Multihole Drilling

a system of boreholes having branches in the form of sharply curved auxiliary shafts from a main well shaft within a producing formation (of oil, gas, and so on). Multihole drilling is used for the production of oil and gas, and also in prospecting for hard minerals. Multihole drilling is expedient in relatively stable producing formations with a thickness of 20 m or more—for example, in monolithic oil-bearing sandstones, limestones, and dolomites or those with intercalated clay and shale beds at depths of 1,500–2,500 m, in the absence of both a gas dome and abnormally high formation pressures. Multihole drilling cuts down the number of conventional wells by increasing the drainage surface of the producing well. In the USSR, powerful curved turbodrills and electrodrills have been designed to drill such wells, methods and facilities have been created for the forced insertion of geophysical instruments, and technological methods and instruments have been developed for drilling and casing the branches.

Multihole drilling was first performed in the USA (Texas, 1930). The branches were drilled by specially designed ball-and-socket and flexible drill pipes driven from the surface. The inadequate strength of the pipes and the complex technology restricted the length of the auxiliary shafts to 30 m. A new principle, the use of bottom-hole motors (turbodrills and electro-drills), was proposed by A. M. Grigorian, V. A. Bragin, and K. A. Tsarevich and first realized in the USSR in 1948, when the first multiple completion wells were drilled by this method. This made possible the use of conventional high-strength drill pipes and an increase in the length of the auxiliary shafts to several hundred meters.

Wells with five to ten branching shafts, each 150–300 m long, are used in the oil-producing regions of the USSR. For this reason, the flow of oil is several times greater than in conventional wells (the total increase in well-drilling cost was 30–80 percent). An important advantage of such wells over conventional wells is the possibility of more complete extraction of oil from the fields. Three multiple completion wells with horizontal shafts that were drilled in 1957 near the city of Borislav yielded 28 to 15 tons of oil per day in a depleted field, which had begun producing in 1914 and had daily standard production rates of not more than 0.1–2.0 tons. With the use of multihole drilling methods, wells may be drilled in strictly specified directions, which is useful for eliminating uncontrolled gas and oil flow (gusher) by drilling special wells to connect with the shaft of the gushing well.

An achievement in multihole drilling was a test well in the Markovo oil field (Irkutsk Oblast) in 1968 with a horizontal shaft length of 630 m and a depth of 2,250 m. The well was drilled at the same rate as a conventional well and was only 23 percent more expensive. The great length of the horizontal portions in multihole drilling made possible the drilling of giant wells, with a large field area conformance and high oil yield rates (this is especially important for the development of almost inaccessible fields, such as in the development of shelves, in swampy areas, and within the boundaries of cities).

In the USSR as of 1974, several dozen oil wells had been drilled successfully by multihole drilling; high-speed multihole drilling of deep horizontal wells of great length (several kilometers) is being developed and tested.