Abstract

In steamflood heavy oil reservoirs, early steam breakthrough occurs mainly due to gravity override of the steam through the upper parts of the reservoir and channeling through the high permeability zones. Poor injection profiles may indicate steam gravity override or channeling that can lead to early steam breakthrough to the producing wells and cause poor sweep efficiency. The sweep efficiency of steamflood projects can be improved by modifying the injection and/or production profiles. In the Duri steamflood, several steam breakthrough zone isolation methods have been tried, both in producers and injectors. In the producing wells, one of the following three methods is usually used: installation of blank casing straddling the breakthrough zone, cement squeeze in the open hole, and spotting micro matrix cement in front of the steam breakthrough zone from inside the screen liner using double cup packers looking down. In the injection wells, one of the following methods is usually used: squeeze cementing after pulling out the injection assembly, squeeze pack using gravel pack sand and Silica Flour (SiF), injection of high temperature polymer, and squeeze cementing through tubing using contaminated cement. Results of the jobs were evaluated to identify best practices. Optimization is then made to determine the proper method to be applied on a particular well condition in order to improve overall job success ratio.

Introduction

The Duri steamflood project, located in Duri field in Central Sumatra, Indonesia is currently the largest thermal enhanced oil recovery project in the world. More than 4000 wells have been drilled so far from a total of 5700 wells planned when all of the areas are developed in 2015. Presently, the production from the field is about 300,000 BOPD with steam injection of about 1,250,000 BCWEPD. The field is approximately 18 km long by 8 km wide with developable area of about 20,000 acres. Structurally, the Duri field is faulted, asymmetric anticline with the degree of complexity increasing westward near the Sebanga fault that bounds the field. Duri field reservoirs occur in sands of Duri, Bekasap, and Bangko formations of early Miocene age. The reservoir are found at drilled depth from about 300 feet to 750 feet at oil-water contact (Fig. 1).

Most of the production from the field comes from Pertama and Kedua sands. These sands generally consist of sand shale sequences. Most of the producers are completed in all sands. With a few exception, generally the injectors are completed in multiple layers, too. Because of this configuration, early steam breakthrough (BT) in a certain layer due to higher permeability, will result in a lower vertical sweep efficiency. Consequently, a significant fraction of the initial oil in place is bypassed. Reservoir simulation study shows that incremental oil recovery after 1.5 pore volume of steam injection (PVI) is minimal.

Furthermore, reducing or terminating the injection of steam into the steamed-out layers and diverting the steam to non-breakthrough layers is the most effective means to improve oil recovery. The economic evaluation of the results shows that starting the rate reduction at 1.0 PVI yielded the greatest value.

Operationally early steam BT also causes serious problems. The excessive steam production, carrying solid, from the breakthrough layer often causes damage to the gravel pack, the screen liner and the production tubing. Also, it often creates severe operating difficulties at the surface facilities as well. Excessive casing pressure and temperature in producing wells, because of the early steam BT, frequently causes killing problem during workover and also results in high casing effluent rate. High casing effluent rate (casing blow) in the producing well can lead to the installation of a choke on the casing vapor collection system (CVCS) line. Although the chokes are an operational necessity, their installation also causes back pressure to the entire sands which reduce production significantly. Production from this casing-tubing annulus (CVCS) through misting and slugging contributes about 5-20% of an area's oil production.

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