Vertical Transmissibility Assessment Across Stylolites in Stratified Carbonate Reservoir - A Field Experience

Abstract

Abstract One of the key uncertainties that impacts reservoir development and waterflood performance in a thick reservoir is vertical transmissibility across the stylolitic intervals. It is not clearly understood whether low vertical permeability or negative capillary pressure across stylolites holds the water from slumping, because both the realizations could achieve good history matches of observed well performances (water cut and pressure except MDT/RFT) in the simulation model. It is, therefore, crucial to calibrate the simulation model using field test results before it can be used to produce a reliable forecast. A comprehensive dualwell vertical interference testing program using a singleprobe formation tester was designed1 and implemented in a stratified carbonate reservoir in Upper Zakum Field. This paper presents the results of a time-lapsed dual-well vertical interference test that enabled assessment of the degree of vertical communication across various sub-zones and quantification of in-situ vertical permeability on a reservoir scale. The testing procedure involves a partial penetration well test followed by the generation of a pressure wave by a partially penetrating well which is monitored in an offset, fully penetrating well via time lapse MDT measurements. Upon completion of the inter-well interference test, a vertical interference test is conducted in the observer well by using a dual-packer, dual-probe MDT tool. Analytic methods were initially utilized followed by construction of a 3D numerical sector model to match the observed data in order to estimate vertical permeability. Results indicate that there are no vertical permeability barriers present except in the bottom part of the reservoir that could account for the water over-ride of oil seen in the test well. Thus, low vertical permeability is discounted and presumably, capillary pressure barriers may be responsible for preventing slumping of injected water within the reservoir. This finding will have a significant impact upon future field development planning particularly with regard to well design (orientation, location and benefit of lateralisation). More importantly, the results can be utilized to calibrate the full field model culminating in a credible dynamic simulation model to produce accurate future prediction cases. Introduction The giant Middle Eastern field is an anticlinal structure consisting of three major reservoirs. The reservoir under investigation is the middle one which is 140 feet thick carbonate of Lower Cretaceous. The reservoir is subdivided into six major sub-zones based on porosity profile (Fig. 1), i.e. the porous units of A-F and each sub-layer is segregated by thin stylolitic S intervals (S0-S5). These S intervals are generally represented by relatively dense characteristics. It is considered that stylolization reduces both permeability and porosity resulting from secondary calcite precipitation. The main subzones were further subdivided by a recent geological study. A total of thirty-six layers exists in the geological model. Production from the field commenced in December 1982 and seawater injection started in early 1984 soon after the pressure depletion was observed. The field was developed with five spot patterns with 2.8 km spacing between the injectors and producers. Pattern size was later reduced to 1.4 km spacing by infill drilling and conversion of producers to injectors.