In this study, a three-dimensional wellbore stability model is presented that takes into account thermal stresses combined with an integrated circulation temperature model for horizontal well drilling, the bottom hole temperature simulation were then validated using field measurements, and compared with results for vertical wells. A subsequent analysis of temperature sensitivity revealed that the heat source term, the length of horizontal section and mud specific heat were the main reasons cause the bottom hole temperature for horizontal wells rises above the static formation temperature. Results from the wellbore stability model show that the temperature variation magnitude in horizontal well is smaller than that in vertical wells, however, the effect of thermal stress on critical mud weight window in horizontal is more sensitive. The wellbore at toe of horizontal section is more stable than that at heel of horizontal section when the bottom hole temperature exceeds the static formation temperature. This research can provide a theoretical reference for enhancing overall operational efficiency and safety for horizontal well drilling.
In oil and gas resource development and exploitation, horizontal wells have been widely used to enhance production by increasing the amount of wellbore that has contact with the target reservoir. Unfortunately, due to high temperature environments that accompany such drilling, severe problems can be encountered such as the instabilities caused by drilling fluids and in-situ stresses.
Many scholars have studied the heat transfer of geothermal systems during drilling operations, using both analytical and numerical methods to estimate the circulating fluid temperature. Analytical methods are generally used for modeling simple drilling systems, for example, with regular wellbore geometry and single temperature gradients (Holmes and Swift, 1970; Ramey, 1962; Edwardson et al., 1962; Tragesser et al., 1967; Kabir and Hasan, 1996). For more complex systems, however, simple analytical methods are unable to accurately model the thermal behavior. Numerical methods are required for studying more complex systems, and to provide a powerful predictive tool that can efficiently solve the governing finite difference equations for unsteady-state heat transfer in both wellbore and formation (Raymond, 1969; Wooley, 1980; Marshall and Bentsen, 1982).
All mentioned above was suit for vertical wells, and some scholars (Perkins and Gonzalez, 1981; Tang and Luo, 1998) presented model predictions of the effect on the near wellbore stresses of different temperature for vertical wells.
More recently, as horizontal well have been widely used, Yoshioka et al. (2007) and Li and Zhu (2010) developed thermal models to predict downhole temperature, pressure and flow rate profiles for horizontal wells, but these models only consider the heat transfer in horizontal section and the reservoir. Kumar et al. (2012a, 2012b) developed a simple analytical model to analyze heat generated from borehole friction and to predict downhole temperatures for extended-reach well drilling operations. Their model applies only to steady-state conditions and therefore does not accurately model the heat transfer processes. Iyoho et al. (2009) discussed the influencing factors on wellbore temperature of horizontal wells with long horizontal or near horizontal sections for mud system design purposes, no theoretical details were revealed.Gonzalez et al. (2004) found that the fracture gradient can be influenced by wellbore temperature through leak-off test. Yu et al. (2001) and Nguyen et al. (2010) modeling the thermal effects on wellbore stability, separately. However, only limited studies presently exist that use numerical methods to study the thermal effect on wellbore stability combining with thermal behavior of horizontal well drilling systems, especially with long horizontal section.
The temperature variation in horizontal wells is much different from vertical wells (Trichel and Fabian, 2011), which cause a different thermal effect on wellbore stability along the long horizontal section, thus, we can't study either the temperature model or thermal effect on wellbore stability independently.
Given the above, the objective of our present research was to develop a combined model that would serve to: (i) numerically simulate the heat transfer processes during high temperature drilling operations in horizontal wells, and (ii) determine the thermal effect on wellbore stability under the true downhole drilling environment with long horizontal wells, based on temperature distribution derived from the simulations.
In this study, an integrated circulation temperature model of horizontal well drilling was established to investigate the heat transfer characteristics of horizontal wells. Thermal stress near wellbore of horizontal well were analyzed combining with the true downhole drilling environment, the thermal effects on the “critical mud weight window” was discussed, providing a theoretical reference for better understanding the thermal behavior and thermal effect on wellbore stability in horizontal well drilling operations.
2. Wellbore temperature of horizontal well and influencing factors
2.1. Mathematical model development
A schematic diagram of the horizontal drilling operation is shown in Fig. 1. The whole drilling system has five distinct regions: (1) drilling fluid flow downward through the drill pipe; (2) drill pipe wall region; (3) drilling fluid flow upward through the annular; (4) formation region, and (5) drill bit region. According to the well trajectory, each region can be divided into three parts: vertical section, curved section, and horizontal section (excluding region (5) above).
To develop the energy equations for describing thermal behavior of the entire wellbore profile and surrounding formation, the following assumptions are made:
(1) Only heat conduction in horizontal direction is considered, as the majority of formations drilled are layered rock. (2) Physical properties of formations, i.e., density, specific heat and thermal conductivity rate are constant; heat conduction only is applied in modeling the formations. (3) Fluid properties are independent of temperature, and wellbore drilling fluids are incompressible and in steady-state flow during each time step. (4) Heat transfer within the drilling fluid occurs by axial convection. Conduction is neglected except when the circulation process is terminated. (5) Horizontal well drilling has a rotational motion drilling, without any buckling.
In applying these equations to model the thermal behavior of the entire drilling system, five different sets of governing differential equations must be defined, one for each of the five regions identified earlier, along with boundary conditions at each interface as determined by flow continuity or other conditions.