The undisturbed geothermal gradient is a key thermal boundary that drives heat transfer processes occurring in oil and gas wells throughout their lifetime. However, the temperature distribution with depth is somewhat uncertain, and this is often assumed to be a linear approximation from the mudline to the bottom of the well. During drilling, the circulating temperature may significantly affect the rheology of the drilling fluids and the cement setting processes. Therefore, erroneous estimates of the wellbore temperature may affect the overall performance of the drilling phase and subsequent well operations. Further, it is important to know the accurate temperature distribution within the formation for assessment of the petroleum prospectivity through source rock maturation and reservoir quality.
This paper presents a numerical methodology to estimate the undisturbed geothermal gradient while drilling in offshore wells. This methodology may also be applied to onshore wells by simplification.
The new approach is based on an in-house axisymmetric wellbore transient thermal model, in which the equations are solved using the finite difference method. The model computes the heat transfer between the well and riser system with the surroundings. However, other computational codes may also be used following the framework presented in this study. The computer code should provide a detailed representation of the geometry of the wellbore, the physical properties of the drilling fluid and formation, the suitable thermal boundary conditions and temporal discretization. The temperatures of the fluid at the inlet of the drillstring and at the bottom hole assembly (BHA), in the annulus A, are used as input to the numerical model that iteratively adjusts the undisturbed geothermal gradient, which generated the temperature recordings while drilling.
The paper comprises cases studies of hypothetical wells drilled in relevant offshore areas in the world, each with their distinctive and variable geothermal gradient, defined by the different rock formations encountered. Uncertainties regarding the thermal properties of the rock were also considered to ascertain the robustness of the code. The water depth of the drilling site was also observed to impact the convergence of the algorithm. The results obtained by the numerical approach are in good agreement with the expected values of the undisturbed formation temperatures.
The novelty of the numerical framework is the ability to provide reliable and satisfactory estimates of the undisturbed geothermal gradient for wellbores with any configuration, lithology and rock properties. These estimates are based on temperature measurements of the circulating drilling fluid at the BHA and account for uncertainty in rock thermal properties; in reasonable time using standard engineering computers.