Several years ago, the Center for Tectonophysics at Texas A&M University formed an Industrial Associates Program (IAP) for the purpose of funding fundamental and applied research on the structural controls on migration, trapping, and production of oil and gas. The Industrial Associates Program provides an important vehicle by which industry can support fundamental and applied research of importance to the petroleum industry while also supporting graduate training.

Recently, the program focus was narrowed. The current goals of the IAP are :

• to determine values and spatial distributions of fluid-flow properties of reservoir-scale faults using field and laboratory measurements together with theoretical models and inverse methods, and
• to develop a mechanistic understanding of the linkage between host-rock lithology, fault development, fault structure and fluid-flow characteristics of faults.

Clearly, such a knowledge-base is important. For example, it is critical for providing reasonable estimates or constraints on quantitative values needed in reservoir simulation models incorporating faults as reservoir heterogeneities. Nevertheless, at the reservoir scale, very limited validated data exist for the hydraulic conductance attributes of faults. Most fluid-flow properties data are from measurements at the core scale, either from samples of faults in outcrop or a few subsurface cores through faults. How does one "scale-up" spatially limited core-scale permeability data, given geologic field studies readily demonstrate that fault structure, hence the associated fluid-flow properties, exhibits significant spatial heterogeneity? Fault structure is far from homogeneous in space, especially if the fault cuts through differing lithologies.

What is needed are integrated geological and engineering studies of faulted reservoirs that: (1) characterize the fluid-flow properties of faults over a range of scales and (2) seek correlations of the fluid-flow properties and their spatial distributions with the observed or inferred geologic characteristics with constraints from mechanistic models of fault development. In order to appropriately study this issue, we have organized an interdisciplinary team consisting of graduate students and faculty from Geology, Geophysics, and Petroleum Engineering at Texas A&M University.

Faulted Groundwater Aquifers as Analogs for Faulted Petroleum Reservoirs

We believe that fault-partitioned, shallow, aquifer/aquiclude systems are natural analogs for fault compartmentalized petroleum reservoirs and, thus, provide an ideal field-laboratory for fault-permeability studies. Studies of faulted aquifers provide a low-cost, high-resolution alternative to oilfield-based studies. Furthermore, given the greater potential for acquiring abundant data with greater ease, we believe that study of faulted aquifer systems may be the only practical approach for modeling faulted petroleum reservoir systems. The fault-partitioned, Hickory Sandstone aquifer in northern Mason County in central Texas is our proposed study site (Map, 135K). The extensive geologic and hydrogeologic data-base gathered over 10 years supports the use of this site as a reservoir analog.

The Hickory Sandstone is a 140 m thick, high porosity, transgressive, fluvial to shallow marine siliciclastic unit comprised of quartzose sandstone and interspersed clay-rich interbeds and is the principal aquifer in central Texas (Stratigraphy). The study area is cut by a system of normal and oblique-slip faults that formed under an overburden of 1 km during the Pennsylvanian Ouachita orogeny (Structure). Shear zones of exposed faults in outcrops of the Hickory sandstone exhibit significant grain-size and porosity reduction. Measurements of spatial and temporal variations of water levels in wells indicate larger faults hydraulically compartmentalize the aquifer (Compartmentalization). In some areas, the presence of numerous small faults dramatically reduces water production rates of the reservoir. Analyses of water-level data from wells straddling faults permit the determination of the average hydraulic conductance or transmissibility values for two faults (Fault Conductance).

Pilot Study: 1996-1997

With financial support from the Exxon Production Research Co., we performed a pilot study during 1996 and 1997. The objectives were: (1) to continuously diamond-core the fault with the best estimate of hydraulic conductance for the purposes of characterizing the texture, mineralogy and core-scale permeabilities of the sheared fault rock, and (2) to determine to what extent the core-scale permeabilities compare with the average fault permeability inferred from the hydraulic conductance value determined at the reservoir scale.

To date, four continuously diamond-cored boreholes (Map (36K); Cross Section (86K)) have been drilled through the fault at depths ranging from 30 to 400 ft (9 to 122 m). These boreholes and core provide constraints on the fault's dip-section geometry, excellent (100% recovery) sampling of the shear zone, and nearly continuous (99% recovery in most recent borehole) stratigraphic sampling of the Hickory section and distributed smaller faults. The four transects of the fault shear zone differ significantly and reflect in part the controls of lithology on shear zone character (NNR-1 (95K); NNR-2 (116K); NNR-3 (44K); NNR-4 (65K)). Geological and petrophysical characterization of the core are on-going, but the data obtained for samples from two shear zone transects demonstrate that the core-scale permeabilities of shear-zone fault rock are significantly less than that deduced for the average fault permeability calculated from the groundwater data.

We recently purchased a wireline, core-drilling system (Figure, 48K) that performed excellently during drilling of the two deep boreholes. Having a dedicated, in-house, drilling system provides significant cost savings and research flexibility in terms of drilling, borehole pressure testing, and installation of borehole instrumentation.

Research Plan: 1998

The IAP research plan for fiscal year 1998 builds upon and extends the scope of the pilot study. We will utilize four, scale-dependent, laboratory- and field-measurement strategies combined with appropriate theoretical fluid-flow and transport models and inversion schemes to determine the spatial distribution of the permeability of selected faults in the Hickory aquifer. We also will attempt to correlate these permeability distributions with observed fault structure and field-based mechanistic models of fault-structure development.

Fault permeability will be determined at three scales:

• the core-scale, using samples from the shear zone of the fault;
• an intermediate-scale, using localized, cross-fault, pressure-transient and tracer tests between closely spaced, instrumented boreholes, and
• the large-scale, using both reservoir-scale pump tests and long-term pressure histories associated with the annual cycle of irrigation pumping and recovery.

Our work in 1998 will focus primarily on further characterization of the pilot study fault (Geologic Map, Site A (138K)), but exploratory studies (funding permitting) will be initiated for faults at two other sites in the study area (Geologic Map, Sites B and C (138K)).

In order to achieve the stated goals, we plan the following specific tasks:

• Continuously diamond core additional closely spaced boreholes through the pilot study fault (Map (47K); Cross Section (91K)) ;
• Characterize the structural elements, textures and mineralogy of the shear zones;
• measure permeabilities (using water for samples with significant clay content) of whole core and sample plugs from the shear zones (samples will be retained for future capillary pressure testing);
• Using continuous core and geophysical well logs, construct a high-resolution lithofacies model for the reservoir in the vicinity of the borehole grouping, and measure core permeabilities for a suite of samples from each lithofacies and determine the associated spatial statistics.
• Instrument two to six boreholes (number depends upon the level of funding) with proven, commercial (Westbay Instruments, Inc.) multilevel groundwater monitoring systems (Cross Section (91K). Each well will have at least four, hydraulically isolated monitoring intervals (two above and two below the fault). Each monitoring interval allows for high resolution (0.003-0.007 psi) measurement of fluid pressures, extraction or injection of fluid samples (essential for tracer tests), and controllable ports for large-volume pumping injection/withdrawal necessary for, cross-fault, cross-hole interference and forced-gradient tracer tests.
• Determine long-term pressure histories by measuring fluid pressure in monitoring intervals and neighboring monitoring wells on a regular schedule during the annual irrigation (3 months) and recovery cycle;
• Perform a "natural-gradient" tracer test during the recovery interval. Three different conservative tracers will be injected in three monitoring intervals on the up-gradient side of the fault. Subsequently, on a appropriate time schedule, fluid samples will be collected from the monitoring intervals and analyzed to determine the breakthrough curves for each tracer in each monitoring interval. In addition, we will conduct several cross-fault, forced-gradient conservative tracer tests in which an interval on one side of the fault is pumped and water reinjected into another interval on the other side;
• Perform a series of cross-fault, pressure-transient tests between the closely spaced, instrumented boreholes. One monitoring interval will be pumped and the pressure histories measured in the pumped and surrounding monitoring intervals. Different zones of the fault will be interrogated by changing the location of the pumped interval. Subsequent to the localized cross-fault, interference tests, a reservoir-scale pump test will be performed using a nearby major irrigation well as the pumping well. During this test we will measure pressures in the instrumented boreholes and other monitoring wells in the region;
• Analyze the various fluid-flow and transport data from the field experiments using several approaches. The reservoir-scale pump test data and long-term pressure histories will be analyzed to provide estimates of the average and large-scale spatial zonations of fault conductance using: (i) existing analytical solutions for idealized reservoirs with permeable faults and (ii) deterministic forward modeling and stochastic inverse modeling using a numerical reservoir simulator. The intermediate-scale characteristics of the permeability of the fault, primarily in the region encompassed by the closely spaced boreholes, will be determined stochastically using an inverse model conditioned by the lithofacies geologic model, the geostatistics of the core-scale permeabilites measured for each lithofacies and the observed pressure-transient and tracer data;
• Drill an additional borehole in the interior of the initial borehole grouping as a means to test the predictions of the inversion and thus provide a measure of validation of the methodology used;
• Diamond-core a borehole through each target fault at Sites B and C (Figure) and instrument each with a multilevel monitoring system Each system will consist of two monitoring intervals straddling the fault with dedicated pressure transducers and data loggers. These boreholes will provide preliminary data needed for subsequent in-depth study of the faults.

Budgetary Issues

The number of member companies supporting IAP will determine the level of effort that can be accomplished. The annual membership per company/organization is $25k. Even though diamond-core drilling, installation of borehole instrumentation and field experiments are expensive, we have retained a modest annual membership fee in order to encourage as many companies to join as possible and there by maximize the leveraging factor. Ideally, we will have at least eight supporting companies for fiscal year 1998. The support of eight companies ($200k annual budget) will permit the following effort:

Membership greater than eight companies will permit additional student support and drilling and installation of additional instrumented multilevel boreholes either at site A or exploratory boreholes through faults in sites B and C.

If membership is less than eight companies, then the effort will be reduced accordingly. With four members, funding would allow:

Tangibles for Member Companies

Membership in the Industrial Associates Program (IAP) of the Center for Tectonophysics provides several important benefits to its members, but under the charter of the IAP by university rules there can be no contractural deliverables. Representatives of member companies have easy and ready access to the findings of the research on a timely basis. The results of the research will be presented at an annual Associates meeting and workshop entailing oral and poster presentations; in addition, written summaries of the year's research results will be made available at the annual meeting. Copies of all theses and dissertations related to the research will be provided to member companies. Furthermore, representatives of the member companies are encouraged to meet with participating students and faculty in order to discuss the research, review and provide insight into the analysis of the data, and provide constructive input into the research plan and its execution. In addition, the IAP provides a vehicle for cooperative research efforts involving personnel of the member companies and the research personnel of the IAP; these cooperative activities are strongly encouraged.

Becoming a Member of the Industrial Associates Program

We are confident that our Industrial Associates Program as outlined above will prove beneficial to the Petroleum Industry and other organizations. We solicit your company's membership and financial support of the program and hope that you will promote our Industrial Associates Program during budgetary planning and decision making for 1998. For additional information concerning the Industrial Associates Program or details of how to activate membership, please contact :