Well-Temperature Monitoring With Thermistor Cables Through Permafrost

Abstract

Abstract Continued drilling and recent oil and gas discoveries in the arctic have increased the need for an improved understanding (for design purposes) of the thermal disturbances in permafrost caused by drilling and production operations. Techniques have been developed for fabricating a multiconductor cable with thermistors for measuring downhole temperatures. These thermistor cables have been successfully placed in the permafrost zone at several arctic well locations b:r strapping the cable to the surface casing as it is being run. This monitoring has provided useful information on the wellbore temperature profile during drilling the thermal behaviour during placing and setting of cement, and the rate of freezeback in the permafrost zone. Thermistors can be used also to measure thermal disturbances resulting from oil or gas testing or production operations, to obtain data for checking the accuracy of thermal simulation programs and, at equilibrium conditions to establish the base of the permafrost and the geothermal gradient through the permafrost. Introduction RECENT OIL AND GAS DISCOVERIES in the arctic have increased the need, for design purposes, for an improved understanding of the thermal disturbances in permafrost caused by drilling and production operations. Permafrost thickness can vary from a few feet to 1,500 feet or more. Permafrost depth and temperature regime are influenced by topography, climate, vegetation and formation type. Equilibrium temperatures of permafrost can range from 5 °F to 32 °F; seasonal temperature changes affect only the upper 50 to 100 feet. The presence of massive ground ice, ice lenses and wedges is generally restricted to the upper 100 feet. Above the permafrost is a thin active layer, 1 to 3 feet thick, which thaws in summer and refreezes each winter. Drilling, completion and production operations in permafrost areas pose unique problems. Ice-consolidated silt, sand and gravel can cause drilling difficulties when thawed. Ice forming in casing annuli can result in casing- collapse. Considerable research is being directed at these and related problems of arctic operations, and several novel solutions have already been developed. Special cements that will set at subfreezing temperatures(1,2), chilled-mud systems(3, 4, 5) foam drilling (6), special surface-casing designs and insulated production-casing strings(5,7,8) are just some of the methods that have been tried or utilized in permafrost areas. A basic requirement in dealing with any well problem in the arctic is a good understanding of the thermal behaviour during drilling and production. In addition, studies of casing collapse caused by ice formation in casing annuli(9) require data on the rate of freezback after drilling or production operations, In order to study thermal disturbances in permafrost, techniques have been developed for fabricating multi-conductor cables with thermistors for measuring wellbore temperatures. In several exploratory wells in the Canadian arctic these cables have been successfully strapped to surface-casing strings, and have provided temperatures during cementing and drilling operations and through the freezeback period after drilling operations had been completed. This monitoring system can also be used to record wellbore temperature data during production operations.