Using Flow Induced Stresses for Steering of Injection Fractures

Abstract

Abstract Both horizontal and vertical rock stress magnitudes are substantially affected by pore pressure gradients generated by production from and water injection into low permeable chalk fields. This is confirmed by solid mechanics stress analyses and by observing fracture propagation in developed fields. Stress changes generated around horizontal injectors placed centrally between two parallel horizontal producers are considered in detail in this paper. It is shown that overburden and sideburden stresses may be altered and controlled to confine propagation of fracturing along the length of a central openhole horizontal injection well. It thereby becomes possible to establish a line drive development based on injection of water at fracturing conditions at reduced risk of premature water breakthrough (patent pending). 1. Introduction Mærsk Olie og Gas AS operates oil and gas fields in the Danish sector of the North Sea on behalf of DUC (Dansk Undergrunds Consortium). The Dan field, which came on production in 1972, was the first oil field to come on production. Experience has been gathered over three decades of production development of the tight chalk of (0.5–2.0 mD permeability range) Maastrichtian (Upper Cretaceous) and Danian (Lower Paleocene) age and has been associated with significant improvements to horizontal well and water injection technology. The conditions are favourable for displacing the oil by water because the injected water is less mobile than the oil. A means of providing sufficient leak-off area for water injection could be adoption of water injection at fracture propagation pressure. Induced water injection fractures in the Dan field can be large, some of more than 1,800 m (6,000 ft) of length have been created (Ovens et al. [1], Larsen et al. [2]). A few of these fractures intersect horizontal production wells resulting in water breakthrough, which has required isolation of production well partitions. To a large extent water injection fractures have been attempted aligned with the horizontal producers. Due to the low permeability of the chalk, a close well spacing is typically required. Traditionally, the fracture propagation direction has been assumed to be governed by a combination of local heterogeneities and the orientation of principal stresses as determined by the regional structural history. It shall be demonstrated here, that indeed orientation of stresses play a significant role in steering the fracture propagation. By controlling production and water injection induced stresses, an active steering of the fracture propagation direction may be achieved (patent pending). Based on Terzaghi's effective stress concept (Terzaghi [3]), the criterion for achieving (tensile) fracturing of the reservoir rock is that the effective rock stress becomes equal to the tensile strength of the rock. This criterion has been demonstrated experimentally (Jaeger [4]) and the theoretical basis has been treated by several authors amongst others Murrell (Murrell [5], see also comment by Jaeger and Cook [6] p. 225). Following a fracture mechanics approach Bruno and Nakagawa [7] derived the strain energy release rate (Griffith [8]) for fluid infiltrated porous rock. According to their analysis, the local pore pressure has an explicit, albeit minute, influence on the strain energy release rate for tensile fracturing. In response Detournay and Boone [9] argued against a direct contribution from the local pore pressure to the strain energy release rate (see also reply by Bruno [10]). Here, it is noted that the contribution from pore pressure is a result of the rock grain compressibility. For cases where the grain compressibility can be ignored (such as the present) the strain energy release rate can be derived directly from the compliance of the rock and the state of Terzaghi's effective stresses.