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Abstract This paper introduces a method to fracture more than 30 zones in a single well per day and also how to fracture low permeability zones when the impermeable barrier(s) to high permeability and high water cut zones are only about 0.5 m thick and not create channels in the cement sheathe.
The above methods are used in a large scale in Daqing Oil Field (about 1,000 wells per year since 1980). The results are quite good, cumulative incremental oil production by these fracturing methods is about 60 million tons; the fracturing costs per ton of incremental oil is about $ 6.
Introduction Daqing Oil Field is a multi-zone heterogeneous reservoir. A well drilled vertically downwards encounters on the average about 150 to 200 pay zones in an interval of about 200 m. (Figure 1), which means the average pay zone and impermeable zone each are only 0.5 to 0.7 m. The thickness of the pay zones varies from 0.2 m to 15 m; the permeability varies from 10 md to 1,500 md. The injection and production rate of high permeability zones is very high and very low for low permeability zones. If all the low permeability zones could be fractured to increase their injectivity and productivity, then a more uniform flood front and production rate between high and low permeability zones could be obtained and thus higher recovery.
However, in Daqing, on the average 30 low permeability zones in a well need to be fractured. With commonly used methods, only a few zones could be fractured in one day, to fracture 30 zones would need a large amount of service work and a long period of time, which would in turn greatly increase the cost. With many impermeable barriers being very thin, to prevent channeling, many zones that need to cannot be fractured.
To solve the above questions, 3 techniques were modified and combined into a new technique. These 3 techniques are: multiple-zone fracturing using packers, limited-entry fracturing and pressure-balanced fracturing. These techniques and how they are integrated into one and the field results are explained in this paper.
Important Aspects of the Technique Multiple-Zone Fracturing Multiple-zone fracturing technique has been used in Daqing since the early 1970s. Multiple (3 to 4) inflatable packers and multiple selective opened mandrels are run in tandem in the well (Figure 2) opposite the zones that need to be fractured. The first mandrel and first 2 packers (from bottom up) are open (or can be inflated) when run in the well and on start of the fracturing job. The zone opposite the mandrel between the upper and lower packer is first fractured. Then a ball is dropped in to trip open the second mandrel, block the lower mandrel and open the 3rd packer. On immediately re-starting the fracturing job, the second zone is fractured, and so on until 3 or 4 zones are fractured. Between fracturing jobs, the tubing string is not pulled or set down, only a tripping ball is dropped down, greatly shortening the waiting time between fracturing jobs. The packers are inflatable ones, after the fracturing job, they automatically retract to their original outer diameter.
Sometimes there is sand settled on top of the packers that prevent pulling the tubing string. To avoid this problem, all the mandrels are joined directly on top of the lower packer, when fracturing, the high volume and velocity of the fracturing fluid will form strong turbulence, which will carry away most of the fracturing sand (proppant), little will settle down on top of the packer. These measures and features (inflatable packers, turbulent flow and direct joining of the mandrel to the lower packer) make it relatively easy to pull the tubing string out of the well after fracturing. This type of fracturing has been performed 2,000 to 2,500 times per year in Daqing for 30 years, which shows that it is quite a reliable technique.
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