A computer implemented method of determining a measure of inter-reservoir crossflow rate to a tested layer of a subsurface reservoir through leaks in cement sheaths of offset wells (12, 42) in the tested layer during a pressure transient test of the tested layer, the computer implemented method comprising the steps of: obtaining a test measure of well pressure during the pressure transient test on the tested layer; determining a test pressure derivative of the test well pressure at sampled instants of measurement during the pressure transient test of the tested layer; receiving estimated values of hydraulic conductivity of the cement sheaths of the offset wells; determining a model wellbore flowing pressure at the layer based on the test measure of well pressure and the estimated values of hydraulic conductivity of the cement sheaths of the offset wells; determining a model pressure derivative based on the test pressure and the estimated values of hydraulic conductivity of the cement sheaths of the offset wells; determining a model inter-reservoir crossflow rates in the offset wells to the tested layer based on the estimated values of hydraulic conductivity of the cement sheaths of the offset wells; comparing the model wellbore flowing pressure with the test measure of well pressure; comparing the model pressure derivative with the test pressure derivative; and if the estimated measures and test measures match within a preset tolerance, storing the estimated values of hydraulic conductivity of the cement sheaths of the offset wells, the model inter-reservoir crossflow to the tested layer, the model wellbore flowing pressure, and the model pressure derivative; and, if not, adjusting the values of hydraulic conductivity of the cement sheaths of the offset wells, and iteratively repeating the steps of determining a model wellbore flow pressure, determining a model pressure derivative, determining a model inter-reservoir crossflow rates in the offset wells to the tested layer and comparing based on the adjusted values of hydraulic conductivity of the cement sheaths of the offset wells until the estimated measures and test measures match within the preset tolerance. obtaining a test measure of well pressure during the pressure transient test on the tested layer; determining a test pressure derivative of the test well pressure at sampled instants of measurement during the pressure transient test of the tested layer; receiving estimated values of hydraulic conductivity of the cement sheaths of the offset wells; determining a model wellbore flowing pressure at the layer based on the test measure of well pressure and the estimated values of hydraulic conductivity of the cement sheaths of the offset wells; determining a model pressure derivative based on the test pressure and the estimated values of hydraulic conductivity of the cement sheaths of the offset wells; determining a model inter-reservoir crossflow rates in the offset wells to the tested layer based on the estimated values of hydraulic conductivity of the cement sheaths of the offset wells; comparing the model wellbore flowing pressure with the test measure of well pressure; comparing the model pressure derivative with the test pressure derivative; and if the estimated measures and test measures match within a preset tolerance, storing the estimated values of hydraulic conductivity of the cement sheaths of the offset wells, the model inter-reservoir crossflow to the tested layer, the model wellbore flowing pressure, and the model pressure derivative; and, if not, adjusting the values of hydraulic conductivity of the cement sheaths of the offset wells, and iteratively repeating the steps of determining a model wellbore flow pressure, determining a model pressure derivative, determining a model inter-reservoir crossflow rates in the offset wells to the tested layer and comparing based on the adjusted values of hydraulic conductivity of the cement sheaths of the offset wells until the estimated measures and test measures match within the preset tolerance.