Title : Infill drilling in the largest clastic reservoir_ a challenge
Abstract:
Introduction
Infill Drilling is done in Greater Burgan field as in any brown field to drain the reservoir systematically and maximize the Oil recovery. For a brown field, the process of infill drilling is a complex as the well density is high and anti-collision issues. Unlike in a green field, pattern drilling is not the easiest option as surface constraints, structural variations, reservoir heterogeneities, unexpected water movement and differential draining of the reservoirs have to be reckoned with. Greater Burgan field comprises of multilayered reservoir .Eocene Burgan Formation, covering an area of over 1250 sq. km. down to the original OWC.
Geological Setting:
The Cretaceous Burgan Formation of the Greater Burgan Field, Kuwait (Fig. 1) is the largest clastic reservoir in the world. Hydrocarbons are trapped in an enormous doubly-plunging Anticline (65 X 25 km), that has three prominent culminations namely Main Burgan, Magwa and Ahmadi. The hydrocarbon accumulation is four-way dip-closed beneath Middle Cretaceous Ahmadi mudstones and has a common OWC of 4472 ft. TVDSS, while small primary gas caps exist in Magwa and Ahmadi.
To accelerate the production from these clastic reservoirs, a number of horizontal wells have been drilled in previous years. The targeted reservoir sands show a lot of heterogeneity and flow barriers. Predicting sand continuity in such a scenario is a challenge. Moreover, velocity anomalies in the shallow sections and structural complexity in some areas brings in a great deal of uncertainty in the depth prediction.
Burgan Formation has a dominantly fluvial to tidal and shallow marine environment of deposition and the reservoir is segregated in to three major units, Upper, Middle and Lower. Greater Burgan field was discovered in 1938 and put on production since 1946, and Burgan reservoir has the largest share in production.
The annual infill drilling program is driven by production target, which is again market driven. The types and number of wells to be drilled is also governed by the number of rigs and material available. However, the cornerstone for the entire exercise is the geological model of the reservoir and the production potential under given constraints. The reservoir is oil bearing down to a depth of 4472 ft. subsea with initial net pay thickness of over 700 ft. with exceptionally high quality reservoir. Initially an infill drilling program on a grid of 500 meters had been adopted effectively for sweeping the fluvial sands, supported by an active bottom and edge water drive. While the homogeneous sand units are uniformly drained, whereas heterogeneities in the reservoir tend to leave bypassed oil behind. Seismic data helps limitedly in capturing the reservoir heterogeneities, but faults are imaged very well. A full field dynamic model had limited success owing to the massive size of the field, Sector models are however more helpful for planning infill wells. With over 2000 wells in the field, a reasonably good correlation between wells can be established.
