Reservoir characterization and volumetric estimation of reservoir fluids using simulation and analytical methods: a case study of the coastal swamp depobelt, Niger Delta Basin, Nigeria

Suites of wireline well logs and three-dimensional (3D) seismic data were integrated to characterise the reservoir and estimate the hydrocarbon in Otigwe field, coastal swamp depositional belt, Niger Delta. The 3D seismic data were used to generate seismic sections through which fourteen faults and two horizons of interest were mapped across four wells. Depth structural map generated from the mapped faults and horizons of interest shows that the trapping mechanism within the field is fault-supported anticlinal structural trap. The four available wells were correlated using lithostratigraphic correlation to establish two reservoir continuities (Reservoir A and B). The estimated reservoir fluid volume at surface condition using reservoir simulation and modelling software is 59 MMstb for reservoir A and 25.70 MMstb for reservoir B. On the other hand, the estimated reservoir fluid volume at surface condition using analytical method is 52.58 MMstb for reservoir A and 18.85 MMstb for reservoir B. Using reservoir simulation and modelling software, the average net-to-gross ratio and shale volume for reservoir A range from 0.86 to 0.89 and 0.11 to 0.14, respectively, while for reservoir B the range is between 0.69 to 0.82 and 0.18 to 0.31, respectively. On the flipside using the analytical method, the average net-to-gross ratio and shale volume for reservoir A is 0.78 and 0.22, respectively. The results from the volumetric estimation of reservoir fluids showed close values using both methods and reservoir A is more prolific compare to B.

HYDROCARBON PLAY ASSESSMENT OF “OSWIL” FIELD, ONSHORE NIGER DELTA REGION

Hydrocarbon play assessment of any field involves the evaluation of the production capacity of hydrocarbon reservoir unit in the field. This involves detail study of the reservoir petrophysical properties and geological interpretation of structures suitable for hydrocarbon accumulation in the field as observed from seismic reflection images. This study details the assessment of hydrocarbon play in OSWIL field onshore in Niger Delta, with the intent of appraising its productivity using a combination of seismic, well logs, petrophysical parameters and volumetric estimation using proven techniques which involves an integrated methodology. Two reservoir windows ‘R1’ and ‘R2’ were defined from five wells OSWIL-02, 04, 06, 07 and 12. The top and base of each reservoir window was delineated from the wells. Structural interpretation for inline 6975 revealed two horizons (X and Y) and eight faults labelled (F1, F2, F6, F8, F10, F16, F17 and F18). Five faults (F1, F6, F10, F17 and F18) were identified as synthetic faults and dip basin wards while three faults (F2, F8 and F16) were identified as antithetic faults and dips landwards. Time-depth structural map at top of reservoirs R1 and R2 revealed structural highs and closures. These observations are characteristics of growth structures (faults) which depicts the tectonic style of the Niger Delta. Results of petrophysical evaluation for reservoirs ‘R1’ and ‘R2’ across the five wells were analysed. For reservoir ‘R1’ effective porosity values of 27%, 26%, 23%, 20% and 22% were obtained for wells OSWIL-04, 12, 07, 06 and 02 respectively with an average of 23.6%, while for reservoir ‘R2’ effective porosity values of 26%, 22%, 21%, 24% and 23% for wells OSWIL-04, 12, 07, 06 and 02 were obtained respectively with an average of 23.2%. This porosity values correspond with the already established porosity range of 28-32% within the Agbada formation of the Niger Delta. Permeability index of the order (K > 100mD) were obtained for both reservoirs across the five wells and is rated very good. Hydrocarbon saturation (Shc) across the five wells averages at 61.6% for reservoir ‘R1’ and 67.4% for reservoir ‘R2’. Result of petrophysical model for porosity, permeability and water saturation reveal that the reservoir system in R1 and R2 is fault assisted and fluid flow within both reservoirs is aided by presence of effective porosity and faulting. Volumetric estimation for both reservoirs showed that reservoir R1 contains an estimate of 455 × 106 STB of hydrocarbon in place, while reservoir R2 contains an estimate of 683 × 106 STB of hydrocarbon in place. These findings impact positively on hydrocarbon production in the field and affirm that the two reservoirs R1 and R2 are highly prospective.

Evaluation of Ultrafast Wave-CAIPI 3D FLAIR in the Visualization and Volumetric Estimation of Cerebral White Matter Lesions

BACKGROUND AND PURPOSETo evaluate an ultrafast 3D-FLAIR sequence using Wave-CAIPI encoding (Wave-FLAIR) compared to standard 3D-FLAIR in the visualization and volumetric estimation of cerebral white matter lesions in a clinical setting.MATERIALS AND METHODS42 consecutive patients underwent 3T brain MRI including standard 3D-FLAIR (acceleration factor R=2, scan time TA=7:15 minutes) and resolution-matched ultrafast Wave-FLAIR sequences (R=6, TA=2:45 minutes for the 20-ch coil; R=9, TA=1:50 minutes for the 32-ch coil) as part of clinical evaluation for demyelinating disease. Automated segmentation of cerebral white matter lesions was performed using the Lesion Segmentation Tool in SPM. Student’s t-test, intra-class correlation coefficient (ICC), relative lesion volume difference (LVD) and Dice similarity coefficients (DSC) were used to compare volumetric measurements between sequences. Two blinded neuroradiologists evaluated the visualization of white matter lesions, artifact and overall diagnostic quality using a predefined 5-point scale.RESULTSStandard and Wave-FLAIR sequences showed excellent agreement of lesion volumes with an ICC of 0.99 and DSC of 0.97±0.05 (range 0.84 to 0.99). Wave-FLAIR was non-inferior to standard-FLAIR for visualization of lesions and motion. The diagnostic quality for Wave-FLAIR was slightly greater than standard-FLAIR for infratentorial lesions (p<0.001), and there was less pulsation artifact on Wave-FLAIR compared to standard FLAIR (p<0.001).CONCLUSIONSUltrafast Wave-FLAIR provides superior visualization of infratentorial lesions while preserving overall diagnostic quality and yields comparable white matter lesion volumes to those estimated using standard-FLAIR. The availability of ultrafast Wave-FLAIR may facilitate the greater use of 3D-FLAIR sequences in the evaluation of patients with suspected demyelinating disease.