Identifying and Remediating High Water Production Problems in Basin-Centered Formations

Through geochemical analyses of produced waters, petrophysics, and reservoir simulation we developed concepts and approaches for mitigating unwanted water production in tight gas reservoirs and for increasing recovery of gas resources presently considered noncommercial. Only new completion research (outside the scope of this study) will validate our hypothesis. The first task was assembling and interpreting a robust regional database of historical produced-water analyses to address the production of excessive water in basin-centered tight gas fields in the Greater Green (GGRB ) and Wind River basins (WRB), Wyoming. The database is supplemented with a sampling program in currently active areas. Interpretation of the regional water chemistry data indicates most produced waters reflect their original depositional environments and helps identify local anomalies related to basement faulting. After the assembly and evaluation phases of this project, we generated a working model of tight formation reservoir development, based on the regional nature and occurrence of the formation waters. Through an integrative approach to numerous existing reservoir concepts, we synthesized a generalized development scheme organized around reservoir confining stress cycles. This single overarching scheme accommodates a spectrum of outcomes from the GGRB and Wind River basins. Burial and tectonic processes destroy much of the depositional intergranular fabric of the reservoir, generate gas, and create a rock volume marked by extremely low permeabilities to gas and fluids. Stress release associated with uplift regenerates reservoir permeability through the development of a penetrative grain bounding natural fracture fabric. Reservoir mineral composition, magnitude of the stress cycle and local tectonics govern the degree, scale and exact mechanism of permeability development. We applied the reservoir working model to an area of perceived anomalous water production. Detailed water analyses, seismic mapping, petrophysics, and reservoir simulation indicate a lithologic and structural component to excessive in situ water permeability. Higher formation water salinity was found to be a good pay indicator. Thus spontaneous potential (SP) and resistivity ratio approaches combined with accurate formation water resistivity (Rw) information may be underutilized tools. Reservoir simulation indicates significant infill potential in the demonstration area. Macro natural fracture permeability was determined to be a key element affecting both gas and water production. Using the reservoir characterization results, we generated strategies for avoidance and mitigation of unwanted water production in the field. These strategies include (1) more selective perforation by improved pay determination, (2) using seismic attributes to avoid small-scale fault zones, and (3) utilizing detailed subsurface information to deliberately target optimally located small scale fault zones high in the reservoir gas column. Tapping into the existing natural fracture network represents opportunity for generating dynamic value. Recognizing the crucial role of stress release in the natural generation of permeability within tight reservoirs raises the possibility of manmade generation of permeability through local confining stress release. To the extent that relative permeabilities prevent gas and water movement in the deep subsurface a reduction in stress around a wellbore has the potential to increase the relative permeability conditions, allowing gas to flow. For this reason, future research into cavitation completion methods for deep geopressured reservoirs is recommended.

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