Assessment of Water Holdup in a Multilayer Carbonate Reservoir

Abstract

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 93480, “An Integrated Approach for Assessment of Water Holdup in a Multilayer Carbonate Reservoir,” by A. Abdelmawla, SPE, C. Dabbouk, D. Urasaki, B. Ateeq, SPE, and A.R. Kutty, Zakum Development Co., prepared for the 2005 SPE Middle East Oil & Gas Show and Conference, Bahrain, 12–15 March. A giant multilayer carbonate reservoir offshore Abu Dhabi has fieldwide lower-permeability stylolite layers between the main producing zones. Interestingly, a water-holdup phenomenon has been observed above stylolite layers, bringing into question the degree of vertical communication between the different reservoir producing zones. This phenomenon can be related to the negative capillary forces in the oil-wet reservoir rock and/or the vertical permeability, kv, across the stylolite layers. An integrated approach is shown for the evaluation of water holdup by investigating both the vertical transmissibility across the stylolite layers throughout the reservoir and the capillary forces caused by oil-wetting characteristics of the carbonate rock. Introduction The subject reservoir structure is an anti-cline faulted by small vertical-throw faults. The reservoir rock is mainly limestone. The producing formation is subdivided into six sublayers (Layers A through F) with stylolite intervals separating these sublayers. Generally, the two uppermost sublayers, Layer A and Layer B, have the highest permeability in the reservoir. Stylolite layers are characterized by higher rock density, low porosity, and low permeability. There is a large degree of heterogeneity between the different sublayers and even within each sublayer. Because of weak aquifer response, the reservoir is produced by waterflooding, using a normal five-spot pattern with current well spacing of approximately 1.4 km. Initially, water was injected from deviated injectors through the whole formation thickness. Because of higher permeability in the upper Layers A and B, water moved faster in these layers, causing water breakthrough at producers. A recent development strategy has been implemented for horizontal re-entry of producers to recomplete them with an approximately 1-km horizontal wellbore in the lower Layers E and F. It has been observed that water is held up above the stylolite layers (in the high-permeability streaks) but slumps slowly downward with time. Several time-lapse saturation logs show this interesting phenomenon. Investigation of Water Holdup The reservoir section is interbedded with five major stylolitic intervals that are correlatable fieldwide (S1 through S5). These intervals appear at the same stratigraphic position in other onshore and offshore Abu Dhabi fields, suggesting a regional significance, possibly indicating events that were essentially synchronous over very extensive areas. Stylolites in the reservoir indicate chemical dissolution of calcite at depth (pressure solution), usually as a result of overburden loading. The stylolite is the irregular, black film or seam that represents the insoluble or relatively less soluble residue resulting from the dissolution of the limestone. Depending on the composition of the host limestone, the stylolite residue mineralogy comprises quartz, dolomite, pyrite, bituminous and organic residues, and various clay minerals.