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High connection gases during drilling but low pore pressures from wireine(post drilling).
06 November 2014
Hi, We just completed an exploration / appraisal well, where we experienced high connection gases while drilling, which resulted in increasing the mud weight till we saw no more connection gases. Results of the post drilling pore pressure measurement taken on wireline showed that the reservoirs were hydrostatic (8.2 - 8.5ppg). Now the well design was high pressured (prognosed high case formation pressures was 11 - 12.95ppg). We started drilling with 12.5ppg and had to raise the MW to 13.5ppg before the connection gases stopped. The drilling assembly was designed with minimal sensors (gamma and resistivity only) due to a long history of losing the entire string when we drilled HP wells and took kicks. This meant we couldn't really determine the pore pressure while drilling as we had no sensors. Although pore pressure monitoring during the drilling (resistivity and gases), was indicative of high pore pressures. My question is has anyone experienced this before and what could be the reason for this.

Thanks
SC
9 answer(s)
rclyde
Senior Drilling Engineer
Wintershall
Total Posts: 8
Join Date: 15/09/11
It is very common to see connection gas still while overbalanced particularly if the formation has some permeability. When the pumps are shut off the downhole circulating pressure is reduced to the static pressure then small amounts of fluid balloon back into the borehole pulling some of the gas from the formation back with it. Even if you do not detect ballooning at surface it is still probably happening albeit with small volumes. If the formation is not very permeable, such as with a shale, small fractures can sometimes develop with the same effect. The accuracy of pore pressure prediction can be greatly improved with better technology and a good pore pressure prediction expert. However the best technology may not be economically feasible and the expertise is often in a short supply. It is always important to differentiate between connection gas and background gas. If the background gas goes up then it is more likely to be due to a gas influx from the formation and the mud overbalance may need to be re-evaluated.
Gordonholm
Operations Geologist
Tullow Oil Plc
Total Posts: 8
Join Date: 17/04/13

Have any morfe info on the geology or are you unable to share that?

Rakesh
Operations Geologist
Reliance Industries Ltd.
Total Posts: 1
Join Date: 12/02/13
It means that the Shale has high pressures and the Reservoir is at hydrastatic pressure.

Please note that the pressure can be estimated (by Dxc or by other petrophysical parameters) only for Shale and can be measured post drill (or in realtime using formation pressure measurement while drilling)in Sand/Reservoir.

Thus, in this case, the realtime pressure monitoring as well as well events indicated high pressures while drilling and Pressure measurement after drilling recorded hydrostatic pressure. 

It is very common to have high pressure Shales and low pressured Sand.
PhilippeFornage
Operation Geologist
SPREADAssociates
Total Posts: 2
Join Date: 18/10/14
Since we are drilling with OBM, people continue to manage gas like we were doing with water base mud... We have to take into account gas diffusion and expansion... PVT... Formulas exist for few centuries only... Two pressure and temperature sensors on BHA, two pressure and temperature sensors on BOP... Cost nothing, bur could save a lot of money.
AndyPua
Drilling Consultant
PT Drilling Services
Total Posts: 60
Join Date: 15/09/14
SC,
Is the pay formation you are drilling Sandstone, Carbonate or Granite ?
Low permeability sandstone does give confusing indication.
Also we need to analyse the gas chromatograph to see whether they are recycled gas or not (which has lesser methane and ethane).

Regards,

Andy
Maitreya0196
Sr. Drilling Engineer
SPREADAssociates
Total Posts: 8
Join Date: 05/09/14
Hi Chike,

What is an effective porosity and permeability of your pay? Also which mud system have you been using?

Gordonholm
Operations Geologist
Tullow Oil Plc
Total Posts: 8
Join Date: 17/04/13
It is possible that this gas was bleeding in from a thin interval which has high pressure but low permeability.  These gas bleeds are low volume so don not show up on flow checks.  Look at the gas ratios, if they are identical then this is typical of a gas bleed.  Important to look at the point where the CGs started.

Cheers

Gordon holm
Companyrep
Drilling Specialist/Well Engineer/Training Consultant
SPREADAssociates
Total Posts: 339
Join Date: 10/01/05

A recent high connection gas HPHT well example, on land, Indonesia drilling in the transition zone, where every time pumps were off we had a 40-60% bottoms up, mud loggers gas reading, supported by observing gas breaking out at the header box. So as an engineer, if we reverse engineer the 'bubbles' breaking out, i.e. using simple PVT calcs to bottom hole conditions, we conclude ml vs litres entering well. Note: A very small volume i.e. less than 50ml entering into these wells should in theory result in gas expansion and gas cut mud at surface!

In our case there was no (0) gas cutting, just bubbles, coming from a HPHT source, the well was not flowing, finger print, return volumes were the same.

Contractor was adamant he was increasing mud weight and I was adamant we were not. After a time out for safety, a few more pumps off, bottoms up where I insisted on a pumps off-swab test, to convince drillers etc, there was no imminent danger downhole, we drilled to TD with same mud weight, had no issues and 40-60% pumps off and connection gas readings (for whatever reason.) that actually reduced and faded away after a few days.

Latent causes as to why, how this happened, Im not sure and we are still trying today to figure out what and why these readings result at surface! In conclusion it's a lot of things and quite complex! However stick to the basic and real indicators where in combination these should affirm when mud wt increase is needed.

Companyrep
Drilling Specialist/Well Engineer/Training Consultant
SPREADAssociates
Total Posts: 339
Join Date: 10/01/05
Document attached may help this is the best I have come across to explain gas in wells.

In the 90's when we started on HPHT wells in North Sea, we were trained by our peers not to react too much, too quickly to gas readings. The Geologist and Supv's also had a normalized formulae that we applied and depending on penetration rates and other factors we only increased mud weight when we were absolutely sure pore pressure was needing this or drilled and/or connection gas reading really confirmed pp was increasing. i.e. Where Drilling trends, gas cut mud, Finger printing on connections also started to show deviating signs of mud weight needed increasing. (Facts are that connection gas can be very miss-leading, and could be a fact of the energy lost from well every time pumps are stopped, formation mud fluids relationships, characteristics, dynamic change that results as mud formation system cools down or heats up on a connection etc (Where different wellbores will act differently all need to pre-empted and better engineered in the first place) etc. Diffusion into well could also result and be in part the cause, note: diffusion that is 10x more in OBM than WBM so I am told. (There is an SPE paper on this.)

Rule no 1 learned in HPHT wells? is to drill with the minimum safe overbalance and only increase mud weight based on the right indicators. So we would not drill out a shoe with too high a mud weight. We would drill with agreed controls in place, listen to the well and increase mud weight only when/as needed. Easier said than done I know, however I have no experience taking kicks while drilling in HPHT wells, so what we did worked then and should do now

We have as an industry learned that increasing mud weight in HPHT wells for the wrong reason is not best practice that only tends to make things worse, more masked and often more confusing.
Documents uploaded by user:
Gas definitions document.pdf
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