Determination of Permafrost Thawing Around Oil Wells

Abstract

A practical procedure is presented for determining the radius of the thawed-permafrost region around a well and finding the temperature distribution in that region. Such information is important in drilling, completion, and production operations in Arctic regions. A new numerical method was developed and used in a computer model to generate solutions for radial thawing of permafrost with axial symmetry, which was shown to be a function of three dimensionless parameters plus dimensionless radius and time. The model parameters plus dimensionless radius and time. The model generated solutions for the range of values of the three parameters for conditions in Alaska's North Slope. The dimensionless radius of the thawed-permafrost region was related to dimensionless time through a simple power-law equation containing two constants. Results generated by the computer model were used to develop correlations giving the relationships between the two constants and the three parameters. The correlations can be used to find the thawed-permafrost radius, and once this information is available, the temperature distribution in the thawed region can also be calculated. For the correlations, the temperature at the well was assumed to be constant. In more realistic situations in which well temperature varies with time, we also describe a simple method of calculating the amount of thawing. OPERATIONS IN ARCTIC POSE NEW PROBLEMS FOR OIL INDUSTRY One of the problems associated with oil-field operations in Alaska is thawing of permafrost around producing oil wells. In such wells, the heat from producing oil wells. In such wells, the heat from well fluids will gradually thaw the permafrost. The thawing may destroy the bond between the cement and the permafrost, cause instability and subsidence of the soil around the well, and place high mechanical stresses on the casing. To prevent damage to the well, permafrost thawing is typically controlled by insulation, refrigeration, or insulation of the top several hundred feet of the well, while the rest of the permafrost is allowed to thaw uncontrolled. Whatever type of preventive measure is used to avoid damage to the well, we need to know the thermal behavior of permafrost in order to design an economic well completion for avoiding thawing problems. In particular, we would like to be able to forecast the particular, we would like to be able to forecast the rate of advancement of the thawing front and the temperature distribution in the thawed-permafrost region under a given set of conditions. Such a forecast requires the solution of one of the classic nonlinear heat-transfer problems known as heat conduction with phase change, or the Stefan Problem. The common feature of Stefan Problems is Problem. The common feature of Stefan Problems is the existence of a moving interface separating liquid and solid phases. The thermal properties of the two phases differ on either side of the moving interface, phases differ on either side of the moving interface, where heat is released or absorbed. Depending on the relative temperature gradients of the two phases, the interface moves either into the solid region (melting) or into the liquid region (solidification). The position of the moving interface is to be determined as a position of the moving interface is to be determined as a function of time, along with the temperature distributions in the liquid and solid regions. One of the first important works that examined the amount of thawed permafrost around producing oil wells was published in 1970 by Couch et al. They used an implicit finite-difference scheme to solve the equations resulting from the weak formulation of the Stefan Problem. The weak formulation assumes that melting or solidification takes place in a temperature interval (for more details see Atthey), rather than at a specific temperature (e.g., 0C (32F) for water), as assumed in the classic formulation. Couch et al. studied the effect of insulation on the radial-vertical heat conduction with phase change around producing oil wells. P. 643

Thaw Front Dynamics and Super Insulated Well for Thermal Recovery in Cold Environments

“…(30). The symbol φ represents the ratio of heat flow in the solid region resulting from the temperature difference (T e -T f ) to heat flow in the liquid region resulting from the imposed temperature difference (T w -T f ) 22 :…”

“…The ratio σ is the heat required to melt a unit volume of solid upon the maximum available heat content of a unit volume of liquid 22 is…”

“…The dimensionless t D time is written as The complete derivation of the thaw-front radius from the unsteady-state heat conduction in an isotropic solid is described by Sengul et al 21,22 . Calculation results are correlated relatively simply and we use the correlation approach.…”

Evaluation of Waste Energy Utilization for Downhole Steam Generation: Application to Alaskan Arctic Environment

“…. (12) In equation (12) Wt is the total amount of energy available to preheat eacti barrel of the injected water at prevailing conditions. This energy must be conveyed by a heat exchanger.…”

“…The determination of the thermal resistances in the wellbore and formation is also discussed by Willhite 11 • For the case where permafrost is present, such as in the Alaskan North Slope, the method presented by Sengul and Brigham 12 is used to estimate the radius of the thawed permafrost in order to calculate the heat losses in the permafrost zones. The combination of the equations presented in the preceding references (9)(10)(11)(12) allows us to describe the heat losses in the wellbore prior to the water entering the direct DHSG unit.…”