Casing Strain Resulting From Thawing of Prudhoe Bay Permafrost

Abstract

A compact five-spot well pattern was completed through permafrost at Prudhoe Bay and circulated with hot fluid, creating thaw zones. Compaction Prudhoe Bay and circulated with hot fluid, creating thaw zones. Compaction and deformation of soil in these zones led to casing strain. Analysis determined that casing strains were caused by large differences in compactibility of soil types. Introduction The development of petroleum reserves in the arctic region of Alaska requires new petroleum engineering technology to deal with permafrost problems. A conventional well drilled through permafrost is subjected to the hazards of external freezeback pressures, internal freezeback pressures, and thaw consolidation (compaction). These three major concerns have been studied extensively by operators in the Alaskan arctic. Solutions to the problems associated with the freezeback of shut-in wells have been published in the past four years. Thawing of permafrost around a wellbore also can lead to casing damage. When ice melts, it is reduced approximately 9 percent in volume. Stresses carried through the ice phase of in-situ permafrost tend to transfer to the soil matrix as the ice melts and the pore pressure diminishes because of this reduction. The soil tends to compact when subjected to a higher stress level. The degree of compaction depends on the type of soil, the stress state in the frozen permafrost, the pore pressure after thaw, and the size of the thawed region. Although most compaction is accounted for by lateral movement of the surrounding frozen permafrost, some vertical consolidation or subsidence also can be expected. This vertical consolidation or movement can damage the casing strings throughout the permafrost section. The original field rules for Prudhoe Bay development required that a producing well be protected from subsidence damage by refrigeration, insulation, or other means. To understand thaw consolidation in the thick permafrost at Prudhoe Bay, BP Alaska Inc. conducted a permafrost at Prudhoe Bay, BP Alaska Inc. conducted a field test to thaw a limited region surrounding a well. The amount of thaw in this field test was equivalent to that created around a heavily insulated producing well. Both casing strain and formation movement were detected with a moderate degree of uncertainty in the field data. One solution to the internal freezeback problem is to replace water-base mud with a gelled, weighted oil-base fluid. The thermal-insulating properties of this fluid are quite good. However, a producing well insulated in this manner would develop a larger thaw radius during its operating life than was created in the BP Alaska field test. Petroleum engineers designing casing programs for field Petroleum engineers designing casing programs for field development were uncertain of the effect of this large thaw zone on casing integrity in a producing well. To assess the significance of the consolidation process for a gelled-oil-insulated completion, a major field test at Prudhoe Bay was conducted. The test site was located in Prudhoe Bay was conducted. The test site was located in Section 11, T10N, R15E, in the center of the Prudhoe Bay field and adjacent to a BP Alaska well where numerous cores were taken in the permafrost. Core analysis provided a complete lithology of the permafrost. The provided a complete lithology of the permafrost. The goal of the field test was to observe casing integrity and to measure the strain of a casing string surrounded by a thaw zone equivalent in size to a zone created by a well after 20 year's production. This paper describes the field test, operational aspects, and results. JPT P. 468