Abstract

Wells drilled with oil-based muds can suffer from severe damage which decreases their overall productivity. During drilling, mud solid parts tend to form an external and internal filtercake along well bore through filtration, while liquid part (filtrate) tends to leak off into the formation. Clean-up process is then necessary to restore the productive zone to a near-natural state before production. Chemical treatment is often used to break external cake and to wash-off damage zone around well bore, so that formation recovers its fluid transfer properties (water, oil).

This paper focuses on the development and application of surfactant-based systems for treatment of wells drilled with oil-based muds. Aqueous based systems are widely used in such cases to effect significant production increases, but they create an additional damage which may even block oil production. A promising issue would be to use a surfactant-based system in a non-pollutant organic base to limit in situ formation of emulsion and water saturation around the well bore.

Though, our research methodology aims to select an organic-based surfactant system able to break the cake formed by filtration of a given oil-based mud system. The demulsifying formulation should be able to bother interface surfactants stabilizing effect to allow water droplets coalescence inside the cake. Through breaking emulsion inside the cake, surfactants may decrease cake cohesion and bore adherence. Careful choice may help preventing the system composed of filtrate, reservoir fluids and treatment fluid from forming a stable emulsion inside the porous medium, which could block oil production. Laboratory data are presented showing the relative effects of the organic phase surfactant system and of a conventional commercial treatment on cake destruction and productivity recovering. Results are held by a precise experimental methodology combining Differential Scanning Calorimetry and Cryomicroscopy. Both techniques characterize emulsion stability and destabilization as a function of wellbore parameters (contact time, overbalance pressure...).

Introduction

Formation damage resulting from invasion by drilling or completion fluids decreases the overall well productivity. Clean-up treatments are often necessary to restore the productive zone to a near natural state. This damage removal process should be carried out immediately after well completion, before production operations begin.

The complex well configurations involve long horizontal sections that are left in open hole configuration for long periods of time. Oil-based muds (OBM) are very well suited to drill these long open hole sections since they provide excellent lubricating properties and high rate of penetration thus limiting torque and drag difficulties. But drilling these long open hole sections results in long contact time of drilling fluid with the producing zone under overbalanced conditions, thus inducing invasion phenomena that may be detrimental for reservoir productivity. This requires low damage properties for drilling and completion fluids.

Oil-based muds may appear to be generally less damaging than water-based muds, filtercake being thinner and fluid loss being reduced and constituted of oil rather than water. However cleaning up wells drilled with OBM is a major issue.

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