Abstract
The thermal regime of a formation in the vicinity of a wellbore affects the stability of the well significantly during drilling operations, especially for deeper or geothermal wells. This paper describes how the thermal behavior of a tubular-wellbore-reservoir system is altered with time during mud loss, as well as its consequent impact on wellbore state of stresses and critical mud weights. A new coupled thermal-poro-elastic model, integrated with a transient tubular-wellbore-reservoir heat flow model, is developed to evaluate near-wellbore stresses and pore pressure redistribution during a mud loss. The results reveal that continuous mud loss destabilizes the near-wall region as the fracture gradient decreases over time. This can intensify the existing fracturing condition and allows the development of new fractures at other locations, which leads to further losses as time progresses. During severe losses, the fracture gradient at the bottom of the well can decrease by over 1 ppg within the first few hours. The time- dependent critical mud weight window and near-wellbore state of stresses under different lost circulation conditions and types of muds (OBM/WBM) are presented to evaluate the effects of different parameters. This model allows prediction of a more realistic operating window during lost circulation by taking additional thermally-induced effects into account.
1. INTRODUCTION
Wellbore instability is a major source of additional costs to drilling operations, and is therefore of paramount importance to avoid when drilling for petroleum production. It is more likely to occur when drilling in hostile environments, such as weak and/or reactive formations; formations with tectonically active beddings [1]; formation with unfavorable lithology sequences; and formation with high pressures and temperatures [2]. In addition, the increasing use of directional drilling and extended-reach drilling (ERD)[3], as well as underbalanced drilling and multilateral drilling[4], has increased the difficulty of maintaining stability of the wells significantly[5,6].
Wellbore stability problems have been extensively studied during the last few decades. Many wellbore stability models have been developed to describe the physics of the systems by considering multiple effects, including but not limited to mechanical effects, pore pressure effects, chemical effects, and thermal effects. Based on the effects that have been taken into account, the wellbore stability models can be categorized as: pure elastic model[7,8,9]; poro-elastic models[10,11]; thermal-poro-elastic models [2,12,13,14] ; chemo-poro-elastic models [15,16,17,18,19]; thermo-chemo-poro-elastic models [20,21], etc.
