Solution 2021

Pagina 1

1 Solution set and marking schedule for Reservoir Engineering Exam 2021 Prepared by Martin Blunt (1) (i) Start from equation: gg gi p B PWc B BGG        1 Then: gig g gi p BB PWcG B B G         1 Plot        g gi p B B G 1 on the y axis and gig BB P   on the x axis. Slope = Wc and the y intercept when x=0 is the gas in place, G. Gp (million scf) P (Mpa) Bg (rb/scf) x=DP/(Bg-Bgi) y=Gp/(1-Bgi/Bg) 0 40 0.0018 58 38 0.0022 5000 319 113 36 0.0029 3636.363636 297.9090909 143 34 0.0035 3529.411765 294.4117647 167 32 0.0042 3333.333333 292.25 From the graph below, the intercept, G is approximately 2.4×108 scf (acceptable range 2-3) and the slope Wc = 0.016 rb/Pa. There evidence of an active aquifer albeit of modest size. (2 marks for method, 4 marks for table, 4 marks for graph and 8 marks for values, including correct units. Lose 2 marks for any values quoted to 3 or more significant figures.) 2 (ii) Recovery factor is 167/238 = 0.70 (2 marks). The gas saturation        G G B BSS p gi g gig 1 = 0.51 (2 marks). From the relative magnitude of the two terms in the material balance equation this is 82% gas expansion and 18% water influx – so a fairly weak aquifer drive (8 marks). The field should be produced simply by dropping pressure leading to a recovery likely to be approximately 90% (2 marks), although there may be a problem of water production later in field life to be mitigated by slower production or only producing from near the top of the field (2 marks). (iii) With carbon dioxide injection, the reservoir volume injected is 𝐼௖𝐵௖ (4 marks). This is added to the water influx since it also contributes to production and pressure maintenance (4 marks) and converted back to a surface volume of gas to obtain (4 marks): 𝐺௣ =𝐺ቆ1 −𝐵௚௜…