Determining Well Control Issues in HTHP Wells by Predicting Equivalent Circulating Density

Well control issues are caused mainly due to pressure instabilities in the wellbore. For example, when the hydrostatic pressure exerted by the mud column is being overcome by the pore pressure, the formation fluids start to invade the wellbore. This well control problem is also known as “kick”. Well control issues can also be caused when there is too much hydrostatic pressure exerted by the mud column which causes the formation to fracture. This formation fractures can mitigate, thus create a flow conduit in the formation where the drilling fluid can permeate and be lost – a common well control problem known as “lost circulation”. Furthermore, high temperature and high pressure formation normally has the issue of narrow bottomhole pressure margin between the pore pressure and the fracture pressure. This characteristic of the HTHP well means that there is a little margin for error when choosing the right mud weight and the mud components that can function properly within this margin. The problem is further complicated during circulation as the mud weight is increased depending on the amount of frictional pressure loss along the flow conduits, now defined as equivalent circulating density (ECD). Predicting the ECD requires circulating fluid temperatures to be simulated, which is built using implicit numerical methods by applying Crank-Nicolson solution. This temperature simulator is used in conjunction with the temperature and pressure dependent rheological properties of drilling fluids to evaluate the ECD. The results show that the bottomhole fluid temperature decreases with increasing circulation time and circulation rate, which consequently increase the ECD, which may lead to formation fracture. Furthermore, geothermal gradient is found to be the most sensitive parameter that may lead to well control problems if a linear gradient is assumed especially in heterogeneous HTHP formations. Therefore, choosing the right drilling fluid constituents that are stable in high temperature and high pressure, and compatible to be used in HTHP formation is critical to ensure well control issues are minimized.