Abstract
Understanding the pressure history of a field brings valuable information that could affect decisions regarding future field development. Sequential formation tests (RFT, MDT, XPT, RDT, Stethoscope and other tools) done while drilling a new well can help describe the dynamics of pressure conditions in the reservoir. In the early life of a field, sequential formation tests describe initial reservoir conditions (gradients, contacts, compartments). Sequential formation tests obtained in wells drilled after the start of production can help the understanding of communication within the field (injector to producer; producer to producer; aquifer/peripheral injection effect; barriers; and connectivity across faults).
Historical sequential formation tests performed on a nearly 30-year-old oil and gas field in West Africa were analyzed and used to identify, confirm and describe a significant pressure barrier at the OOWC (e.g. "tar mat" that gives up to 1000psi difference across contact). Systemizing sequential formation test data by time and area allowed estimation of the pressure barrier extent and its effect on communication between peripheral injectors and the oil zone. Pressure breaks were then plotted on well logs and located in conventional core, where possible, to determine a possible geologic explanation for the pressure barriers.
Smaller pressure barriers are attributed to lithologic changes associated with either facies changes or secondary dolomitization. However, the largest pressure barrier appears across the OOWC. Visual inspection of 5 conventional cores across the field, confirms the presence of 6-9 ft. of solid hydrocarbons, interpreted to be a "tar mat," across the OOWC, except in the northwest portion of the field. Core analysis of samples within the tar mat zone shows unaltered, high permeability (as per testing procedure all cores were cleaned with solvent before testing). The pressure barrier is likely due to the presence of soluble, but highly viscous, long chain hydrocarbons that effectively create a no flow barrier across the OOWC.
Wells with a long period of water free production correspond to areas with a predictable strong pressure barrier at the OOWC. Conversely, wells located in an area with a poorly defined barrier at the OOWC had quicker water break-through. Thus, we conclude that peripheral water injection perforated below the OOWC is less effective in areas where the tar mat is present.
