In the challenging North Sea environment, as a result of the typically low number of wells, there is usually a single opportunity for executing a successful fracturing operation in a depleted well in a mature field. A thorough understanding of reservoir geomechanics becomes critical to candidate selection, particularly in cases where there is significant depletion in the reservoir zones underlying the target zones. Ordinarily, detailed core rock mechanics would be used to significantly reduce the uncertainties in the rock mechanics, and understanding of any barriers to fracture height extent (i.e., the presence or lack of a barrier), if indeed there is core material available. In cases where there is not, the range of uncertainties become difficult to narrow down, which has lead to the development of a unique workflow where drill cuttings may be used instead.

Rock Property Determination from Drill Cuttings involves embedding a selection of cuttings in resin for mineralogical and lithological analysis. Rock mechanical properties of selected grain samples are then measured by nano-indentation, integrated with the mineralogical analysis, and finally finite element modeling is performed to up-scale the rock mechanics to formation level, generating a continuous log, allowing for understanding of not only the target reservoir units, but the important over and underlying formations, often not sampled in coring programmes.

In a three-well project for Field A, core material was available for Well 1, and cuttings were available for Wells 2 and 3. Rock Property Determination from conventional core rock mechanics testing was completed for Well 1 where core material was available, and thereafter the core plugs from these tests were sent for Rock Property Determination from Drill Cuttings, along with carefully selected subsets of cuttings for each of Well 2 and Well 3. Testing was done blind, and the core rock mechanics test were used as a verification method for the drill cuttings test procedure.

Finally, the test results from both standard core plug and drill cuttings were used to calibrate the rock mechanics properties for the respective candidate wells, to take forward into final fracture designs and campaign planning. The rock mechanics from cuttings material was in line with the core rock mechanics data, except for where there was discrepancies between the petrophysical interpretation and the mineralogical analysis, since the petrophysics is used to build an upscaled geomechanics model.

When comparing geomechanics models calibrated with cuttings rock mechanics data versus non-calibrated geomechanics models, having some calibration points versus none, contributes to early optimization of the fracture design phase, both technically and operationally and in some cases can actually rule out candidates as being suitable for hydraulic fracturing.

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