document

Abstract

High-density formate brine-based drilling fluid systems have emerged as viable 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 damage and 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 than the 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 workover 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 completed in 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 where one 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 workover/completions operations in the North German gas fields. This paper discusses fifteen drilling and workover/completions operations and the extensive infrastructure for fluid mixing and reclamation that have developed for these environmentally friendly and 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 early 1996.