Gas-in-riser events can lead to rapid unloading if not timely controlled in a proper manner. When gas influx enters a wellbore with non-aqueous muds (NAMs), the ability of gas being dissolved in NAMs increases the difficulty in gas kick detection and significantly alters gas migration and unloading behavior from the predictions based on water-based muds (WBMs) assumptions. In this study, a new mathematical model for riser gas management in NAMs is developed. In this model, the desorption of dissolved gas influx from NAMs is accounted for as an instantaneous process using a solubility-based mass transfer submodel. The effects of surface backpressures and circulation rates on the unloading behavior in both WBMs and NAMs were studied. This model was validated using data obtained from a drift-flux model (DFM) based simulator. Results show that with the same amount of free gas in the risers at the mudline level, the severity of unloading is significantly more severe in the cases of NAMs. Applied backpressure can effectively control the desorption of the gas influx from the mud, and the unloading occurs later and at shallower depth with higher backpressure. The behavior of unloading tends to be independent on the time when backpressures are applied but highly dependent on the magnitude of the backpressure and the circulation rates. The new two-phase model can accurately simulate riser gas kick events utilizing a simplified approach with improved numerical stability, making it more applicable for real-time riser gas management.

Prediction of reservoir performance during water displacement process is a routine procedure for homogeneous reservoirs but complicated in heterogeneous reservoirs. The Stiles method has consequently been used over time in the industry for such reservoirs. This method, however, is evidently time consuming and tedious as the varying permeability values are reordered and lumped. This paper applies the Welge procedure to a stratified reservoir without reordering or lumping of permeability with the aim to ensure improved productivity by more proper planning, more efficient use of resources and determination of the economic viability of the project. This paper also seeks to formulate a procedure that can judiciously handle even larger sets of permeability values to produce more accurate result A fractional flow equation was derived for any number of layers to generate a single fractional flow curve (FFC). Injection inflow into a layer was determined using the layer capacities and this facilitated calculation of times to breakthrough and times to attain a particular saturation after breakthrough. A formula was then derived to determine the oil produced at any instant. A software was designed for the entire procedure to ensure faster and more accurate predictions. Results showed that heterogeneities had no effect on the microscopic displacement and thus the fractional flow curve remained unchanged whether the reservoir was heterogeneous or not. Heterogeneities affected only the total flow in the distinct layers and thus times to attain specified average water saturations. The results of oil recovery obtained were compared to those obtained using the Stiles method to demonstrate that this method is faster without loss of accuracy.

Real-time prediction of riser gas behavior is of great importance in well control. Single bubble models have, thus far, been used to describe gas-in-riser events and define riser equilibrium. These models have however not considered the transient nature of desorption of gas influx from non-aqueous fluids (NAFs) during migration or circulation in a riser. This paper uses a modified drift-flux model (DFM) to more properly describe gas-in-riser events by incorporating time-dependent mass transfer processes in NAFs. In this paper, we modified the DFM to account for the gas-liquid mass transfer due to the time dependent desorption of the gas phase. The advection upstream splitting model (AUSMV) hybrid scheme was used to solve the model. The time dependent mass transfer is calculated using a kinetic model developed based on recent experimental data. The capability of this model to improve riser gas management is demonstrated using a case study and the simulations are compared to when mass transfer between gas influx and NAF is not considered. Results also show that the severity of unloading and depth of the riser equilibrium can be underestimated if a time dependent desorption is not considered. The concept of riser equilibrium has been, thus far, developed without due consideration of mass transport of gas phase in the mud. This paper factors in the time-dependent desorption of the gas phase in the mud for a more realistic prediction of riser gas unloading events.

Existing well control prediction techniques have not yet found wide acceptance in slim holes. This study investigates the two-phase transient pressure behavior when a gas kick occurs in slim holes. This paper presents an improved two-phase model that will account for the change in annular pressure transients. The two-phase model is based on the drift-flux model. Newly developed drift-flux correlations with specific target to narrow annuli are applied. An explicit scheme is adopted to solve the two-phase model. Results show the high inaccuracies that occur from using conventional models instead of this new model. The velocities of each phase are seen to increase tremendously with assumption of slim holes. This in turn causes significant increase in the equivalent circulating densities (ECDs) and underestimation of bottom hole pressures. On the other hand, the hydrostatic pressures are seen to decrease as well leading to a counterbalance. Pressure signatures for slim holes are seen to shift to the right consequently affecting the void fraction distribution and other fluid properties. Case studies are performed in the simulations to examine the effect of varying annulus ratio on results for constant influx and mud rate. From the results, as annulus ratio goes beyond 0.8, the changes in bottom hole ECDs become much more substantial. The understanding of hydraulics in narrow annuli is vital for real-time pressure prediction during a gas influx. This study provides an improved simulation tool for more accurate prediction of influx behavior in slim holes.