Calculating the Density of Water at Reservoir Conditions: A Comprehensive Guide

Introduction:

When it comes to oil reservoirs, understanding the density of water is crucial for accurate estimation of formation properties and fluid behavior. In this article, we will explore the methods used to calculate the density of water at reservoir conditions, including the impact of salt content and pressure.

Method 1: Using Saturated-with-Gas Curves

For oil reservoirs below the bubblepoint, the 'saturated-with-gas' curves should be used for water considered to have no solution gas. This method is based on the assumption that the density of water remains constant at a given temperature and pressure. By knowing the density at atmospheric pressure and reservoir temperature, you can use Fig. to find the increase in specific gravity (density) when compressed to reservoir pressure.

Method 2: Using No-Gas-in-Solution Curves

Alternatively, for oil reservoirs above the bubblepoint, the 'no-gas-in-solution' curves should be used. This method takes into account the solubility of gas in water and is based on the assumption that the density of water changes with pressure.

Method 3: Estimating Salt Content from Formation Resistivity

The salt content can be estimated from the formation resistivity, as measured from electric-log measurements. By knowing the equivalent weight percent NaCl at standard conditions, you can use Fig. to calculate the density of formation water at reservoir conditions.

Method 4: Calculating Density from Salt Content and Temperature

The only field datum necessary is the density at standard conditions, which can be obtained from the salt content by use of Fig. . By extrapolating the weight percent NaCl to reservoir temperature and reading the new density, you can calculate the density of formation water at reservoir conditions.

Method 5: Using Correlations

When laboratory data or actual water samples are unavailable, the density of formation water at reservoir conditions can be estimated roughly (usually to within +/−10%) from correlations. Figs. provide a comprehensive guide for estimating the density of water using this method.

Method 6: Experimental Verification

Experimental verification of computer predictions is essential in cases where any error could have significant impact. This involves comparing calculated densities with measured values and adjusting the calculations accordingly.

Calculating the density of water at reservoir conditions is a complex task that requires careful consideration of various factors, including salt content, pressure, and temperature. By understanding the different methods used to estimate density, engineers can make more accurate predictions of formation properties and fluid behavior. Whether using saturated-with-gas curves, no-gas-in-solution curves, estimating salt content from formation resistivity, calculating density from salt content and temperature, or relying on correlations, accurate estimation of density is crucial for successful oil reservoir management.

References:

Density of water in gcm3