Evaluation of methods for restart of wells with low reservoir pressure

Abstract

A pre producing well which has been rendered unable to restart production present challenges which is not in need of consideration when faced with a well which has been intentionally killed with the use of high density fluids. The procedures main goal is to reduce the hydrostatic column pressure in the wellbore in order to obtain a suitable differential pressure with regard to the low available reservoir pressure and subsequently to trigger production flow. The evaluated methods are the use of light weight hollow glass spheres (HGS) additive, bullheading with the use of a lightweight substitute for gas and finally the use of a plunger swab procedure.

The applicability of the HGS additive presented a wide range of utility, especially in conjunction with bullheading as the injection procedure where the HGS could be a substitute for gas. Available HGS products have initial densities ranging from ρHGS=600 kg/m3 to ρHGS=320 kg/m3 and crush strengths ranging from 13793 kPa to 68966 kPa. Several mixture compositions were established for different cases with a base fluid density of 800 kg/m3 in a 6 5/8” (6,1” ID) tubing where a 30% solution of HGS5000 could cover a differential pressure of 40 bar in a well with TVD = 5000m. Backflow evaluation with the use of bullheading showed that the mixture would be displaced from the wellbore in 309 minutes with a final production rate of 196 m3/d and a differential pressure of 9,8 bar.

The swabbing procedure was a great success with regard of restarting a killed well with ρkillfluid=1200 kg/m3. Backflow considerations established that pre shut in production flow could easily be regained, even with relative low plunger submergence. An operation with a drowned wellbore presented a high submergence requirement in order to be successful. Submergence depths of 309m, 495 and 1500 meters in a 2000 meter wellbore were considered for a 5 ½” tubing, were the latter did prove successful. The first two depths were the maximum suggested work tension and 80% of break strength for the most hardened swab line from Camesa.