Due to geological characteristics and low permeability of Western Siberia hydrocarbon reservoirs, hydraulic fracturing is the most effective method for improving hydrocarbon production. During recent years the massive fracturing treatments in this area have led to a reduction of well candidates for this type of production enhancement technique. Currently the majority of selected wells for fracture stimulation are considered "risky" because of significant water cut increase after stimulation. The question arises - How to reduce water production while enhancing oil production?
This paper reviews 26 case histories exemplifying the use of Relatively Permeability Modifiers (RPM) in hydraulic fracturing to enhance oil recovery and control water production. Also results of laboratory studies are included to support the RPM-fracturing concept and placement techniques.
The Western Siberian Oil and Gas Basin is second in the world in hydrocarbon reserves after the Persian Gulf Basin. The proven recoverable Western Siberia Oil reserves are an estimated 13.8 billion tons. Total area of the Western Siberia Basin is 3.5 millions km2 (for example the total area of France and Germany together are 0.9 millions km2). Hydrocarbon production from Western Siberia makes up 70% of the oil and 90% of the gas production of Russia (Figure 1). The main hydrocarbon reserves are concentrated in relatively young terrigene sediments, Mesozoic and Cainozoic formations, which formed in the last 200 million years. However, oil and gas production is also associated with Paleozoic formation. According to the Russian Ministry of Energy, Western Siberia will maintain a leadership position in oil and gas production up to 2020, although its contribution to Russia oil will drop to 55–58% (from 70%) by that time. The majority of oilfields are confined within the central part of Western Siberia -Khanty-Mansy County, while the northern part is predominantly gas and condensate production - Yamal-Nenetskiy County.
Development of Western Siberia oilfields began back in 1950–1960 with the discovery in 1953 of the first oilfield in the Berezovskiy region in lower reach Ob River. To date, there have been about 626 oilfields discovered. The majority of these fields are mature reservoirs with very high water production. In the largest oilfields that govern total oil production, about 60% of total reserves have already been extracted. Water cut has increased to 80–85% (Figure 1, Table 1) 1. In reality this means that with every 1 ton of oil produced, 5.5 tons of water are produced - with a required 7 tons of water injected into the formation (to maintain formation pressure) 1. If it is assumed that water cut increase will be 1% per year, and then in 5–8 years water cut in Khanty County will reach 90% and oil production cost will double (Figure 2) 1. Significant water cut affect not only oil cost (and thus net price) but it is a primary factor in the high rate of pipeline blowouts which have a negative environmental effect on Western Siberia. The major cause of pipeline blowouts (98%) is pipeline corrosion. High fluid velocity and water content in pipelines reduce pipe lifetime by half.
Nevertheless, and despite the development of major fields since the middle of the last century, hydrocarbon production is increasing every year(Figure 1). For example, it is expected that Khanty County will produce 289–302 millions tons in 2007 and 294–310 millions tons in 2008. High production levels will be achieved through newly drilled wells (Priobskoe field) as well as applications of production enhancement technologies, where hydraulic fracturing occupies the greatest part.
Traditionally wells with a risk of water production increase were not considered as fracturing candidates. Even a few years ago these wells made up the major part of low productivity wells (rate <10m3/day). In order for fracture stimulation to be a valid option for these marginal reserves it is required to perform water control and fracturing simultaneously5.