A method to determine a drilling fluid density limit for maintaining a target amount of rock failure in a drilling operation, comprising: obtaining a geomechanical model of a portion of a subterranean formation traversed by a target borehole (300, 304, 308, 313), wherein the geomechanical model comprises a formation pressure dependent on subterranean formation depth; the method being characterised in that it further comprises: generating, by a computer processor using a stress model (104, 135) dependent on the formation pressure, a depth of damage (140, 202, 204, 206, 302, 306) model of the target borehole (300, 304, 308, 313), wherein the depth of damage (140, 202, 204, 206, 302, 306) model describes a relationship between a drilling fluid pressure and a depth of damage (140, 202, 204, 206, 302, 306) resulting from the drilling fluid pressure, wherein the depth of damage (140, 202, 204, 206, 302, 306) represents a radial distance across a modeled amount of yielded rock (301, 305) surrounding the target borehole (300, 304, 308, 313), wherein the modeled amount of the yielded rock (301, 305) is calculated using the stress model (104, 135) according to a pre-determined failure criterion; and calculating, by the computer processor using the depth of damage model and the geomechanical model, a target drilling fluid density as a function of the subterranean formation depth, wherein the target drilling fluid density is a lower limit of the drilling fluid density that maintains the depth of damage (140, 202, 204, 206, 302, 306) at a target level for the target borehole (300, 304, 308, 313), wherein the target drilling fluid density corresponds to a depth gradient of the drilling fluid pressure, wherein the target borehole (300, 304, 308, 313) is drilled based at least on the target drilling fluid density. obtaining a geomechanical model of a portion of a subterranean formation traversed by a target borehole (300, 304, 308, 313), wherein the geomechanical model comprises a formation pressure dependent on subterranean formation depth; the method being characterised in that it further comprises: generating, by a computer processor using a stress model (104, 135) dependent on the formation pressure, a depth of damage (140, 202, 204, 206, 302, 306) model of the target borehole (300, 304, 308, 313), wherein the depth of damage (140, 202, 204, 206, 302, 306) model describes a relationship between a drilling fluid pressure and a depth of damage (140, 202, 204, 206, 302, 306) resulting from the drilling fluid pressure, wherein the depth of damage (140, 202, 204, 206, 302, 306) represents a radial distance across a modeled amount of yielded rock (301, 305) surrounding the target borehole (300, 304, 308, 313), wherein the modeled amount of the yielded rock (301, 305) is calculated using the stress model (104, 135) according to a pre-determined failure criterion; and calculating, by the computer processor using the depth of damage model and the geomechanical model, a target drilling fluid density as a function of the subterranean formation depth, wherein the target drilling fluid density is a lower limit of the drilling fluid density that maintains the depth of damage (140, 202, 204, 206, 302, 306) at a target level for the target borehole (300, 304, 308, 313), wherein the target drilling fluid density corresponds to a depth gradient of the drilling fluid pressure, wherein the target borehole (300, 304, 308, 313) is drilled based at least on the target drilling fluid density. generating, by a computer processor using a stress model (104, 135) dependent on the formation pressure, a depth of damage (140, 202, 204, 206, 302, 306) model of the target borehole (300, 304, 308, 313), wherein the depth of damage (140, 202, 204, 206, 302, 306) model describes a relationship between a drilling fluid pressure and a depth of damage (140, 202, 204, 206, 302, 306) resulting from the drilling fluid pressure, wherein the depth of damage (140, 202, 204, 206, 302, 306) represents a radial distance across a modeled amount of yielded rock (301, 305) surrounding the target borehole (300, 304, 308, 313), wherein the modeled amount of the yielded rock (301, 305) is calculated using the stress model (104, 135) according to a pre-determined failure criterion; and calculating, by the computer processor using the depth of damage model and the geomechanical model, a target drilling fluid density as a function of the subterranean formation depth, wherein the target drilling fluid density is a lower limit of the drilling fluid density that maintains the depth of damage (140, 202, 204, 206, 302, 306) at a target level for the target borehole (300, 304, 308, 313), wherein the target drilling fluid density corresponds to a depth gradient of the drilling fluid pressure, wherein the target borehole (300, 304, 308, 313) is drilled based at least on the target drilling fluid density.