Abstract

The results of a study of the effects of the temperature and pressure conditions in high temperature/high pressure wells on drilling fluid equivalent circulating density and consequently bottom-hole pressure are presented in this paper. High temperature conditions cause the fluid in the wellbore to expand, while high pressure conditions in deep wells cause fluid compression. Failure to take these two opposing effects into account can lead to errors in the estimation of bottom-hole pressure. The rheological behavior drilling fluids is also affected by the temperature and pressure conditions.

A simulator called DDSimulator was developed to simulate the wellbore during circulation. A Bingham plastic model was employed to express the rheological behavior of the drilling fluids studied, with rheological parameters expressed as functions of temperature and pressure. The Crank-Nicolson numerical discretizing scheme was employed in the DDSimulator for the evaluation of the wellbore temperature profile. The results of the simulation show that higher geothermal gradients lead to lower bottom-hole pressure. The inlet pipe temperature did not have a significant effect on the bottom-hole temperature and pressure, and higher circulation rates result in lower bottom-hole temperature and higher bottom-hole pressure.

Introduction

Drilling fluids are in general complex heterogeneous mixtures of various types of base fluids and chemical additives that must remain stable over a range of temperature and pressure conditions. The properties of these complex mixtures such as equivalent static density (ESD) and the rheological properties of the fluid mixture along with the wellbore geometry determine pressure losses in the system while circulating. It is often assumed that these properties and thus the equivalent circulating density (ECD) are constant throughout the duration of drilling activities. This assumption can prove to be quite wrong in cases where there is a large variation in the pressure/temperature conditions, such as in high pressure-high temperature (HPHT) wells.

As the total vertical depth increases, there is an increase in the bottom-hole temperature, as well as the hydrostatic head of the mud column. These two factors have opposing effects on ECD. The increased hydrostatic head causes increase in ECD due to compression. The increase in temperature on the other hand, causes a decrease in ECD due to thermal expansion. These two effects are most often assumed to cancel each other out. This is not always the case, especially in HPHT wells.

Errors in the estimation of ECD can have a disastrous effect when drilling through formations with a small gap between pore pressure and fracture pressure. In such cases, the margin of error is very small and thus, the ECD must be estimated accurately. Disregarding pressure and temperature effects in this case can lead to greater probability for the occurrence of kicks, and blow-outs due to under-balanced pressure or lost circulation and formation damage due to over-balanced pressure.