## Driller’s Method or Wait and Weight Method – What is The Practical Well Control Method for You?

Driller’s method and wait and weight method (engineer’s method) are widely used to circulate wellbore influx while maintaining bottom hole pressure constant. There are a lot of opinions regarding which method is the best for well control operation therefore this article will discuss about pros and cons of both methods.

### Driller’s Method

The driller’s method requires two circulations to kill the well. The first circulation is to circulate influx out of the well with original mud weight. The second circulation is to kill the well with kill weight fluid. During the first circulation, the bottom hole pressure remains constant due to maintain drill pipe pressure constant while circulating. For the second circulation, in order to maintain constant bottom hole pressure, casing pressure is held constant while circulating kill mud to the bit. Once the kill mud passes the bit, the drill pipe pressure will be held constant until the kill weight mud is on surface and there is no sign of influx in the annulus.

### Wait and Weight Method

The Wait and Weight method requires only one circulation. The influx will be circulated out while the kill weight mud is displaced into the well simultaneously. While pumping the kill fluid from surface to the bit, drill pipe pressure schedule must be strictly followed. After that the drill pipe pressure is maintained constant until the kill mud returns back to surface.  Some people call the Wait and Weight method as “Engineer’s Method” because there are more calculations compared to the Driller’s method.

## Pressure Profile of Drillpipe and Casing Pressure while killing a well with wait and weight method

This topic will cover about pressure profile both drill pipe and casing pressure while kill a well with wait and weight method.

Drill Pipe Pressure

Firstly, we will take a look at the drillpipe pressure. When kill weight mud is displaced in the well, drill pipe pressure drops as per the drill pipe schedule. Once kill weight mud is to the bit, drill pipe pressure is maintained until the well is killed. A pressure profile will look like the following chart.

## Drill Pipe Pressure Schedule Calculation for Wait and Weight Well Control Method

This example demonstrates how to determine drill pipe pressure schedule for wait and weight method.

Well Information

Current mud weight = 9.5 ppg

Pump output = 0.1 bbl/stroke

Well depth = 9000’MD/9000’TVD

Drill string capacity = 0.0178 bbl/ft

Surface line volume = 15 bbl.

Shut in casing pressure = 700 psi

Shut in drill pipe pressure = 500 psi

ICP = 1600 psi at 30 spm as kill rate

Please follow steps below to determine the drill pipe pressure schedule (step down chart).

## Formulas for Wait and Weight Well Control Method

There are some well control formulas that you need to know for wait and weight method.

1. Kill Weight Mud

KWM = OMW + [SIDPP ÷ (0.052 x TVD)]

Where;

KWM is kill weight mud in ppg.

OMW is original mud weight in ppg.

SIDPP is shut in drill pipe pressure in psi.

TVD is true vertical depth of the well in ft.

## Kill Weight Mud

Kill Weight Mud or Kill Drilling Fluid Density is the mud weight required to balance formation pressure. The kill weight mud may be pumped into the well at different time depending on kill methods (Driller’s method, Wait and Weight, Bull head, etc).

How to determine kill weight mud?

## Oilfield Unit

With the following equation, you can determine this figure.

Kill Weight Mud (KWM) = Current Mud Weight + (SIDPP ÷ 0.052 ÷ Well TVD)

Where,

Kill Weight Mud (KWM) in ppg

Current Mud Weight in ppg

SIDP stands for “Shut In Drill Pipe Pressure” in psi.

Well TVD is true vertical depth of the well in ft.

Example:

Drilling with 9.5 ppg mud and current depth at 9,500’MD/9,000’ TVD. The well takes influx. Operation is stopped and the well is shut in.

Shut in drill pipe pressure = 500 psi.

Shut in casing pressure = 700 psi.

What is the kill weight mud?

The kill weight mud required to balance formation pressure:

Kill Weight Mud (KWM)  = 9.5 + (500 ÷ 0.052 ÷ 9,000)

Kill Weight Mud (KWM) = 10.6 ppg (round up number)

## Metric Unit

With the following equation, you can determine this figure.

Kill Weight Mud (KWM) = Current Mud Weight + (SIDPP ÷ 0.00981 ÷ Well TVD)

Where,

Kill Weight Mud (KWM) in Kg/m³

Current Mud Weight in Kg/m³

SIDP stands for “Shut In Drill Pipe Pressure” in KPa.

Well TVD is true vertical depth of the well in m.

Example:

Drilling with 1,140  Kg/m³ mud and current depth at 3,000 m MD/2,700 m  TVD. The well takes influx. Operation is stopped and the well is shut in.

Shut in drill pipe pressure = 3,450 KPa

Shut in casing pressure = 4,500 KPa

What is the kill weight mud?

The kill weight mud required to balance formation pressure:

Kill Weight Mud (KWM)  = 1,140 + (3,450 ÷ 0.00981 ÷ 2,700) Kg/m³

Kill Weight Mud (KWM) = 1271 Kg/m³ (round up number)

Why do we need to use SIDPP?

When you take a kick (wellbore influx), the density of fluid in the annulus is very hard to predict because it is a mixture between drilling mud and influx. Therefore, you don’t know exactly what the density in the annulus is. Moreover if you take gas kick, shut in casing pressure will increase over time, and you will NOT be able to identify the right shut in pressure to determine formation pressure.

Looking at the drill pipe side, there is only one pure fluid column so you know exactly its density. With the precise mud density, you can apply hydrostatic pressure concept in order to get the formation pressure and kill weight mud (the equation is showed at the beginning of the topic).

Note: If your drilling string has a solid float, you will not be able to read SIDPP right away. So you need to bump the float to get SIDPP. You can read more detail about it via this > float bumping procedure.