In cyclic steam operations, it is advantageous to know the bottomholetemperature, (BHT) for production problem analysis. A low BHT indicates lowbitumen mobility, with a resulting limited inflow. If BHT is high and inflow islimited, production is being limited by some other constraint.
The BHT can be measured directly with downhole thermocouples, or it may beestimated from surface conditions. The advantage of estimating BHT is thatinstrumentation costs and operational problems can be reduced. It is difficultto estimate, due to the transient nature of cyclic steam operations. Variableflow rates and conditions, heat transfer coefficients, overburden temperaturesand other factors combine to make the estimation complex.
In this study, different estimation techniques were compared to measured BHTvalues over several cycles in a shallow McMurray well. A regression coefficientwas then applied to compare the techniques. The estimation techniques fell intotwo principle groups; analytic solutions and geothermometry. The analyticsolutions examined varied from a simple correlation between well-headtemperature (WHT) and BHT, to the application of a transient wellbore heat-lossequation. The geothermometry techniques represented various relationships oftemperature to dissolved silica; sodium and potassium and sodium, potassium, and calcium.
It was found that the best estimation techniques were the silica geothermometerand a detailed analytic solution. A large daily variation between estimated andmeasured values existed, precluding their use on an instantaneous basis.Application is therefore limited to estimating an average when severaltemperature points are available over a stable production period. Thecorrelations cannot be confidently applied in analyzing well problemsassociated with rapidly declining of fluctuating production.
In the PCEJ in-situ project an attempt was made to estimate the bottomholetemperature (BHT) by two general methods: analytic solutions of heat-loss andgeothermomery- The purpose of the estimations was to analyze the producingcycle when rapid decline occurred. The formation temperature in thisapplication of cyclic steam varies with time from 250 °C to 80 °C while thebitumen changes from highly fluid to virtually immobile. A low BHT may indicatethat a new injection cycle should start while a high value would indicate someother constraint such as well bore plugging may have affected production andsome remedial treatment was required.
The more detailed analytic solutions investigated are based on the transientheat loss equations presented in literature1,2 for constant rate, single phase production of injection. More complicated numericaltechniques3, which allow for two phases and pressure drop were notapplied because of the limitations of the measured data and the primary concernwas the cooler part of the cycle when the fluid was mainly in the liquidphase.
Geothermetric formulas are semi-empirical based on the temperature dependenceof solubility. Dissolved solids in the produced water will depend on themineralization in the formation rock and original condition of the water aswell. Each general area would have a slightly modified equation to representthe temperature relationship. The formulas tested were previously found to begood in the Clearwater in the Cold Lake area4. These were applieddirectly without modification for the different conditions in the McMurray.