Tender trap for core fluids
  from: Offshore Engineer
  by: Pascal Bartette and Philippe Cravatte
  Thursday, October 02, 2008

Hailed as a groundbreaking advance in coring technology, the Liquid Trapper is designed to allow seamless and cost-effective sampling of both reservoir rock and fluids. Reservoir Group’s Pascal Bartette and Corpro’s Philippe Cravatte discuss the innovative tool’s provenance and potential.

Formation evaluation service providers are among the first people involved in oilfield exploration to obtain information from the reservoir. The quality and reliability of the acquired data is an absolute priority as it will serve to shape subsequent activities on the same well, and ultimately in the field.

In the evaluation of a formation, the coring services provide the first tool (core barrel) to penetrate the reservoir. The core barrel delivers rock samples (a core) from the reservoir.

The core is recognised as an essential part of any meaningful formation evaluation programme. It provides the only direct measurement of the reservoir characteristics and allows the visual examination of the reservoir rock. Coring technologies have progressed significantly over the recent years. New technologies were brought to the market in terms of downhole equipment, core processing and on-site data acquisition – all with the aim of improving continuously the quality and cost-effectiveness of coring operations.

Among these new solutions, a recently patented technology called the Liquid Trapper emerges as one of the most innovative coring services, breaking new ground in the acquisition of critical petrophysical and reservoir engineering data.

The tool is aimed at addressing one of the biggest shortfalls of coring services, which is the loss of formation fluids during the coring process. To put this in a technical context, during the core pull out trip to surface, the mud hydrostatic pressure decreases gradually and the gas comes out of solution inside the core. The expanding gas causes the expulsion of liquids from the core. When using standard core barrels, the expelled core fluids are lost for ever!

This phenomenon has always been considered a limitation of conventional coring, particularly in situations where the knowledge of oil and water saturations is critical. Over the years the industry has made many attempts to overcome this challenge but the technologies developed so far have not been successful in offering improved services that are both reliable and cost-effective.

Considering the ever-increasing industry imperative to produce hydrocarbons through secondary and tertiary recovery, Corpro identified a very specific and urgent requirement from the oil industry for a new technology aimed at trapping the liquids escaping from the core.

It is with this objective in mind that Corpro’s research and development group based in Aberdeen, designed the Liquid Trapper. The tool was conceived to allow a seamless and cost-effective sampling of both reservoir rock and fluids while satisfying quality, health and safety and environmental requirements.

During coring, the Liquid Trapper operates as a conventional barrel. However, its distinctive features become apparent during the trip to surface, as pressure drops and gas liberates from the oil and expands. The gas drives out part of the liquids present inside the core, the volume of which depends on the reservoir rock and fluid characteristics. The expelled fluids flow into the core barrel inner tube annulus and, with a conventional barrel, disappear in the mud system. This is where the Liquid Trapper differs.

The Liquid Trapper consists of a specifically designed liner assembly which, through a inflatable seal system, ‘traps’ the liquids escaping from the core. The Liquid Trapper inner tube system is pre-filled with a selected fluid before it is run in hole. The type of pre-filling fluid depends on the information targeted from the core. If required, the fluid can be doped using a mud tracer. It is kept inside the inner tube by a plug, which consists of a ceramic plate that breaks as soon as the corehead tags bottom of the wellbore. The specifically designed ceramic plate shatters into minuscule pieces to avoid any obstacle to the core entry.

The inner tube of the Liquid Trapper is made of stacked modules, 1m in length and each made up of closed ‘cells’. The cells allow capturing of the expelled fluids and segregation of oil, water and gas thanks to their gravity difference. These cells isolate 1m sections of the core, thanks to specifically designed seals which prevent liquids from escaping. The seals still allow gases to get away upward through a modified inner head. Gases are discharged into the mud system, thus eliminating trapped gas hazards.

The Liquid Trapper coring process is absolutely standard. However, at completion of the run, a ball is dropped to the seals. Ultimately, oil and water expelled from the core are trapped in the closed cells made up by the annulus between inner and outer barrel walls.

On recovery, the core barrel is laid down in the rig-processing site using core cradles as per standard procedure. Then the Liquid Trapper cells are cut in 1m sections, sealed and stored vertically in special core containers. The segregation of oil and water can still continue during storage and transport to the laboratory.

In the laboratory, the fluids in the cells surrounding the core are collected, the volumes are measured and fluid analyses are carried out. Following fluids collection core analyses are conducted and among other things the fluid saturation of the core is determined. By adding the collected free oil (or water) expelled from the core to the measured saturation of the adjacent core, realistic saturation figures are obtained. By applying the usual corrections for, for example, oil shrinkage and stress, fluid saturation data can be obtained that are representative for the reservoir.

The Liquid Trapper service improves confidence in terms of the evaluation of hydrocarbon and water saturations.

In the exploration and appraisal stage of a reservoir the Liquid Trapper adds value to the determination of the oil-water contact. The hydrocarbon typing, together with the possibility to identify changes in the hydrocarbon composition over the cored interval, might be of help for the reservoir engineer in building reservoir models.

The collected volumes of the captured and thus movable oil and the properties of the adjacent rock give the production index of zones on a 1m x 1m basis. Beyond that the Liquid Trapper also yields information that might be of help to decide on the number and locations of MDT, RFT and production tests.

Further, the possibility to pre-fill the core barrel with selected fluids and the specific barrel design features allow the cutting of high quality core with in-situ wettability.

The tool can be run in both the exploration, appraisal and production stage of a field. However, its most common utilisation to date is in mature fields for the determination of remaining oil ahead of secondary and tertiary recovery projects.

In conclusion, in light of the increasing commercial imperative to recover stranded reserves, the core and fluid samples collected with the Liquid Trapper will continue to enhance the ability of operators to make robust decisions based on accurate and reliable data collected directly from the reservoir. OE

About the authors
Pascal Bartette is chief executive officer of the Reservoir Group, a leading provider of downhole tools, technologies and associated subsurface services. He has nearly 30 years’ international experience in the oilfield service industry, becoming managing director to lead a management buyout of Corpro Systems in 1999 and helping secure 20-fold growth over the next eight years. Last year he led a new MBO at Corpro to provide the platform for establishing the Reservoir Group, which now encompasses an international group of companies.

Philippe Cravatte is chief operating officer and technical director of Corpro, which specialises in coring services, core handling and on-site core measurements. He has extensive experience in the coring and downhole tools sector and was part of the 1999 management buyout team. He plays a key role in developing the product and service portfolio at Corpro – now part of the Reservoir Group – and also oversees its operational management.

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