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User Guide: Mud to Cuttings Ratio (MCR)

By Mike Morgenthaler

Mud to Cuttings Ratio (MCR) Guide

The Mud to Cuttings Ratio (MCR) is an essential metric used by mud engineers and solids control operators to quantify the amount of drilling fluid retained on discharged drill cuttings. Monitoring MCR is critical for:

  1. Cost Control: Minimizing base oil/fluid loss to the environment or disposal system.
  2. Environmental Compliance: Meeting regulatory limits on oil retention (e.g., in offshore drilling).
  3. Equipment Optimization: Assessing shaker screen selection and centrifuge performance.

The MCR represents the ratio of the volume of liquid (mud) adhering to the cuttings relative to the volume of dry solids. A higher MCR indicates wetter cuttings and more wasted mud.

Input Parameters

To compute the MCR, you must input two primary measurements depending on your chosen unit system:

1. US Customary Units

  • Cuttings Weight (1 qt): The weight of a 1-quart volume of wet cuttings sample, measured in pounds (lbs).
    • Typical Range: 2.5 to 5.4 lbs.
  • Mud Weight: The density of the active drilling fluid, measured in pounds per gallon (ppg).
    • Typical Range: 7.2 to 20.0 ppg.

2. Metric Units

  • Cuttings Weight (1 L): The weight of a 1-liter volume of wet cuttings sample, measured in kilograms (kg).
    • Typical Range: 1.13 to 2.45 kg.
  • Mud Weight: The density of the active drilling fluid, measured in kilograms per cubic meter (kg/m³).
    • Typical Range: 863 to 2397 kg/m³.

Step-by-Step Field Measurement

  1. Collect Cuttings Sample: Retrieve a representative sample of cuttings directly from the shaker discharge or centrifuge outlet.
  2. Fill the Cup: Fill a standard 1-quart cup (US) or 1-liter container (Metric) completely with the cuttings. Tap the cup gently to settle the cuttings and level off the top.
  3. Weigh the Sample: Weigh the filled cup and subtract the empty cup tare weight. Record the net cuttings weight (nw).
  4. Record Active Mud Weight: Take a fresh mud weight reading (mw) using a mud balance.
  5. Calculate: Input these values into the Cutpoint app.

Calculations & Formulas

The MCR calculation is derived from the Void Fraction (vf) of the cuttings mixture. The void fraction represents the portion of the cuttings volume that is occupied by the adhering liquid mud.

1. Void Fraction Calculation

The void fraction (vf) is calculated using the following API-derived relationship:

vf = (4 × nw - mw) / (21.7 - mw)
  • nw: Cuttings weight (lbs per quart / kg per liter).
  • mw: Active mud weight (ppg / kg per cubic meter).

2. Mud to Cuttings Ratio (MCR)

Once the void fraction (vf) is established, the volumetric ratio of adhering mud to dry solids (MCR) is calculated as:

MCR = (1 - vf) / vf

The result is formatted as a ratio of MCR : 1.

Example:

If you measure a cuttings weight (nw) of 3.5 lbs and a mud weight (mw) of 12.0 ppg:

  1. Calculate the void fraction (vf):
    vf = (4 × 3.5 - 12.0) / (21.7 - 12.0) = 2.0 / 9.7 ≈ 0.206
  2. Calculate MCR:
    MCR = (1 - 0.206) / 0.206 ≈ 3.85

This means 3.85 parts of mud are lost/discharged for every 1.0 part of rock cuttings by volume.