Al-Khalij: The Quest For Oil In A Highly Complex Carbonate Field

Abstract

Abstract Al Khalij could be viewed as the archetypal complex carbonate field. Laterally sealed by a stratigraphic closure, the reservoir monocline consists in a layercake of alternating good and poor quality rock whose fabric has been intensively reworked during multiple phases of diagenesis. Additionally, the oil column is relatively thin and average water saturation above free water level exceeds 85%. Al Khalij development challenge can thus be formulated as: "How to efficiently recover a large oil accumulation trapped with much larger amounts of water in the capillary transition zone of a highly heterogeneous reservoir of uncertain boundaries overlying an active aquifer?" To meet a challenge of such magnitude, a phased development was undertaken and completed recently, nine years after kick-off. Even so, the expected recovery factor remained low and the reservoir model unmatched. This paper describes the extensive work program implemented to better understand early-time reservoir behavior and find ways to increase recovery. Starting with a "back to the rocks" approach, a wide range of studies and additional measurements were undertaken, culminating in full field reservoir simulations. Innovative modeling and interpretation techniques were implemented to extract maximum information from formation pressure and pressure build-up measurements. Where key uncertainties remained, specific solutions were sought in terms of enhanced data acquisition and monitoring programs, from petrophysical measurements on full size cores to injection PLTs in oil producers. Integrated static and dynamic syntheses reviewed all resulting information to better assess critical reservoir heterogeneity levels. A specifically designed dual-porosity simulation model was built to properly represent the smallscale heterogeneity impact, and successfully history matched. In less than two years, a full field redevelopment plan was defined that is expected to double the recovery factor. The innovative acquisition, interpretation and modeling techniques developed in the process could fruitfully be applied to other complex fields. Introduction Al Khalij field, operated by Total, is located 110 km offshore Qatar, within the Block 6, with a water depth of approximately 60 m. The field was discovered in 1991 after the interpretation of 2D seismic acquired in 1989, and developed in a phased way through horizontal wells since 1997. The field being marginally eruptive, production wells are activated by a wide range of centrifugal pumps. The increasing knowledge gathered on the field has lead to reassess its potential since exploration time, leading to a stabilized oil production plateau since 2005. Al Khalij reservoir presents some striking features (Ref. 1, 2). It is quite wide (some 250+ km2) and thin (in most areas under 50 m) monocline (Fig. 1). Oil-bearing reservoirs are Cenomanian limestones of the Mishrif formation, capped by Turonian Laffan shales. These carbonates were deposited in a shallow shelf environment and range from lagoonal muddy deposits to Rudist shoal facies. The uppermost part of the reservoir series is partially eroded as a consequence of a regional uplift of the platform northwards. Moreover, the monoclinal trap reveals a stratigraphic component as a lateral seal is provided by muddy deposits developed westwards. The resulting rock structure is a layer cake of "matrix" low-permeability (circa 10 mD) and average porosity (circa 20%) carbonates alternated with "drain" high permeability (in the Darcy range) and high porosity (circa 30%) ones, with pressure communication throughout the different units (Fig. 2). It is filled with fluids mainly in the capillary transition zone, with water above WOC considered mobile (Fig. 3). Finally, in terms of production, it exhibits good well productivities, but a production behavior marked by both a fast water breakthrough and steady pressure depletion in the well's drainage area.