Underbalanced Drilling: Praises and Perils

Abstract

Underbalanced drilling (UBD) has been used with increasing frequency to minimize problems associated with invasive formation damage, which often greatly reduce the productivity of oil and gas reservoirs, particularly in openhole horizontal well applications. UBD, when properly designed and executed, minimizes or eliminates problems associated with the invasion of particulate matter into the formation as well as a multitude of other problems such as adverse clay reactions, phase trapping, precipitation, and emulsification, which can be caused by the invasion of incompatible mud filtrates in an overbalanced condition. In many UBD operations, additional benefits are seen because of a reduction in drilling time, greater rates of penetration, increased bit life, a rapid indication of productive reservoir zones, and the potential for dynamic flow testing while drilling.UBD is not a solution for all formation damage problems. Damage caused by poorly designed and/or executed UBD programs can rival or even greatly exceed that which may occur with a well-designed conventional overbalanced drilling program. Potential downsides and damage mechanisms associated with UBD will be discussed. These include the following.1. Increased cost and safety concerns. 2. Difficulty in maintaining a continuously underbalanced condition.3. Spontaneous imbibition and countercurrent imbibition effects. 4. Glazing, mashing, and mechanically induced wellbore damage. 5. Macroporosity gravity-induced invasion. 6. Difficulty of application in zones of extreme pressure and permeability.7. Political/career risk associated with championing a new and potentially risky technology. We discuss reservoir parameters required to design an effective underbalanced or overbalanced drilling program, laboratory screening procedures to ascertain the effectiveness of UBD in a specific application and review the types of reservoirs that often present good applications for UBD technology.

A Simple Analytical Approach to Simulate Underbalanced- drilling in Naturally Fractured Reservoirs—The Effect of Short Overbalanced Conditions and Time Effect

“…Most of these parameters (overbalanced pressure, wellbore radius, and fluid properties) are assumed to be known. The relation between r D and t D is defined by equation (10). Then, for each time step (t), r d is calculated and the fluid loss volume (V m ) is determined by equation (4).…”

Numerical Modelling of Cuttings Transport with Foam in Inclined Wells

“…(2,3) illustrate the effect of gas injection rate on the cuttings concentration and bottomhole pressure respectively. The gas injection rate has significant effect on the cuttings transport process.…”

“…Field applications have proven that UBD techniques have many advantages including minimized formation damage, increased drilling rate, improved formation evaluation while drilling, minimized lost circulation, reduced occurrence of differential pipe and logging tools sticking, enhanced detection of all producing zones, and enhanced earlier production from reservoir [3][4][5][6][7][8][9][10][11].…”

Underbalanced Drilling And Completion Operations to Minimize Formation Damage-Reservoir Screening Criteria For Optimum Application

“…A detailed discussion of underbalanced drilling has been presented in the literature (1)(2)(3)(4)(5)(6)(7)(8)(9) . By rigid definition, an "underbalanced" condition is generated any time the effective circulating downhole pressure of a drilling, completion, stimulation or workover fluid (the pressure exerted by the hydrostatic weight of the fluid column, plus whatever pump pressure is applied to the fluid system to circulate or inject it, and the associated frictional pressure drop) is greater than the effective pore pressure in the formation adjacent to the sand face.…”