Abstract

This paper describes the use of an innovative high-performance water-based drilling fluid system in south Europe. The novel drilling fluid allowed to successfully drill a sidetrack in a deep, high pressure-high temperature (HP-HT) well, in an area where low toxicity oil-based drilling fluids had been used in the past, due to the adverse drilling conditions.

Comprehensive laboratory testing carried out to customize the drilling fluid formulation during the well planning phase, was complemented by further testing conducted during drilling operations. An obvious advantage of this system is its simplicity. It is composed mostly of liquid additives that allow a quick and easy preparation at rig site. The system is also compatible with the full range of mud densities, while exhibiting consistently superior lubricity characteristics.

A tailored version of the system for this sidetrack well in southern Europe, required a heavy weighted fluid (up to 2.06 SG) to provide sufficient hydrostatic pressure for well control. The formulation included a high temperature rheology modifier that improved fluid stability at temperatures up to 160°C at total depth.

A high performance lubricant was added to the system to improve drilling rate, while using an advanced steerable drilling system. Lubricity coefficients were monitored and recorded daily at rig site using a standard industry accepted ring and block type lubricity tester. The lubricity values fell in the range traditionally associated with oil-based mud usage. The successful sidetrack operations were completed with very good performances.

Introduction

Drilling fluids can influence all processes of well construction and operation, since incompatibility between the selected fluid and geological conditions yields many undesirable consequences in terms of drilling issues, quality of the production zone and economical performances. This is even more "true" when operating in complex drilling conditions, such as those described in this paper.

In particular, one of the first applications of an innovative high performance water-based mud (HPWBM) outside the United States - it had been previously used only once in Europe - is reported herein, which made it possible to successfully drill a challenging sidetrack within a deep, high pressure-high temperature (HP-HT) well, in hard and tectonically-stressed formations.

The HPWBM, optimized to the specific field conditions, met several standards for effectiveness: high lubricity, stability at high temperatures, little tixotropy, resistance to solids contamination. The fluid represented a big step forward in reducing the environmental impact, while providing performances comparable to those of low-toxicity oil-based muds used in the area for the last 20 years, as the only means to overcome the difficult drilling conditions.

The reservoir is characterized by a white light brown dolomite of the Triassic age, and has two main hydrocarbon zones, an upper and a lower one. The sidetrack's target was the upper reservoir, which is an isolated fault block in the west area of the field resulting from a series of tectonic discontinuities, that had remained undrained by the previous operations. The depth of target reservoir was between 5500 and 6000 m, with a temperature of up to 160°C and pressures over 1000 kg/cm2 at total depth.

In the offset wells, these demanding conditions, combined with the high formation hardness and the significant tectonic stresses, had leaded to such drilling issues as high torque, low rate of penetration (ROP) and wellbore instability.

Instead, in the case described in this paper, the usage of the HPWBM, in combination with an advanced steerable drilling system, enabled the sidetrack to be successfully executed without incidents.

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