Multiphase flow modeling of asphaltene precipitation and deposition

Asphaltene precipitation in reservoirs during production and Enhanced Oil Recovery (EOR) can cause serious problems that lead to reduction of reservoir fluid production. In order to study asphaltene tendency to precipitate and change in flow rate as a function of distance from wellbore, an equation of state (Peng-Robinson) based model namely Nghiem et al.’s model has been employed in this study. The heaviest components of crude oil are separated into two parts: The first portion is considered as non-precipitating component (C31A+) and the second one is considered as precipitating component (C31B+) and the precipitated asphaltene is considered as pure solid. For determination of the acentric factor and critical properties, Lee-Kesler and Twu correlations are employed, respectively. In this study, a multiphase flow (oil, gas and asphaltene) model for an asphaltenic crude oil for which asphaltene is considered as solid particles (precipitated, flocculated and deposited particles), has been developed. Furthermore, effect of asphaltene precipitation on porosity and permeability reduction has been studied. Results of this study indicate that asphaltene tendency to precipitate increases and permeability of porous medium decreases by increasing oil flow rate in under-saturated oil reservoirs and dropping reservoir pressure under bubble point pressure. On the other hand, asphaltene tendency to precipitate decreases with pressure reduction to a level lower than bubble point pressure where asphaltene starts to dissolve back into oil phase. Moreover, it is observed that precipitation zone around the wellbore develops with time as pressure declines to bubble point pressure (production rate increases up). Also, there is an equilibrium area near wellbore region at which reservoir fluid properties such as UAOP (Upper Asphaltene Onset Pressure) and LAOP (Lower Asphaltene Onset Pressure) are constant and independent of the distance from wellbore

Application of the transient rate decline analysis for determining average reservoir pressure in naturally fractured reservoirs

Based on the fracture-pore dual medium model proposed by Warren and Root et al, the solution of the pseudosteady-constant production fractured system by Mavor and Cinco-Ley and the solution of the pseudosteady-constant pressure matrix system by Daprat are extended, and a new method is presented for estimating average reservoir pressure of naturally fractured reservoirs using the transient rate decline analysis. The specific method is taking logarithm of the matrix production equation and fracture production equation to obtain two straight lines; then the slope and intercept of the straight lines are used to determine storage capacity ratio, total storage capacity and reservoir drainage area, and average reservoir pressure at last. Examples show that the new method is easy and practical, and gives result similar to that of the material balance method and direct Tiab synthesis method. Key words: naturally fractured reservoir, average reservoir pressure, pressure transient testing, decline curve analysis, dual medium, pseudo-steady state flo

Multiphase flow modeling of asphaltene precipitation and deposition

Asphaltene precipitation in reservoirs during production and Enhanced Oil Recovery (EOR) can cause serious problems that lead to reduction of reservoir fluid production. In order to study asphaltene tendency to precipitate and change in flow rate as a function of distance from wellbore, an equation of state (Peng-Robinson) based model namely Nghiem et al.’s model has been employed in this study. The heaviest components of crude oil are separated into two parts: The first portion is considered as non-precipitating component (C31A+) and the second one is considered as precipitating component (C31B+) and the precipitated asphaltene is considered as pure solid. For determination of the acentric factor and critical properties, Lee-Kesler and Twu correlations are employed, respectively. In this study, a multiphase flow (oil, gas and asphaltene) model for an asphaltenic crude oil for which asphaltene is considered as solid particles (precipitated, flocculated and deposited particles), has been developed. Furthermore, effect of asphaltene precipitation on porosity and permeability reduction has been studied. Results of this study indicate that asphaltene tendency to precipitate increases and permeability of porous medium decreases by increasing oil flow rate in under-saturated oil reservoirs and dropping reservoir pressure under bubble point pressure. On the other hand, asphaltene tendency to precipitate decreases with pressure reduction to a level lower than bubble point pressure where asphaltene starts to dissolve back into oil phase. Moreover, it is observed that precipitation zone around the wellbore develops with time as pressure declines to bubble point pressure (production rate increases up). Also, there is an equilibrium area near wellbore region at which reservoir fluid properties such as UAOP (Upper Asphaltene Onset Pressure) and LAOP (Lower Asphaltene Onset Pressure) are constant and independent of the distance from wellbore