Measuring behind casing hydraulic conductivity between reservoir layers

In pressure transient testing a first formation layer of a subsurface reservoir to evaluate productive capability of the first formation layer, a method of forming a measure of the rate of flow from a second formation layer through well casing cement in an annulus between a well casing and borehole walls of the subsurface reservoir between the first and second formation layers comprising:

• conducting a pressure transient test of the first formation layer to obtain pressure test data for the first formation layer, the pressure transient test of the first formation layer comprising physically blocking casing flow of fluid from the second formation layer, the pressure test data for the first formation layer comprising time and well pressure measurements acquired during the pressure transient test of the first formation layer;
• obtaining rock, fluid and geometric properties of the plurality of the formation layers;
• determining, from the pressure test data for the first formation layer, a first test wellbore flowing pressure at a first time during the pressure transient test of the first formation layer;
• determining, using the pressure data for the first formation layer, a first test pressure derivative of wellbore flowing pressure corresponding to a pressure derivative of the wellbore flowing pressure during the pressure transient test of the first formation layer;
• determining, using the rock, fluid and geometric properties of the plurality of the formation layers, a first estimated behind casing hydraulic conductivity corresponding to behind the casing flow of fluid to the first formation layer during the pressure transient test of the first formation layer;
• determining, based on the estimated behind casing hydraulic conductivity, a first postulated wellbore flowing pressure corresponding to a wellbore flowing pressure of the second formation layer;
• determining, based on the estimated behind casing hydraulic conductivity, a postulated pressure derivative;
• determining that the first postulated wellbore flowing pressure matches the first test wellbore flowing pressure and the postulated pressure derivative matches the first test pressure derivative of wellbore flowing pressure;
• and
• in response to determining that the first postulated wellbore flowing pressure matches the first test wellbore flowing pressure and the postulated pressure derivative matches the first test pressure derivative of wellbore flowing pressure:
• storing the estimated behind casing hydraulic conductivity as a measure of the capacity of the cement in the annulus between the well casing and borehole walls between the first and second formation layers to transmit fluid to the first formation layer during the pressure transient test of the first formation layer;
• and
• forming the measure of the rate of flow from the second formation layer through well casing cement in the annulus between the well casing and the borehole walls of the subsurface reservoir between the first and second formation layers based on the measure of the capacity of the cement in the annulus between the well casing and borehole walls of the subsurface reservoir to transmit fluid to the first formation layer during the pressure transient test of the first formation layer.