Integrated experimental and computer modeling approach to understand permafrost thaw subsidence induced oil well instability for Alaska North Slope oil wells

Abstract

Hydrocarbon reservoirs in the Arctic region of Alaska have been developed by various oil and gas producers for several years. Most of them are overlain by massive layers of permafrost soils which extend to a thickness of up to 2300 feet. Production and injection wells in such regions have experienced design and operational challenges due to heat loss from the wellbore and subsequent thawing of the permafrost soils. Thawing is a phase change of ice to water resulting in volumetric reduction of the frozen soil due to pore space contraction and segregated ice thaw, causing a major problem of thaw subsidence. Thaw subsidence affects the stability of the well, causing buckling and structural distress along the length of the wellbore within the thaw susceptible permafrost zones, thus damaging the well casing. Two different experimental approaches, one-dimensional consolidation and three-dimensional physical scale test, were employed to study thaw subsidence mechanisms in three different types of soils; namely, clay, silt and sand. The main objective of these experiments was to understand the well-soil system and the changes occurring within it with time, which will further increase knowledge of the interaction between the wellbore and the soil in Arctic regions during progressive thaw. Due to a lack of data and information, several areas were selected for multiple experimental approaches, including lateral pressure development, soil strain and strain within well casing, to study the frictional effects along the wellbore and pore-pressure response within the soil. Along with the experimental work, two different models were built in COMSOL Multiphysics™. The first model focused on thermal analysis of the thawing and refreezing behavior of ice-rich permafrost for drilling and production operations, while the second model focused on mechanical analysis, to study and understand the generation of the vertical and horizontal loads and stress-strain characteristics of the ice-rich permafrost. Simulations focused mainly on obtaining data for lateral pressure development, well stress-strain and temperature.