How to do Quick-Look Shaly Sand Log Analysis with Confidence
- Mark Deakin (Petrophysics Pty Ltd)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- SPWLA Asia-Pacific Regional Conference, 1-4 March, physical event not held
- Publication Date
- Document Type
- Conference Paper
- 2020. held jointly by the Society of Petrophysicists and Well Log Analysts (SPWLA) and the submitting authors
- 16 in the last 30 days
- 16 since 2007
- Show more detail
The operations petrophysicist faced with the daunting task of producing fit-for-purpose Quick-Look results in the freshwater, stacked shaly sands of S.E. Asia will understandably feel challenged. The methods he/she adopts typically fail to provide the necessary information to distinguish hydrocarbon zones and permeability. Rather than using porosity/conductivity log analysis - a tool intrinsically unsuited to the task at hand - this paper sets out simple, cross-disciplinary techniques which cross-check and harden Quick-Look log analysis. Commonly available special log, non-log and generic data are employed with the method to harden results, with or without core.
A variable fluid density porosity is used to compute an apparent salinity curve from standard Archie and the maximum salinities used in a manner analogous to a Pickett plot.
Zones above FWLs, if any, are established by a single valid pressure; mudgas, shale corrected density-neutron separation or subtle departures of logged Rt over Ro. If FWLs remain uncertain the most promising zone can be assumed to be above a FWL.
With salinity and FWLs fixed the apparent standard Archie m and Rwa curves are computed below FWLs which quantify the presence of non-Archie (excess) conductivity. These are now predicted from conventional logs and used in this paper's quick look Shaly Archie formulae. The variable m and Rw are also used to compute the continuous Waxman Smits Qv and Dual Water Model shaly sand inputs, similarly to Juhasz Qvn (Juhasz 1981). If available, standard output NMR curves may be employed to compute these inputs. We have now established water saturated resistivity, a critical but usually neglected requirement.
Standard output NMR is now used for Swi, independent of Rt, Rw, m, n or shaly sand effects. Alternatively, sidewall, offset or generic rock typed core data provide Swi via established relationships. If neither NMR or core is available generic vshale, porosity and tvdss provide permeability, Swi and full Saturation Height functions, where Sw>Swi. Under certain conditions dielectric curves may be used.
With a resistivity independent Sw now available the Quick-Look process adjusts the saturation exponent n for the saturation equation. If n values are non-feasible the assumption of FWL is revisited.
A final pass allows interactive adjustment of salinity, FWL, Qv and n for Sw. Bulk volume water then calculates Timur Coates permeability which allows full circle reconciliation of the entire process against any available permeability, core, sidewall, offset or generic.
This process is coded to a log analysis command file and completed within 3-4 hours. The diversity of inputs used affords a degree of confidence simply not possible with shaly sand log analysis.
The paper recommends optimal data acquisition to enhance these Quick-Look techniques.
|File Size||2 MB||Number of Pages||16|