In 1993 Mobil Erdgas Erdöl GrmbH drilled four short radius wells in Northern Germany. The wells were drilled in two reservoirs having complex geologies and faced technological challenges in a low budget.
Three wells were gas-storage wells in a depleted oil reservoir near the city of Hamburg. Two of these were designed for water production in order to increase the storage volume and one at the top of the structure was designed for producing and injecting gas.
Because of given surface locations, fixed targets and complex geology of this highly fractured reservoir, short radius drilling and horizontal lengths of more than 400 m were necessary to achieve the different goals of the wells. External casing packers (ECP) had to be used to ensure complete gas shut off after cementing the 7" liner. The short radius curve and the complete horizontal section was reamed for running stimulation tools later on, Geology required small targets, precise directional work with complex wellplans and correct placement of casing shoes and ECPs.
The fourth well was planned as a low budget oil production well in the shallow oil field Voigtei. Cost reductions were achieved by using a specially equipped mining rig, a simple but adequate drillsite, cementation performed with a Mobil-owned pump truck and running of used and inspected casing. The biggest challenges were to drill 430 m of horizontal section at a vertical depth of only 270 m and to stay within the thin pay zone of the Lower Cretaceous Sandstone, which required a TVD limitation of 1.5 meters. To fulfill this objective a nearbit inclinometer as part of the MWD was used successfully.
The short radius curve was drilled in 8 3/8" hole diameter and cased with a 7" surface string. A specially designed pull down assembly hooked to the rig's top-drive was used to run the casing through the short radius to horizontal. Drag problems while drilling the horizontal section were overcome by additional WOB from the pull down. The horizontal section was logged with the tools connected to the drill string and cased with a 4 1/2" slotted liner.
The well could prove that shallow oil-wells can be drilled inexpensively by using existing - but for the oil field untypical - technology.
Short radius horizontal drilling technology has existed for several years, but its application still faces many challenges. Special tools are required to pass through the short radius curvature and BHAs react differently from standard directional tools. Limitation of doglegs in the horizontal section is a major requirement due to torque and drag considerations.