Fractured Reservoirs: Integration Is the Key to Optimization

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized to be experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction New technologies in the last decade (most notably wellbore-image logs) have shown that the rocks in most hydrocarbon-bearing reservoirs contain faults and fractures. This observation has generated an intense interest in understanding "fractured reservoirs." As a result, numerous industry and academic groups are working on different facets of the problem. In addition to research efforts by most major oil companies, these include the Rock Fracture Project at Stanford U., the Rock Deformation Research Group at Leeds U., and the Fractured Reservoir Characterization Group of the Petroleum Engineering Dept. at the U. of Texas. Fractured reservoirs are defined here as reservoirs whose productivity and performance are controlled or strongly affected by faults and fractures occurring in the field. For a reservoir manager, four issues need to be addressed.Is there any evidence that faults and fractures are affecting the performance of my reservoir?If so, what is a description of their important features?How can that information be used to build a quantitative 3D model for performance prediction?How can the predictive model be applied to optimize production or ultimate recovery from this reservoir? Evidence of Fault/Fracture Control on Production " Effective" fractures are defined here as those fractures that impact reservoir production by enhancing flow, by partitioning the field, or by allowing early breakthrough of water or injected fluids. The ability to predict the presence of effective faults and fractures allows the reservoir engineer to collect appropriate data and to make good decisions while drilling the initial discovery and delineation wells on a prospect. Recent advances in seismic wavelet recording and processing [multicomponent recording, amplitude variation with offset (AVO)] hold the promise of being able to recognize a preferred anisotropy to wave propagation in the rocks that reflects an oriented arrangement of liquids in an existing fracture system.1 Seismic lines show major faults, and dip-attribute mapping combined with fault-interaction analysis based on structural principles allows prediction of an increasing amount of detail about the sealing nature of the fault zones.2 While drilling, interaction of the bit and the mud system with the in-situ stress in the rock determines borehole stability, completions strategy, and potential pipe failure throughout the life of the well. Recent work on small mud losses (less than 0.25 bbl in some cases) shows that, as a permeable fracture or fault zone is intersected by the wellbore, careful observation can document effective fractures, not just catastrophic mud loss.3 Other wellbore-based data sets that can be useful in identifying faults and fractures are structural logging of conventional core, rubble zones and core loss in fault-damage intervals, and image logs for viewing damage intersected by the bit. As a field is put on production, injection and production anomalies combined with transient-pressure testing give indications of reservoir compartments and allow better analysis of fluid-flow anisotropies.4 Detailed geochemistry of hydrocarbons can indicate compartments, and analysis of produced waters can differentiate in-situ water from aquifer fluids that have been coned through conductive faults and fractures. Combining these data with the various data types mentioned earlier can indicate that the existing effective-fault and -fracture network in a reservoir is strongly impacting performance. Description of Effective-Fracture Systems Describing the effective-fracture network in a reservoir must be done by integrating relevant data from wells (wireline logs and conventional cores), from well tests (production logs, interference tests, fluid typing), and from fieldwide observations (reservoir-scale 3D seismic and outcrop analogs). With these data, fracture types (extensional, compressional, shear, induced), geometries, properties (distributions of porosity and permeability), interaction with matrix, and deformation history can be determined and the effective-fracture system can be defined. Modern image-log data sets provide spectacular views of the borehole wall (visual and acoustic images) and the near-well rock volume (electrical images).5 These logs are capable of imaging features that are only a few millimeters in size. Fractures are often visible on these images (Fig. 1) and are easily measured in terms of physical and geometrical attributes (distribution attitude and aperture6). New sonic devices (dipole shear) use Stonely-wave propagation downhole to detect fluid-filled fractures that extend farther away from the wellbore.