Overcoming Geological and Fluid Uncertainty through Real-Time Reservoir Mapping
Authors: Al-Dhaferi Yousef R , Al-Khaldi Nasser , Al-Sultan Athbi , Samaroo Rajeev Brent , Jawad Waleed Ahmed 
Published: May 2016

Reservoir mapping is growing rapidly, especially for complex reservoir systems, due to the global need for enhancing fields' production. For mature fields, the uncertainties in the geological structure and water encroachment in the active reservoirs are two major concerns in field development programs. Both play important roles in reducing the overall hydrocarbon production potential of wells and can result into a drastic recovery drop due to premature drainage of the reservoirs.

Al-Khafji Joint Operations has drilled several wells in its main offshore field targeting a fluvial channel depositional environment that consists of alternating beds of sandstone and shale and reveals high lateral heterogeneity in reservoir position as described by existing log correlations. Moreover, some wells show a water signature within the main hydrocarbon-bearing interval although the addressed water contact is much deeper. These wet intervals are attributed to water fingers and can hardly be correlated throughout the field.

Remote identification of the reservoir characterization and successful wellbore placement relative to the water encroachment would extend the production life for those related producers. Placing and completing horizontal wells within this complex reservoir system without the plan to drill pilot wells poses a particular challenge. The existence of laterally complex heterogeneity will make it very difficult to place the well even a short distance away from an existing offset well.

Deep directional resistivity measurements of up to 100 ft radially from the wellbore have helped to overcome the challenges of placing wells and characterizing the reservoirs within such structural, stratigraphic, and fluid uncertainties and added to the completion strategy. Practicing this technique has helped to eliminate pilot wells without impacting the success of the delivery of these wells, thus increasing the time and cost efficiency of the overall drilling operations. Oil columns ranging between 20 and +80 ft thickness with underlying water contacts were mapped along with reservoir and fault geometry. The reservoir mapping application therefore facilitated improved understanding of the overall reservoir; structure, fluids, and lateral heterogeneity.

Introduction

The main oil-bearing zones in the field occur in the upper and lower parts of the main target formation. The gross productive reservoir section exceeds 1,000 ft true vertical thickness (TVT) in some areas of the target formation in the field, and the stratigraphic analysis indicates that these productive intervals are subdivided into 23 major stratigraphic units: 7 in member 1 and 16 in member 2. The units were identified by systematic vertical changes in bedding architecture occurring above successive, laterally persistent shale.

Both members show coarsening-upward log characteristics and can be interpreted to be an upward-shallowing environment from marine shale (fine sediments) to sandstone or limestone (coarser/porous sediments). Although each of the identified units is reasonably laterally continuous in the field, the thickness of some of the sandstone units varies, sometimes into pinch outs. As the sandstones show three types of log signature, such as fining-upward, coarsening-upward, and cylindrical morphologies, and the members do not yield any marine fossils, the depositional setting is regarded as a tidally influenced fluvial channel environment. Frequent lateral changes in lithofacies characterize these units. Based on the sedimentary facies distribution, it is suggested that the fining-upward and cylindrical log patterns in the sandstone correspond to channel facies, and that coarsening-upward log profiles are indicative of crevasse splay sands within the channel thalweg (Beydon 1988; Alsharhan and Nairn 1997).

Fig. 1 illustrates the lateral heterogeneity of the various members across the main formation targets in the field, members 1 and 2.