Once you know how much pit gain you got, you are able to determine influx height. Please take a look at the figure below for more understanding.

The influx height can be calculated by this following equation.

**Influx height = Pit gain ÷ annular capacity**

Where: influx height in ft,

Pit gain in bbl

Annular capacity in bbl/ft

The annular capacity can change depending on how big of your pipe and your hole.

**Note:** if you would like to get more understanding regarding annular capacity, please check this link, **Annular Capacity Calculation**.

Let’s get more understanding about it via this example.

Pit gain = 25 bbl

Hole size = 8.5 inch

BHA OD = 6 inch

BHA length = 420 ft

Drill pipe size = 5 inch

**Firstly, you need to know annular capacity between BHA and hole.**

Annular capacity, bbl/ft = (8.5^{2} – 6^{2}) ÷ 1029.4 = 0.0325 bbl/ft

You can calculate how much annular volume between hole and drill collar via this following equation.

Annular Volume, bbl = BHA length x annular capacity

Annular Volume, bbl = 420 x 0.0325 = 14.8 bbl

Because you gain a total of 25 bbl, there will be some influx above the BHA.

Influx above the BHA, bbl = Total pit gain – Annular volume between hole and drill collar

Influx above the BHA, bbl = 25 – 14.8 = 10.2 bbl

**Secondly, after knowing that 10.2 bbl of influx above the BHA, you need to convert this volume into height so as to determine height of the influx.**

Determine annular capacity between drill pipe an hole.

Annular capacity, bbl/ft = (8.5^{2} – 5^{2}) ÷ 1029.4 = 0.0459 bbl/ft

Determine height of 10.2 bbl influx across 5” DP.

Height, ft = Volume, bbl ÷ Annular Capacity, bbl/ft

Height, ft = 10.2 ÷ 0.0459 = 222 ft

It means that there is 222 ft of influx across DP so you can determine the total influx height:

**Height of influx = 222 + 420 = 642 ft**

**Ref book: ** **Formulas and Calculations for Drilling, Production and Workover, Second Edition**