In the broad sense, unbalanced-drilling is not only an effective technology of exploiting low pressure and low permeability formation and exhausted formation, but also a drilling and completion technique for finding and exploiting formations which are different pressure systems. Unbalanced-drilling pressure difference can be obtained by use of single-phase fluid, aerated fluid, tiny glass bead, and tiny plastic bead. If unbalanced-drilling pressure difference is obtained by different ways, drilling cost will be much variable. Under allowable formation pressure, single-phase and low-density fluid was injected from surface. When it was circulated in the annular space, it can stimulate formation fluid into annular space so that we can obtain the designed unbalanced-drilling pressure difference, working-on process, and theory of annular pressure system. How to select single-phase fluid injected from surface, and to obtain of initial unbalanced-drilling pressure difference, calculation and control technique of characteristic factor of annular multiphase fluid dynamics, control and calculation of injection quantity of formation fluid, control technique of unbalanced-drilling casing pressure and standpipe pressure, were discussed and analyzed in this study in detail. This technology has been applied to Northern China Oil Field successfully.


During drilling processing, when the effective hydrostatic pressure is less than formation pressure, it will cause formation fluids entering wellbore and thus can be circulated to surface. This technology is called Unbalanced-drilling (UBD). This technology is an effective method for exploiting low pressure and low permeability reservoir and protecting reservoir. For normal pressure or high-pressure formation, using UBD will have the advantages of exploiting formation and improving production rate. There are three types of gas injection of UBD.

Standard Drill Stem Injection

Using standard drill stem configuration, gas and liquid mixed at the surface, two-phase flow through drill stem and bit then entering the annular space to obtain unbalanced pressure difference. Using this type of injection, MWD cannot be applied during UBD processing.

Parasite Pipe Injection

Installing a certain size injection pipe inside of the middle casing pipe, liquids is pumped into the pipe from stem and then mixed with the gas which is from parasite pipe in order to obtain the unbalanced pressure difference. This type of injection can keep the annular space to be aerated continually. Because inside of drill stem is pure liquid phase, MWD is applicable. But this injection requires a lot of initial work, when well depth increasing, the formation below the injection point may be polluted by single-phase fluid.

Concentric Pipe Injection

After well cemented with middle casing pipe, concentric pipe is put down, gas will be injected from concentric pipe and middle casing pipe annular area. Liquids injected from stem will mix with gas at bottom hole to obtain unbalanced pressure difference. Basically, this type of injection has the same advantage and disadvantage like parasite pipe injection.

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