Chemical and Physical Characterization of Aphron-Based Drilling Fluids

Abstract

Abstract The depleted reservoirs in many of the world's mature fields contain hidden problems that can make further development unprofitable. Some zones are typified by water-wet sands that frequently trigger costly seepage losses and differential sticking. Others are characterized by laminated sand and shale sequences, which create the conditions for slow, dangerous and unduly expensive drilling when conventional rig equipment is used. If operators turn to underbalanced drilling as an alternative, the extra time and equipment required for safe operation can seriously degrade project economics in some applications. Aphron-based systems are engineered drilling-fluids that aid in well construction by controlling losses in depleted, high-permeability sands while stabilizing pressured shales. 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. Once formed, these micro-bubbles, or "aphrons," differ from the bubbles produced in air or foam drilling in two significant ways. First, aphrons resist coalescence into larger bubbles. The aphrons are attracted to lower-pressure regions in the formation but remain discreet from each other, forming a strong network of individual micro-bubbles. Second, aphrons are tough and stable. They are comprised of a core of air surrounded by layers of a proprietary polymer and a tensoactive additive. The authors will detail the chemical and physical characteristics of aphron-based drilling fluids that make them uniquely qualified to solve some operational problems. 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 multiply 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. 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 depleted reservoirs in major drilling campaigns in North and South America. 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 is composed of two fundamental elements1: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.