Measurement of Hydrostatic and Hydraulic Pressure Changes During HPHT Drilling on Erskine Field
Authors: Swanson B.W. , Elliott G.S. , Meier J.L. , Easton M.D.J. 
Published: December 2007

Abstract
A novel drill collar mounted pressure and temperature gauge tool has been used to measure and record temperatures and pressures near the bit while drilling the 8.1/2" hole section of a North Sea Central Graben HPHT well on the Erskine field in Block 23/26b of the UK sector. The tool was run on the first well in the development with the intention of improving drilling performance through a better understanding of downhole pressures in HPHT wells. The memory type pressure and temperature gauges were present during a series of experiments designed to test currently held theories and models of hydrostatic and hydraulic pressure changes in HPHT wells. The tool was also present when the 8.1/2" section was drilled and recorded temperatures and pressures during loss and gain flow incidents typical of HPHT hole sections in the Central North Sea.
This paper presents data from the tool which show:–The effects of swab and surge pressures while drilling and tripping.–The effect of rotation on ECD.–The effect of wellbore temperature changes on measured bottom hole pressures.–The transmission of pressures in heavy oil based mud and the effects on leak off test accuracies.–Bottom hole pressures during stripping operations.–Bottom hole pressure variation during loss and gain events.–The need for downhole measurements to understand fully events that may be difficult to interpret from surface.
These observations have resulted in operational procedures that have produced significant reductions in time and costs when drilling the HPHT section on subsequent wells.

Introduction
The Erskine field, situated in the North Sea Central Graben block 23/26b, is the UK North Sea's first HPHT development. Drilling of exploration and appraisal wells on Erskine has historically proven to be difficult with average kick frequencies of two per well, with at least one kick on every well. The reason for the demanding nature of the wells had previously been identified as the narrow window between pore- and fracture-pressure in the overpressured HPHT section. In this section the window between pore and fracture pressures narrows significantly and drilling becomes a fine balance between loss and gain - to and from "ballooning" shales in the formation.
During the detailed planning of the Erskine drilling campaign it was decided to measure the effect of drilling variables (flowrate, pipe movement etc.) on downhole pressures and temperatures. These tests were intended to provide a better understanding of ECD's while drilling and a means of managing bottom hole pressure to reduce drilling problems in the critical section of this and subsequent wells. To this end a PWD (Pressure While Drilling) tool was installed in the drillstring while drilling out the intermediate casing shoe on the first Erskine HPHT development well.
The well used for the tests was an 'S' shaped development well drilled form the heavy duty jack up rig Santa Fe Monitor. The directional profile and casings set at the time of the test are shown in Figures 1 and 2. The 10" intermediate casing had been set 60 ft into the Lower Cretaceous Unit, within the Erskine transition zone, at a depth of 15015 ft. The expected pore and fracture pressures are shown in Figure 3 and the expected pressure "window" for operations is presented in Figure 4. This representation was designed to help manage hydrostatic and hydraulic pressures while drilling. P. 169^