Subsidence-Induced Well Failure

Abstract

Summary This paper describes the influence of reservoir compaction and surface subsidence on well casing damage, including compression, buckling, shear, and bending failure mechanisms. Casing damage from shear stresses within the overburden is identified as a primary failure mode for many reservoir conditions. This is consistent with several field observations. Introduction The weight of overburden sediments above a producing formation is supported partially by the rock matrix and partially by the pressurized fluid within the rock pore space. When fluid pressure pressurized fluid within the rock pore space. When fluid pressure is reduced, more of the load is transferred to the rock matrix, and the formation is compacted. This subsurface compaction also can sometimes produce surface subsidence, with significant displacements in both the vertical and horizontal directions. When significant subsidence occurs, it can cause serious financial and environmental consequences. For example, during 1935-65 the surface above the Wilmington oil field in California subsided almost 33 ft [10 m]. The costs to elevate, protect, and repair various facilities exceeded $100 million by 1962. The subsidence caused casing failure in hundreds of wells. By 1987 about 13 ft [4 m] of seafloor subsidence was measured above the Ekofisk gas field in the North Sea. The entire project to raise platforms and to protect storage facilities at project to raise platforms and to protect storage facilities at this important offshore complex exceeded $400 million. Casing failures have occurred in more than two-thirds of the wells. Several researchers described the effects of compaction and subsidence on well casing compression and buckling failure, including Yudovich et al., Wilson et al., and Bradley and Chia. However, these studies have n(m considered well failure associated with shear and bending deformations. The purpose of this paper is to analyze a wider range of potential well-failure mechanisms, to describe the locations for these failures, and to compare analytical and numerical model results with field observations. Subsidence-induced Reservoir Displacements When the lateral dimensions of the reservoir are very large compared to its vertical thickness, most of the subsurface compression associated with fluid withdrawal can be assumed to occur in the vertical direction. The vertical compaction, h, of a reservoir may be described with the following approximate expression. (1) where h = original reservoir thickness, Cm = uniaxial compaction coefficient of the material, and p = change in reservoir pressure. Subsidence refers to the surface displacement that may be produced by subsurface reservoir compaction. The degree to which produced by subsurface reservoir compaction. The degree to which this subsurface compaction is transferred to surface displacement depends on the areal extent of the reservoir and the depth of burial. A large, shallow reservoir, for example, will produce much more extensive surface subsidence than a small, deeply buried one with identical material properties and pressure drawdown. Analytical Solution. Surface displacements resulting from subsurface compaction may be obtained by applying the nucleus-of-strain concept from continuum mechanics described by Geertsma. The volumetric strain at a point, caused by a local reduction in pore pressure, is treated as a center of compression in an elastic halfspace. This produces a corresponding displacement at the free surface. The total surface deformation caused by a varying pressure reduction within an arbitrarily shaped reservoir is given by integrating the contribution of all these compression points over the reservoir volume as follows.