CASE HISTORY - HDC Mark II - OBM Barite Dissolving

Case History July 2002 South China Sea

Summary: This well was a gas producer with a key zone designed to produce 25 mmcfd. This was never achieved as the well only produced 10 mmcfd before it began decreasing. After pumping 13.0 ppg OBM to kill the well, 250 bbls of acid was pumped. Production dropped to below 1 mmcfd. Besides damage from acid, it was also believed some of the barite from the OBM had settled and covered some of the perforations. A year later in an attempt to recover some production, almost 70 bbls of HDC Mk II was bullheaded into the well over 26 hours. Production increased immediately to 7 mmcfd.

This is a dual string gas producer drilled offshore Terengganu in 1999. The well was completed as a dual string completion to isolate a higher pressure reservoir at the bottom from intermediate production zones above.

Due to communication between the completions and lack of heavy brine, the well was killed and suspended in 13.0 ppg OBM. On re-entry, it was found that most of the perforations (in both zones) were partially buried under settled barite and OBM.

In the upper zone, coiled tubing could not be used to attempt a wash out, so the zone was acidized with 350 bbls of SWIK Halliburton formulation.

The zone was originally designed to produce 25 mmscfd. At the time of acidizing, the zone was producing between 10 and 7 mmscfd and falling. After acidizing, the production dropped to less than 4 million, dwindling to less than 1.5 mmscfd by June 2002 with over 200 bpd water.

HDC Mark II was used in an attempt to recover some of the buried perforations, and reverse some of the HCL damage. Due to the high volume of acid used (over 300 barrels) it was felt that attempting to reach the complete step out radius of the acid impact on the initial treatment stage was too expensive on an experimental basis.

The HDC Mark II job design was a staged bullhead operation through a cement unit. The job design was based on staged displacements of HDC Mark II over three hour intervals in a “dissolve” – “wash” – “dissolve” sequence to induce removal of barite from the lower perforations and flowing through them as the chemical depleted. The final stage consisted of displacing the entire volumes into the formation and static soaking for 12 hours. The entire operation was completed in 26 hours.

At the end of 26 hours, a partial nitrogen gas lift was used although the well began cleaning up naturally. Within 24 hours of lifting, the well was producing 4.5 mmscfd, going up to 6 mmscfd within 96 hours and over 7 mmscfd after five days with 80 bpd water and 5 cubic meters per day of condensate. Production has continued in excess of 7 mmscfd on a 19% choke through to the last tests held 45 days after the well was stimulated.

The condensate production results indicate a clear response from the previously buried perforations although the actual gas production source remains questionable.