ABSTRACT Recently, underbalanced drilling has been utilized as a technique to minimize invasive formation damage in both horizontal and vertical wells. While it is widely accepted that underbalanced drilling greatly reduces the propensity for formation damage, particularly with respect to the potential for whole mud losses and invasion of mud solids, the potential for formation damage still exists in situations where the original saturation in the reservoir (either hydrocarbon or water), is less than the irreducible saturation of the phase utilized in drilling and the capillary pressure and wettability characteristics can cause spontaneous imbibition. This often occurs when drilling in an underbalanced mode in low permeability gas reservoirs that have abnormally low initial water saturations. Although fluid flow is occurring from the formation, circulating drilling fluids are in dynamic continual contact with the formation face. At abnormally low initial saturations, there exists the ability for strong, spontaneous imbibition effects which can, in some situations, counteract pressures that are far greater than the apparent underbalance pressure occurring during the drilling process. If the formation contains potentially sensitive clay materials, deflocculatable fines, the potential for emulsions or permeability reductions due to aqueous phase trapping, these phenomena could possibly occur and reduce permeability. 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 these phenomena as a fully exposed formation face which remains unshielded by any type of bridging or sealing filter cake is exposed for potential imbibition effects at all times. In addition, if any overbalanced pulses occur during drilling or completion, the potential for severe and damaging fluid and solids losses to the unprotected damage scenarios based on the results of specific laboratory tests and reviews new techniques designed to evaluate and pre-screen underbalanced drilling operations prior to costly implementation in the field.
INTRODUCTION Various authors have documented why concerns with formation damage in horizontal well applications often outweigh those observed in vertical wells. Mechanisms of damage common to both horizontal and vertical wells would include:Fluid-fluid incompatibilities such as reaction of invaded mud filtrate with in-situ fluids (oil or formation brine) to form scales, insoluble precipitates, asphaltic sludges or stable emulsions.
Rock-Fluid incompatibilities - contact of potentially swelling (i.e., smectitic clay) or deflocculatable (i.e., kaolinite clay) minerals by non equilibrium aqueous phase solutions may have the potential to severely reduce near wellbore permeability.
Solids invasion - The invasion of artificial solids contained in the drilling fluid (i.e., weighting agents or artificial bridging agents) or the invasion of formation solids (microfines) generated by the milling action of the drill bit on the formation. The permanent entrainment of these solids in the formation can have a severely reducing effect on permeability in some situations.
Phase trapping/blocking - This phenomena is the invasion and entrapment of high oil or water phase saturations in the near wellbore region and can have a substantially reducing effect on oil or gas productivity, particularly for certain types of formations.
Chemical Adsorption/Wettability Alteration - Most drilling fluids contain a variety of chemical additives to improve mud performance and character. In some cases these additives may be incompatible with the formation fluids or rock, or exhibit a high propensity for physical adsorption. This can result in a number of undesirable phenomena such as permeability reductions due to physical polymer adsorption, or wettability alterations due to surfactant adsorption.
Fines Migration - The actual internal movement of formation fines or loosely attached in-situ formation particulates can be a concern in certain reservoirs where high, uncontrolled fluid loss is apparent at highly overbalanced drilling conditions.