Abstract

Research has identified a novel water-based drill-in fluid for drilling and completing geopressured horizontal wells. This fluid has a unique combination of properties which make it especially suitable for geopressured applications. They include the use of calcium and/or zinc bromide as a base brine, minimal concentration of calcium carbonate as bridging material, low plastic viscosity, tight fluid loss control, good filter cake properties, and excellent return permeability. This drill-in fluid has been used successfully to drill a 1,200 foot production interval, 4.75 inch diameter wellbore in the Gulf of Mexico with a system weight of 13.2 lbm/gal, bottom hole temperature of 185 F., and a 1400 to 1700 psi overbalance. The system functioned very well in both the drilling and completion operations. Fluid rheology was easily maintainable and the hole conditions were excellent without torque or drag problems. Initial production data suggests that the well is producing at expected rates with low drawdown pressure.

Introduction

The year 1995 has witnessed widespread use of horizontal wells by Shell Offshore, Inc. (SOI) and Shell Western E&P, Inc. (SWEPI). At present, the drilling and completion of horizontal wells requiring a fluid density of 12.5 lbm/gal or less, has become routine practice in the Gulf of Mexico. As horizontal well technology requiring low-density systems has gained acceptance within the Shell groups, there is a need to develop non-damaging drill-in fluids for geopressured horizontal wells, requiring densities ranging from 12.5 lbm/gal to 16 lbm/gal.

Due to the demand for higher density drill-in fluids, an effort was undertaken at Shell E&P Technology Company (SEPTC) in concert with service companies to develop fluids that met both drilling and completion requirements. More than half a dozen service companies were contacted to submit drill-in fluids for various target reservoirs. These fluids were then evaluated to determine if they met SEPTC established criteria for both drilling and completion. The drill-in fluid that met these requirements was then recommended to the operational engineers based on both technical and economic reasons.

A horizontal sidetrack well with 1200 feet of 4.75 inch diameter open hole was planned for drilling using a 13.2 lbm/gal nondamaging drill-in fluid in the Gulf of Mexico during October of 1995. Previous experience indicated the 13.2 lbm/gal system would generate an overbalance of 2400 psi over the formation pressure, thereby presenting a challenge for a slide drilling operation utilizing a water-based system. Well data is found in Table 1.

In this report, the laboratory development and field application of a 13.2 lbm/gal water-based drill-in fluid is summarized.

BACKGROUND OF DENSE BRINE FLUID SYSTEMS

Ideally, a high-density drilling/completion system should be compounded with a heavy brine, minimal soluble bridging solids concentration, and low viscosity polymer additives. Density adjustments to the system should be made with brine rather than particulates to maintain an ultra-thin filter cake and low viscosity profile, but with high low shear rate viscosities. The most commercially available dense brines are calcium chloride, calcium bromide, and zinc bromide. However, utilization of these brines as a base for drilling fluids has been limited. Generally, water-soluble polymers used for viscosity and filtration control do not perform well in calcium bromide or zinc bromide brines. Therefore, the basic components necessary for successfully compounding a high density drilling fluid, 12.5 lbm/gal to 16 lbm/gal, exhibiting high, low shear rate viscosities, shear-thinning capabilities, and low filtrates were not readily available for calcium or zinc bromide brine blends.

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