SLIMHOLES: reducing the cost of exploration

The test rig used to drill slimholes is about the size of a workover unit. Note the slender drill pipe sections in the rack.

The exploration and production of petroleum and gas deposits is extremely expensive. However, slimhole drilling and completion technology can assist by obtaining the data required from the exploration phase at a greatly reduced cost and providing cheaper development wells in some circumstances.

The main cost savings result from the reduced drilling diameter (slimholes), which range from 3 in. to 5 in. The cost reduction achieved depends on the drill site but is typically between 25-50%.

In a project supported by the EC's Thermie program, Security DBS (Belgium) and Forasol-Foramer (France) designed and tested at full scale a high performance drillstring for small diameter operation. This was followed by the construction of a complete rig, specifically dedicated to slimhole drilling. This has been tested on several wells, under different geographical and geological conditions.

Joule-Thermie program

The Joule-Thermie program (1995-1998) aims to improve the EU's energy security by supporting innovative technologies which ensure durable and reliable energy services at affordable costs and conditions. In addition, emphasis is placed on technologies which reduce the environmental impact of energy production and use, particularly in relation to CO2 emissions. The Thermie part of the program includes support for demonstration projects as well as other activities such as strategy, dissemination, preparatory, accompanying and support measures, concerted actions and technology stimulation for SMEs.

In the hydrocarbons part of the program attention focuses on oil exploration, production, transport, and natural gas. In addition, new drilling fluids and cements, which are well adapted to the new hole structure, were investigated together with special coring, logging and testing equipment. Also, attention was given to safety aspects and the protection of the environment.

Euroslim project

The project focused on four key points including the drillstring, the rig, muds and cements, and specialized exploration equipment. All of these elements have been engineered, manufactured and tested at full scale on several test wells.

• The drillstring: Drilling and coring in small diameter are not new. The mining industry currently drills or cores small or very small diameter holes, sometimes up to a depth of 15,000 ft. The drill pipes which are used by the mineral industry are performant - they rotate very close to the hole wall - which results in the pipes being guided along the hole.

  In the oil industry (or more generally in sedimentary formations), the clearance between the drillstring and the hole wall is much bigger. This is due to reasons related to the mud circulation. The result is that the above-mentioned guidance or bearing effect is partially lost.

  As a consequence of this, it was necessary to develop drillpipes strong enough for transmitting significant power to the drill bit or the coring head, even in cases where the guidance is completely missing.

  Two other important specifications were the tightness of the connections and the polyvalence, (the drillstrings should be capable of drilling bigger hole dimensions). The new strings, SH111 and SH66, were built and tested on different well sites in France and Gabon, in sizes of 4 3/4 in. and 3 3/8 in. The tests were successful and confirmed the design.

• The rig: In order to achieve the cost reductions deriving from slimhole drilling, a dedicated rig was engineered and built. This was adapted to allow for the targeted application, new drillstrings, slimhole drilling parameters, small weights and volumes, a new crew organization, and environmental aspects. The target was to drill exploration and delineation wells up to a final depth of 3,500 meters in 4 3/4 in. and 3 3/8 in.

  Both urban area and remote areas were considered. This flexibility required the footprint area to be reduced to 850 sq meters. In addition, the rig had to be capable of being transported either by a 10-tonne truck or by helicopters with a 2,500 kg payload.

  Taking into account the new drillstring characteristics, the mast and hoisting capacity needed to be 100 tonne and the main drawworks required a power rating of 600 hp. The speed of the power swivel ranges from zero up to 600 rpm, according to the needs of the small diameter drillbits and coreheads. The rig consists of a performant and instrumented wire-line winch, dedicated for coring and other inhole operations.

  As far as the drilling parameters are concerned, the most outstanding feature is the very high precision control (achieved by numeric SCR) of the main functions: the drawworks, power swivel, coring winch, and pumps. As a result of this, the weight on bit accuracy, for example, is below 200 daN. The mud flow ranges from 50 liters/min for small diameter coring, up to 2000 liters/min for the superficial drilling phases.

  The rig only requires 13 people. This is achieved by means of integrated organization. For example, mud monitoring and cementing are performed by the same person. In order to enhance the ergonomy, the main drawwork is equipped with remote controls. The braking is achieved by an electromagnetic brake and a DC motor control unit. The rig is fitted with an hydraulic tong and a pipe handling system. All of the engines are soundproof.

  Operation of the rig can handle waste treatment. Dry cuttings are delivered at one side and clear water at the other. No waste pit is present. Like the drillstrings, this performant equipment has been successfully tested on several wells in different environments.

• Muds and cements: During the Euroslim project, the characteristics of different solid-free drilling fluid formulations were examined. Optimal concentrations for the viscosifying polymer and the fluid loss additive were determined. Specially designed hydraulic tests were performed to determine the evolution of the annular pressure losses with the flow rate and the drillstring rotation. The field results compared very well with the theoretical predictions.

Different cement formulations were field tested as well, taking account of the small annular existing between the casings and the formations.

Specialized exploration equipment

Among the specialized exploration equipment developed for this project are the following:

• Coring equipment: The wire-line coring concept has been applied successfully. This cost-efficient method uses a sampling tube which is linked by means of latches with the drillpipes when cutting the core. When it is full, the sampling tube is unlatched and pulled up through the drillpipes. This results in significant savings in tripping costs.

The slimhole core barrels have the same mechanical performance as the drillstring itself. They form a modular system for coring in 9-meter and 18-meter sampling length. Low fluid invasion bottom discharge core heads allow good quality coring. Fibreglass disposable inner tubes are available in SH111 and SH66.

• Logging and testing equipment: Some existing equipment has been adapted to the small diameters by specialized companies. These performed well in the field.

• Kick detection: The existing kick detection methods have been adapted to the slim well configurations. Kick detection software has been developed and implemented.

Exploration and delineation costs are a major concern for oil companies. The Euroslim integrated slimhole system is an alternative, low cost, efficient solution. It provides exploration managers with the data they require at cost savings of 25-50% of more traditional methods.

Euroslim consists of new, heavy duty downhole equipment, together with a dedicated surface installation, new muds and adapted exploration downhole tools. In some cases, the new method will allow development wells to be drilled at lower costs.

Acknowledgment:

Forasol-Foramer, Security DBS, and Elf-Aquitaine combined resources to undertake this program. The Institut Français du Pétrole also made an important contribution. Further information on this project can be obtained from Jonathan Shackleton, the Centre for Marine and Petroleum Technology. Fax: +44 (0) 1224 706601. E-mail: [email protected].

Copyright 1997 Oil & Gas Journal. All Rights Reserved.