Under downhole conditions, a lot of things can change between when the reservior is in its original state and then during production through to its depleted state. This includes the change in state of the fluids in the reservior.  So, obviously, it depends on the fluid type and the composition, but also the temperature, pressure etc.  This is to a large extent described by PVT analysis. But think also about the mobility of the fluid components through the reservior: gas/CO2/oil mobility and the formation pore throat geometries.  (Hence fracking can make a major difference to not just to the flow, but also the fluid -type mobility.) The abundance of associated gas and CO2 will be determined by the PVT curves for the fluid, but their ability to move will depend on the relative permeabilities. Another issue is the dissolved gas aspect of the formation water, and as production progresses more formation water may be produced, or come nearer the producing zones.  So changes in pressure may release dissolved gasses. And not to mention the microbiological effects that changes in pressure and fluid interactions cause.  So ... lots of things are going on down there - most of which we only think we understand, but (in my case) actually we are only guessing.

If your reservoir is above the CO2 critical temperature (304.25 K, 31.10 °C, 87.98 °F) and critical pressure (72.9 atm, 7.39 MPa, 1,071 psi), as it is likely to be, and the oil is light oil (conditions vary), the CO2 is very likely to be in solution into the oil phase. I agree with the general comments by Harald above, but reservoir temperature variations are probably less influencial than reservoir pressure variations (given a certain gas and oil composition, and CO2 abundance in the reservoir). By lowering the reservoir pressure (more near the producing borehole, and less away from the borehole) during production, CO2 will redistribute between the liquid phase and the gaseous (natural gas) phase, and this in different degrees radially away from the borehole. At this point, the relative permeability of the gaseous phase relative to oil and water are going to play an important role. A well done reservoir simulation would provide good indications of the fluid behaviour in the given reservoir.

If in a group of quantum wells (which in this case is true given the distribution of observations on gas diffusion by average density above the oil surface conditional on any parameter like temperature or pressure or surface tension) similar measurements are abserved, then given your tolerance level the PVT analysis gives a stable reservoir distribution it can be said, Econophysically using String Matching Field Theory. We hope this is of some help. 

SKM

For NH, SM

The composition of the separator gas will depend on the separator conditions and, as Harald notes above, if the reservoir falls below bubble point the compositions and proportions of gas and oil that simultaneously enter the wellbore. This can easily be modelled using software.

Other reasons are man induced, such as gas being injected downhole to help lift the oil, of gas break through, which is when a gas injected into the reservoir for enhanced oil recovery reaches the producing well. Sometimes additional reservoirs become connected if a sealing fault breaches as the pressure differential increases during depletion, but this is rare and can be detected by a change of slope in the pressure decline (or P/Z plot for gases).

It has nothing to with string theory or even the critical point of CO2, merely the relative volatility of CO2 compared with the other components.

I can send you a paper on this where the composition are accurately calculated for a reservoir under simple depletion.

Brian

PS Check the apparent change is genuine and not a mis-measurement!

Dear Dr. Botker and Dr.Moffatt,

                                                       Assuming that a part of Dr. Moffatt'a comments were in response to my answer I would like to clarify that this Resevoir Physics project usinh String Field Theory ande Econophysics was originally started I think it was in 2010 when Prof. E.Tjaland on NTNU,Oslo expressed interest in my visiting NTNU to work on Reservoir Physics, simultaneousli in 2013 I submitted a paper for the Indian Ststical Institute Conference in Giridih, India on the problems of the newly formed state of Jharkhand which is rich in mineral resorces like coal, so I wrote a joint research paper on the String Theory and Econophysics problem of coal mining assuming a blasting potential to invent the Genetic Algorithm with the developed String Matching Field Theory which after a number of papers which were submitted to the Indian Statistical Institute Conference and then to Prof. E. Tjaland and to the Royal Swedish Academy of Sciences and to the Journal of Physics A & D (different papers out of the project) and were archived by the journals and later on also published secondarily on www.researchgate.net/Soumitra K. Mallick. The research is going on and also in collaboration with my son Sandipan Mallick who is at present an undergraduate student and researcher at NSHM College of Pharmacy and MAKAUT in Kolkata and Dr. Nick Hamburger of HN and Megatron Consulting, Fairfax, Virginia who is Dr. Ing. we have found that the String Field Matching Genetic Algorithm for the Physical*Economic*Financial Efficient Mining Function requires an initiating Balsting Potential and results in a Quantum Mechanical Stock and Flow with genetic utility and arrow of time assumptions. Since the only physical properties we have used are diameter-depth ratio  of the quantum mines (wells) no surface tension, viscosity, specific gravity etc.. hence the initiating blasting potential is necessary for the six step meta heuristic String Field Matching algorithm. There are other quantitative physical properties we have derived for the system which includes a systems biology in the provision of drinking water for the miners. The gaseous and solid matter diffusion problem can be solved Econophysically by the topoloical twist contained in the geometic diameter-depth ratio solution which gives rise to cellular compactification for systems classification and systems integration (Mallick (2014, 2016)) and differential topologies of the material particles in the wells. We can conjecture that the distribution of matter density including CO2 can shape the partitions prabilistically in terms of the Copenhagen intrepretation. We hope you find some solutions.

SKM for SKM, NH, SM

That is a wonderful answer Soumitra, and I'm not challenging your theories, but I think they are vastly more complex than needed for this issue. There are simple PVT programs that can model phase behaviour and solve this.

Why would you want to crack a nut with a hydraulic press when a simple nutcracker works perfectly well?

Brian.

 As the reservoir will deplete, reservoir pressure will decrease resulting in more and more heaviers coming out in the associated gas and a change in the composition of associated gas. 

(... I agree with Brian)