This paper describes the use of a numerical simulator for study of productivity improvement by application and proper performance of UBD in wells, which are faced with near-wellbore damage. Minimizing formation damage in the near-wellbore region and thus ensuring proper well performance for enhanced productivity of the well also results in increased recovery factor. Formation damage caused by conventional overbalanced drilling (OBD) can be reduced or eliminated through UBD, which may ensure the original formation productivity is maintained. A single horizontal well numerical model has been built that simulates the UBD and OBD processes and their impacts on production of hydrocarbon under a variety of reservoir conditions. The paper describes study methodology, models, simulation results and analysis results. It also summarises the formation criteria for successful UBD operations. For the purpose of expressing rate versus time, the production rate as a series of steady state production with increasing radius of drainage is considered. This dimensionless analysis of productivity (PID) provides a simple way to describe the productivity trend with different reservoir and fluid properties and well geometry. Several hundred cases have been numerically simulated for different reservoirs and drilling operations. Dry gas, gas condensate reservoir, dead oil reservoir and black oil reservoir have been considered. Several sensitivity analyses such as impact of OBP pressure and OBP time and effect of horizontal well length have been simulated. The results clearly show that proper candidate selection is the key to success of UBD projects. This clearly demonstrates that UBD operation is good choice for some reservoirs but not for all. The results of this study provide a set of reservoir criteria that can be used as guidelines to evaluate potential UBD candidates.
Formation damage mechanisms that may result from drilling operations and reduce productivity include:
Invasion of solids such as barite or fragments from cuttings generated by the drill bit, which may block pore throats.
Drilling fluid/mud filtrate invasion.
Formation fines migration caused by high fluid-loss rates.
Relative permeability effect such as phase trapping of water-based mud (WBM) or oil based mud (OBM) filtrate near-wellbore.
Chemical incompatibility of invading fluid with the in-situ rock matrix and in-situ fluids.
Near-wellbore wettability alteration and surface adsorption effect.
Though the majority of the formation damage happens only in both OBD and UBD operations.
Phase trapping happens when water or oil -based drilling mud filtrates enter into the near-wellbore region because of leakoff during overbalanced drilling operations or due to spontaneous imbibition in some situations during UBD. Phase trapping can result in permanent entrapment of a portion or all of the invading fluid, causing adverse relative permeability effects and reducing oil or gas permeability in the near-wellbore regions.
The basic mechanism of a phase trap in a low permeability gas saturated matrix is illustrated by Bennion et.al in Figure 1.