07122-09

Goal
The goal of this project is to test whether the composition of natural gas in tight gas sand reservoirs can be used to track and model gas migration and distribution in these reservoirs. Specifically, the project will examine the feasibility of using gas composition to understand: 1) gas migration processes in tight gas reservoirs; 2) controls on gas distribution in the reservoir; 3) controls on pressure distribution in the reservoir; and 4) lateral and vertical compartmentalization of the reservoir.

Performers
Colorado School of Mines (CSM), Golden, CO 80401
University of Oklahoma, Norman, OK 73019
U.S. Geological Survey, Denver, CO 80225
University of Manchester, Manchester, ENGLAND M139PL

Background
The process by which gas migrates into the large tight gas sand reservoirs of the Rocky Mountain basins is unknown. Possible mechanisms include: (a) gas diffusion upward through a series of moderately permeable seals; (b) gas forcing its way upward by fracturing intermediate seals; or (c) gas migrating up conduits such as faults or fracture systems and then diffusing laterally. Each mechanism has implications for modeling the distribution of gas within a given field and for locating and quantifying the natural gas resource within a basin. An understanding of which of these models is applicable could enhance efforts by industry to optimally develop their Rocky Mountain tight gas fields and discover new fields more efficiently. It may also lay the groundwork for geophysical approaches to the direct detection of gas accumulations in tight gas sands.

Each of the possible models noted above should leave a distinctive record in terms of the distribution of gas composition. For example, vertical diffusion through a series of semi-permeable seals should lead to considerable fractionation of gas species, manifest in terms of 13C and D-H isotopes. Lighter compositions should be found in shallower horizons as 12C and hydrogen diffuse more rapidly through seals than 13C, deuterium and larger molecules such as higher molecular weight hydrocarbon gases and CO2. Alternatively, if gas fills the reservoirs through via natural fractures and rapid filling of successive compartments, the fractionation should be much less pronounced. Finally, if gas is channeled along faults and fractures and then migrates laterally, we should see fractionation laterally away from the channels and very likely substantial compositional differences between fault blocks.

This project will consist of four parallel research efforts designed to test these concepts: (1) documenting the detailed composition of natural gas in several tight gas sand fields to determine the variability in bulk hydrocarbon composition and major gas components (C1 – C4, CO2 and N contents), the isotopic composition (13C, D-H and 15N) of these gases, the trace and radiogenic gas compositions (He, Ne, Ar), along with pressure data; (2) documenting the composition of the gas in past geologic time by analyzing the composition of gas trapped in fluid inclusions; (3) experimentally determining the composition of gas generated from possible source rocks at varying levels of thermal maturity in order to constrain the composition of gas migrating into the tight-gas-sand reservoirs through the sampling of gases during hydrous pyrolysis experiments; and (4) applying computer models of gas migration based on percolation theory to simulate the distribution of gas compositions.

The simulations will be compared to the present-day gas compositions (from the analyses of gas samples) and to the past compositions (from the analyses of fluid inclusions). Through these comparisons, possible mechanisms of gas migration and filling will be tested and a determination will be made as to what models of gas filling apply to Rocky Mountain tight-gas-sand reservoirs.

Approximately 1000 samples will be collected from a number of tight gas sand fields (e.g., Jonah Field in the Greater Green River Basin, Wyoming; Mamm Creek – Rulison – Parachute – Grand Valley Fields in the southern Piceance Basin, Colorado; and Greater Natural Buttes Field in the Uinta Basin, Utah). Samples will be taken from mud gas, drilling production tests, and produced gas streams where the completed wells are producing from restricted zones over time.

Samples of source rock (shale and coal) will undergo hydrous pyrolysis and the gas produced at various levels of thermal maturity will be sampled and tested. Cuttings samples from twelve wells will be used for fluid inclusion analysis via mass spectrometer chemical profiling. All project tasks will be carried out by a research team led by CSM and including scientists from CSM, University of Oklahoma, and University of Manchester.

The deliverables for the project will include: 1) periodic technical reports and a final technical report; 2) a dedicated web site with project information, data, and interpretations; 3) natural gas compositional data; 4) results of hydrous pyrolysis experiments; 5) gas migration modeling results; and 6) scientific papers for publication in peer-reviewed journals.

Potential Impacts
The results of this project may lead to improved models for gas migration and distribution in Rocky Mountain tight gas sand basins. Such models could help E&P companies more accurately map and quantify the gas resource in their acreage and exploit this resource more efficiently. Better definition of reservoir compartments could enable improvements in infill drilling and field development plans, and a reduction in the environmental footprint of drilling operations.

The understanding resulting from this research will provide a basis for testing the “basin-centered gas” model—currently the foundation for U.S. gas resource estimates in the Rocky Mountains—resulting in more accurate estimates. A new model that explains how tight gas sand reservoirs fill could lead to new methods for the direct detection of gas migration fronts at the top of tight sand gas accumulations. Such direct detection methods, comparable to “bright spot” seismic detection in the Gulf of Mexico, could significantly improve the future success of gas exploration efforts in the Rocky Mountains.

Project results should have direct application to tight gas sand exploration in Laramide basins of the Rocky Mountain region, including the Piceance, Uinta, and Greater Green River basins. However the models should be generally applicable within any low-permeability gas system in which the top of the gas column is not defined by a conventional seal. Such reservoirs exist in many parts of the United States, including the periphery of the Gulf of Mexico and the Appalachian Basin.

Economic benefits will depend on the number of new wells that are drilled based on results of this project. An increase in production of natural gas from tight gas sand reservoirs would increase domestic gas supply and generate increased tax revenues, royalties, and regional economic benefits.

Accomplishments
Work on this project began in August, 2008. At the present time, there are no major accomplishments to report.

Current Status
Work has begun on two initial tasks -- the development of a Project Management Plan with a work breakdown structure that concisely addresses the objectives and approach for each task with all major milestones and decision points, and the development of a Technology Status Assessment describing the state-of-the-art of the proposed technology. The key tasks to be undertaken following the submission of the Project Management plan and Technology Status Assessment are outlined below.

Gas Composition in Tight Gas Sand Fields. Approximately 1000 natural gas samples will be collected from tight gas sand fields, including Jonah Field (Greater Green River Basin), Mamm Creek – Rulison – Parachute – Grand Valley Fields (Piceance Basin), and the Greater Natural Buttes Field (Uinta Basin). Mud gas samples, production test samples, and produced gas samples will be collected and analyzed. The bulk composition of all samples will be analyzed for the following components: hydrocarbon gases from C1 to C6, CO₂, and N₂. The distribution of these gases will be mapped across the fields.

In addition, a subset of approximately 100 samples will be selected and analyzed for compound-specific 13C and D-H isotope analysis on the hydrocarbon gases and CO₂. These samples will be selected to provide a geographic and stratigraphic distribution across the fields. Analytical results will be mapped across the fields.

Finally, a subset of approximately 50 samples will be selected for N2 isotope and noble/ radiogenic isotope gas analysis, including He, Ne, and Ar. These samples will also be selected to span the geographic and stratigraphic range of each field. Analytical results will be mapped across the fields.

Hydrous Pyrolysis on Source Rock Samples. Samples will be selected of immature equivalents of possible source rocks for gases in the tight gas sand fields in this project. These samples will undergo hydrous pyrolysis at the U.S. Geological Survey. Gases will be analyzed for bulk hydrocarbon, CO₂, and N₂ composition. Gases from hydrous pyrolysis will also be analyzed for compound-specific 13C of hydrocarbon compounds and CO₂ and 15N of nitrogen gas composition.

Fluid Inclusion Gas Analysis. Twelve wells will be selected and cuttings samples obtained for fluid inclusion analysis. These wells will be either the same as those sampled for mud gas or production test gas, or will be nearby those wells. Mass spectrometer chemical profiles will be developed for each of the 12 wells sampled. This will include analyses of: C1-C13 hydrocarbons; paraffins, naphthenes, and aromatics; organic acids; inorganics (CO₂, He, N₂, O₂, and H₂S); and mud additives. Mass spectrometer results will be compared to results from analysis of mud gas, well test gas, and produced gas.

Modeling of Gas Composition with MPath. Migration of gas into the fields will be simulated with MPath. Different assumptions regarding permeability through intra-reservoir seals, the fracture strength of the intra-reservoir seals, and the vertical permeability of the faults will be tested in order to try to match predicted bulk hydrocarbon and isotopic compositions to measured values. Assumed parameters will be adjusted until a match is achieved. The project team will test whether different migration models provide unique geochemical signatures.

Project Start: August 25, 2008
Project End: August 24, 2010

DOE Contribution: $670,417
Performer Contribution: $346,000

Contact Information:
RPSEA – Kent Perry (kent.perry@gastechnology.org or 847-768-0961)
NETL – Virginia Weyland (Virginia.Weyland@netl.doe.gov or 281-494-2517)
CSM – Nicholas Harris (nbharris@mines.edu or 303-273-3859)