Reservoir dynamic behaviour

Keywords compressibility, primary-, secondary- and enhanced oil-recovery, drive mechanisms (solution gas-, gas cap-, water-drive), secondary gas cap, first production date, build-up period, plateau period, production decline, water cut, Darcy s law, recovery factor, sweep efficiency, by-passing of oil, residual oil, relative permeability, production forecasts, offtake rate, coning, cusping, horizontal wells, reservoir simulation, material balance, rate dependent processes, pre-drilling. 

Introduction and Commercial Application The reservoir and well behaviour under dynamic conditions are key parameters in determining what fraction of the hydrocarbons initially in place will be produced to surface over the lifetime of the field, at what rates they will be produced, and which unwanted fluids such as water are also produced. This behaviour will therefore dictate the revenue stream which the development will generate through sales of the hydrocarbons. The reservoir and well performance are linked to the surface development plan, and cannot be considered in isolation different subsurface development plans will demand different surface facilities. The prediction of reservoir and well behaviour are therefore crucial components of field development planning, as well as playing a major role in reservoir management during production. 

This section will consider the behaviour of the reservoir fluids in the bulk of the reservoir, away from the wells, to describe what controls the displacement of fluids towards the wells. Understanding this behaviour is important when estimating the recovery factor for hydrocarbons, and the production forecast for both hydrocarbons and water. In Section 9.0, the behaviour of fluid flow at the wellbore will be considered this will influence the number of wells required for development, and the positioning of the wells. 

---

Reservoir simulation is a technique in which a computer-based mathematical representation of the reservoir is constructed and then used to predict its dynamic behaviour. The reservoir is gridded up into a number of grid blocks. The reservoir rock properties (porosity, saturation, and permeability), and the fluid properties (viscosity and the PVT properties) are specified for each grid block. 

Introduction and Commercial Application Section 8.0 considered the dynamic behaviour in the reservoir, away from the influence of the wells. However, when the fluid flow comes under the influence of the pressure drop near the wellbore, the displacement may be altered by the local pressure distribution, giving rise to coning or cusping. These effects may encourage the production of unwanted fluids (e.g. water or gas instead of oil), and must be understood so that their negative input can be minimised. 

This section will look at formation and fluid data gathering before significant amounts of fluid have been produced hence describing how the static reservoir is sampled. Data gathered prior to production provides vital information, used to predict reservoir behaviour under dynamic conditions. Without this baseline data no meaningful reservoir simulation can be carried out. The other major benefit of data gathered at initial reservoir conditions is that pressure and fluid distribution are in equilibrium this is usuaily not the case once production commences. Data gathered at initial conditions is therefore not complicated... 

For micro-pores, molecular dynamics calculations can be used to find the pressures at which pores of simple shape fill and empty In meso-porous materials capillary condensation can occur and the behaviour is then better described in terms of the theory of capillarity combined with percolation theory. For macro-porous materials, such as oil reservoir rocks, capillary forces can dominate the displacement of one fluid by another. Percolation or pore blocking which is the shielding of large pores by smaller pores can occur in all of these processes and can make a significant difference when the processes are analysed theoretically. 

Figure 9.10(a) compares velocity profiles obtained in a dynamic NMR microscopy experiment performed at 30°C using 1.6 x 10 Da poly (ethylene oxide) (PEO) dissolved in water at a range of concentrations between 0.5% and 4.5% (w/v) where the solution is forced through a Teflon capillary with internal diameter 700 jum [17, 18]. A transition from Newtonian to non-Newtonian behaviour is observed as the concentration increases, an effect that is consistent with a measured value of (p of around 0.5%. Pressure heads of up to 21 atm were used to drive the polymer solution from the header tank reservoir through the capillary. A power law fit to the high concentration velocity profiles yields an exponent of 0.4, similar to that found in laser Doppler anemometry experiments using polyethylene melts [127]. 

Macro-scale experiments involve a special apparatus that allows foam floods to be performed at reservoir conditions of temperature and pressure in an integral two metre length of rock sample, that is, in porous media samples one order of magnitude longer than the meso-scale. In addition to being a first step in scale-up, this allows the study of dynamic foam behaviour that would be impossible in short core samples. 

---