Cemented Completions and Through-Tubing Coiled-Tubing Drilling Enable Significant Cost Reduction in Mature Gas Fields

Abstract

A novel well concept to unlock reserves from mature gas fields in Northern Germany has been developed. This concept combines cemented completions with through-tubing coiled-tubing drilling to enable significant cost reductions using ultra slim hole drilling in sour gas bearing, Upper Permian Zechstein dolomite reservoirs. Once gas reservoirs mature, drilling of conventional infill wells can quickly become economically unattractive. Often this leaves resources untouched and it limits the economic life span of a field. To improve the economics of infill drilling in deep and mature gas fields significant cost reductions are necessary. These cost reductions can be achieved by changing the proven, yet costly, casing scheme to an ultra slim hole well concept. Besides unfavorable economics another challenge while drilling with conventional technology in mature fields can be the reduced inflow performance caused by formation damage. This challenge can be overcome by under- or at-balanced drilling, which is enabled by through-tubing coiled-tubing drilling. Despite improved efficiencies gained from knowledge by drilling many offset wells, the estimated gas volumes are not sufficient to justify drilling of new wells with the established and conventional well design. Therefore, the operator prepared an advanced ultra slim hole well concept. The casing shoe setting depths remained unchanged, however the hole sizes are reduced significantly. The openhole reservoir section is changed from 5.875-in to 2.5-in and this section is drilled with coiled-tubing and through the installed completion. The size of the completion is selected to be 3.5-in and it is cemented in a 4.125-in hole. In this application, the cemented 3.5-in completion eliminates an entire 7-in liner that would be necessary in the conventional casing scheme. The remainder of the ultra slim hole well is drilled with a 5-in drilling liner, a 7-in intermediate casing and a 9.625-in surface casing. This needs to be compared with the conventional casing scheme comprising of an 18.625-in surface casing, a 13.375-in intermediate casing, a 9.625-in production casing and a 7-in liner. The reduction in cost is estimated to be in the order of 40%. The presented concept can enable significant cost reductions and by applying this ultra slim hole concept further infill drilling in mature gas fields can become more economically attractive. Moreover, formation damage can be overcome by underbalanced drilling, which is enabled by drilling through-tubing with coiled-tubing. The synergies created by combining cemented completions with coiled-tubing drilling are presented in this paper.