Cutback Calculations

Introduction

Cutback calculations can be done using two different approaches. The first is the volume density approach using Equation 8 and Equation 9 as shown in earlier examples. The second method, which is more accurate, uses Equation 15 below and the weight percentages of the dry salts in the starting fluid and final cutback fluid to calculate the fraction of a barrel of starting fluid that should be diluted with fresh water to get one full barrel at the target density (d_f). The weight percent values for each single salt fluid are provided in the density and composition tables in Chapter 6 in the section titled, "Single Salt Fluid Composition and Blending Tables."

Cutback calculations are based on weight percent salt on a 100% basis dissolved in a fluid of a particular density. All the examples in this section assume the cutback is being done with fresh water. The goal is to determine the volume of starting fluid of density (d₁) that, when brought up to a final volume of one barrel, will give the correct final density (d_f). The formula for a cutback is given below in Equation 15.

EQUATION 15.

Example I demonstrates the use of weight percentages and Equation 15 to calculate the volume of starting fluid needed to cut an 11.6 lb/gal calcium chloride fluid back to 10.9 lb/gal. The weight percent values used in the example below are taken from Table 39.

Cutback Tables

A faster way to solve the cutback problem is by using a cutback table. Table 11 is the cutback table for calcium chloride; the density of the starting fluid (d₁) appears across the top and the final target density (d_f) is shown down the leftmost column. The values are in barrels of starting fluid that should be brought up to a final volume of one barrel by adding water. Cutback tables, such as the following one (Table 11), are provided in Chapter 6. They are organized by fluid type and density in the section titled, "Single Salt Fluid Composition and Blending Tables".

EXAMPLE I. Cutting Back a Single Salt Fluid with Fresh Water

If, for instance, you are starting with 11.4 lb/gal (d₁) CaCl₂ and want to make 800 bbl of 10.2 lb/gal (d_dil) CaCl₂ fluid, you will need to follow the steps below.

To determine the quantity of 11.4 lb/gal fluid needed to make 800 barrels of 10.2 lb/gal fluid:

1. Locate the density of the starting fluid, in this case 11.4 lb/gal, along the top of table 11.
2. Run down the 11.4 column until you intersect the 10.2 lb/gal row in the leftmost column.
3. Read the volume as the fraction of a barrel, 0.595 bbl. This means that for every one barrel of 10.2 lb/gal fluid, you will need to start with 0.595 bbl of 11.4 lb/gal fluid.
4. Multiply that number by the number of barrels you want in the end.
5. Add the amount of fresh water required to increase the fluid volume to the final desired volume.

To make 800 bbl of 10.2 lb/gal fluid, it will take 476 barrels of 11.4 lb/gal CaCl₂, diluted with fresh water up to a final volume of 800 bbl.

TABLE 11. Calcium Chloride (CaCl₂) Cutback Table (bbl/bbl)

Calculated to make one barrel of final density fluid.