At the workshops that I'm facilitating, I hear more-and-more these days about RDF (Reservoir Drilling Fluid). I am (kind of) familiar with DIF (Drill-in Fluids) which we were using years ago, but RDF is a fairly new one on me.
My questions are:
- What's the difference between DIF and RDF (if any)?
- What circumstances would make it worthwhile to use RDF?
- What circumstances tend to make it not worthwhile to use RDF?
- What tests and financial-modelling can be applied to determine the cost vs. benefit of these fluids?
- Are there any compelling case histories that show the benefits (or otherwise)?
- If RDF are 'so good' then why isn't everyone using them?
Dave, good question. This will be a fairly long answer as there are many facets to each part.
First, there is no substantial difference between 'DIF' and 'RDF' They each convey the same meaning and intent- to drill the well with a mud system that has been designed to minimize damage to the reservoir as it is drilled. I use RDF term as that was the designation that my former employer-M-I SWACO, used for the business line. Habits are hard to break.
RDF's are usually employed in wells that will be completed using open-hole methodology. Standalone Screen, Pre-drilled liner, or a Sand Screen with gravel pack. The driver for RDF's in these completions is these are often long horizontal or extended reach intervals and often much of their permeability lies in the first part- often called 'heel' of the well. If we don't do something to ensure that this area will not be excessively damaged as we drill it, the well may become uneconomic. A well designed RDF should deliver a low skin completion to the client. This is something that can be demonstrated in the Formation Damage testing that is often done to qualify an RDF formulation for the target interval.
If the well is going to be cased and perfed, then an RDF is not called for regardless of what else is done in the well completion. It's the job of perforation to extend past the damaged zone and reconnect the reservoir to the well bore. Note- uncontrolled losses of mud to the interval in this case may make the completion moot. So, that isn't what I am discussing here.
A good question is how is an RDF different from a standard drilling fluid?
The RDF has to deliver the same drilling properties that the standard drilling fluid does. Chief among them-
Drilling properties- rheology
Well Bore Stability
But in addition the RDF must
Form a thin, impermeable filter cake on the face of the reservoir that can easily be removed with flow in the production direction.
This filter cake must also be reactive to chemistry that will completely destroy it or promote its disaggregation.
The filtrate should not adversely impact the wettability of the reservoir
It should not form emulsions with connate brines or crudes.
It should not create scales or precipitate with in-situ brines or gases.
So you can see that we ask a lot more of an RDF than we do a standard drilling fluid.
The RDF properties and compatibility with the reservoir are verified by a series of tests that employ the techniques of thin section analysis to determine max pore channels, x-ray diffraction data to understand formation minerology-particularly with respect to clay and shale content, various lab bench tests to determine if the initial formulations are suitable for further testing. And, finally a full scale formation damage test against natural core material under simulated reservoir conditions to finally ascertain the damage this mud formulation will inflict on the reservoir as it is drilled. The outcome of the this testing is a Return Flow percentage and often is the final determinant for the wrap up of testing, or used as a guide to design further testing.
RDF's or DIF's have been employed for decades and there is a huge body of documentation as to their effectiveness. I would recommend an SPE literature search thru OnePetro for relevant papers.
There are lot of reasons why someone might not use an RDF. THey are going to be more expensive than a standard drilling fluid so that might be one reason. What it is important to understand is that if you are planning an open hole completion, an RDF is your best chance to minimize damage to the reservoir, and will repay it's additional cost in reduced skin damage and increased production over the life of the well.
Thanks for asking this question. I actually get variants of it a lot. Hope the information provided is useful.