To maximize return on investment, many operators choose to re-fracture existing wells as an economic alternative to drilling new wells. While it may improve ultimate recovery, re-fracturing of wells with multiple producing zones can be challenging. Several techniques are available in the market place; each having its own limitations and benefits. The technology discussed in this paper, already implemented in the Western Hemisphere, uses a guar-based gelled fluid for temporary zonal isolation in the annulus. The need for a cost-effective solution for re-fracturing, without using a packer or other downhole isolation tool was identified in the Changqing oilfield in China. The objective was to prove the concept of using a gelled annular isolation technology for re-fracturing in this oilfield for applicability in the Asia Pacific region.
Two candidate wells were selected for re-fracturing based on previously completed methodologies and observed production declines. A two stage vertical well was selected for proof of concept. The original completion used a 5 ½-in. casing, applying multi-stage sand jetting fracturing. The application of the gelled polymer system for re-fracturing utilized the 5 ½-in. casing as an outer tubing and implemented a 3 ½-in. inner string to create an annular space for isolation. The gelled fluid was pumped into the annular space between the inner tubing string and the original casing to isolate existing perforations in the two stages. After the re-fracturing treatment was completed, the gel was broken down and the inner string removed.
Before the job was pumped, laboratory testing and simulation concluded that the gelled fluid would be placed within the pump time of the job and hold for the expected time frame of the fracturing operations. This testing provided detailed operational guidelines on placement time, pump rate, fluid shear rate at bottom hole temperature within the constraints of the wellbore architecture before committing to pump the job. The fluid was successfully placed and maintained isolation during fracturing stages. The inner string was removed from the hole following the breaking of the isolation fluid.
The novelty of this work included the application of using a guar-based gelled fluid as a temporary annular isolation pill in a re-fracturing application in China. In addition, this work proved that an inner completion string could also be temporary and therefore removed after the job was completed to ensure that maximum access to the reservoir was achieved. Finally, this is the first cost effective and simple re-fracturing methodology introduced to this area of the Asia Pacific region.