Potential reservoir formation damage was avoided when curing up to 87.4 m3/hr (550 barrels per hour (bbl/hr)) losses of drilling fluid in a carbonate reservoir. In addition to traditional lost circulation assessment and treatment consideration, self-degrading fibers were used as part of the lost circulation system, and these preserved the reservoir from any consequential formation damage.
The treatment procedure consisted of pumping a given volume of treatment pill through bypass ports present in the drilling string and displacing it down to the loss zone (located 56 m below the bit). Managed pressure drilling (MPD) was used to minimize hydrostatic pressure above the said loss zone during pill placement (statically under-balanced mud weight). Since drilling was meant to continue after the treatment, the pill had to be squeezed to and through the reservoir to prevent loss from re-occurring when drilling resumed. The only available solutions at the time of need were either a thixotropic acid soluble cement plug (TASCP) or, the proprietary degradable fiber. Preference was given to the degradable fiber since it involved less rig time and does not need any subsequent dissolving treatment. An appropriate spacer was pumped ahead and behind the degradable fiber to prevent intermixing of incompatible fluids. The treatment was pumped using the rig mud pumps.
The loss rate registered prior to the treatment was 87.4 m3/hr (550 bbl/hr) at a pumping rate of 2650 l/min (700 gal/min). The equivalent circulating density (ECD) was 1.22 SG (10.2 ppg). Out of 19 m3 (120 bbl) of prepared degradable fiber pill, 15.6 m3 (98 bbl) were pumped and displaced into the reservoir, leaving the estimated top of the pill at 5850 m measured depth (MD). The top of the loss zone was estimated to be at 5856 m TVD/MD. The bypass port was then closed. It was then observed that the loss rate reduced to 3.65 m3/hr (23 bbl/hr) when circulating the hole clean at 5800 m TVD/MD and maintaining the same ECD of 1.22 SG (10.2 ppg) with the help of MPD equipment; pumping down string at 3028 l/min (800 gal/min) and boosting the marine riser at 757 l/min (200 gal/min). This pill was designed to self-degrade after 4 days. The pill lasted for 5 days, and the loss rate came back to its original level, providing evidence that the fiber had self-degraded as expected. MPD helped minimize further loss through the reduction of hydrostatic overbalanced pressure. Later, openhole wireline logs were run and did not reveal any change in expected porosity or permeability.
This paper presents a case study in which the introduction of degradable fiber through a bypass port in the bottomhole assembly (BHA) cured severe loss of nonaqueous fluid (NAF) in a deepwater exploration well without damaging the formation. This case provides evidence that properly designed fiber-based pills can be used in the reservoir section without any major consequences on the well production potential.
