1 SPE Distinguished Lecturer Program Primary funding is provided by The SPE Foundation through member donations and a contribution from Offshore Europe The Society is grateful to those companies that allow their professionals to serve as lecturers Additional support provided by AIME Society of Petroleum Engineers Distinguished Lecturer Program

Core Analysis: A Guide to Maximising Added Value Colin McPhee Senergy (GB) Limited Society of Petroleum Engineers Distinguished Lecturer Program

3 Why core matters… Core…. –“confirms lithology and mineralogy –calibrates estimates of fundamental rock properties –shows how fluids occupy and flow in pore space –supplies mechanical properties for faster & safer drilling and better completions” “Logs cannot characterize a reservoir if knowledge of the rock is absent” “a struggle to convince management that the project benefits from the knowledge gained” Bob Harrison, JPT Technology Focus, August 2009

4 Why core analysis matters - volumetrics Oil initially in place OIP Gross rock volume GRV Net to Gross N/G Porosity  Water saturation Sw Formation volume factor Bo Logs, welltests, CORE Logs, CORE PVT GeophysicistGeologistReservoir EngineerPetrophysicist

5 Why core analysis matters – reserves Recovery factor depends on technical and economic factors Recovery factor is partly defined by formation’s relative permeability –from CORE Welge fractional flow equation

6 Core data – do we get value? The “ground truth” for formation evaluation But…. Lab –variable lab data quality and method sensitivity –poor lab reporting standards End user –inadequate planning and inappropriate design Have undermined value from core analysis

7 Core data – do we get value? Review of > 20,000 SCAL measurements 70% of legacy data is unfit for purpose ~ $10,000,000 data redundancy cost Examples of unreported lab artifacts –porosity, Sw, and capillary pressure Impact on hydrocarbons in place

8 Example - Archie water saturation, Sw tortuosity constant a=1 unless core says otherwise formation water resistivity saturation exponent from core true formation resistivity from logs porosity exponent from core porosity Logs – calibrated by core

9 Porosity error – excess brine Correct for excess brine in annulus between core and coreholder test sleeve Otherwise … porosity too low

10 Porosity errors - impact Lab B: -7% error in log 

11 Excess brine – resistivity tests Ambient ‘m’ and ‘n’ –core must be fully saturated –excess brine on plug surface –Formation factor (F): resistivity (R 0 ) too low –‘m’ too low –Resistivity index (I) Rt unaffected –‘n’ too high

12 Correcting for excess brine Formation factor (F) tests at stress on tight sand mean - 30% error in ambient ‘m’

13 Correcting for excess brine Resistivity index (I) tests at ambient +15% error in ambient ‘n’

14 Grain loss– material balance - = Before testAfter test Grain loss

15 Grain loss correction Grain loss correction required Water saturation (v/v) Resistivity Index -20 saturation unit error in Sw

16 Impact of errors on OIP Uncertainty analysis –North Sea reservoir –20%  and 20% Sw –100 MMbbl OIP –+20% error in input data Largest impact – , m and n (core)

17 Pc curve distortion Mercury injection capillary pressure (MICP) Pre-1994: tests on 50 – 80 ml plugs Now: most tests on < 10 ml “chips”/end trims Pc curve (Sw versus Pc) problems –Use Hg-filled pore volume (> 20,000 psi) –clay destruction in small pores –distorted Pc curves