New Fault Permeability and Capillary Entry Pressure Relations for Production History Matching of Matured Fields
Faults are important elements in the production behaviour of hydrocarbon reservoirs. In the last few years compartmentalisation of deepwater reservoirs in the Gulf of Mexico and North Sea has become an important issue and faults have been identified as one of the key contributing factors. In some fields, production data show that faults are not significant baffles. In other fields, production shows that faults are major baffles, retaining thousands of psi production-induced pressure differences without breaking down, even in cases where fault displacement is much less than the reservoir thickness, and faults appeared to be non-sealing prior to production. Historically, the oil and gas industry has been challenged to predict these differences in cross fault flow, and in reservoir simulations it has been modeled by ad-hoc history-match tuning parameters. Given this uncertainty in fault modeling, and the current industry need to simplify and accelerate reservoir modeling, the current trend is to simply ignore small intra-reservoir faults, potentially leading to significant production forecast downgrades.
The industry-standard Shale Gouge Ratio (SGR)-based fault permeability and transmissibility relations yield too much cross fault flow when tested in reservoir simulation models for fields with extended production histories. Conversely, the same functions may yield too little cross fault flow in some high permeability reservoirs recently developed in deepwater West Africa. This paper proposes new fault permeability and capillary entry pressure functions that include variables for the initial reservoir permeability, the effects of cataclasis, the formation of phyllosilicate-fault-framework rocks and fault rock capillary entry pressure. Initial “Close-the-loop” tests of the new function in fields with extended production histories have yielded good history matches in multiple wells.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009