Near-Wellbore Formation Damage Effects On Well Performance - A Comparison Between Underbalanced And Overbalanced Drilling

Abstract

Abstract Prediction of formation damage that occurs while drilling horizontal wells is a critical point for optimising an oil field development. The economic impact of near wellbore drilling-induced damage and clean up efficiency has led to make significant progress in both experimental and numerical studies in order to assess wellbore flow properties during oil production. As a result, a numerical model has been developed to study the impact of various parameters related to the properties of drilling fluids on the well inflow performance. This paper describes a numerical approach developed in this model to simulate near wellbore formation damage due to underbalanced (UBD). It is generally expected that UBD will be of benefit from preventing formation damage. However, this benefit can be lost for various reasons. For instance, an overbalanced pressure can be applied on the formation during short periods of time for various operational reasons and it can cause a severe formation damage due to the absence of external cake protection and huge filtrate invasion. Another possible cause of formation damage in UBD is related to spontaneous imbibition which induces water blocking, experienced while drilling tight gas reservoirs. A methodology for the modelling of possible formation damage during UBD is presented. Cross flow phenomena due to spontaneous imbibition is taken into account to model the filtrate invasion from the well to the porous media as the well is producing. Introduction It is well known that formation damage due to drilling fluid has a huge impact on wells oil and gas productivities, especially for open hole completed horizontal wells. During overbalanced drilling (OBD), mud and mud filtrate penetrate the near-wellbore formation due to this overbalanced pressure altering near-well flow properties. As a result, well productivities are dramatically reduced. It is generally recognized that underbalanced drilling (UBD) can be used to minimize problems associated with invasive formation damage. When correctly performed, UBD reduces or eliminates invasive formation damage, improves access to reserves and provides potential for reservoir evaluation while drilling. Additional benefits of UBD are the reduction in drilling time, high rates of penetration and increase of bit life. Underbalanced Drilling has recently proven its efficiency in numerous situations where serious problems were encountered with classical drilling techniques. For example, heavy losses formations or depleted zones are ideal candidates for drilling underbalanced. During normal UBD, the negative pressure drop between the wellbore and the formation prevents drilling fluids to enter the formation. However, this benefit can be lost in at least two situations due to possible sources of failure such as temporary overbalanced condition or spontaneous imbibition. When drilling underbalanced, the fluid system is not designed to contain cake-building solids as in the case of overbalanced drilling, thus if underbalanced condition can not be maintained 100% of the time, severe losses can occur which may result in formation damage. Overbalance condition may result from bit trips, running in or other situations where wellbore intervention is necessary. Spontaneous imbibition is particularly important in tight gas reservoirs. Even though UBD has many advantages over OBD, it is necessary to quantify production improvement and possible formation damage, which is an important factor in the evaluation of economic feasibility of UBD projects1–6. This paper presents a numerical model allowing to calculate well productivity reduction due to possible formation damage during UBD such as temporary overbalanced drilling or spontaneous imbibition. Modelling of well performance in OBD has already been presented7,8. Both internal and external cakes are considered in the model, and filtrate invasion is calculated using a two-phase flow equation. Polymer absorption/retention, phase trapping, wettability alteration, etc. are globally represented using an hysteresis of relative permeabilities. Non-uniform formation damage along the well is represented by variable specific skin factors through an optimisation procedure. The change of well performance can therefore be calculated using a flow simulator by taking into account these variable skin factors.