Schlumberger

Case Study: Identify Orinoco Heavy Oil Reservoirs and Water-Producing Zones

PDVSA uses MR Scanner service’s deep radial profiling to distinguish invasion from irreducible water saturation in Venezuela

Challenge: Differentiate water-producing zones from heavy oil reservoirs in a Venezuela well drilled with water-base mud.

Solution: Characterize the invading mud filtrate by running MR Scanner expert magnetic resonance service to obtain continuous fluid-typing data at multiple depths of investigation (DOIs). The mobile fluid type is identified with a novel simultaneous inversion that calculates changes in saturation in the radial (4th) dimension.

Result: Clearly distinguished irreducible water saturation from the invading free water for a visual identification of reservoir quality and a better estimate of total reserves in the Orinoco fields.

Heavy oil obscures porosity determination

PDVSA planned to conduct extensive reservoir characterization in a well drilled with water-base mud in the Orinoco heavy oil belt. Nuclear magnetic resonance (NMR) logging has long been recognized in this environment as providing useful information for the identification of potential water-producing zones. However, the characteristically short NMR signals of heavy oil cause conventional NMR tools to invariably fail to fully characterize the total formation porosity, which can bias fluid saturation calculations.

MR Scanner service acquires NMR data at multiple DOIs

MR Scanner expert magnetic resonance service was run to obtain reliable NMR data that was used to factor out the poor signal quality resulting from the presence of heavy oil. The continuous NMR fluid characterization data collected at multiple DOIs was processed with a simultaneous 4D inversion that essentially stabilized the small variations in heavy oil volume to better estimate the free fluid and total NMR porosity, without sensitivity to water salinity.

Data visually distinguishes invasion from irreducible water saturation

MR Scanner service’s diffusion (D) and longitudinal relaxation time (T1) data (Tracks 2 through 4) were analyzed to separate the water signal from the oil signal. As shown by increasing DOI in Tracks 5 through 7, the apparent free-water signal (above 100 ms) progressively decreases until it practically disappears at 4 in [10 cm] into the formation. This water signal originates with the water-base mud filtrate, displacing oil in a reservoir that is otherwise at irreducible water saturation. For the sand and shale interval above X,050 ft, where the resistivity barely reaches 20 ohm.m, this characterization is of interest because similar low-resistivity intervals have produced water in the Orinoco fields.

Visual confirmation of this condition is readily apparent in the series of D-T1 maps from X,040 ft. A considerable amount of irreducible water (below 100 ms) can be distinguished from the diminishing free-water signal. The reservoir is at irreducible water saturation, despite its low resistivity. The degradation in reservoir quality is evident, with little oil being displaced at this depth.


Download: Identify Orinoco Heavy Oil Reservoirs and Water-Producing Zones (0.95 MB PDF)

Related products and services

 
 
 

Water Differentiated by Sources to Map Invasion

The 4D inversion of MR Scanner continuous radial data distinguishes free water originating as water-base mud filtrate from irreducible water saturation in potential heavy oil reservoirs. In the lower high-resistivity sand, there is little irreducible water, whereas the low-resistivity sand above X,050 ft remains at irreducible water saturation.D-T1 maps of the low-resistivity reservoir at X,040 ft show the decrease in filtrate invasion with NMR DOI.
PrevNextZoom1 of 2

Measuring Properties Independently from Rock Type

Investigate the formation at multiple depths in a single pass.
MR Scanner expert magnetic resonance service measures 3D profiles of porosity, permeability, and fluid type and movability that are unbiased by the reservoir matrix, which is typically highly variable in carbonates.
Visit MR Scanner page