The Evolution and Application of Formate Brines in High-Temperature/High-Pressure Operations

Abstract

High-density formate brine-based drilling fluid systems have emerged asviable reservoir drilling and completion fluid options for deephigh-temperature wells. Upon their commercialization in the early 1990's, thesesystems were shown to possess a unique combination of properties that could beexploited 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 recentapplication 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 sagproblems experienced with water-based drilling fluids used in technically andeconomically demanding high-temperature environments. Furthermore, the formatebrine-based systems were shown to improve well productivity.

Through an examination of 15 deep gas wells in Germany and otherformate-based applications, the authors review the performance of formatebrines as reservoir drilling, completion and workover fluids. Particularattention is given to drilling and eventual well performance, as compared tooffset 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 NorthGermany. The gas fields of North Germany have high temperatures (~300°F /~150°C) and are considered as HT/HP wells with the usual problems associatedwith 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 lowformation damage potential. Other criteria the fluid had to meet were goodhydraulics (low solids content) and low corrosivity. The selected system bestmatching the criteria was a biopolymer-formate-based fluid that wassubsequently applied successfully. From this initial application and the recentpositive experiences of other operators, the formate fluids have evolved anextensive and ongoing use for both drilling and workover/completions operationsin the North German gas fields. This paper discusses fifteen drilling andworkover/completions operations and the extensive infrastructure for fluidmixing and reclamation that have developed for these environmentally friendlyand cost-effective operations.

Up until the time that formates were applied, recurring problems had beenlimiting successful operations. The problems routinely encountered with thetemperature limited, calcium carbonate weighted, water-based polymer drillingfluids used previously included inadequate solids suspension, poor solidstransport, stuck pipe, and tight holes. Costs were brought under control as themigration to the use of formate-based fluids combined with the emergingbiopolymer technology eliminated most of the problems encountered previously,starting with the first well in Northern Germany¹ drilled in early1996.