document preview

Summary

For the Elgin–Franklin UKCS fields, drilling of the high pressure/high temperature (HPHT) phase is critical due to the narrow margin between formation pore and fracturation pressures. This problem is accentuated by the extreme temperature (up to 205°C—400°F) and pressure (up to 1,200 bar—17,400 psi). A new generation of down-hole pressure while drilling (PWD) tools allows a better real-time monitoring of the bottom-hole pressure, but cannot withstand the extreme temperature of the entire HPHT phase. Furthermore, down-hole and surface pressures are dependent on complex hydraulic phenomena which make predictions challenging. Consequently, it was decided to perform hydraulic tests before drilling the HPHT section of the well 22/30c-G4 (first Elgin development well), in order to quantify the surface and down-hole pressures and temperatures on several operating conditions: circulating (with and without pipe rotation), tripping and flow check. Gelation tests, trapped pressure as well as pressure transmission tests were also performed. Objectives of these tests were to better understand some complex down-hole physical phenomena, to prepare practical guidelines, before drilling HPHT sections, to check reliability and accuracy of new real-time PWD tools, and to collect accurate well data for further hydraulic software packages validation or improvement. These procedures led to great benefits, both in terms of safety and performance, when drilling the HPHT phases of the well: results were considered as highly positive. This paper details down-hole measurement procedures and results, operational guide lines, tools performances, lessons learnt, and explains how to cope with sources of possible discrepancies between actual values and software predictions.

Introduction

When drilling the HPHT section of the Elgin–Franklin wells, we must cope with the very narrow margin between the formation pore and fracturation pressures. Therefore it is necessary to maintain the down-hole mud pressure inside this “safe operating window,” in order to avoid any kick or losses problems, which could have drastic consequences due to the very high gas pressure. Prediction of this pressure is very challenging because of the numerous phenomena that influence this value:

• Mud density function of pressure and temperature.^1-3
• Pressure losses function of flow rate, pipe rotation,^4-7 pipe eccentricity,^8-11 mud rheology, well geometry.
• Pipe eccentricity function of well geometry, well deviation, and pipe rotation.
• Mud rheology function of temperature and pressure.
• Static pressures function of mud gelation properties.

The stand pipe pressure (SPP) cannot be used to monitor the bottom-hole pressure because it is mostly dependent on the pressure losses inside the drill string assembly, but could be used to monitor the down-hole rheological behavior of the mud.¹² Nevertheless the down-hole pressure must be calculated from ?_es and annular pressure losses, and is still dependent on accuracy of hydraulic formula and geometrical data.

Down-hole memory gauges help to quantify dynamic pressure and temperature, ¹³ and to construct flow charts, but it is impossible to monitor the down-hole pressure in real-time.

Development of a new generation of pressure while drilling tools allows the monitoring of the bottom-hole pressure and the optimization of drilling practice,^14-16 but they have still temperature and pressure limitations (175°C, 20,000 psi). Therefore it is necessary to check the real-time pressure while drilling (RTPWD) tool reliability at high temperature conditions with a heavy mud (2170 kg/m³), in order to decide if we can use it, with confidence, to optimize parameters while drilling the HPHT section. Moreover, PWD tools cannot send measurements to surface in static or tripping conditions, thus, software predictions are still necessary in these conditions, as well as in operation planning.

For these reasons, Elf Exploration UK decided to conduct “hydraulic tests” before drilling the HPHT section of well 22/30c-G4.

The primary goals of these tests were to define operational guidelines for drilling the HPHT section, and to check reliability and accuracy of the real-time PWD tool. The additional goals were to collect data to better understand some complex down-hole hydraulic phenomena, as well as to further validate and improve software packages. The test program was successfully carried out within the rig-time allocated, and the quality of data collected was very high.

This paper focuses on surface and down-hole measurements, on operational guidelines, and discusses briefly the source of discrepancies between measured and predicted values.

Well Specifications and Test Program

Well Specifications.

The well G4 is the first development well of the Elgin field situated in block 22/30 in the Central Graben basin in the UK sector of the North Sea. Targets are

Top target:

• Franklin ‘C’ sands, with top at 5416.5 m D_tV =5416.5 m (D_m?5562.9 m).
• Expected pore pressure is 110.7 Mpa at D_tV=5416.5 m, with a temperature up to 191°C at this depth.

Bottom target:

• Pentland, with top at D_tV=5776.5 m (D_m ?5923 m).
• Expected pore pressure is 115.0?117.5 Mpa at D_tV =5776.5 m, and 1187?1212 Mpa at D_tV=6176.5 m .
• Expected temperatures are 189?199°C at D_tV=5776.5 m, and 198?208°C at D_tV=6176.5 m.

The water depth is 92 m. The fracturation gradient is 2330 kg/m³. One problematic zone (the Kimmeridge clays) is situated at a vertical depth between 5130 and 5370 m. The problem of the Kimmeridge clays is the uncertainty on the native pore pressure gradient (between 2150 and 2200 kg/m ³), and the possibility of a ballooning effect. Table 1 shows details of the drill string assembly, casing sizes, and well trajectory, at time of tests. Tests sequence was conducted inside the 9 in. 7/8 liner–10¾ in. tie back at D_m=5092 m, before starting to drill the 8½ in. HPHT section and using a 2170 kg/m³ Synthetic Based Mud (SBM). The main operations before starting the test sequence were:

• Drill out shoe track with 1660 kg/m³ mud (at 50°C).
• Drill 3 m of formation, circulate bottoms up.
• Perform leak off test (LOT) with 1680 kg/m³ mud (at 50°C).
• Displace light mud with heavy mud (2170 kg/m³ at 50°C).
• Perform two LOTs with 2170 kg/m³ mud (at 50°C).