Effect of Natural Repressurization of Thawed Permafrost on Strain of Surface Casing for Prudhoe Bay Infill Wells

Abstract

Summary When hot oil is produced through closely spaced wells that penetrate a permafrost zone, large thawed regions can be created. Thawing initially produces a sudden, large decrease in permafrost pore pressure; this results in compaction of soil and strain in the casing. Induced pipe strains initially increase with time as the thaw front expands. Later, water flows into the thawed region from above and below the permafrost and repressures the pore space. Repressurization limits the induced casing strain, thus permitting the selection of adequate permafrost casing from a number of conventional casings. Introduction Heat lost from Prudhoe Bay wells warms the adjacent permafrost, causing it to thaw. As the ice in the pores of the permafrost melts, the volume decrease causes an immediate and severe drop in the pore pressure. The change in pore pressure causes the thawed permafrost to compact and subside, inducing compressive and tensile strains in the well casings.' For a given permafrost lithology, the magnitudes of the induced strains depend primarily on two parameters: the extent of thaw, and the pore pressure decrease. Infill drilling on existing Prudhoe Bay pads can result in substantial savings in gravel pad and flow line costs when compared with extending the pads or building new ones. When new wells are placed near existing ones, however, the thawed zones around individual wells can merge to form a much larger thawed region. The larger thawed region tends to increase casing strains and surface subsidence. These effects are offset, however, by the tendency for the fluid in the thawed permafrost to be naturally repressured by the influx of water from the surface and from below the permafrost. This paper describes new procedures which permit the calculation of water influx and the resulting pressures within the thawed zone. The water influx is significantly influenced by the extent (area) of thaw and the rate of growth of the thawed area. The resulting casing strains, in turn, depend on both the extent of thaw and the amount of water influx. The mechanical problem is thus intimately linked with the heat transfer problem. To evaluate the new procedures, pore pressures calculated using the new method were compared with pore pressures measured during a Prudhoe Bay 5-spot field test conducted in 1973-1974. Agreement between measured and calculated pressures was good. The mathematical method was then used to calculate expected behavior for two base cases of infill drilling. Studies quantified the sensitivity of repressurization to lithology variations. A substantial amount of repressurization was found to occur, especially near the top and bottom of the permafrost. Repressurization reduces casing strain, makes the selection of an acceptable surface casing tractable, and thus makes infill drilling feasible from the thaw and casing-strain point of view. THAWING OF PERMAFROST AROUND WELLS In ARCO's operating area at Prudhoe Bay, wells are typically drilled along pairs of parallel rows. The spacing between rows is approximately 175 feet. Currently, wells are drilled along each row on 120-foot spacings. After a first round of infill drilling, well spacing along each row would be reduced to 60 feet. After a second round, well spacing would be reduced to 30 feet. Extents of thaw were calculated for two well configurations, both for parallel rows with row-to-row spacings of 175 feet. One configuration used a well-to-well spacing of 60 feet; the other, 30 feet. The projections of thaw for these two configurations were calculated using a 2-dimensional model. P. 181^

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