Offshore Platform Dan F

30 Years with Horizontal Wells in the North Sea

Wednesday 16 Aug 17

Horizontal Wells

Horizontal wells are wells with high-angle deviation from vertical (with an inclination of generally greater than 85°) drilled to enhance reservoir performance by placing a long wellbore section within the relatively thin, but extended reservoir. The advantages of horizontal wells among others include: Increased production rates because of the greater wellbore length exposed to the pay zone, reduced water and gas coning due to the reduced drawdown in the reservoir for a given production rate, thereby reducing the pressure drop around the wellbore etc. There was relatively little horizontal drilling activity before 1985. The Austin Chalk play was responsible for the boom in horizontal drilling activity in the U.S. Today, horizontal drilling is considered an effective reservoir-development tool. (

2017 is the year where we celebrate 30 years with horizontal wells in the Danish North Sea. A game changer for the Danish oil production that has provided better access to the reservoir and as a result access to more oil and gas. Today around 80% of Danish Underground Consortium’s oil production comes from horizontal wells.

Maersk Oil has for many years been internationally recognized as one of the front-runners within one of the oil industry’s most important technologies today – the drilling of horizontal wells. The horizontal well made it possible to significantly increase the production from the thin, low permeable chalk reservoirs in the Danish sector of the North Sea. This due to the option of giving the horizontal wells a much longer contact length in these reservoirs. Through the course of the last 30 years, this has amounted to more than 300 horizontal wells on oil fields such as Dan, Kraka, Gorm, Tyra, Roar, Halfdan, Valdemar and Harald. A record of horizontal wells reaching as far as 12.3 kilometers has been drilled.

The journey began in 1987 when Maersk Oil planned to drill the first horizontal well.  The original plan was to drill from the Dan E satellite wellhead platform by reusing one of the existing six well slots here. However, a study undertaken at that time demonstrated that several wells drilled closely together had less wave resistance than the resistance of the multiple single wells. In light of this study, the number of wells on the new Dan F wellhead platforms under hook up could be increased and three additional slots were added outside the vertical face of the wellhead platform jacket, this allowed the three further wells to be drilled from the existing wellhead platform. An innovation much needed, as Maersk was facing limitations with their current wellhead platforms and the number of well slots available. A new wellhead platform would have taken at least two years to build and would have involved high costs for three wells plus an expensive new wellhead platform.

"This was truly ground breaking for the Danish oil production that lead to an increase in the oil potential of 15 to 20 % more than expected. "
Jørgen Gross-Petersen, Advisor at DHRTC

Former Senior Facility Engineer at Maersk Oil and now advisor at Danish Hydrocarbon Research and Technology Centre (DHRTC) Jørgen Gross-Petersen were one of the main persons behind Maersk’s planning of the first horizontal wells: “We made the first horizontal test wells in the Dan field. I had the idea to arrange three new add on slots outside of one of the wellhead platforms on Dan F. By placing the wells right next to each other, we could benefit from a shadow effect, which meant that the platform with the expanded number of wells could withstand the extreme waves in the North Sea.” 

“The three new horizontal wells were a massive success and as a result we ended up adding 12 and 13 wells on each of the new Dan F wellhead platforms. This was truly ground breaking for the Danish oil production that lead to an increase in the oil potential of 15 to 20 % more than expected. It really made it possible for us to increase the production from the chalk layers of the North Sea substantially. The horizontal wells were especially suitable for the Danish fields because the oil-bearing layers are narrow and cover large areas. A lot of vertical drills would therefore have been required in order to recover the oil. Ultimately, more than 100 new add on wells were drilled from the existing wellhead platforms.” Explains Jørgen Gross-Petersen.

Read more about the oil history of Denmark.