 So, for this problem, we're going to be calculating our cumulative oil production, which is denoted as NP. So, in this case, we're given 10 million stock tank barrels originally in place in the reservoir. We have an initial pressure in the reservoir, which is 3,000 psi, and after so much time of production, our pressure at a certain time is going to be 2,000 psi. So, as production occurs, your pressure in your reservoir will deplete as reservoir fluids are leaving the system. So, we have two BO values, one for 3,000 psi and 2,000 psi, so our formation volume factor of oil at 3,000 psi is 1.087, and at 2,000 psi it's 1.090. Our initial water saturation is 0.20, so as long as this is only oil in the reservoir, our oil saturation will be 0.80. So, our compressibility of water, which is CW, is going to be 1.5 times 10 to the negative 6, 1 over psi, and our compressibility of our formation is going to be 1 times 10 to the negative 6, 1 over psi. And so, the first thing we're going to do is we're going to calculate the cumulative oil production using volumetric method, which is going to be equation 4.58d in lesson 4, and so that equation is as follows. So, your cumulative oil production, do you know what it is, NP, is going to be equal N times BO minus BOI divided by BO. So, the BO is going to be at 2,000 psi, and the BOI, which is your initial formation volume factor, is going to be at 3,000 psi. So, we can just do that right here. So, we're going to have 10 times 10 to the 6 BO, which is going to be 1.090 minus 1.087 divided by 1.090. And so, when we do this, we're going to get the cumulative oil production at this point is going to be 27,522.94 STB. So, this right here is a result of fluid expansion. So, that's the drive mechanism, as you'll read about in lesson 4. So, this is one of the drive mechanisms for oil production. So, in this case, it is fluid expansion. So, that's why your difference in your BOs, your formation volume factors, is the reason you're going to actually be getting production in this case. So, now we can also do this for not volumetric. So, basically, we're going to be accounting for our compressibility of our water. We're going to be considering the compressibility of the formation. So, this equation is going to be similar to equation 4.65B in the note. And this equation is going to be as follows. So, your cumulative oil production is going to equal N times the quantity of BO minus BOI plus 0.2, which is going to be your SWI right here. So, that's right, SWI. So, this is going to be SWI multiplied by your compressibility of your water. So, I'll just cancel that off. This is just going to be SWI multiplied by your compressibility of your water. And then, you're going to add your compressibility of your formation. This is all going to be divided by 1 minus SWI. And then, we're going to multiply this quantity by our change in pressure. The reason we do this is because these compressibilities are in terms of 1 over PSI. So, this is basically at this different pressure, how are my compressibilities changing? So, these compressibilities given are for initial conditions. So, your compressibility will change as a result of your pressure in your reservoir. And this is just accounting for all of that. So, this is just a more accurate way of determining or calculating your cumulative production. And this is all divided by B.O. So, in this case, we're going to have cumulative oil production is going to equal 10 times 10 to the 6 times 1.090 minus 1.087 plus 0.2 times 1 times 1.5, 1.5 times 10 to the negative 6 plus 1 times 10 to the negative 6. This is going to be divided by 1 minus SWI. So, that's going to be 0.2. And then, this whole quantity right here will be multiplied by your delta P's. So, 3,000 minus 2,000 PSI. And then, we're going to divide this all by B.O., which is going to be 1.090. So, when we do this, we find that our cumulative oil production is going to be 42,431.19 STB. It should be around there. And so, from these cumulative oil productions, we can also find what our recovery factors may be. And so, like I said, is the only drive mechanism when we're considering here is just a fluid expansion, which is one of your lowest recovery from drive mechanisms. So, calculate recovery factor. Recovery factor is just simply your NP over your N. So, your cumulative oil production divided by your original oil in place, stock tank conditions. And so, if we consider for volumetric condition, our oil production was 27.522, and our original oil in place was 10 times 10 to the 6th. So this will give us our recovery factor of about 0.27%. And so, for our recovery factor using the non-volumetric method, we're going to have the 42,431 divided by 10 times 10 to the 6th. This recovery factor is going to be about 0.424%. So these seem very small, which they typically are, like I said, for this drive mechanism. But as you can see, 0.424% and 0.27% is quite a bit of a difference when we're talking about a difference in over 10,000 barrels of oil. When that's right here, that's 25% of that alone. So it is important to understand, like, this is a way to visualize one of the drive mechanisms of oil production. This doesn't account for, like, really pressure drop at the well bore compared to the reservoir, like we talked about with Darcy's Law and, like, pseudo-steady and steady state. This is just purely about a drive mechanism. This is a material balance.