Fluid Contacts : Log or Pressure Gradients

[soman703]

Hello everyone,

I have a question please. What is the reliable method among log or pressure gradients for determining fluid contacts?

In Heterogeneous reservoir why do we trust contact obtained from pressure gradient compared to logs? How does this selecting contact change from heterogeneous to homogeneous & sandstone to carbonates?

In theory, log is a response of matrix while gradient is a response of fluid. And generally log gives us contact while gradient gives us free water or free oil level.


Can anyone shed light on this issue please?

Thanking you all,
Soman Ahmed

[kader_007]

Hi Ahmed the right answer to your query is "it depends"
Contacts from logs are deduced from resistivity logs (usually you have three trends over there for shallow medium and deep formation resistivity the difference between the 3 are used to highlight an invasion like process).
So here is the critical point your resistivity reading will always be affected by formation nature hence the heterogeneity you were highlighting (shale content, mineralogy, chlorine etc ) while pressure points are mainly driven by the rock type (overburden) and fluid types that will give you a pore pressure.

So yes resistivity is used but better cross checked with formation pressure points both should speak the same language, if not, pressure points are taken as main indicator and formation resistivity is further analysed by the petrophysicist for a better understanding.

The point there is that to have the engineer, the geologist and the petrophysicist sit together to see the CPI of the well - processed logs (or even raw logs) and chose properly where to have the pressure points picked to have a sound number of points on which you build your trends/gradients.

Hope this answers your question.

Regards,
Kader

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[i-anuar]

if you have enough reliable pressure points you could derive FWL while from logs OWC depth can be estimated. if you have vertical barriers FWL's should be used with care if you have single well and pressure measurements just from one zone, esp if pressure measurements only in upper zone above vertical barriers and you don't have another well where other zone pressures/contacts evaluated.
As was suggested best to work on this together with geologist & petrophysicist etc...

[soman703]

Dear Kader & Annuar,


Thankyou so much for detailed replies.
Very helpful!!

Best Regards,
Soman Ahmed

[temr]

One more point in sandstone resestivity is a good tool but with no shale
in shaley sands resestivity do not work well there are reserviors where logs reed water but after perforation you get pure oil
In carbonates resestivity does not work in low porosity and tie intervals ( from 0 to 6% approx)
So to range the methods form top to bottom i would sugest following
WellTest
MDT
CapPressure(Core)
Logs.
So there is no clear answer what to use for example if you have 100 wells with logs and only 1 well with MDT you can not use only 1 well MDT results. You need to combine and QC for you fluid contacts should always be match to you history production data

[soman703]

Thankyou Temr bro.

Also one more thing, can core data be also used in any way to find contacts?

Also has anyone observed that when we take pressure measurements near the dip, gradient response is not so accurate there?

[kader_007]

Thankyou Temr bro.

Also one more thing, can core data be also used in any way to find contacts?

Also has anyone observed that when we take pressure measurements near the dip, gradient response is not so accurate there?

First of all happy new year to all of you...

Back to your question now, core analysis has various scopes either for routine or special core analysis.

Among other things we get capillary pressure (Pc) trends and core images (uv exposed).

Usually cores are taken in different depths within the same well, this will help figure out among your depth of interest how the things evolve.

You'll have different Pc curves for different depths (from which you can estimate the hight above free water level and somehow the transition zone height) for your specific well.

From images you can see if there are changes in rock types along the depth, if the main rock type is present and clean over all the depths, if there are vertical changes/stratigraphic barriers,etc.

Knowing your geology you can make assumptions whether this well is representative of the area or not to use it as indicator for free water level and then cross check with other data as logs and pressure points.

And again this comes from an integrated work so get the right people working on this, along with the Reservoir Engineer the team should at least have a Petrophysicist and Reservoir Geologist even if not at full time at least consultancy.

Cheers,
Kader

Envoyé de mon SM-A500F en utilisant Tapatalk

[soman703]

Thanyou so much sir.

Very detailed and useful reply.