Gel-cement, a water shut-off system: qualification in a Syrian field

Abstract

Abstract Water production in the North-east of Syria has increased significantly in recent years. As a result costs per barrel of oil have increased and the field's production is currently constrained by the facilities capacity. PLT surveys combined with a reservoir study showed that good-quality sands were not properly swept by the water, probably due to poor connectivity in the reservoir. It was anticipated that these un-swept sands could contribute to production if the watered out sands were shut-off. A newly developed gel-cement has been used to shut-off the watered out sands in a cost-effective manner. The gel-cement system combines the properties of two shut-off techniques:Cement for mechanically strong perforation shut off.Gel for excellent matrix shut off. The gel, used as "mix water" of the cement, will be squeezed into the matrix creating a shallow matrix shut off. The cement will remain in the perforation tunnel as a rigid seal. This system showed superior shut off performance in the laboratory compared to normal cement squeeze techniques. Selective perforation of the hydrocarbon zones will re-establish the oil production. The shut off zones can be re-opened later in the well's life when artificial lift has been installed. In the first field trial 84 meters of perforations (gross) were squeezed of with the gel-cement in a single attempt. After re-perforation of the top and the middle zone the well produced at a strongly reduced water cut, i.e. 25–33% compared to 60–62% before the treatment, and an increased oil production, i.e. 3000 bopd compared to 1000 bopd before the treatment. The oil production declined to 2000 bopd over a year. The water cut gradually increased over that period to 56%. Introduction Water drive, either natural or through water injection, is probably the most important recovery mechanism for oil production from oil-bearing rocks. Consequently, water is produced together with the oil. Generally, oil production decreases with the maturity of an asset while the water production increases. For 1999 the worldwide daily water production, associated with oil production, has been reported as 33 million m3 or roughly three barrels of water for every barrel of oil1. The United States petroleum industry generates 2.4 billion m3 of water annually2. This amounts to an average 7–8 barrels water per barrel of oil. Water production within the Shell Group has roughly increased from 350,000 m3/day in 1990 to over 1,000,000 m3/day, today3. The costs associated with handling produced water are typically proportional to the amount of water produced. Consequently, costs per barrel of oil produced continue to increase with increasing water production. Ultimately, individual wells or complete fields are abandoned when cash flows turn negative because of excessive water production. The heterogeneous geologic nature of most oil reservoirs, however, provides opportunities to prevent or reduce excessive water production. In layered reservoirs water production can be managed by either controlling the injection profile in the injectors (if water is injected) and/or by selectively producing different layers in the producers. In cemented liners, perforating is used to connect the well to additional layers. However, the reverse process is not as straightforward. Different remedial treatment options are available in the industry (see Table 1 for an overview):Cement new liner inside the already perforated liner and selectively re-perforate.Expandable clad4 to selectively shut-off intervals.Expandable composite sleeves5 to selectively shut-off intervals."Self-selective" chemical treatment that is bullheaded into all open perforations6,7.Full blocking gels to selectively shut-off the intervals that produce water by filling the matrix of the rock with a rigid gel9,10.Squeeze cementing of existing perforations and selectively reporforate8. Success rate is 50%. Low success may be attributed to small pressure differential required to re-open the perforation.Sequential application of full blocking gel with cement tail in. Lai et al.12 report economic success rates of 85% of this technique for gas shut-off in Alaska.