The ability to predict the impact of faults on locating the remaining Hydrocarbon (LTRH) is critical to optimal well placement, reservoir management, and field development decisions, particularly relevant for cost effective management of North Sea assets. Tools and techniques to realistically differentiate between sealing and non-sealing faults have presented a great challenge to the industry. This paper discusses the results of an integrated study that incorporates detailed geology and reservoir engineering to understand production behaviour of a complex faulted high pressure high temperature (HPHT) field in the North Sea. The fault architecture divides the field into 5 lateral compartments. Historically, fault transmissibility from lateral connectivity between compartments and changes of this property with depletion was recognized as a key subsurface uncertainty.
Oil-bearing Pentland and Skagerrak are key producing reservoirs of interest; Skagerrak reservoir with an average reservoir permeability of 50mD is the focus of the study. The initial reservoir pressure and temperature are 12500psi and 3400 F respectively. Production started in 1998 from well 22/24D-10 (southern fault block) and after producing slightly more than 1MMstb, rapid decline in reservoir pressure (~6000 psi) signifying no pressure support was observed. In 1999, a flattening of the pressure that extended to 2006 was observed. From Material Balance work, flattening of pressure was not expected until below bubble point if there is no change in connected Stock Tank Oil Initially in Place (STOIIP). Therefore, one hypothesis is that the observed pressure flattening could be as a result of cross fault flow that changed the connected dynamic STOIIP as a result of draw-down during production. Another hypothesis is that recharge could be through the aquifer. This study shows that fault seal failure is the most likely mechanism for pressure support.
Three main techniques used for investigating dynamic fault seal breakdown are presented. This includes proprietary Petrel FTM plug-in tool, production analysis and deconvolution. Static evaluation of faults using the Shell tool suggests initial sealing nature at initial conditions and the ability for the fault to breakdown given high enough pressure differential. Production analysis identified the weak faults. Deconvolution of the rate and pressure history reveals signature consistent with breakdown of a fault. The distance extracted from deconvolution is consistent with that from static evaluation. Also, 4D seismic signal is consistent with all interpretation of fault seal breakdown. Result shows that the first three compartments in the southern part of the field have been depleted and that there is across fault flow at or below 6000psi capillary threshold pressure.
It will be shown that using well test analysis technique; dynamic fault seal failure can be properly understood. It is hoped that this paper will guide and improve a petroleum engineer's ability to account for dynamic nature of fault Transmissibility Multipliers during dynamic simulations.
