistivities of the salts formed in the Palmer recombination scheme to obtain the reciprocal of the water resistivity. In the Jones method, a series of conversion factors to change the primary and secondary salinity and alkalinity to equivalent quantities of NaCl is used. Dunlap and Hawthorne proposed a method whereby the various ions reported in the chemical analysis are converted to an equivalent amount of NaCl. This was done by multiplying each ionic constituent in a given chemical analysis by a conversion factor to obtain the equivalent concentration in ppm of NaCl. The conversion factors were determined by measuring resistivities of a number of simple solutions containing only two types of ions, one of which was always either sodium or chloride. For the method presented by Dunlap and Hawthorne, the average of the absolute values of the per cent deviation between the measured and calculated resistivities was 3.3 per cent. For the Tixier method and the Jones method, the averages of the absolute value of the per cent deviation were 7.8 and 4.6 per cent, respectively. The principal advantage of the Dunlap and Hawthorne method over those methods reported by Tixier and Jones is speed and convenience. Dunlap and Hawthorne checked the accuracy of their method by comparing measured and calculated resistivities of formation waters from 26 wells in the Gulf Coast region. Water resistivities ranged from 0.097 to 3.52 ohm-m at 68F.A review of the literature shows an apparent need for a more direct method of calculating water resistivities from chemical analyses without the aid of tables, conversion factors or resistivity concentration plots. The purpose of this study was to develop a method whereby resistivities of brines from oil-producing formations can be calculated directly from one equation.

Discussion

One approach to calculating water resistivities from chemical analyses is to develop a single equation based on data obtained over a wide range of chemical concentrations. This technique was utilized in the development of this investigation. Data reported by Hawkins et were selected as the basis for correlating the water resistivity with the chemical analysis. These data were obtained from a variety of oil producing formations in East Texas, South Arkansas and North Louisiana. The chemical analyses and electrical resistivity measurements reported by these investigators represent 422 samples of oilfield brines from 120 East Texas fields in Railroad Commission of Texas Districts 5 and 6. These samples represent water from all of the major petroleum productive formations in the area. Other data include 267 samples from 74 oil fields in South Arkansas and 199 samples from 80 fields in North Louisiana. Resistivities were measured at 75F.

JPT

P. 1396ˆ