How We Drilled To 4520m In 89 Days
By Kurt Reif, Operations Manager, Strada Global.
At the time of writing this, Strada are in the middle of final testing for our patented Fluid Hammer Operating System (FHOS). We believe that once testing is complete, we will be able to demonstrate that our unique approach has a significantly higher drilling rate in hard rock, deeper drilling capability and better safety due to downhole condition controllability.
We have good reason to be confident in these claims; using our experience and skills we have been able to drill in 200 MPa granite rock at a rate of up to 25 metres per hour with a crew of three people in Australia. Then, in Finland, we achieved the fantastic result of drilling 4520m in 89 days. It is this process that I am going to describe below.
So how did we do it?
The Finland project was drilled using conventional percussion drilling methods, however we utilised custom made drill bits, stabilised retainers and string stabilisers to ensure hole integrity and minimise deviation. A considerable air package was used to provide sufficient pressure and volume for air percussion drilling.
Two wells were initially planned in the Finland location, both with an intended depth of 7500m. The project required the drilling of two holes to support a new geothermal plant — upon completion, one hole would be used to pump water down through the bedrock that would act like underground heat exchanges. The heated water would be forced back up the second hole to a heat plant system on the surface to provide clean, environmentally friendly energy at a fraction of the cost of fossil fuels. It was estimated that the heating plant would produce up to 40 megawatts of geothermal heat, which is up to 10% of the region’s heating demand.
The geothermal project utilised multi-stage, telescopic drilling methods to reach the desired depths. The first stage used a Patriot 180 hammer, and 26″ (660 mm) retained bits with PCD (Polycrystalline Diamond) buttons to drill down to 300m. The surface section was cased, and pressure cemented using 18 5/8″ K55 casing.
The second (Intermediate) stage started with a 448mm (17.6″) P180 PCD drill bit, gradually decreasing in 2mm increments to reach 3000m at a hole size of 430mm. Average bit-life through this zone was 236m, with the best run of 385m between 1500–1885m (VD). Including the surface section plus the drilling of the 3000m intermediate section, it took a total of 55 days. Next, 3000m of 13 5/8” casing was installed, and pressure cemented in place. However, due to pressure losses with communication between OTN 2 and OTN 3, it took another 14 days to resume drilling.
The third stage used a Patriot 125 hammer specially fitted with fully retained PCD drill bits to ensure strict hole diameter was maintained throughout drilling. Bit sizes started at 313mm to finish at 308mm at 4520. Average bit-life through this zone was 217m per bit, with a best performance of 384, between 3311m and 3695m. During drilling activities, there was a maximum of 17 primary air compressors, and 5 secondary booster compressors to deliver the required cfm (cubic feet per minute) to clear the hole of drill cuttings.
Once drilling depths exceeded 3500m, there were ever-present challenges to lift the drill cuttings from the hole, and at times the drill pipe became stuck, due to insufficient hole cleaning. However with some collaboration between the client and the drilling contractor, these challenges were overcome to produce an outstanding result.
Strada was only contracted during the first phase of this project, but during this time our system delivered an average rate of penetration of 10–25 metres per hour and drilled up to 300 meters per day in the hardest rock formations in Finland (up to 520 MPa hardness). We finished at 4520m on the 16 November 2016 with just 89 days of drilling.
While we have not had this result verified by an independent body, we are not currently aware that anyone else in the world has ever achieved a drill depth at this speed.
