Underbalanced drilling has been utilized as a technique to minimize damage due to whole mud, fluid filtrate and solids invasion into high permeability and fractured formations and formations which exhibit a high degree of sensitivity to invaded fluids. While it is generally accepted that the process of underbalanced drilling reduces the potential for formation damage, the possibility of invasive formation damage still exists in certain reservoirs where the initial saturation (either hydrocarbon or water) is substantially less than the irreducible saturation of the phase which is being utilized in the drilling or completion process. This situation could commonly occur when drilling in an underbalanced mode in gas reservoirs that are in a desiccated state and exhibit abnormally low initial water saturations, or in oil and gas reservoirs in a portion of the reservoir significantly above the oil-water or water-gas transition zone water saturation. Although dynamic fluid flow is occurring from the formation during the drilling process, the circulating drilling fluids are in continual contact with the formation face. At the low initial liquid saturations which exist in these reservoirs, there is an ability for strong countercurrent spontaneous imbibition effects which can, in some situations, counteract pressures that are far greater than the apparent underbalance pressure which is present during the drilling or completion process. If the formation contains potentially sensitive materials such as defloccuable fines or swelling clays, the potential for the formation of in-situ emulsions or the possibility of permeability reductions due to the establishment of near wellbore aqueous phase traps, these phenomena could occur and cause potential reductions in productivity of the well.The majority of underbalanced drilling fluid systems are not designed with ultra-low fluid loss in mind as extensive mud losses to the formation are not contemplated However, the fact that a stable filter cake is not developed during underbalanced drilling, due to continual inflow from the formation face, could increase the severity of losses to the formation if underbalanced onditions are not maintained 100% of the time during drilling and completion. Overbalanced pulses may occur due to mud pulsed logging programs, bit trips or mechanical problems which may occur during the drilling process of most horizontal wells. This results in the majority of underbalanced horizontal wells being overbalanced for some fraction of the time during the drilling and completion process. A discussion of mechanical damage due to near surface glazing effects is also presented as a possible mechanism of damage which can occur during underbalanced drilling. The paper elaborates on potential damage scenarios based on the results of specific laboratory tests which have been conducted and reviews a variety of new techniques which have been designed to evaluate and pre-screen fluids and procedures prior to costly and possibly ineffective implementation of underbalanced drilling in the field.
Various authors1 have documented why concerns with formation damage in horizontal well applications often outweigh those observed in vertic.al wells. These would include:
Greater exposure time
Greater potential depth of invasion.
The majority of horizontal wells are open-hole completions, resulting in potential flow impairment even with very localized shallow damage.