Thursday, August 17, 2017

Vertical Exploration Well : Case Study

Elf Aquitane embarked on a series of trials to determine whether coiled tubing could be used to drill slimhole wells, cutting exploration drilling costs. The goal of the first well was to demonstrate that a CTU can drill a vertical well sufficiently fast, cut cores and test formations. Elf envisions initially drilling these slimhole wells with a single openhole section-avoiding the need for casing - with the surface casing set using low-cost, water well rigs.

This first trial involved the re-entry of well Saint Firmin 13 in the Paris basin. The plan was to use the CTU to set cement plugs across the existing perforations at 2120 ft and then drill a 2105 ft vertical section of 3 7/8-in. diameter. Directional measurements using a coiled-tubing conveyed survey were to be taken every 500 ft. Then a 50-ft itnerval was to be cored and logged. Finally, a zone was to be flow tested by measuring pressure between two straddle packers.

The trial was carried out by Dowell Schlumberger using a trailer-mounted CTU with a reel of about 6000 ft (1830 m ) of 1 1/2-in tubing. To avoid the need for costly modifications, standard surface hardware, like injector head with stripper and BOP stack, were used. A workover rig substructure was installed over the existing wellhead to act as a work platform.

The operation encountered difficulties at the outset-not with the drilling but with the integrity of the well 30-year old casing. After cement plugs were set, the would not hold the 360 psi aboye hydrostatic pressure required to withstand the anticipated formation pressures. Because of this, drilling depth was limited to 2955 ft which allowed limestone coring but did not extend to a high-pressure aquifer.

The drilling BHAs employed a high-speed, low torque motor with PDC bits. For coring, a high-torque motor was used. The drilling and coring assemblies were made to hang vertically by incorporating heavy drill collars into the BHA, creating a pendulum assembly. At the start, the deviation at the casing shoe was 2o and, as expected, the BHA did not build angle at 2362 ft and 2795 ft, the deviation angles were 2 3/4 o and 2 1/4 o respectively.

During drilling, the rates were comparable to those drilled by conventional rigs at work in the area. This showed that a CTU can drill vertical wells a commercial rates. Two cores were cut and retrieved with good recovery-meeting the second objective of the trial.

Because the program had to be revised to avoid high-pressure zones, no oil-bearing formation could be tested. To prove testing technology and meet the third objective, a drawdown test was carried out on a zone between 2221 ft and 2331 ft . The FSTS Formation Selective Treatment System was deployed with its two packers straddling this zone. The formation was successfllly isolated and , if it had been a reservoir, would have produced into the coiled tubing.

In addition to proving that coiled tubing can be used to drill wells, the trial pointed out how procedures could be changed and where future hardware development is required. For example, rate of drilling could be increased by incorporation of measurement-while-drilling tools to make directional surveys, improving surface handling and weight-on-bit (WOB) control techniques and better optimization of the BHA.

  • Equipment needs - The Elf job utilized a workover rig substructure. In the future, a purpose-built substructure will be employed. Standing 10 ft off the ground and over the wellhead, this substructure will act as the drill floor to make or break the BHA and also to support the injector head. 

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