UNDERBALANCED DRILLING AND FORMATION DAMAGE - IS IT A TOTAL SOLUTION

Underbalanced drilling has been utilized as a technique to minimize da mage due to whole mud, fluid filtrate and solids invasion into high pe rmeability and fractured formations and formations which exhibit a hig h degree of sensitivity to invaded fluids. While it is generally accep ted that the process of under-balanced drilling reduces the potential for formation damage, the possibility of invasive formation damage sti ll exists in certain reservoirs where the initial saturation (either h ydrocarbon or water) is substantially less than the irreducible satura tion of the phase which is being utilized in the drilling or completio n process. This situation could commonly occur when drilling in an und erbalanced mode in gas reservoirs that are in a desiccated oil and gas reservoirs in a portion of the reservoir significantly above the oil- water or water-gas transition zone. Although dynamic fluid flow is occ urring from the formation during the contact with the formation face. At the low initial liquid saturations which exist in these reservoirs, ther is an ability for strong countercurrent spontaneous imbibition e ffects which can, in some situations, counteract pressures which is pr esent during the drilling or completion process. If the formation cont ains potentially sensitive materials such as deflocculatable fines or emulsions or the possibility of permeability reductions due to phenome na could occur and cause potential reductions in productivity of the w ell. The majority of underbalanced drilling fluid systems are not desi gned with ultra-low fluid loss in mid as extensive mud losses to the f ormation are not contemplated. However, the fact that a stable filter cake is not developed during underbalanced drilling due to continual i nflow from the formation face could increase the severity of losses to the formation if underbalanced conditions are not maintained 100% of the time during drilling and completion. Overbalanced pulses may occur due to mud which may occur during the drilling process or most horizo ntal wells. This results in the majority of underbalanced horizontal w ells being overbalanced for some fraction of the time during the drill ing and completion process. A discussion of mechanical damage due to n ear surface glazing effects is also presented as possible mechanism of damage which can occur during some underbalanced drilling operations. The paper elaborates on potential damage scenarios based on the resul ts of specific laboratory tests which have been conducted and reviews a variety of new techniques which have been designed to evaluate and p re-screen fluids and procedures prior to costly and possibly ineffecti ve implementation of an underbalanced drilling operation in the field.