Aphron-based Drilling Fluid: Novel Technology for Drilling Depleted Formations in the North Sea

Abstract

Drilling depleted reservoirs is fraught with a host of technical and economic problems that often make it unprofitable to further develop some mature fields. Most of the problems center around uncontrollable losses and differential sticking. Frequently, less expensive drilling fluids will be used in a particular interval, even though it may have the propensity to damage the formation. The reasoning holds that such fluids will offset the high costs of losing more expensive muds to the formation. If operators turn to underbalanced drilling as an alternative, the extra time and equipment required for a safe operation can seriously degrade project economics in some applications. A specialized invasion-control drilling fluid has been developed to drill reservoirs prone to lost circulation. This fluid combines certain surfactants and polymers to create a system of micro-bubbles or aphrons that are encapsulated in a uniquely viscosified system. Aphron based systems are engineered drilling fluids that aid in well construction by controlling losses in depleted, high-permeability sands while stabilizing pressured shales or other formations. One of the more attractive features of an aphron-based system is that it does not require any of the extra equipment used in air or foam drilling. There are no compressors, high-pressure hoses or connections to add costs and safety concerns. The system uses conventional fluid-mixing equipment to form tough, flexible micro-bubbles. This paper describes the development and application of the specialized micro-bubbles-based drilling fluid for controlling downhole mud loss in a depleted reservoir in the North Sea. The key issues of this project were excessive overbalance drilling conditions (> 5,000 psi) leading to the risk of highly expensive lost circulation and open perforations in the upper producer, requiring temporary sealing during drilling. The well was successfully drilled to TD without any drilling fluid losses. The authors will detail the laboratory methods used to generate appropriate formulations, the operational procedures, and field application. Introduction The drilling problems associated with the depleted reservoirs intrinsic to many of the mature fields throughout the world often make further development uneconomical. The water-wet sands that typify many of these zones propagate seepage losses and differential sticking, both of which are extremely expensive to correct. Uncontrollable drilling fluid losses frequently are unavoidable in the often large fractures characteristic of these formations. Furthermore, pressured shales are often found interbedded with depleted sands, thus requiring stabilization of multiple pressured sequences with a single drilling fluid. Drilling such zones safely and inexpensively is very difficult with conventional rig equipment. Such problems have led some operators to forgo continued development of these promising, yet problematic, reservoirs.1 Excessive overbalance pressure generated when using conventional drilling fluids is thought to be the primary cause of lost circulation and differential sticking when drilling these wells. The equipment required to manage aerated muds or drill underbalanced is often prohibitively expensive, and meeting safety requirements can be an exhaustive effort. Furthermore, these techniques may fail to provide the hydrostatic pressure necessary to safely stabilize normally pressured formations above the reservoir. Recently, a new drilling fluid technology based on aphrons - uniquely structured micro-bubbles — was employed to successfully drill a depleted reservoir in the North Sea. The use of aphron-based drilling fluids has proven to be a successful and cost-effective alternative to drilling underbalanced. Description of Aphron Structure An aphron comprises two fundamental elements2:A core that is commonly, but not always, spherical. Typically, the core is liquid or gaseous.A thin, aqueous, protective shell with an outer hydrophobic covering. The aqueous shell contains surfactant molecules positioned so that they produce an effective barrier against coalescence with adjacent aphrons.

Mathematical modeling of the Colloidal Gas Aphron transport through porous medium using the filtration theory

“…This method improves drilling performance by reducing drilling fluid invasion into the formation while drilling operation (Brookey and Rea, 2003) (White et al, 2003) Using…”

“…From the other hand, the costs which must paid for compressors and equipment employed in foam drilling operation eliminated in case of Aphron fluid drilling operation (White et al, 2003)There is no corrosion issues of drill string and related equipment in case of using Aphron drilling fluids, since most of air in the system is trapped in Aphron shell (Bjorndalen and Kuru, 2008).…”

How Aphron Drilling Fluids Work

“…This occurs naturally during the course of product addition using conventional drilling fluid mixing equipment, and there is no need for highpressure hoses and compressors such as those utilized in underbalanced air or foam drilling. 6 The surfactants in the fluid convert the entrained air into highly stabilized bubbles, or "aphrons." However, in contrast to a conventional air bubble, which is stabilized by a surfactant monolayer, the outer shell of an aphron is thought to consist of a much more robust surfactant tri-layer.…”

Herschel–Bulkley rheological parameters of lightweight colloidal gas aphron (CGA) based fluids

“…Finally, aphrons with cores formed of a waterin-oil emulsion are called colloidal emulsion aphrons (Deng et al, 2005). Currently, the use of CGA based fluids is increasing rapidly in the petroleum industry because these fluids exhibit properties that are desirable in many field operations (Brookey, 1998;Gregoire et al, 2005;Ivan et al, 2001;Ramirez et al, 2002;Rea et al, 2003;White et al, 2003). The potential of aphrons as components of drilling fluids rests in their ability to form a solid free, tough, and elastic internal bridge in pore networks or fractures to minimize deep invasion by means of air microbubbles (Belkin et al, 2005;Growcock et al, 2003).…”