HP/HT Gas-Condensate Well Testing for the Shell Onyx SW Prospect
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 108665, "HP/HT Gas- Condensate Well Testing for Shell's Onyx SW Prospect," by Arild Fossa, Expro; Derek MacKenzie, SPE, Norske Shell; Odd Steinveg and Eric Henderson, Expro; and Bart van den Bosch, SPE, Halliburton/Shell, prepared for the 2007 Offshore Europe, Aberdeen, 4–7 September.
Complex data gathering occurs over a shorter time with much stricter environmental and safety constraints than ever before. Coupled with increased needs for more-accurate reservoir data for prospect evaluation, emphasis is placed on upfront planning and improved technical performance together with extensive use of advanced fluid-data-gathering methodologies. The latest technologies in testing gas-condensate wells were used on this job. Experiences were used later as the basis for other gas-condensate prospects, including those in the Russian sector of the Barents Sea.

Introduction
Onyx Southwest (SW) exploration Well 6406/9–1 was the second well drilled under License PL 255 offshore Norway. The first well, Well 6406/5–1 in the Tott East prospect, was drilled in 2002 to intersect sands of the Middle-to-Lower Jurassic Garn, Ile, and Tofte formations.
Well 6406/9–1, was planned as a vertical exploration well for a possible high-pressure/high-temperature (HP/HT) regime. Although the HP/HT conditions were marginal, the test was designed and executed as if under full HP/HT conditions. Maximum surface pressure with reservoir fluids to surface was considered to be less than 690 bar. Bottomhole static temperatures were anticipated to be in excess of 150°C.
The main objective was to investigate the stacked Ile, Ror/Tofte, and Tilje formations by conducting a multizone drillstem test (DST), with each interval being tested separately. Testing of two zones with a third was planned as a contingency to cover formations of interest. Results from well logging narrowed the selection to a two-zone DST.

Test Objectives
For the Onyx SW exploration-well formations, the primary test objectives for each reservoir were to assess the productivity and to obtain representative fluid samples. Obtaining reservoir pressure and temperature; determining skin and permeability-thickness product, kh; and identifying possible reservoir boundaries and heterogeneities for the formations were secondary objectives of the well tests. These secondary objectives would aid safety and equipment selection for future wells, as well as provide the starting point for further reservoir analysis. The kh and skin could not be attained satisfactorily for HP/HT wells in any other way because alternative methods have poor performance under such well conditions. The kh and skin parameters were selected to further aid analytical reservoir-simulation work and to enable improved use of the productivity data. Detection of possible reservoir boundaries and heterogeneities was included as a secondary target for planning a future field-drainage strategy. To achieve the test objectives, a set of special requirements was established to obtain the desired data quality. These requirements were downhole shut-in, testing the well with seawater as the packer fluid, and use of downhole memory gauges and single-phase sampling.