This section is aimed at summarising the responsibilities and input that can be obtained from each member of an integrated team. It is placed early in programme development to encourage teamwork.
4.2.1 Petrophysics
The petrophysicist has two roles to perform: firstly as core analysis programme focal point and secondly, to co-ordinate petrophysical aspects of core analysis data acquisition. The
petrophysicist has the nominal task of organization and implementation of the coring and core analysis programme. It is recommended that the petrophysicist put together a trial core analysis programme, i.e. a “strawman”. By passing the strawman core analysis programme to each PE team member and other disciplines a consensus can be reached. This may require one of more multi- disciplinary team meetings so that the synergy of the integrated team can be used. The petrophysicist must also ensure that the core analysis programme is consistent with other aspects of the petrophysical data acquisition programme such as the wireline log evaluation programme. The logging suite should be a guide to the core analysis programme. Resistivity logging, sonic logging, and density logging can all be calibrated over the cored interval by core analysis. Pay attention to potential mineralogy identification using spectral gamma ray logs, which can be better quantified with calibration from mineralogy obtained from core.
The petrophysicist selects core analysis measurements relevant to the purpose of calibrating logs and determining input parameters into log interpretation models. For examples:
• basic rock properties – porosity, permeability and fluid saturations; • capillary pressure measurement for saturation calculation;
• stressed measurements if the formation is poorly considered or in anyway stress sensitive; • electrical properties for resistivity log interpretation along with cation exchange capacity; • acoustic rock properties for AVO calculations.
4.2.2 Geology
The core analysis plan should be consistent and be closely interrelated with the geological core analysis plan. Some data are common to both analyses and measurement duplication should be avoided.
Geological input is critical to guarantee that measurements are performed on samples that are most representative of the formation, especially for special core analysis. This may involve determining the number of significant rock types and also roughly estimating the reserves that may be contained in each rock type. The combination of rock typing, i.e. facies identification, is part of the process of geological core analysis. Table 4.1 reviews techniques and information typically obtained in geological evaluation. More detail is contained in the manual "Geological Core Analysis" by L. C. van Geuns and J.A. Okkerman (in preparation).
Geological input should include some of the following: • core descriptions;
• facies analyses which is critical to the sampling for special core analysis measurements; • mineral identification (interaction with petrophysicist may ultimately yield mineral identification
from logs);
• investigating diagenesis as well as analysing the structure of the rock fabric (such variations can play an important part in interpreting core analysis data). Thin sections and grain size analyses are important;
• lithology, depositional characteristics and age of the formations present for geological characterisation of the reservoir.
The geologist makes a preliminary static reservoir model. The ultimate quantification of the reservoir model is accomplished through the interaction of geologist, petrophysicist, reservoir engineer and other disciplines which should determine the goals of the special core analysis programme.
Information
• Depositional environment • Rock type/gross lithology • Net/Gross • Bedding/structure • Degree of consolidation • Grain size/sorting • Fossils • Hydrocarbon shows • Core orientation • Fracture orientation • Visual record • Hydrocarbon shows • Grain characteristics • Porosity indication • Microscopic distribution of minerals • Quantification of
microstructure and porosity • Mineralogy
• (Clay) Mineralogy
• Mineralogy depth profile and approx. matrix density • Quantitative determination of
amounts of U, Th, and K. Used in spectral gamma ray log evaluation
• Chemical analysis of cap and source rock Technique Macroscopic description • Slabbed core • Plugs/sidewall samples/cuttings • Paleomagnetism
• Fracture analysis by 'goniometer' • Visible light photography
• U.V. photography
Microscopic description
• Thin section microscopy • Scanning Electron Microscopy
(SEM) with enhanced image analysis
Compositional analysis
• Energy Dispersive X-Rays with SEM
• X-Ray Diffraction • Mineralog from Core
Laboratories
• Natural Gamma Ray Spectroscopy (NGS)
Geochemical analysis
Table 4.1 Information from geological evaluation
4.2.3 Reservoir engineering
Reservoir engineering application of core analysis data is in providing input to computer
simulation of reservoir performance. The required input is usually focussed on capillary pressure and relative permeability and the experimental conditions that are needed to ensure that the measured data are representative of in-situ conditions, particularly, wettability. However, the reservoir simulation itself should be used to determine the greatest sensitivities in core analysis data and so identify those parameters for which core analysis measurements are critical. This is accomplished by running sensitivities to various input parameters obtained from core analysis using simulation. Sensitivity analyses are performed to investigate the effects of variations in geology and possible recovery process options for reservoir development.
Sensitivities might be run on any of the following parameters:
• relative permeability parameters including endpoint saturations and the shape of the relative permeability curve;
• capillary pressure; • critical gas saturation; • pore volume compressibility;
• flooding tests (such as hot water or steam flooding).
While it is unreasonable to run all sensitivities, selective sensitivity analysis can quickly clarify the economic impact of determining core analysis parameters.
The output from reservoir simulation is used in the economic justification of the core analysis programme using value of information as discussed in Chapter 2.
4.2.4 Other disciplines
Many other disciplines are involved in a core analysis programme development:
• Drilling engineering provides advice on drilling parameters and on coring bit and core barrel choices. New coring bits are continually being developed with the latest being low invasion, high density and anti-whirl bits. Selection of core bits is covered in Core Handling Manual, Chapter 2, and more discussion is available from section 4.4. Organisation of a pre-drilling meeting should be done in close cooperation with drilling engineering.
• Production technology input is sought when core analysis programmes are to be designed to answer questions associated with well injectivity, sand control and rock strength issues pertaining to well-bore integrity, rock mechanical properties for fracture design, sieve analysis for gravel sizing and mineralogy for acid stimulation. An important area is cost estimation involving production technology. Different options frequently require different facilities which have a significant bearing on cost and value of information calculations.
• Geophysicists should be consulted for input relating to measurements involving acoustic velocity measurements and use of core measurements in calibrating seismic data. The analysis of such samples for mineral constituents that can affect acoustic response often plays an important role.