Abstract

Underbalanced drilling is one of today's fastest evolving technologies in the oil and gas industry. Gasified drilling fluid is most commonly used in underbalanced drilling of pressure-depleted reservoirs. Nitrogen gas, which is inert and not flammable, has been the predominant choice in underbalanced drilling operations, with relatively few operations using natural gas or compressed air.

Nitrogen consumption incurs a significant (20% to 30%) portion of the overall underbalanced drilling operation cost. One option to reduce nitrogen consumption would be to recycle it. This operation involves the recovery of the return gas from the separation unit for re-compression and re-injection into the well. In most cases, the return gas is still predominantly nitrogen unless the well is a prolific gas producer. For successful implementation of gas recycling, it is essential to maintain consistent quality and rate of the return gas. The feed gas for the compressors must be sufficiently dry to ensure that any water or hydrocarbon vapors do not condense out during the compression cycle. Secondly, the return gas rate must be stable so that the gas can be re-compressed and re-injected into the well at a specified injection rate and pressure.

Northland Energy Corporation with Porta-test International Inc., designed and fabricated a system to recover and re-circulate the return gas for underbalanced drilling. A comprehensive yard test was designed and implemented to test the prototype system. Initial results indicate this system is ideal for field applications, though some refinement will be necessary to realize the full benefit of such a system.

Introduction

One of today's fastest evolving technologies in the oil and gas industry is underbalanced drilling (UBD). Preventing drilling damage is seen to be a major contributor to well performance, especially when dealing with high cost horizontal wells. Long "pay zones" will naturally require longer drilling time, thereby extending exposure to damaging conditions.

When properly designed, underbalanced drilling programs allow the circulation pressures to be within a specified range, normally just below the formation pressure. Reduced differential pressures can prevent fluid loss and reduce fines migration, thus reducing the damage to the producing formation. Furthermore, underbalanced drilling can allow drilling in highly fractured formations or pressure depleted formations which may otherwise experience fluid loss and numerous other drilling problems (i.e., stuck pipe).

In most of the applications for underbalanced drilling in Canada, low formation pressures have not allowed the use of single phase liquid systems as the hydrostatic pressure would have been above the reservoir pressure. Consequently, to achieve lower pressures requires that the drilling fluid be lightened by entraining a gas in the drilling fluid. The gas is injected into the circulating drilling fluid and then recovered from the well into a closed separator where solids, liquids and gases are separated and metered.

To date, the majority of Canadian underbalanced drilling operations have been undertaken using cryogenic nitrogen with liquid nitrogen supplied from air separation plants. Unfortunately many of the prime underbalanced drilling targets are located far from a source of liquid nitrogen. This results in significant costs for the transport of liquid nitrogen.

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