Reservoir/Well Repair and Environmental Impact Management of Surface Discharge at Cold Lake

Abstract

Summary In Dec. 1988, a 1000-m3 [6.300-bbl] surface discharge of bitumen was discovered near an abandoned oil-sands evaluation well at Cold Lake, Alta. The discharge was initiated when high pressures in the Clearwater formation breached an ineffective cement seal in the abandoned wellbore. This paper focuses on the successful reservoir repair of the well and the effective management of environmentally sensitive issues. The technical accomplishments and lessons learned in the cleanup and disposal of the surface discharge, reservoir/well repair, and evaluation of groundwater contamination are discussed. Introduction On Dec. 8, 1988, a bitumen spill (surface discharge) was discovered adjacent to the abandoned oil-sands Evaluation Well C7-13. The spill covered 1 ha [2.5 acres] of muskeg and was estimated to hold 1000 m3 [6,300 bbl] of bitumen and 2000 M3 [12,600 bbl] of water. Fig. 1 is a diagram of the spill, showing the abandoned well's surface location and the breach. Although several factors contributed to the spill, the main cause was an incomplete cement seal in the abandonment plugs of Well C7-13. Our well problems began in March 1988, when unexpectedly high bottomhole pressures (9 MPa [1,300 psi]) were encountered while coring at the base of the Clearwater formation. While maintaining well control with 2000 kg/m3 [16.7 lbm/U.S. gal] mud, Well C7-13 was abandoned from its total drilled depth to surface by placing three balanced, openhole cement plugs in the wellbore. placing three balanced, openhole cement plugs in the wellbore. Because the cement and mud had the same slurry density and the open hole could not be conditioned before cementing, we believe that at abandonment, channels or voids existed throughout the openhole cement plugs. Upon exposure to high pressures from nearby cyclic-steam-stimulation (CSS) operations, diluted drilling mud eventually was pushed from these voids and channels to form a continuous flow path to surface. The Clearwater formation fluids then flowed up the abandoned wellbore to just below the base of the surface casing, from where they fractured to the surface breach location, 100 m (330 ft) northeast of the Well C7-13 wellhead. The formation fluids intersected three major and continuous aquifers en route to surface. Fig. 2 shows the abandoned wellbore and illustrates the suspected flow path that the fluids took to surface. Note that Well C7-13 was a directionally drilled well, with its bottomhole location displaced slightly more than 50 m [165 ft) from its surface location. The nearest operating wells are more than 330 m [1,100 ft] away. Because vegetation at the site had full seasonal development, the spill is believed to have occurred after Aug. 1988. This observation correlates well with the second cycle steaming of adjacent wells from Aug. 10 to Sept. 14, 1988. Several shallow cores were taken at the spill site to determine the extent of muskeg contamination. In nearly all cases, the bitumen and the water were confined to the surface. The only significant penetration occurred at the breach location, where water and bitumen broke through to surface. This paper focuses on the successful reservoir repair of Well C7-13 and the effective management of environmentally sensitive issues. First, we give background information on CSS operations at Cold Lake, and then we discuss the repair of the abandoned Evaluation Well C7-13, the evaluation of groundwater/aquifer contamination, and the spill cleanup and waste disposal.