Darrell Hoyer
Union Texas Petroleum Holdings, Inc.
ABSTRACT
A technique to quantify natural fracture porosity in coalbeds using the Dual Laterolog tool has been developed. The magnitude of fracture porosity has a direct relationship to reservoir permeability and the viability of a coalbed methane recovery project. While resistivity logs are often recorded in wells that encounter coalseams, analysis is typically limited to inferences of permeability.
Dual Laterologs have been used qualitatively for fracture detection in highly resistive carbonates and other low porosity formations for many years. Recent modelling of the deep and shallow Laterolog curves in fractured formations with high matrix resistivity illustrates the effects of fracture frequency, length, aperture magnitude and fluid conductivity on the Dual Laterolog curves. The Dual Laterolog response is applied to evaluate natural fracturing in coalseams.
Matrix blocks in coal have micropores which store methane molecules but are too small to contain water and other liquids. The lack of a continuous phase of water in coal matrix results in highly resistive matrix blocks. The natural fractures inherent to most coals are generally saturated with relatively fresh water which are conductive paths for induced electrical currents. Fracture porosity is a combination of fracture frequency and aperture magnitude. Calculation of fracture porosity is a function of deep and shallow Laterolog response and the conductivity of the invading fluid. Saline drilling fluids are used to invade the fractures and enhance the response of the deep and shallow Laterolog curves.
The microresistivity pad devices, which are often run concurrently with Dual Laterologs, display low resistivity anomalies in zones that are fractured. Thin bed resolution of microresistivity devices is superior to the Dual Laterolog tools. A correlation between Dual Laterolog fracture porosity and microresistivity is made in thick coalseams which enables fracture porosity estimates to be made in thin coalseams.
Examples are presented of Dual Laterologs with microresistivity devices from wells using saline drilling fluids in the Fruitland Coal, San Juan Basin. The examples illustrate variation in the fracture porosity of various coal seams within a single wellbore and between coal seams in nearby wellbores. Slight changes in fracture porosity result in significant changes in coalbed permeability. Information gained about in situ fracture porosity from Dual Laterologs and microresistivity logs is cost effective and can be used to improve coalseam completions and reservoir computer models.