What is the difference between these basic terms

[abcde123]

Hi everyone, I know that it may seem very easy and basic, but I am facing some problems understanding GOR and solubility. I know the equations of GOR and Rs, but what I want to know is the basic physical difference. If solubility is the amount of gas that can be dissolved in oil for given pressure and temperature and GOR is the amount of gas produced divided by amount of oil produced, then won't these two terms be almost same?
Thanks in advance.

[DAH7542]

Dear abcde123,

Your definitions are right, as for your question, GOR and Rs are only equal above bubble point pressure (Pb), i.e. for reservoir pressures greater than Pb gas will be in solution (at reservoir conditions) so the produced gas is the gas that came out from solution in its way to the surface; once the reservoir pressure declines below bubble point due to fluid extraction, gas is liberated within the reservoir (forming a gas cap) and GOR, as measured at the surface, exceeds Rs, so more gas will be produced due to higher gas mobilities. The gas that has been liberated from solution is known as free gas. Free gas produced at any time and measured at reservoir conditions can be calculated as: Qg = Qo(GOR-Rs)Bg (in consistent units system); this equation clearly shows that for reservoir pressures below Pb, where GOR=Rs, no free gas is produced.

Refer to Dake's book Fundamentals of Reservoir Engineering (fig. 3.4) or any other RE book to get a better picture of this.