High-density formate brine-based drilling fluid systems have emerged asviable reservoir drilling and completion fluid options for deep high-temperature wells. Upon their commercialization in the early 1990's, these systems were shown to possess a unique combination of properties that could be exploited to engineer high-density fluids that would comprise minimal solids, maintain rheological stability at high temperatures, minimize reservoir damageand satisfy environmental requirements. This paper describes the evolution of these systems and their recent application in the drilling of deep wells in Northern Germany and other areas. Not only did the formate-based systems exhibit faster penetration rates thanthe water-based fluids used previously; they also eliminated the solids sag problems experienced with water-based drilling fluids used in technically and economically demanding high-temperature environments. Furthermore, the formate brine-based systems were shown to improve well productivity. Through an examination of 15 deep gas wells in Germany and other formate-based applications, the authors review the performance of formate brines as reservoir drilling, completion and work over fluids. Particular attention is given to drilling and eventual well performance, as compared to offset wells drilled and/or completed with alternative systems. Introduction In late 1995 there was a need for a spotting fluid on a well being completedin the gas-bearing zone of the Schneverdingen sandstone formation in North Germany. The gas fields of North Germany have high temperatures (~300°F /~150°C) and are considered as HT/HP wells with the usual problems associated with HT/HP environments. A series of spotting fluid systems was tested whereone of the main criteria of the spotting fluid was that it should have low formation damage potential. Other criteria the fluid had to meet were good hydraulics (low solids content) and low corrosivity. The selected system best matching the criteria was a biopolymer-formate-based fluid that was subsequently applied successfully. From this initial application and the recent positive experiences of other operators, the formate fluids have evolved an extensive and ongoing use for both drilling and work over/completions operations in the North German gas fields. This paper discusses fifteen drilling and work over/completions operations and the extensive infrastructure for fluid mixing and reclamation that have developed for these environmentally friendlyand cost-effective operations. Up until the time that formates were applied, recurring problems had been limiting successful operations. The problems routinely encountered with the temperature limited, calcium carbonate weighted, water-based polymer drilling fluids used previously included inadequate solids suspension, poor solids transport, stuck pipe, and tight holes. Costs were brought under control as the migration to the use of formate-based fluids combined with the emerging biopolymer technology eliminated most of the problems encountered previously, starting with the first well in Northern Germany1 drilled in early1996.
fax 01-972-952-9435. AbstractDrilling 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 fluidmixing 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.
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.
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