One of the factors in improving oilfield development efficiency is the acceleration of well construction. Since 2004, when SPD began drilling at the West Salym field, average well drilling time has reduced more than twofold. As the experience is accumulated and new technical solutions are introduced, well completion time is also being reduced.
Initially, well completion in West Salym took between 9 to 14 days depending on the well complexity. Completion included such basic steps as scraping, drifting, circulation with completion fluid, logging, casing and perforation, which is opening up of a net interval with the use of special guns installed on a tubing string.
Swabbing (i.e. reduction of the level of fluid inside the well using special tools) was applied to create a 60 bar underbalance pressure to reduce the effect of the fluid used to control the well. Thereafter, the well was killed with brine, an electric submersible pump was run in-hole, X-mas tree installed and the well hooked up to the production header. To improve the tubing conveyed perforating (TCP) process, SPD engineers decided to remove the swabbing operation. The required underbalance is created while the perforator is lowered into the well by partially filling the well with liquid up to a predetermined level. This allowed testing the well for flow immediately after perforation to obtain data for the selection of the optimal ESP size. Improvement of the process has reduced the completion time to 7–9 days. However, SPD specialists did not stop at this and decided to combine several operations into one to further speed up the completion time. The principal idea was to perforate the pay formation after the installation of downhole equipment, excluding the killing operation and allowing the well to produce oil right away.
The new scheme is outlined as follows:
- Scraping, drifting and displacing well with natural completion fluid (crude oil);
- Running perforation guns with a time delay hydraulic firing head to the depth defined based on GR correlation logs and suspension in position with a mechanical anchor set on the bottom of the gun;
- Running the ESP in the hole, positioning at a safe distance above the guns and actuating the hydraulic firing head by pressuring the annulus to 80 bars. The firing heads have a time delay (90 min);
- Starting up the ESP to pump down the well and achieve underbalance. Thanks to this, by the firing time the underbalance necessary for oil inflow is achieved.
This technology of underbalanced perforation below the ESP provides a number of benefits: eliminating skin damage of the well bore area caused by killing the well after perforation during conventional completion, achieving an optimum underbalance to clean perforation channels and reducing completion time. This perforation technology allows to do away with an expensive and not always safe process of killing the well, especially where it taps into several formations with a big differential in formation pressures. More than 30 wells in SPD have been completed with this technique of perforation with an ESP at the time of writing. The experience gained in the process and improvements to this technology have reduced completion time to an average of 5.5 days. The record time of completing a well in the Salym oil fields was achieved in May 2008 at 3.56 days.