This paper provides theoretical analyses of hydrocarbon reserve that can be added by Underbalanced drilling (UBD). For simplicity, the analyses are based on the characteristics of solution-gas-drive reservoirs and deepwater reservoirs. The concept is also applicable to other types of reservoirs.

This paper addresses UBD-added hydrocarbon reserve in two aspects: a) minimizing formation damage so that lower reservoir pressure can be achieved at the same level of abandonment production rate at the end of field development; and b) producing hydrocarbons from those zones in deepwater that are not accessible with overbalanced drilling (OBD) or nearbalanced drilling (NBD).

Our analyses indicate that UBD can add reserve very significantly. An example calculation shows that UBD can increase oil reserves by 7% for a skin factor reduction from 10 to 0 in a low pressure, thin oil reservoir. The same example also indicates that the field development time can be shortened by 38% if the field is developed with UBD. Replacing the high-skin OBD or NBD wells with UBD wells in a depleted field can increase reserves and prolong the economic life of the field. Our analyses of formation pore and fracturing pressure gradient curves from a deepwater field confirm the results of previous studies in that UBD can add deepwater hydrocarbon reserve very significantly.

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