core data

[yasiobasio123]

Hello
can anyone share conventional core analysis programme, plug selection, how plugs are selected for RCAL, thanks

[DAH7542]

In the company I worked for several years, this was the common core plug selection procedure (applicable to clastic reservoirs):

Basic core analysis:
- 1plug per foot for horizontal perm and helium porosity measurements at at least two Net Overburden pressures (one being the current/overall NOP)
- 1 plug every 3feet for vertical perm and helium porosity measurements
- Saturation measurements using Retort method in horizontal plugs

Based on a Sedimentological study (if available) or a quick core visual description, facies were determine to define the Special Core Analysis program. For each geological facies, horizontal perm distributions were constructed leading to the identification of P10, P50 and P90 horizontal permeability values. The following SCAL program was applied once to each facies and each P10, P50 and P90 horizontal permeability values whenever possible:

Special core analysis:
- XRD, SEM, Unsteady State Relative Perm (restored state cores usually used), Capillary Pressure (and Pore Size distribution if Mercury Injection Pc was used), Electrical properties (such as Formation Factor and Resistivity Index), Pore compressibility, Critical velocity (for poorly-cemented sandstones only).

If I remember well, the application of this whole set of tests was around USD 500,000 for a 100 to 150ft-length core. Sometimes, Budget forced the reduction of this program, and some other times, low perm facies didn´t allow to perform all tests.

[yasiobasio123]

@ DAH7542, thanks a lot for this.
how to go for plug selection for a well to be drilled? how one can know that which spots are good for plugs? pls reply

[DAH7542]

The objective of coring should be to get a good and representative coverage of the vertical/horizontal succession of facies within your reservoir. Having said that, and replying to your second question, whenever possible I suggest to core and test the worse facies ("worse" meaning low perm and phi) along with the productive "better" facies. This is because the whole succession of facies make up the reservoir and when it comes to model it, there should be enough information to characterize the entire system. In summary, ideally you will end up with a core, or set of cores, covering the entire thickness of the reservoir from base to top. These cores are tyically recovered at more than one location (areally) in the reservoir, allowing the paleo-evironmental reconstruction of the depositional system. In fact, such kind of sedimentological interpretations can be done earlier (i.e. soon after field discovery, when there aren't many cores) by studying outcrops and/or studying sedimentary processes at a basin level. This sort of information is useful for spotting the next core to be recovered.

Nevertheless, I don't know any rule of thumb to get the right number of cores needed and the right spatial location of these cores in the reservoir. This, I would say, depends on a judicious assessment of the reservoir (degree of heterogeneity, which is linked to the depositional environment), budget, time, objectives of the coring program, level of detail for modelling (if considered), and so on and so forth.

[Yengineer]

If the well is on drilling then you plan to core the whole producing interval (or as much as feasible) especially on new areas.
When it comes to plug selection for analysis a lot of factors come in play.
How big is the formation and how much did you cored out of it.
Are the cores in good shape or have they been fractured/damaged from transportation and handling.
You then correlate the cores with open hole logs and choose to experiment with plugs cut from the most contributing/representative layers/depths (if the core is in good shape and allows it)
The experiments that you choose to perform have to do mostly with your objectives. Routine - phi, k_abs or special - wettability, end points of saturations, relative pearmibility etc.

[yasiobasio123]

Thanks a lot for this.
One more question please:
What values of Interfacial tension and contact angle (theta) should one use while calculating the height above free water level using air brine drainage Pc?
And what is difference between free water level (although i know where Pc=0) and OWC? How one pick that this is OWC?
Please reply.

[DAH7542]

- What values of Interfacial tension and contact angle (theta) should one use while calculating the height above free water level using air brine drainage Pc?
Answer: use IFT = 72dynes/cm and theta = 0degrees, with these values IFTxCos(theta) = 72x1 = 72. This is the standard value to be used as reported in different books such as CARLSON - Practical Reservoir Simulation, p. 516.

- And what is difference between free water level (although i know where Pc=0) and OWC? Answer: the FWL is the interface between those two immiscible fluids (lets say oil and water) as if there wasn't any porous media (i.e. Pc=0). Since real reservoirs are porous, a capillary rise of water occur above the FWL (in case of water-wet rocks) resulting in the OWC (where still Sw=100% but Pc is greater than 0 or quite similar to 0 if pores are big enough and the density differene between fluids significant).

- How one pick that this is OWC?
Answer: RFT, MDT data (in general formation testers) allows you to identify the FWL (given that the reservoir is under capillary/gravity equilibrium) since this corresponds to the interception (crossing) of the oil and water gradients. From there, and knowing the capillary rise from Pc lab measurements you can estimate the OWC. Well logs are of course a valuable tool.

[yasiobasio123]

@DAH7542

I’m using the eq. below to calcualte the heifht above free water level, is this the right one I am using?

Reservoir Height above FWL = Pc_lab*(IFT_res*cosθ_res / IFT_lab*cosθ_lab) / (gw - ghc)

Ok I use use IFT_lab = 72 dynes/cm and theta_lab = 0 as lab values.

Should I use IFT_res = 30 dynes/cm and theta_res = 30 deg ?? (please correct if wrong, I'm using air-brine Pc_lab)

Also let me know the equation m using is the right one for HAFWL?

Many thanks

[DAH7542]

Dear yasiobasio123,

The eq. you're using is ok if units are consistent. As a reminder: if height avobe FWL is in feet, Pc in psi and density difference (gw - ghc) is in lb/cf, then you should multiply by 144.

- Should I use IFT_res = 30 dynes/cm and theta_res = 30 deg ?? (please correct if wrong, I'm using air-brine Pc_lab)
Answer: these values are ok only if the reservoir you're studying contains oil and water. See the table I attached in a previous reply if you have a gas/water system in the resevoir.

[yasiobasio123]

@ DAH7542 thanks again

Using this equation, Pc_lab*(IFT_res*cosθ_res / IFT_lab*cosθ_lab) / (gw - ghc)
I am calculating the height above free water level.
Units being used are:
Pc_lab: psi
IFT: dynes/cm
gw is water grad.: psi/m
ghc is oil grad.: psi/m

Guide if these are correct, thanks.