Welcome to Petroleum Geology Forums

This is a free online community that aims to bring petroleum professionals and geologists together and share valuable knowledge. Registration is easy so become a member now for instant free access.
  • Petroleum Geologists can stay up to date with industry related topics and exchange ideas and concepts.
  • Upstream Oil and Gas Consultants get a chance to share their expertise and gain exposure to land future projects.
  • Geology students and graduates can join the discussion and get into contact with potential future employees.

  >> Register Now





Post new topic Reply to topic  [ 8 posts ] 
Water Gradient 0.42 psi/ft 
Author Message

Joined: Sat Apr 14, 2012 12:56 am
Posts: 15
Post Water Gradient 0.42 psi/ft
Hi All,

I have a water gradient in a field which was from MDT points, is 0.42 psi/ft. This got me thinking since fresh water gradient is 0.433 psi/ft, and is it possible to have 'lighter' water in the formation than fresh water?

From DST sample it was converted to 0.45 psi/ft gradient, assuming the clean up was good and we obtained the formation water with correct salt amount in it.

The MDT was obtained on more than 5 points yielding good gradient. is it possible that we have subnormal pore pressure in the formation? or is there other explanation...

thanks guys :)


Tue Oct 23, 2012 8:36 am
Profile

Joined: Tue Apr 05, 2011 8:38 am
Posts: 68
Post Re: Water Gradient 0.42 psi/ft
Yes, you can have a water gradient of 0.42 psi/ft. I have seen plenty of water bearing zones with such a gradient.
It does not mean that your reservoir is subnormal.
A reservoir with subnormal pressure will have a pressure offset (at Mean Sea Level for offshore wells) lower than 14.7 psia, when you extrapolate the water pressure trend line back to MSL.
In other words, the water pressure points (or the trend line) will fall below the normal hydrostatic pressure line (0.433D + 14.7), where D is the depth in feet TVDSS.


Fri Oct 26, 2012 12:05 pm
Profile

Joined: Sat Apr 14, 2012 12:56 am
Posts: 15
Post Re: Water Gradient 0.42 psi/ft
Ko ko kyi, again thanks for your explanation, i was not thinking clearly then ;)

Do you have any idea of why the gradient is so low? its not subnormal reservoir, but still on how come the gradient is lighter than fresh water?

Sorry if the question is rather basic, really thank you for your sharing...


Fri Oct 26, 2012 12:28 pm
Profile

Joined: Tue Apr 05, 2011 8:38 am
Posts: 68
Post Re: Water Gradient 0.42 psi/ft
One possible reason is that your water zone may have some dissolved gas inside, like hydrocarbon gas or carbon dioxide.
All formation water have some amount of gas in it. There is published data which indicate water samples from water bearing zones do have gas chromatograph readings (fingerprint) with some hydrocarbon peaks.
Another possible reason is that if the well has been drilled using oil base mud or synthetic oil base mud, the MDT pressure may be seeing a somewhat lower pressure gradient in the invaded zone (as the MDT measures the pressure of the fluid in the invaded zone) due to oil filtrate invasion. In theory, the pressure in the invaded zone should have equalized with the formation pressure away from the wellbore by the time of MDT operation. However,due to capillary effect in the invaded zone, the MDT may be seeing a lower pressure gradient in the invaded zone. There is an SPE paper by Schlumberger on this phenomenon of pressure tests in a water bearing zone invaded by oil filtrate from the drilling mud.


Fri Oct 26, 2012 2:49 pm
Profile

Joined: Sun Jul 22, 2012 6:33 pm
Posts: 2
Post Re: Water Gradient 0.42 psi/ft
Thanks kokokyi for the explanations. However; there is another possibility of uncertianity coming from the TVDss calculation?


Wed Oct 31, 2012 4:45 pm
Profile

Joined: Sat Apr 14, 2012 12:56 am
Posts: 15
Post Re: Water Gradient 0.42 psi/ft
Can you elaborate more what about the tvdss and on how will affect the gradient? I was thinking on top of my head if the tvsss was wrong then it will affect all points right?

Thanks


Thu Nov 01, 2012 1:30 am
Profile

Joined: Sun Jul 22, 2012 6:33 pm
Posts: 2
Post Re: Water Gradient 0.42 psi/ft
See below ex:

Tvdss Form. Pressure
ft psi
4500.000 3242.967
4509.000 3246.235 0.420 psi/ft your case
4513.000 3248.578


4500.225 3242.967
4508.923 3246.235 0.433 psi/ft case of water grad=0.433
4512.851 3248.578


Thu Nov 01, 2012 11:14 am
Profile

Joined: Tue Apr 05, 2011 8:38 am
Posts: 68
Post Re: Water Gradient 0.42 psi/ft
Yes, TVD errors can affect pressure measurements, as the pressures are referenced to TVDSS depths. This is especially true for deviated wells, where TVD depths are computed using survey data.
However, I have seen gradients of 0.42 psi/ft in vertical exploration wells.
Of course, there is also the possibility of uncertainty in the depth measurement, even for vertical wells.
To make things more complicated, if pressures are taken using LWD (Logging While Drilling) tools, the uncertainty on depth measurement is even higher because of using Driller's Depths.
The uncertainty in TVDSS has an impact on the determination of field-wide fluid contacts or Free Water Level.


Thu Nov 01, 2012 2:24 pm
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 


Related topics 
 Topics   Author   Replies   Views   Last post 
There are no new unread posts for this topic. Estimation of pore pressure and overburden gradient

rahuldinno

1

566

Thu Dec 05, 2013 9:30 am

O_Stevens View the latest post

There are no new unread posts for this topic. water resistivity source

spherical

9

1285

Fri Dec 28, 2012 6:22 am

teganiqbal View the latest post

There are no new unread posts for this topic. Calculation of pore water resistivity

Kris_10

4

2249

Wed Aug 29, 2012 1:50 pm

Kris_10 View the latest post

There are no new unread posts for this topic. Water Saturaion in Oil zones

Sabee

3

440

Tue Jun 04, 2013 4:05 pm

bezoarb View the latest post

There are no new unread posts for this topic. How to calculate separate saturations of gas,oil,water

iheanyichukwu

0

1705

Wed Jan 22, 2014 5:40 am

iheanyichukwu View the latest post

 



Search for:
Jump to:  


Contact || © EPGeology.com. || Powered by phpBB

phpBB SEO