This paper describes the successful application of Control Pressure Drilling (CPD) in the Kirchweidach geothermal project located in Bavaria, Germany. Wells located in the field and other offset wells in the area had faced drilling problems including severe mud losses and differential sticking in the reservoir formation. The project consists of a pair of wells, one of them to be used as producer and the other one as an injector. Which well will be used as producer or injector will be decided after the well tests. The first geothermal well was drilled using Controlled Pressure Drilling equipment in the reservoir section from the beginning; the underbalanced borehole pressure was achieved by pumping various rates of nitrogen and fresh water with polymers which can significantly reduce NPT and formation damage. For the second well it was decided to use the controlled pressure drilling equipment after mud losses appear.

Extensive engineering and development of procedures were carried out prior to spud the first well. The GT 1 well was originally drilled to 4503 m MD using two runs in the 9½" reservoir section and then tested. This was short of the original planned TD set at 4719 m MD. Two additional runs increased the TD to 4937 m MD. The primary purpose of employing underbalanced drilling methods was to avoid total loss scenarios and reduce formation damage by losing cuttings into the formation. Additionally, CPD techniques allowed reduced differential sticking problems, potentially faster ROP and less total drilling days.

CPD greatly reduces formation damage. Due to the fluctuation from conventional to underbalance conditions, some reservoir invasion inevitably occurred. This will be greatly reduced by virtue that there were no solids from drilling fluid additives. Nevertheless, some cuttings injection is likely to have occurred. This was further remedied by continuous and prolonged flowing of the reservoir. The planning, development and execution phases of the CPD project are detailed in this technical paper.

The GT2 well and GT2a side track were drilled conventionally as mud losses remained during drilling at manageable levels (15 m3/hr).

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