New tools/techniques give positive production increases - Statistical Data Included

World Oil, Feb, 2000 by Murray Williams, John Ramalho

Shown here are five new technologies that can improve field exploration, drilling and production operations, increase downhole tool efficiency and enhance project management. Two of the improved practices have been implemented in the field, the management enhancement scheme is being put into practice, the new tool material has undergone extensive comparative testing, and a wrapup from the Petroleum Technology Transfer Council (PTTC) summarizes important technologies aimed at independent producers.

The five products/services covering these applications include: 1) how a vessel-mounted, underbalanced drilling/separation system helped an operator drill and complete fluid-sensitive chalk/sand reservoirs offshore the Netherlands; 2) comparative testing of a polyurethane sealing material to prove better sour-gas resistance; 3) how to direct new management concepts to show tangible increases in actual project values; 4) positive benefits for sand control using a new downhole, expandable sand screen (ESS) system; and 5) a listing of nine topics from recent regional workshops sponsored by PTTC that are available in more detail online.

Underbalanced system helps drill North Sea chalk/sandstones

Murray Williams, UBD Co-ordinator, Expro Surface & Environmental Systems; and John Ramalho, Phase 2 Coordinator, NAM

Case-history results from drilling upper chalk formations and the actual sandstone producing reservoir for Netherlands North Sea operator Nederlandse Aardolie Maatschiappij B.V. (NAM) show positive economic/operational results. The Expro Group supplied the underbalanced drilling (UBD) surface fluid handling equipment and site supervision to the project team, which comprised the operator and third-party contractors. This discussion covers the background of UBD, the system used by NAM and results of the two applications in actual wells.

Re-introduction of UBD. Underbalanced drilling (UBD) is seeing a popular comeback in almost every producing area of the world. However, resistance to the "new" technology is widespread. Although this resistance is primarily due to the misconception of higher safety risks, there is also a culture change as we move away from traditional methods.

Until design/acceptance of the rotary drilling rig in 1901, all wells were drilled underbalanced, i.e., hydrostatic pressure of the drilling fluid was intentionally lower than pressure of the formation being drilled. In the absence of control mechanisms, these drilling practices led to the infamous "gushers" of hydrocarbons when the reservoirs were breached. At this time, the well would be capped once the gusher had subsided, and the remaining hydrocarbons were pumped or flowed to surface.

The inefficiencies and safety concerns of these processes led to introduction of the rotary rig, which meant companies could circulate drill fluids to the bit. The variety of mud weights available ensured that drill fluid pressure was higher than formation pressure--this set a trend and overbalanced drilling became normal practice. Control systems were installed--thus satisfying safety measures, and productivity was optimized for most reservoirs.

The present interest in UBD is due to a number of developments, and it is demonstrating its worth as our understanding of reservoirs improves. Of course, its reintroduction does not come without vast technical improvements and innovative design changes to surface equipment which incorporate separation process technology, rotating control devices and directional drilling systems, which enable us to maintain stringent safety levels. Specifically, UBD's reintroduction will prolong life of older reservoirs and allow development of new fields by reducing life-cycle costs.

System description. The Expro Group's system, as shown in Fig. 1, is designed to ensure that drilling fluids are directed from the rotating control head at the top of the BOP stack to the supplier's surface control equipment. Well effluent flows through large-bore pipework into a high-pressure mud/gas separator where free gas is separated and flows to the flare via the backpressure control valve. This backpressure valve, ultimately, controls downhole pressure.

[Figure 1 ILLUSTRATION OMITTED]

This maximizes stability of reservoir pressure even when a liquid slug enters the vessel. In this event, the pressure increase will be minimal, thus reducing the likelihood of an overbalanced condition being created downhole. The gas is routed via a knockout separator to one of the burner booms, flare stack or, in some cases, back to an existing production installation.

Liquids and solids flow via a hydraulically actuated level-control manifold--which controls liquid level in the mud/gas separator--to the solid/liquid removal vessel. Parallel to this level-control manifold, a high-pressure sampling manifold is fitted to obtain geological rock samples from the liquid stream on a continual basis.

The solid/liquid removal system extracts solids from the liquid phase using a bank of cyclones. This vessel is positioned downstream of the level-control manifold. Extensive choke tests showed that choke life was too short to be practical if high-pressure drops with concentrated slurries were expected. Overflows and undertows from the liquid/solids separator are then directed for further degassing and separation.