High Temperature & High Pressure (HTHP) Mud P-?-T Behavior and Its Effect on Wellbore Pressure Calculations
Authors: Haige Wang , Yinao Su 
Published: April 2008

Abstract
Drilling and completing wells in HTHP environments is difficult and dangerous. The paper has studied the effects of formation temperature gradient, wellbore inlet mud temperature, and mud type on the equivalent static density (ESD). The calculation model of ESD under HTHP conditions has been presented. Computing results show that temperature gradient has large effect on ESD. With increasing of temperature gradient, ESD decreases, and the ESD difference between surface and bottom hole increases. While with the increasing of surface temperature or mud inlet temperature, ESD decreases, but this has not much influence on ESD difference. Moreover, the paper has analyzed the effects of ESD variations on wellbore static pressure, flowing pressure, Equivalent Circulating Density (ECD) and surge and swab pressure. The results show that errors resulting from ignoring variations in mud properties are small in relatively shallow wells. However in HTHP wells, especially in those wells with narrow safe mud margins available, to ensure the wellbore stable, changes in mud density due to elevated temperature and pressure and its influence on wellbore pressure system must be taken into accounts.

Introduction
Drilling and completing wells in HTHP environment faces with difficult and dangerous. Despite these challenges, interest in these wells has remained high and the number of HTHP wells has grown steadily. Reservoir pressures in excess of 68.9 MPa, and bottomhole temperature excess 150° has been exploited in many parts of the world, particularly in seeking for gas. Extreme high temperature and high pressure in USA, China South Sea, North Sea wells have been successfully drilled into reservoirs where temperature may exceed 250°, temperature gradients over 4°/100m, bottomhole highest pressure exceeding 110 MPa, requiring mud weights in excess of 2.22 g/cm3. Much attention should be paid when drilling and completing such HTHP wells. One particular problem is that mud weight at down hole temperature and pressure can be significantly different from those measured at the surface in HTHP deep wells. It was reported the specific gravity of the base fluid under 395° at depth of 16000 ft is 0.68, while the same fluid returned to surface temperature and pressure has a gravity of 0.79,a 14% decreases in base fluid density[1]. So it is very important to search for the relationship of mud weight changing with well depth, in order to reduce high temperature and pressure risks, increase safety and efficiency.
Most of the hazards of drilling HTHP wells are related to overpressure formations. An over pressured formation becomes a major problem when the formation fracture pressure is close to that in the over pressured zones. These problems are clearly illustrated in the Elgin and Frankin oil field in the North Sea. While the reservoir pressure is up to 112.5Mpa, and temperature exceeding 200 °C, these conditions are compounded by the small margin-approximately 9650 KPa between formation pore pressure and fracture pressure in the lowermost intervals, a 9650 KPa hydrostatic pressure window at 4750 m corresponds to a small increment in mud weight of 0.22 g/cm3. The narrow mud weight window, sometimes less than mud circulating pressure, makes drilling confronting with challenges. This results in drilling conditions where kicks are easily taken, and where fractures can be inadvertently initiated, resulting in drilling fluid losses that are difficult to control, and difficult in mud logging and cementing operation. In extreme cases can result in well bore abandonment. Therefore, it is absolutely necessary to take into account wellbore temperature and pressure on mud properties for HTHP wells with narrow operational margin.