Question

The (water/oil/quartz) capillary pressure curves for a sandstone reservoir are shown in the figure below. Suppose a drainage experiment was conducted on the same sandstone but with oil and gas, what will the capillary pressure be at 60% oil saturation? what will the residual oil saturation be? ơ.-72 dyne/cm P 1 g/m θ (water/oil/quartz)s 470 35 dyne/cm ơ.- Po 0.82 g/ml θ (oil/gas/quartz): 650 10 (psi) 0 20 40 6080 100 s. (96)

Answer 1

Petrophysicis ts us e cap pressure water s aturation, adjusted for height above the free water level, an d residual oil saturation (Sor) to help calibrate log derived water s aturation in oil and gas reservoirs above the transition zone, and to help detect depleted reservoirs. I will not help calibrate SW in p artially depleted zones Accumulation of hydrocarbon in a reservoir is a drainage process; capillary pressure graphs show the drainage curve. Fluid flow process in which the saturation of the nonwetting phase increas es. Mob ility of nonwetting fluid p hase increases as nonwetting ph ase saturation increas es upper curve on image at right. 30 10 03 04 05 07 0.8 0.9 Water Sturation, Sw The resulting graph of pcvs SW is known as a saturation-pressure curve and can be plotted on a depth plot of log data or results by setting pc 0 at the base of transition zone on the logs. This assumes a uniform porosity-permeability regime, which is seldom encountered in real life, so more complicated methods are needed to superimpose the saturation values from multiple Po curves Cap pressure curves areavailable at certain depth in the reservoir. The saturation from curve is selected from the graph with respect to the sample's position above the water contact. These saturations are then plotted with respectto the sample depths onto the log a na lysis depth plot. It ha s been traditional to look at the minimum water saturation on a cap pressure curve and to ca ll it irreducible water saturation [SWir). The true irreducible water saturation is much higher than the minimum on the graph because we are so close to the contact. The true irreducible saturation is defined by the height versus SW curve for each sample, and not by the minimum SW. If porosity, permeability, pore geometry, grain size, sorting vary in a reservoir, you need a height versus SW curve for each rock type, and a reliable method for identifying those rock types by using a log analysis algorithm or curve shape pattern A typical method to differentiate rock types is to use raw log or analysis results ranges to segregate rocks into various categories. Then an appropriate cap pressure curve can be applied to each rock type, where ever it occurs in the reservoir interval. In hig her permeability rock, the cap pressure curve quicky reaches an asymptote and the minimum saturation usually repres ents the actual water saturation in an undepleted hydrocarbon resevoir ab ove the trans ition zone. In tight rock, the as ymptote is s eldom reached, so we pick saturation values from the cap pressure curves at two heig hts (or equivalent Pc values to rep res ent two extremes of reservoir condition