Shale Gas Methods, Free and Adsorbed Gas in BGF/Section, Good and Poor Haynesville Wells, Petrophysical Characterization using a Nuclear Spectroscopy (ECS) Multi-Clay Model and Geological Analysis by a Maximum Likelihood System

PRESENTER: Bob Everett
Petrophysicist

DATE: Wednesday, November 4th, 2009

TIME:12:00 PM
(Cocktails at 11:30)

PLACE: Fairmont Palliser
(date tentative)

Abstract:

What am I going to tell you?
• The sources of shale gas are often over-pressured shales
• A relationship exists between Total Organic Carbon (TOC) and kerogen
• TOC can be determined from GAMLS, core, NS (ECS etc) and TOC_Magara
• TOC can be converted to BCF/sq mile if pressure, temperature, and the interval (thickness) are known
• The relationship derived between TOC and sonic-resistivity was derived from a well in another field that is not nearby and is being applied to the example well
• Only production will tell if this works but it sure seems to…
• The Appendices give the details: descriptions, methods, and equations.
What’s in it for me?
• Concepts and methods you can try
• Equations in Appendices you can use
• Tested in clastic and carbonate gas shales
• What is my motive? Talk about ideas; Take home methods: Ask for slide

Details

Clay minerals usually comprise a large component of shales and siltstones, and sometimes sandstones, and clay minerals can influence estimates of porosity, permeability, and water saturation made during petrophysical analysis of electric well logs.

A method is described for estimating Adsorbed and Free Gas in shale gas zones. The method is not new nor unique but is a composite based on literature available plus a methodology that has been established computing some shale gas reservoirs.

Mineralogy is used to estimate the Free Gas volume. Mineralogy can be either estimated from elemental Si, Ca, Fe, and S from a down-hole electron capture spectroscopy tool, or via a forward modeling procedure, or both, integrated and/or compared.

Although several assumptions are necessary to implement this procedure, the methods are perhaps based less on serendipity than other methods used. The inputs can be readily adjusted and the outputs checked for credibility and consistency. In particular, a series of "balance" checks are made which serve as warnings if outputs are unreasonable.

An integrated six-step approach has been developed that helps to minimize errors and to maximize understanding when performing petrophysical studies involving single or multi-well projects. This approach can be used even when the use of Adsorbed and Free Gas is not part of the work flow. The seventh step is the calculation of Adsorbed gas.
The first step is to perform a probabilistic clustering analysis. This step uses a well log suite selected by the analyst (typically, bulk density (RHOB), neutron porosity (NPHI), gamma ray (GR); optionally, deep resistivity (Rt), Photoelectric factor (PEF), compressional travel time (DTC), silicon (Si), calcium (Ca), iron (Fe), sulphur (S), total organic carbon (TOC)). Following a key initialization procedure, each sample (depth) is probabilistically assigned to each of several "modes" (electrofacies). The second step is to interpret the lithology of each of the modes. Core data is used as available, but a "ModeAssign" routine permits automatic assignment of the modes to lithologies (sandstone, limestone …). The third step is to estimate the mineralogy of each mode, based on all information available. The fourth step is to compute (forward model) the wireline logs using the assumed mineralogy, available fluid composition data, and a mineral response log properties database, plus some empirical relationships. The fifth step is to examine the "fit" between: 1. the down hole logs and the computed logs, and, 2. any available core data (porosity, permeability, grain density, and mineralogy) and modeled estimates of these parameters. Also, certain "balances" that must be satisfied (e.g., is Sw < 1.0 ? (i.e. is Ro < Rt?); is VWB < BVW?; is the “free” porosity zero, in shales and tight rocks?; etc.) are examined. The sixth step is to iteratively adjust inputs as needed to satisfy the requirements of step five. The seventh step is to calculate TOC from DT, compare to core TOC and to use an estimate of kerogen wt% "as a mineral" to solve for Free Gas and then for Adsorbed Gas.

The role of clay minerals in the above procedure is handled by including the ability to adjust: 1. the type of clay minerals and the weight percentage of each (and thus the effect on density, porosity, and neutron log response), 2. the CEC of each clay type (and thus the effect on Sw), and 3. the effective surface area of each clay (and thus the effect on permeability).

Biography:

R. V. (Bob) Everett, P.Eng. is a Consulting Petrophysicist and Professional Engineer with over 40 years experience in petrophysical analysis. He is a specialist in the integration of Neutron Spectroscopy having spent some early years at Schlumberger-Doll Research working on shaly sand analysis. Bob has also worked as a consultant for Texaco, Unocal, and Z&S/Dresser Atlas/Baker Hughes, and the University of Texas at Austin on a GRI Tight Gas Sand project. Some of the many plays he has worked on are offshore West Africa carbonates, West Texas Carbonates, Alberta Devonian Reefs, and Alberta shallow gas Sands as well as Shale Gas plays. He is particularly interested in the mineralogical and chemical aspects of petrophysics involving shaly sands, tight gas sands and shale gas reservoirs and has been directly involved with the development of mineralogical and geochemical logging tools. He has a BASc in Mechanical Engineering from the University of British Columbia.