A study of underbalanced drilling application for granite basements in Vietnam

Abstract

Underbalanced drilling (UBD) has gained popularity during the recent years as it provides a method to prevent formation damage, minimize lost circulation risks, and increase the rate of penetration. However, drilling with a bottomhole pressure less than the formation pore pressure will often increase the risk of borehole instability due to the shear or compression failure of the rock adjacent to the wellbore. The extent of rock failure is related directly to the pressure in the annulus between the drillpipe, collars and the wellbore which can only be calculated through modelling multiphase flow in the drilling system. The relationship between rock failure and wellbore hydraulics becomes more complex due to the appearance of the influx formation fluid in UBD. Therefore, the aim of this research is to describe methods to solve the complex interaction of wellbore stability, rock yielding, collapse, wellbore hydraulics, and production capacity during UBD operations. To achieve the aim, analytical and numerical solutions have been codified into two programs WELLST, and UBDRILL. Commercial software packages such as ABAQUS, PERFORM, HYMOD were also used to model the process. Field data from granite basement reservoirs of Basin X, Vietnam were used as the parameters input into the model to calculate. This research includes: • An analysis of the influences of time dependence, thermal and hydraulic diffusivity, wellbore pressure changes, inclination and azimuth variation, poroelastic and thermo-poroelastic deformation, cooling and heating effects on wellbore stability in UBD. • An analysis of pressure, temperature, fluid properties distribution in the annulus and inside the drillpipe while UBD. • An analysis of the suggested liquid gas rate window (LGRW) which gives field engineers flexibility in the selection of liquid and gas injection rates on the drilling site when UBD is applied. • An estimation of production capacity in UBD operations. These results were obtained by analyzing the field data of granite basement formations and clastic formations of Basin X, Vietnam which is under compression in a strike-slip environment.