NOVELTY - The method involves conveying a carrier (5) through a borehole (2). NMR measurement is performed on a volume of interest (9) in an earth formation (4) to provide a relaxation time constant using an NMR tool (10) arranged at the carrier. Information describing solid particles in pores is received. Effect on the measured relaxation time constant is quantified by using a processor due to invasion of solid particles using the received information.    USE - Method for estimating effect on NMR measurements of invasion of solid particles i.e. fine carbonate particles, into pores of an earth formation penetrated by a borehole for various applications such as hydrocarbon production, geothermal production, and carbon dioxide sequestration applications.    ADVANTAGE - The method enables quantifying effects on NMR measurements, so that output of an NMR tool can be corrected or uncorrected NMR tool output can be compensated for during interpretation by an analyst. The method enables tuning frequency of transmitted radio-frequency energy to investigate the volumes of interest with different diameters around the NMR tool since a linear relationship exists between hydrogen proton resonance frequency and strength of a static magnetic field, thus improving accuracy in determining other formation properties from distributions of measured transverse and/or longitudinal relaxation time constants.    DETAILED DESCRIPTION - The information comprises size of each of particles, surface relaxivity of each of particles, and amount of space where particles occupy in the pores. The relaxation time constant is a transverse relaxation time constant and a longitudinal relaxation time constant. An INDEPENDENT CLAIM is also included for an apparatus for estimating effect on NMR measurements of invasion of solid particles into pores of an earth formation penetrated by a borehole.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a NMR tool arranged in a borehole penetrating an earth.    Borehole (2)    Earth formation (4)    Carrier (5)    Interest (9)    NMR tool (10)