document

Abstract

The use of low-solids, water-based reservoir drilling fluids has generally been restricted to fields with temperatures of less than 300°F, primarily due to the limitations of fluids in more aggressive environments. The Miano and Sawan fields in Pakistan are both classified as high temperature (up to 340°F BHST), and some wells have been recognised as requiring sand control for optimal production. The operator of this field is dedicated to the application of water-based muds in all activities to reduce the environmental impact of operations.

Based on these criteria and extensive laboratory testing for thermal stability and expandable screen (filter cake quality) compatibility, a formate-based, low-solids, reservoir drilling fluid was selected. The laboratory performance of the fluid will be confirmed by the performance in the world’s first field application of high-temperature formate drilling fluid run with expandable screens.

Introduction

Oil companies have been drilling for oil & gas in Pakistan for many years. Primarily land wells, the variable geology and variable well conditions have made exploration for and development of hydrocarbon deposits a challenge for the Operators that have been active within Pakistan. This operator has been actively drilling in the area for 12 years, with some notable successes – in particular the discovery of the Miano and Sawan fields in the Middle Indus Basin of Pakistan. With bottomhole static temperatures (BHST) in excess of 340°F,  both fields are considered high-temperature developments. Temperatures in excess of 300°F present specific challenges to the drilling fluids used in the lower sections of these wells, as many of the additives used are not thermally stable under long-term or repeated exposure to temperatures in excess of 300°F. In addition to this, the operator is dedicated to using only water-based drilling fluids as part of their commitment to reducing the environmental impact of their drilling operations. Many of the additives used to develop viscosity and control fluid loss in water-based drilling fluids - such as xanthan gum, starch, poly anionic cellulose (PAC), carboxymethyl cellulose (CMC) – are widely recognized as being unstable from prolonged exposure to elevated temperatures.

A number of the wells on both the Sawan and Miano fields require some form of sand control in order to prevent premature sandout of the wells. Expandable sand screens have been selected as the preferred method of sand control. As openhole completions are preferred, it is essential that the drilling fluid used to drill the reservoir section does not damage the formation and that the filter cake generated can either be removed chemically or can be back-produced with acceptably low drawdown pressure after the well is completed. As expandable sand screens are deployed, it is also considered desirable that the reservoir drilling fluid (RDF) can pass easily through the sand screen in the event that the well (or at least the openhole section) is filled with conditioned drilling fluid to control fluid loss while running and opening the sand screens. Additional complications are expected because of the high permeability of the reservoir sand (500 – 3000 mD) and the depleted nature of the reservoir (6.8-lb/gal reservoir gradient).

Drilling Fluid Selection

The use of an openhole completion and sand screens marked a change in completion philosophy, where wells are typically completed by cementing liner and perforating underbalance. Water-based drilling fluids have been used to drill the well from surface to total depth, with high-temperature synthetic polymers used to provide fluid-loss control and viscosity in the lower zones where the bottomhole static temperature (BHST) exceeds 300°F (150°C). The drilling fluids for the reservoir section have traditionally been weighted using barite.

The application of expandable sand screens for these openhole completions required a change in philosophy towards the design and engineering of the reservoir drilling fluid. Expandable sand screens have a limited tolerance for the size and concentration of solids that pass through the filter medium. This can cause the screen to plug during screen running operations or when production is initiated.