An operator in Ecuador produces from the Napo "U" sandstone formation with permeability ranging from 90 to 2,000 mD. A lateral aquifer supports the oil production coming from this zone. Water breakthrough in this formation is common in intervals with high permeability streaks causing the water cut to increase substantially, thus affecting the oil producing zones. Besides the onset of water, additional oil decline results from high pressure drawdown and high clay content (including kaolinite) in the formation. The primary damage mechanism is identified as fines migration, a common problem in wells in Ecuador.
The main challenge is to increase oil production through stimulation in wells with high water cut and a strong aquifer, without increasing the water cut. A treatment was proposed that would remove the damage in the intervals producing oil and "good" water without stimulating the interval where water had already broken through. It consisted of combining a modified organic clay acid (OCA) and a viscous disproportionate permeability modifier (VDPM) to maximize zonal coverage and minimize water production.
The first stimulation treatment using the modified OCA and the VDPM enabled the treatment to be diverted to the intervals where water had not broken through. Post-treatment results showed production had doubled and water cut remained the same as it was pre-treatment. Production then remained stable after more than 3 months. A nodal analysis model predicted that the well had a skin of approximately 10 prior to the treatment, and this skin was reduced to nearly zero following the treatment.
A subsequent stimulation treatment performed in a horizontal openhole completion using coiled tubing increased oil production by more than ten times the initial production. It was not known where in the openhole the water breakthrough had occurred in this well, but through the use of the VDPM, the water cut was reduced.
An additional 13 wells with similar damage have been treated successfully using the same fluids resulting in an additional 2,100 BOPD of production. The combination of effective diversion, conformance control and modified OCA is an effective means of selectively increasing oil production in mature fields without increasing the water cut.