Light Annular MudCap Drilling - A Well Control Technique for Naturally Fractured Formations

Abstract

Abstract Light Annular MudCap drilling (LAMCD) is a safecost effective drilling technique that may be used to drill through massively fractured formations or where Karsts are prevalent, especially when productive fluids are sour.It is an adaptation to regular Mud Cap Drilling (MCD), used to drill formations such as the Austin Chalk. The technique was first developed and used in the Cogollo formation in South America and has recently been used to drill sour formations in Kazakhstan. The LAMCD technique allows the driller to control and monitor the amount of sour gas migrating into the annulus. Drilling through formations containing multiple large fractures using conventional well control techniques is impossible unless the fractures are sealed. In many cases where extremely large fractures or karsts (vugs/caverns) have been encountered, lost circulation technology has proven to be unsuccessful.1–7 If the fractured formation being drilled is a pay interval, plugging the fractures is counter-productive. The LAMCD well control/lost circulation technique entails drilling with the choke completely closed while using a rotating control device (RCD) to seal the annulus. A sacrificial fluid, usually fresh or brine water is injected down the drill pipe. The drilled cuttings are transported and deposited into the fractures or vugs. The annulus is periodically injected with a viscous fluid that has a density slightly less than the pore pressure equivalent, hence the name "Light Annular MudCap".This controlled displacement process results in a predictive positive casing pressure. The casing pressure will increase as hydrocarbons migrate up the annulus. The value of the casing pressure qualitatively corresponds to the height of the hydrocarbon migration. Once a predetermined casing pressure/height is reached, LAMC fluid is pumped into the annulus until all the hydrocarbon is displaced back into the formation. After all the hydrocarbons are purged from the wellbore, the casing pressure will return to a constant and predictable value. LAMCD is a well control/lost circulation technique that allows drilling through massive lost circulation zones without downtime associated with lost circulation and conventional well control. Penetration rates are increased significantly as well. This paper describes the application of Light Annular MudCap Drilling as well as the details of the method.Well control and equipment issues are addressed. Simple equations for estimating pressure and volume requirements during LAMCD are presented. Introduction The presence of fractured formations has plagued drillers since the beginning of rotary drilling.The conventional way to address the problem is to use lost circulation material or drill "blind" through the zone and subsequently set casing across the loss zone. Many operators target reservoirs that have extensive natural fracturing because of the prolific production associated with fracture porosity. Consequently, it is not desirable to permanently plug the hydrocarbon bearing fractures.