Disposal of Oily Cuttings by Downhole Periodic Fracturing Injections, Valhall, North Sea: Case Study and Modeling Concepts
- Z.A. Moschovidis (Amoco Production Research) | D.C. Gardner (Amoco Production Research) | G.V. Sund (Amoco Norway Oil Co.) | R.W. Veatch Jr. (Amoco Production Research)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1994
- Document Type
- Journal Paper
- 256 - 262
- 1994. Society of Petroleum Engineers
- 5.4.1 Waterflooding, 4.1.2 Separation and Treating, 6.5.3 Waste Management, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 4.3.1 Hydrates, 1.11 Drilling Fluids and Materials, 1.6 Drilling Operations, 4.1.9 Tanks and storage systems, 2.2.2 Perforating, 3 Production and Well Operations, 5.1.5 Geologic Modeling
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This paper presents the results of a pilot study for the injection of cuttings at great depth [8,094 ft total vertical depth (TVD)] in Paleocene/Eocene shale by periodic fracturing injections in two wells in the Valhall field in the North Sea. Injection is necessary to meet environmental requirements (<1 vol% of oil in the cuttings disposed in the sea) that went into effect Jan. 1, 1993. Valhall is an Upper Cretaceous chalk oil reservoir 180 miles offshore southern Norway. Amoco operates the field for Amerada Hess Norge A/S, Elf Petroleum Norge A/S, and Enterprise Oil Norge Ltd. Simulations were performed with a new conceptual model for disposal of cuttings (called the "disposal domain"). The actual pressure response during cuttings injections supports the model predictions. The cuttings are contained within a limited region of the formation (the disposal domain) where a system of hydraulic fractures have been created. The same model then was used to estimate the capacity of the formation to contain cuttings without increasing the treating pressure (and the closure stress) beyond the capacity of the tubulars.
In late 1990, the Norwegian State Pollution Control Agency (NSPCA) announced a tightening of regulations for offshore disposal of drilling cuttings effective Jan. 1, 1993. The allowable oil content of cuttings disposed to the sea was reduced from 6 to 1 vol%, a limit not achievable with existing cleaning technology. With cooperation from NSPCA, Amoco Norway and Amoco Production Research have developed a unique cuttings injection method that appears capable of handling the remaining Valhall development drilling needs. This study shows that the slurrification of cuttings on the Valhall platform (Fig. 1) and their disposal by injection is technically feasible and provides a cost-effective and environmentally safe solution. More than $550,000 is saved per well by cuttings injection compared with onshore disposal.
A multiple, parallel approach was adopted by Amoco in preparation for the changing environmental regulations. The approach included evaluation of (1) transport onshore for treatment, (2) use of non-oil-based muds, (3) use of a rotary kiln for offshore processing, and (4) cuttings injection.
Extensive studies showed that cuttings injection was preferable when environment, safety, and economics were considered. Cuttings injection included two options: deep (tubular injection in or just above the reservoir) or shallow (annular injection at 3,050 ft). We chose deep cuttings injection for several reasons: (1) well availability because of casing collapse, (2) lower preparatory cost, (3) less risk of future shallow drilling problems near the shallow casing-shoe cluster, and (4) a better safety margin for fracturing to surface.
Before investing in this disposal option exclusively, an extensive testing program was initiated to ensure that multiple injections would work. The following were the goals for the extensive testing program.
1. To observe pressure response when fracturing in the Paleocene/Eocene with a heavy, viscous fluid.
2. To develop, test, and calibrate a model suitable for periodic cuttings injections.
3. To test the surface facilities.
Cuttings injection testing in the Valhall field was initiated in three phases (Fig. 2).
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