Drilling Case Studies

Well Design

Tullich Oilfield, North Sea

The development of the Tullich field in the North Sea presented an Earth-stress challenge for the production of oil from the Late Paleocene, deepwater, Balder B2 Sands. PSI’s stress from seismic interpretation method determined significant variations in SHD across the field and these variations were used in horizontal wellbore design.


Geomechanics comprises the physics of the rocks and the stresses acting on them. PSI specialises in the pre-drill prediction of stresses for petroleum exploration, drilling and production.


SH Direction (SHD) and Magntiude (SHM) can be critical for planning inclined and horizontal wells. Even if these parameters are known at some points in a discovery well, field development is greatly enhanced, particularly in weak rocks, if SHD and SHM can be determined from ubiquitous seismic in 3D during the planning stage.


Drilling Hazards

Oliver-2, Northwest Shelf, Australia

There are lower stresses and geological hazard avoidance options which PSI’s 4DGeoStress variable wellbore directions and inclinations offer for pre-drill planning. This is illustrated at the Oliver oil and gas field, Northwest Shelf, Australia.


Oliver-1ST1 is in part fractured by Late Triassic and Early Tertiary normal and thrust faults adjacent and subsidiary to the main bounding Miocene to Recent normal fault. Above and within the 170m Mid Jurassic oil and gas column, repeated faulting has caused areas of major disruption within the limey shale overburden section and this presented drilling challenges.


There is limited public information on Oliver-2. It was a vertical well spudded on 23 October 2009. On 19 November logging was completed at 3233m and preparation was underway to run the 7” liner prior to commencing a testing program. On 4 December the well was plugged and suspended after a second attempt to run the liner below 2100m. Hole problems were predicted on PSI’s pressure depth graph from 2700-2900m.


If the well had been drilled at an inclination to the fault, wellbore instability would have been reduced and PSI’s new 4DGeoStress software shows the stress levels would have been lower. A multilateral off the inclined well could provide a lower cost appraisal to intersect the OWC and the northwest limb of the field.





Pore Pressure, North Sea

The Valhall oilfield in the Norwegian North Sea near the Norway/Denmark boarder hosts an overpressured chalk reservoir. The field has experienced five phases of high pore pressure (PP) (Recent, Mid Miocene, Mid Eocene, Late Campanian and Early Cretaceous) two of which exceeded the fracture gradient. This is effectively a Fluid Expansion Over-Pressure mechanism controlled mainly by horizontal pressure increase (SH) and then decrease.


One of the primary 4DGeoStress software attributes is the recognition of a series of vertically stacked anticlines or depositional thins formed during Compressional Pulses. If the section remains normally pressured, these thins will compact uniformly except over local obstructions. If fluid escape is blocked PP can rise and can tend to thicken a thinned unit over the anticlinal high during burial or a Compressional Pulse. The ratio of thickening to the undeformed part of the isochore is therefore a measure of the over-pressure. Further, if the unit has later burst in the form of a PP volcano as it did during the Mid Miocene and Late Campanian at Valhall, PP was greater than Sh/SV (the fracture gradient at that time) and is therefore representing that maximum PP reached. 4DGeoStress will identify the abnormally pressured horizons and plots a quantified PP curve alongside the SH, Sh and SV curves on a Pressure Depth Graph.



Post-Drill 2D Stress vs Pre-Drill 4D Stress

Snorre Oil Field, North Sea

There are two components to geomechanics: the physical properties of rocks and the stresses acting upon them. It is not possible to make reasonable judgements about geomechanical issues without first knowing the stress direction, magnitude and history affecting rocks in a basin.


“Anything short of the correct description of the earth stresses will render any sand production exercise flawed, irrespective of how well the rock properties are understood” (Geomechanics International, 2009).


PSI’s patented method of deriving stress from reflection seismic interpretation yields a complete 4D structural and stress history including present day stress direction and magnitude, pre-drill. All other stress estimation methods use point data measured from boreholes and extrapolate the stress between them, often inaccurately. These are post-drill, expensive, time consuming drilling risks which expose exploration companies to unnecessary, high severity drilling environments. PSI’s detailed pre-drill stresses deliver timely, accurate and reliable data when they are most required.


Pre-drill stress determination is ideal for field development and imperative for remote wildcat well design.