Initiation of injection planes in a well. A method of forming at least one generally planar inclusion in a subterranean formation includes the steps of: expanding a wellbore in the formation by injecting a material into an annulus positioned between the wellbore and a casing lining the wellbore; increasing compressive stress in the formation as a result of the expanding step; and then injecting a fluid into the formation, thereby forming the inclusion in a direction of the increased compressive stress. Another method includes the steps of: expanding a wellbore in the formation by injecting a material into an annulus positioned between the wellbore and a casing lining the wellbore; reducing stress in the formation in a tangential direction relative to the wellbore; and then injecting a fluid into the formation, thereby forming the inclusion in a direction normal to the reduced tangential stress.