Conventional methods for gas bearing zone identification are based on the comparison of the neutron log porosity to the density log porosity. In low porosity, varying matrix formations the gas detection is often difficult.
The method proposed in this paper is a new method which identifies gas bearing zones by using a Difference Ratio of Equivalent Elastic Modulus of formation rock ( EDR ). The ratio is calculated by the combination of sonic log, density log and neutron log. The formula used to calculate the ratio is as follows:
EDR = (Ecw - Ec) / Ec
in which , Ec is the equivalent elastic modulus of the formation of interest in dyne/cm2, Ecw is the equivalent elastic modulus of the same formation of interest 100% water saturated in dyne/cm2.
When Ecw is greater than Ec, EDR is greater than zero and this indicates gas bearing zone. On the contrary, when Ecw is smaller than Ec, EDR is smaller than zero and this indicates a non-gas bearing zone.
The method has been used to interpret some sand and carbonate gas reservoirs of China. The results presented in this paper show that higher resolving presented in this paper show that higher resolving power of gas bearing zones, better interpretation power of gas bearing zones, better interpretation accuracy, and "quick look" estimates can be readily achieved.
Equivalent Elastic Modulus
The velocity of elastic wave propagated in formation rock depends on the elastic modulus and bulk density of the rock.
According to the theory of elastic dynamics, the velocity of compressional wave propagated in the rock is:
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