Pool, Wilfred (NAM) | Geluk, Mark (Shell Int. E&P) | Abels, Janneke (Shell International E&P) | Tiley, Graham John (Shell International E&P) | Idiz, Erdem (Shell Global Solutions International) | Leenaarts, Elise
In 2008 Shell obtained two licenses for unconventional gas exploration in the Skåne region of southern Sweden, with a total size of 2500 km2 (600,000 ac). The objective was the Cambro-Ordovician Alum Shale, one of the thickest and richest marine source rocks in onshore northern Europe.
The licenses covered the Höllviken Graben and the Colonus Shale Trough. In both areas the Alum Shale had been encountered in older wells, with a thickness of up to 90 m and TOC values up to 15%. Maturities of up to 2% Vre were considered encouraging for a shale gas play. Relative high quartz contents suggested good fraccability of the shales. All data was obtained through public sources. Identified risks were the uncertain timing of hydrocarbon generation and the position of the licenses adjacent to the Trans-European Suture Zone where several phases of fault movement have a risk for actually retaining the hydrocarbons.
The derisking strategy for this opportunity was based on both technical and non-technical aspects. Aim was to collect geological and geophysical data to constrain depth and thickness of the shale and to identify potential dolerite dykes. In addition, well data were needed to establish rock properties and gas content. The external environment, especially concerns from the people in Skåne regarding the visual impact of activities and potential impact of drilling activities on the aquifers and on the tourism industry have resulted in extensive engagements with stakeholders and specific requirements around seismic acquisition (low impact), site preparation and operations (e.g. small rig, different lighting).
80 km of 2D seismic was acquired in 2008 and three wells, with a final depth of around 1000 m, were drilled in 2009 to mid 2010. The Alum shale was fully cored and the well sites have been restored. Thickness, richness and maturity of the Alum were as predicted although the basin was shallower than previously anticipated. Canister desorption tests, however, indicated that the shales have only low gas saturation. This significantly increased the risk for a viable shale gas play and therefore the licenses were not renewed after the initial 3 year period.
Pressure build-up due to fluid thermal expansion in sealed annuli of HP/HT wells can have serious consequences such as casing failure or tubing collapse. To determine whether mitigation was required for a HP/HT development, annular pressures in an appraisal well were studied with a dedicated field test, which consisted of running a pressure/temperature memory gauge in a casing/casing annulus of a well and testing the well several times during a 3-month period, after which the gauge was retrieved and the data were read out.
First of all, comparison of the magnitude of the observed annular pressures with the burst and collapse ratings of the casings, shows that annular pressure build-up is a serious consideration in casing design. Such design is to be based on theoretical models for annular pressure build-up. The data acquired with the test serve to validate these models.
The data demonstrated that in the lower temperature range (20 to 40°C), on average, pressure development in the annulus agreed reasonably well with theoretical model predictions, based on thermal expansion of the annular fluids and casings, and ballooning and compression of the casing strings. The influence of these factors could be established by analyzing the transient pressure response of the annulus. At higher temperatures the theoretical models overestimate pressure build-up. This is probably to be attributed to the properties of the completion fluids differing from the properties of the base fluid, water. Estimates on the basis of pure water properties can be considered a worst-case estimate for pressure build-up. Leak-off of the annular fluids, which was seen to dominate pressure development during a previous test in a well with a cement shortfall between casings, did not play a significant role in this fully cemented and sealed annulus.