Influence of Inelastic Rock Properties on The Stability of a Well-Bore

Abstract

This paper was prepared for the 45th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Houston Tex., Oct. 4–7, 1970. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after, publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. The rock around a well-bore is subjected to a stress field which can-be considered to be composed of four parts:the virgin stress in the rockthe change in this stress field caused by creating the openingthe fluid pressure inside the wellthe pressure caused by the flow of the fluid towards the hole. An additional component can also be developed if oil withdrawal causes compaction of the oil bearing formations, leading to subsidence of the overlying rock mass. Whether the well will be stable or not depends upon the behaviour of the rock in this stress field. The stress distribution around a circular well-bore has been calculated by Paslay and Cheatham for an elastic system with radial symmetry and steady state fluid flow into the well-bore. They showed in particular that the rock strength required around the hole can be influenced very significantly by a change in permeability of the rock in the immediate surroundings of the hole. permeability of the rock in the immediate surroundings of the hole. Rock failure is most likely to occur in this zone. Once the rock strength is exceeded it is necessary to include the behaviour of the broken zone in the analysis. As an initial step we can consider the situation of a vertical cylindrical hole with internal fluid pressure Pi in a rock mass where the virgin stress in the horizontal plane is P in all directions. The radius to which the rock will break* can be calculated from the equilibrium equation:and the expression for the behaviour of the broken rock :