Method for directly detecting free gases in stratum

Abstract

The invention relates to a method for detecting free gases in a stratum. In the method, an exploratory well and a development well are used. The method comprises the following steps of: carrying out well logging in the exploratory well and the development wall to obtain well logging data; then calculating the Lame constant and the shear modulus of an underground rock stratum according to the well logging data; taking the Lame constant, the shear modulus and the density as hydrocarbon detection factors and determining threshold values of the hydrocarbon detection factors; and directly detecting gases, such as free natural gas, carbon dioxide and the like with economic values, in the stratum, by using the hydrocarbon detection factors and the threshold values thereof. The method disclosed by the invention is applied to certain coal bed gas exploration area and used for detecting superficial free natural gas reservoir stratums which interdependent and coexistent with coal bed gas. The method disclosed by the invention is used for detecting superficial free gas reservoirs by using the well logging data observed in the well and has the advantages of low observation environment noise, reliable data and easiness in implementation. When the invention is implemented, field engineering cost is not increased generally; even the field engineering cost needs to the increased, the increased cost is the acquisition cost of transverse wave speed at most. Therefore the invention has the advantages of low cost.

Description

technical field [0001] The invention relates to a method for directly detecting free gas in formations. Specifically, the invention relates to a method for obtaining logging data by using exploration wells and development wells, and using these logging data to calculate the Lame constant and shear rate of underground strata. Modulus, using Lame constant, shear modulus, and density as hydrocarbon detection factors, determining their threshold values, using these hydrocarbon detection factors and their threshold values ​​to directly detect free natural gas, carbon dioxide and other gases in the formation Methods. Background technique [0002] Natural gas, carbon dioxide and other gases in the formation are important natural resources, natural gas is an important energy source, and carbon dioxide is an important chemical raw material. [0003] The gas in the formation may be dissociated or non-dissociated. Free gas refers to the gas that can flow freely between the interconne...

AI Technical Summary

Problems solved by technology

The reservoirs of this type of natural gas reservoirs are usually tight sandstones with small porosity and low gas saturation, so that the existing technologies for exploring conventional natural gas reservoirs are often not suitable for exploring such natural gas reservoirs

For example, because the sandstone of this type of natural gas reservoir is tight and the porosity is small, even if the gas completely fills the pores of the sandstone, there is little difference between the elastic characteristics of the gas-saturated tight sandstone and the tight sandstone without gas. It is not enough to cause obvious changes in seismic wave reflection characteristics; and because of shallow burial and serious surface noise interference, therefore, when the existing seismic exploration technology (including special seismic technology for exploring conventional natural gas reservoirs) is used to explore shallow free gas reservoirs, poor effect

For another example, due to the tight sandstone, small porosity, and low gas saturation of this type of natural gas reservoir, and the plugging effect of the mud cake formed on the borehole wall by drilling mud, the interpretation of conventional logging data is very important for this type of natural gas reservoir. is less effective

Although countries rich in coalbed methane resources (such as the United States, Canada, Australia, etc.) have been committed to the exploration and development of shallow free natural gas reservoirs, there has been little progress so far due to the absence of suitable detection technologies for this type of natural gas reservoirs.

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