Characterization of Naturally Fractured Carbonate Reservoir: Lisburne Field, Prudhoe Bay, Alaska

W. C. Belfield

A synthesis of data from cores, production logs, well tests, and well performance is used to provide both a preliminary description and an estimate of parameters necessary for numerical simulation of a naturally fractured, multilayered carbonate formation under primary depletion.

Core fracture analysis indicates the presence of open fractures immediately beneath an unconformity surface that marks the top of the formation. The interpretation that these fractures provide substantial permeability is confirmed by spinner logs, which indicate large decreases in productivity short distances below the unconformity, and injection surveys, which show large quantities of injected fluid being accepted by the formation just below the unconformity. Rapid pressure response between adjacent wells during an interference test is attributed to this fracture system. The fracture system is believed to be spatially related to the unconformity surface. Other fractures recognized in core are partially open and appear to be discontinuous on a core scale. These fractures are believed o enhance matrix permeability rather than act as fractures associated with a dual-porosity system.

Production is both from fractures and from vertically extensive, better quality matrix zones. Partitioning the total permeability-thickness (kh) value between these two modes of productivity is a problem. Matrix kh can be estimated by using log porosity and a core-derived porosity/permeability transform. Justification for this approach is found in approximately linear semilog plots of permeability vs. porosity of full-diameter cores measured under overburden stress.

Total kh values (determined from pressure transient tests) are cross-plotted with productivity indices for a series of wells. A straight-line fit to the data is considered to be representative of Lisburne well productivity. Use of this plot permits an estimate of total well kh for any well in the field, given a calculated productivity index. Corresponding apportionment of fracture kh and matrix kh can then be made for the well. This approach greatly expands the data base available for inferring effective fracture or fracture kh distributions beyond that determined from limited core and well-test data.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.