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hi all ,

i have a question regarding water resistivity source, as you might know this has high impact on water saturation calculation..

i currently have two sources of rw, the inverse archie from a 30ft interval thats tested water, and a water sample from DST.. my question is which one is more reliable, since i have two different rw value, and i dont have any direct sw and rw calculation from core to calibrate my sw model..

i read that the dst is very prone to contamination, but i have no experience in this..
any help is appreciated, thanks

Since you will be computing water saturation based on resistivity logs, I would recommend to use the Rw derived from resistivity log in a water bearing interval. Draw a Pickett plot over that 30ft interval where the DST tested water. The main issue with Rw derived from such a method is that you have to assume the value of cementation exponent m. However, a regression trend on the water bearing points, if they have enough spread in porosity values, will give you a reliable value of Rw. If the points have similar porosity, you may have to assume a fixed m value to estimate the Rw value.
Water salinity or resistivity measured on DST water samples normally tend to be different from that derived from resistivity logs. The DST water sample is the mobile water which is free to flow. On the other hand, the resistivity log is responding to all the conductive water in the formation, i.e. free water, clay bound water and capillary bound water (which cannot move). Therefore, it will be more representative to use an Rw value derived from resistivity logs, since you will be using the resistivity logs to compute water saturation.

Thanks for the quick reply. I will try your suggestions and try things out..

Related to this topic, our field are currently in appraisal and of course one of the uncertainties that we are trying to reduce is water resistivity, what do you think the best source of water resistivity? I am thinking of acquiring direct rw and sw from core.. Will that be enough for testing the log sw model? Any other suggestions?

Thanks guys..

If you are going to compute your water saturation based on resistivity logs, you will always have uncertainties on the Rw value used. It is difficult to obtain reliable and representative formation water samples, either by means of DST's or wireline formation testers. What you get will always be the mobile water. Furthermore, depending on the type of mud you use to drill the wells, you will have certain amount of contamination with the mud filtrate. Even in wells which are producing water, the salinity of the water sample collected at the surface can be different from that of the actual formation water or the water sample collected bottomhole. To make it worse, there are even cases where the water in the water leg of a formation has salinity (i.e. resistivity) different from that of the connate water in the hydrocarbon bearing zone.
To overcome this problem of uncertainty in water saturation computation, one approach is to use capillary pressure based saturation, using a Saturation Height Function. To do this you will need capillary pressure data and reliable estimation of formation porosity and permeability.

Ko ko kyi, again thanks for the prompt response!
Yes it would be great to have that cap pressure measurement, but unfortunately from the current cap pressure measurement for this particular field, which is a complex turbidite system, we have a very wide range of curves so its really hard to groups the result, in addition to the system not having clear water contact due to compartmentalizations across te field.

Back to your response regardih different salinity from the h/c bearing zone and the water leg, so maybe having both cored and analyzed for sw and rw will be great right?

Thanks for your input ko ko kyi, i am relatively new to this, and its getting more and more interesting!

It will be good if you can get a representative connate water sample in the hydrocarbon bearing interval. You will need to dope the mud to detect filtrate invasion into your core. You will have to centrifuge out the capillary bound water to measure the salinity and resistivity of the connate water. You can also perform Dean Stark measurement to get Sw values in the cored interval. With this information, you will be able to calibrate your Sw computed from resistivity logs. However, there may still be some uncertainty left in your computation. It is always good to do uncertainty analysis of your computed Sw values and give a range for the Sw.

Dear Ko Ko Kyi,
Right now i have two models, ref case, and upside, using two Rw, from DST and inverse archie. In the future appraisal wells i will just recommend taking representative water sample using MDT if we are using water based mud, or if we will use oil based, we should take a representative core sample and do a dean stark...

of course it will depends on what we will found next time.. thanks Ko ko kyi for your response!

You will find it quite difficult to obtain representative formation water sample using the MDT in a water bearing zone, because of the contamination by mud filtrate which essentially water. This mud filtrate will mix with the formation water and you will have to pump out with the MDT for a relatively long time to get a good formation water sample. Even then, you will still have some amount of filtrate contamination. It will be good if you measure the resistivity and salinity of the mud filtrate so that you can differentiate between filtrate and formation water sample. This way you can also check the level of contamination by mud filtrate.

Yes thank you kokokyi for the reminder and guidance, hopefully everything is on track and we can get the appraisal plan going.

Thanks again!

A catalog of water resistivities has been compiled in an effort to evade the continuous frustration of searching through innumerable well files, published reports, or outdated unpublished data files. Over 6,500 water resistivity values are included in this compilation covering Wyoming, and Harding and Fall River Counties, South Dakota. Most of the values were located in the well files at the Wyoming State Oil and Gas Commission and the South Dakota Department of Water and Natural Resources.