38. Types of recorded gases

1) Cuttings gas (formation gas)

It is the gas liberated from the drilled cuttings enters the wellbore mud. These are the gases continuously recorded and plotted against depth and used to represent the formation content. There are some factors that control the size of this formation gas shows.

Factors affecting the size of the gas show:

1. Rate of penetration; ROP

2. Differential pressure; P

3. Porosity ; Ø

4. Hole size; G

5. Flow rate; Q

6. Depth; ft or mtrs

The ROP controls the concentration of gas in the mud for a given flow rate and is therefore the primary factor causing a variation of gas readings.

The P and Ø control the degree of flushing.

The hoe size is G an important factor affects the size of the gas show, the larger the hole size, the more the cuttings, the more the gases liberated from these cuttings entering the mud.

The Q affects the gas concentration but as the flow rate is usually constant for a bit run this is not as important factor as a change in ROP.

As the Depth increases the gas shows should increase due to the increase of expansion that occurs.

2) Background gas

Under normal drilling conditions, it is common for a relatively small amount of gas to be continuously in evidence. This “background gas” can originate from a previously drilled section, which contained a show and bleeds an amount of gas into the mud.

Background gas is often methane with little or no wet gas. However, continuous high levels of background gas often suggest that the well is being drilled very close to balance (formation pressure is very close to mud head) and may indicate that a greater mud weight is required.

3) Trip gas

It is quite common for an increase in the mud gas reading to occur at the first bottom’s up circulation after a trip has been made. This occurrence referred to as “trip gas”.

In the process of “coming out of the hole”, the bit is being pulled through a mud filled cylinder of a diameter only slightly greater than the bit itself. As the bit is pulled through this cylinder formed by the hole wall, a swabbing action on the formation takes place and a momentary reduction in hydrostatic pressure occurs as bit is travelling upward. This enables the formation pressured gases to bleed into the hole each time the string is moved up. The resultant is an accumulated amount of gas at the bottom of the hole.

The amount of this gas depends on the following:-

1. Differential Pressure (Mud Weight / Formation Pressure)

2. Pipe Movement Speed

3. Mud Properties; viscosity

4. Annular Size

Under normal conditions trip gas will be indicative of in creasing formation pressure especially when the amount of trip gas increases with depth and each successive trip.

4) Connection gas

Similar to the trip gas, a connection gas may appear at the first bottom’s up circulation after a connection has been made. The reason of this is the reduction of the hydrostatic head when pumps are shut-off loosing the effect of the E.C.D, along with the upward pipe movement that causes another negative swabbing pressure. This connection gas is used as a helpful guide towards drilling situation.