Enhanced Reservoir Description in Carbonate and Clastic Reservoirs
Authors: Movahed Zohreh 
Published: December 2007

The reservoir is composed of a mixture of dolomite, limestone, anhydrite and shale interstratified with sandstones member. The sandstone is predominantly poorly consolidated and quartz rich. Much of the sand is medium grained although coarser sand is common in the lowermost thick sandstone units. Both anhydrite and carbonate cements are present within the sandstone with the anhydrite dominating in the uppermost units. The basal sand syones are often shaly and silty. The sandstone porosity value range from 9% to 26% with typical values being from 22% to 24% substantially better than the interbedded carbonate units with typical value of 12% to 15%. Permeabilities over 2 Darcies have been measured in this field. The carbonate can have higher permeability, calculated1 from flow tests, as a result of fractures within the carbonate.

Borehole imaging provided is a new way of the characteristics of reservoirs drilled with oil-base-mud. Complicated structures were resolved utilizing the dip data gathered with such techniques. Fractures were characterized for their aperture (open or closed), intensity of fracturing, and directional attributes.

In addition to structural details, the Oil base mud imaging provided quantitative resistivity of invaded zone measurement in oil-base-mud environment. It helped to understand invasion profiles, which generally is a function of permeability, in carbonate and sandstone reservoirs and helped reservoir engineers and petrophysicists to understand response of reservoir formation tester measurements for formation pressure and reservoir fluid mobility.

Introduction

The structural style of the fields in this area is complex. Precise information on the structural dip and fault pattern in the subsurface is mandatory to plan development / infill wells successfully. The areas that appear to have low dips turn out to have steep dips, hence cause the wells to miss the targeted reservoirs or oil column even if they have penetrated the reservoir. To get to the reservoir, such wells have to be sidetracked in the right direction so that the missed reservoir or oil pool could be penetrated. Geologists assess vertical and lateral changes in the reservoir by identifying and characterizing large-scale depositional events and sequence stratigraphic boundaries across fields. Using borehole-image data, they also define and determine the orientation of smaller depositional features to understand stratigraphically controlled reservoirs. Dip Analysis done using manually computed orientations of geological features from the images provide the attitude of the structural and the depositional framework. In the present study, logged intervals of studied well present a medium dipping sequence with dip azimuth towards Southwest. Fracture analysis is one of the most important objectives of borehole imaging in the study well. Borehole images were used to identify open and closed fractures.

Sand Count analysis performed using the high-resolution azimuthal data from the borehole image was successful in identifying the thin sand beds, which are generally overlooked by the conventional logs. The method is capable to provide a more accurate estimate of net reserves. Facies or rock type classification, using the artificial neural net technique was successful in replicating rock classification. An average quantitative resistivity of invaded zone was derived by laterally averaging all 20 resitivity curves from borehole2 image log. Separation borehole image resistivity curve from the Induction curves, caused by invasion of the formations with the oil base mud. Generally the zones with larger invasion have higher mobility and those with minor invasion have lower mobility. In some cases however the average quantitative resistivity of invaded zone may provide at least an indication of the fluid type, ie the water saturation.