Chapter 2 reservoir rock properties and core anlysis

Vietnam National University - Ho Chi Minh City University of Technology Faculty of Geology & Petroleum Engineering Department of Drilling - Production Engineering Course Reservoir Engineering Trần Nguyễn Thiện Tâm Email: trantam2512@hcmut.edu.vn 12/11/2017 Reservoir Engineering References Holditch Resevoir Engineering (Schlumberger) 12/11/2017 Reservoir Engineering Chapter Reservoir rock properties 12/11/2017 Reservoir Engineering Contents  Reservoir  Porosity  Permeability  Saturation 12/11/2017 Reservoir Engineering Reservoir A subsurface body of rock having sufficient porosity and permeability to store and transmit fluids 12/11/2017 Reservoir Engineering Porosity The fraction of total volume that is available for the storage of fluids Vb  Vma Porosity     Vb Vb Vp 12/11/2017 Reservoir Engineering Porosity Rock matrix 12/11/2017 Reservoir Engineering Pore space Pore-Space Classification Total porosity, ϕt Total Pore Space t  Bulk Volume Effective porosity, ϕe Interconnected Pore Space e  Bulk Volume 12/11/2017 Reservoir Engineering Comparison of Total and Effective Porosities  Very clean sandstones: ϕt = ϕe  Poorly to moderately well -cemented intergranular materials: ϕt ≈ ϕe  Highly cemented materials and most carbonates: ϕe < ϕt 12/11/2017 Reservoir Engineering Permeability Permeability is a property of the porous medium and is a measure of the capacity of the medium to transmit fluids 12/11/2017 Reservoir Engineering 10 Capillary Pressure Procedure:  The core is then replaced in the apparatus, the pressure is increased, and the procedure is repeated until the water saturation is reduced to a minimum 12/11/2017 Reservoir Engineering 52 Capillary Pressure The data from such an experiment are shown in Figure 4-5 Two important phenomena can be observed in Figure 4-5 12/11/2017 Reservoir Engineering 53 Capillary Pressure First, there is a finite capillary pressure at 100% water saturation that is necessary to force the nonwetting phase into a capillary filled with the wetting phase This minimum capillary pressure is known as the displacement pressure, pd 12/11/2017 Reservoir Engineering 54 Capillary Pressure If the largest capillary opening is considered as circular with a radius of r, the pressure needed for forcing the nonwetting fluid out of the core is: 2  cos   pc  r This is the minimum pressure that is required to displace the wetting phase from the largest capillary pore because any capillary of smaller radius will require a higher pressure 12/11/2017 Reservoir Engineering 55 Capillary Pressure As the wetting phase is displaced, the second phenomenon of any immiscible displacement process is encountered, that is, the reaching of some finite minimum irreducible saturation This irreducible water saturation is referred to as connate water 12/11/2017 Reservoir Engineering 56 Capillary Pressure Figure 4-6 is an example of typical oil-water capillary pressure curves In this case, capillary pressure is plotted versus water saturation for four rock samples with permeabilities increasing from k1 to k4 12/11/2017 Reservoir Engineering 57 Capillary Pressure It can be seen that, for decreases in permeability, there are corresponding increases in capillary pressure at a constant value of water saturation 12/11/2017 Reservoir Engineering 58 Capillary Hysteresis Pore spaces of reservoir rocks were originally filled with water, after which oil moved into the reservoir, displacing some of the water and reducing the water to some residual saturation When discovered, the reservoir pore spaces are filled with a connate water saturation and an oil saturation All laboratory experiments are designed to duplicate the saturation history of the reservoir 12/11/2017 Reservoir Engineering 59 Capillary Hysteresis The process of generating the capillary pressure curve by displacing the wetting phase, i.e., water, with the nonwetting phase (such as with gas or oil), is called the drainage process The process of generating the capillary pressure curve by displacing the nonwetting phase (such as with oil) with the wetting phase (e.g., water), is called the imbibition process 12/11/2017 Reservoir Engineering 60 Capillary Hysteresis The process of saturating and desaturating a core with the nonwetting phase is called capillary hysteresis Figure 4-7 shows typical drainage and imbibition capillary pressure curves The two capillary pressuresaturation curves are not the same 12/11/2017 Reservoir Engineering 61 Initial Saturation Distribution in a Reservoir The height h above the freewater level 144 pc h  h = height above the free-water level pc = capillary pressure Δρ = density difference 12/11/2017 Reservoir Engineering 62 Example The capillary pressure curves for a sandstone reservoir are shown in the Fig Estimate the height, in feet above the free water table, where Sw drops below 100% and where it is equal to 45% Oil and water densities are 50 and 65 lb/ft3, respectively Sw % 12/11/2017 Reservoir Engineering 63 Leverett J-Function Capillary pressure data from various core samples can be correlated with the J-function The J-function is represented as: 0.217 pc k J (Sw )   cos   J(Sw) = dimensionless function dependent on saturation of the wetting phase; pc = capillary pressure, psi; σ = interfacial tension, dynes/cm; θ = contact angle, degrees; k = rock permeability, md; and ϕ = porosity, fraction 12/11/2017 Reservoir Engineering 64 Example Calculation of the Leverett J Function The set of four oil/water capillary pressure curves shown in Table 2.5 was measured for four cores from the same reservoir The oil/water interfacial tension and angle of wettability are σ = 72 dynes/cm and ϕ = 45°, respectively Calculate and plot the Jfunction curve 12/11/2017 Reservoir Engineering 65 Example 12/11/2017 Reservoir Engineering 66