Patent ID: 6694262
Filing Date: 2004-02-17
Classification: G01V

Abstract:
A method for identifying fractures in coal seams using measurements of drilling fluid resistivity, drilling fluid loss, surface area of the filter used to measure drilling fluid loss, well bore hole diameter, drill bit size, thickness of the coal seam, measurements of shallow, medium and deep resistivities of the coal seams taken from geophysical logs, said method comprising:a) a first screening step selecting only geophysical logs exhibiting resistivity of drilling mud greater than 1.0 ohm-meter; b) selecting at least one geophysical log using a second screening step against the geophysical logs selected in step a) wherein a bore hole caliper size through a target zone of each said selected geophysical is less than 200% of bit size; wherein for each selected geophysical log, the following steps are performed; c) calculating a first ratio from a value of medium resistivity derived from the data of said selected geophysical log divided by a value of deep resistivity derived from the data of said selected geophysical log; d) calculating a second ratio from a value of shallow resistivity derived from the data of said selected geophysical log divided by said value of deep resistivity; e) determining a depth of invasion from said first and second ratios; f) calculating a fluid affected volume from the square of the sum of a radius of a well bore bit size plus said depth of invasion, the square of the sum being multiplied by pi and by a thickness of said target zone; g) calculating a volume of invaded coal from said fluid affected volume less the product of the square of said radius of a well bore bit size, times pi, times said thickness of said target zone; h) calculating volume of invading fluid from a drilling fluid loss measurement of said selected geophysical log well log corrected for time and filter surface area; j) calculating a fracture porosity as the ratio of said volume of invading fluid divided by said volume of invaded coal.