Unconventional Petrophysical Analysis in Unconventional Reservoirs Putting the Puzzle Together in Gas Shales Lee Utley

“Intuitively, it is my belief that this magnitude of money could be better spent on other projects.” Executive with Mitchell Energy in his recommendation for attempting the first completion in the Barnett Shale ‘discovery’ well (Slay #1) - 1982

“Why are we spending all this money to find out how much gas is in the Barnett? If we really want to know what will happen in Johnson County, we just need to drill some damn wells! Engineering executive with Mitchell Energy upon finding out the magnitude of our planned spending on coring and analysis to reevaluate the gas content of the Barnett - 1999

Introduction

Has this happened to you? Somebody just dumped some stuff in your office Large stack of logs Several CDs/DVDs of digital data Core reports Several maps and cross-sections You are told that your company wants to get into this Barnett Shale play everyone is talking about so you need to figure this out.

Problems

General Goals Evaluate the resource Areal extent Thickness Type of hydrocarbon Possible production mechanisms Barriers to economic production Evaluate the resource

Specific Goals to Achieve Using Log Analysis Gas Content Analysis of ‘conventional’ formations Maturity Total Organic Content Porosity Water saturation Lithology Rock Properties Fracture types

Why is this so hard to do? Old logs with limited information Little or no core data Complex lithologies cause problems with typical methods TOC calculation is difficult at best Porosity determination is complicated by presence of TOC

Useful Core Data Geochemical analysis (Ro, TOC, etc…) Porosity Water saturation Gas content (including adsorption isotherm information) Mechanical properties

Gas Content

Gas Storage Sites Sorption – TOC Pore space Open natural fractures Most gas is stored in the pore space and the TOC. Fracture storage is usually minimal and probably can’t be quantified.

Calculation of Gas Content For sorption, relate TOC to gas content – usually through Langmuir parameters. Don’t forget about non-methane adsorption For pore space, use conventional gas-in-place equations. TOC and porosity are two of the biggest keys in looking at gas shales.

‘Conventional’ Analysis

Why look at ‘conventional’ areas Production pathways ‘Unfavorable’ porosity Stimulation barriers Uphole ‘bail-out’ zones

Maturity

Log Indicators of Maturity Resistivity Density – Neutron Separation Use averages of these values in very well defined geologically correlative areas to compare to core vitrinite reflectance data.

Use resistivity as a predictor (OGJ – Morel – 1999)

Use Old Resistivity Logs Too Use resistivity inversion modeling to get old ES logs and induction logs up to modern standards – compare apples to apples 1940’s 1980’s Modern

Density – Neutron Separation Lower Vitrinite Reflectance Gas Shale Well One Higher Vitrinite Reflectance Gas Shale Well Two

TOC

Four main methods Use average TOC from published accounts and apply it to every well Density log regression Delta log R Passey, et al – AAPG 1990 Neural Networks

Porosity

Standard Porosity Transform Core matrix numbers exclude organic material. Normal log presentations show very high apparent porosities. These porosities are closer to the volume of pore space and organic material combined.

Basic Porosity Equation Rock contribution Fluid contribution

Porosity Equation with TOC Rock contribution Fluid contribution TOC contribution

Solved for Porosity

Water Saturation

What are the correct parameters? = S w n w f m R t

Pickett Plot

Calculate Water Saturation

Lithology

Two most common methods Probabilistic methodology Integrated neural network solution

Neural Network Solution

Rock Properties

Standard Rock Mechanic Equations

Use Lithology to Correlate with Rock Properties Neural Network of Young’s Modulus in Two Permian Basin wells using a Fort Worth Basin Model Rock Properties Computed Young’s Modulus Neural Network Computed Young’s Modulus

Fractures

Imaging Logs Fracture Size Direction(s) Complexity Open/Closed Induced fracture direction (stress field)

Barnett Shale Case Study

Core Data Acquired Conventional and pressure cores – Extensive data suite Porosity Water Saturation TOC XRD Canister desorption Adsorption isotherms Capillary pressures CEC

Integrate Core Data

Train a Volumetric Neural Network

Apply integrated solution to all wells

Fort Worth Model Applied to Permian Basin Well

Comparison

Conclusions

Gas shales can be effectively analyzed Maturity, TOC, and porosity are some of the keys to gas shale analysis and can be determined from logs. Even without extensive core data, gas shales can still be analyzed, at least in a relative sense. Other gas shales can be evaluated from log data and core data using these techniques. An integrated study is required for full evaluation.

Unconventional Petrophysical Analysis in Unconventional Reservoirs Putting the Puzzle Together in Gas Shales Lee Utley