Abstract

Air/gas drilling is a technique of Underbalanced drilling (UBD) used for increasing rate of penetration in relatively dry formations. Fluid conversion from air/gas to foam frequently causes drilling complications to drilling engineers. This is mainly due to the lack of understanding of liquid loading problem and limited knowledge of liquid-carrying capacity of gas. It is highly desirable to gain the insight of the problem and develop techniques to minimize drilling complications.

Based on Turner's theory of liquid loading in gas production wells, we have established in this study the minimum kinetic energy criterion for flowing gas to carry liquid up the borehole in air/gas drilling. A theory for estimating the minimum required gas injection rate to remove formation liquid influx has been established. A calculation procedure has been developed for estimating the minimum required gas injection arte to remove a given rate of fluid influx from the formation. Engineering charts have been generated for predicting the liquid-carrying capacity of gas that is injected into the borehole at various rates. This paper provides drilling engineers necessary knowledge and a useful tool for minimizing complications in air/gas drilling.

Introduction

Underbalanced drilling (UBD) is defined as drilling operations where the drilling fluid pressure in the borehole is less than the pore pressure in the formation rock in the open-hole section. By using light drilling fluids, the driller intentionally keeps the borehole pressure not balancing the formation pore fluid pressure. The light fluids used in UBD are usually air, gas, foam, and aerated water. However, oil, water, and even weighted mud can be used for UBD in areas where formation pore pressure gradients are higher than the hydrostatic pressure gradient of water.

UBD is an integrated suit of technologies resulting from the merging of the traditional air/gas drilling techniques (for solving drilling problems) developed in 1950s and the aerated liquid and foam drilling methods (mainly for solving reservoir problems) developed during the past 20 years (GRI, 1997). Some experts estimate that about 30% of current oil and natural gas recovery drilling and completion operations utilize the UBD technology (Lyons et al., 2001).

The major advantages of UBD include increased rate of penetration, minimized loss of circulation, and reduced formation damage. UBD applications are limited by borehole problems such as instability and complications, safety, and cost.

This study focuses on reducing complications in air/gas drilling operations. Historically, the drilling complications are related to hole-cleaning issues resulted from inadequate gas injection rate for carrying drill cuttings. It appears that using larger gas compressors can solve this problem (Guo and Ghalambor, 2002). However, it has been generally recognized that liquid accumulation in bottom hole can also cause complications during air/gas drilling wet formations.

Liquids (water and/or oil) from wet formations accumulate at bottom hole when the air/gas injection rate is not high enough to carry them to the surface. Accumulation of the liquids increases bottom hole pressure, which compresses gas and reduces gas velocity, resulting in reduced carrying capacity of the gas and, in turn, causing solid and more liquid accumulation at bottom hole. This cycle will create drilling complications such as mud ringing and pipe sticking. Adding foamers (surfactants) to the gas stream can ease this problem to certain extend. If the liquid production rate is significantly high, additional gas injection capacity is required, or the air/gas drilling needs to be converted to foam drilling. However, it is unclear as what constitutes the "significantly high". Converting to foam drilling will result in much lower rate of penetration, while waiting for compressors of higher capacity will also reduce the overall drilling performance due to added non-rotating time. A guideline is highly desirable for drilling engineers who are making decisions on whether or not to convert to foam drilling.

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