This paper summarizes the conceptual development, reservoir simulation, and field testing of a new miscible EOR recovery technique at Prudhoe Bay. Two distinct variations of this technique have been evaluated.

The vertical Miscible Injectant Stimulation Treatment (MIST) process involves completing a production well at the bottom of a thick, continuous, watered-out interval. A large slug of miscible injectant (MI) is injected, followed by a small slug of chase water. The MI sweeps rock not contacted by previous MI injection and drives EOR oil to offset producers. It also creates a mobilized oil bank in the vicinity of the production well. The well can then be recompleted at the top of the reservoir and returned to production service, or used for some other purpose.

It is particularly advantageous to utilize this vertical drive process in conjunction with horizontal wellbores. In the lateral MIST process, a horizontal lateral is drilled along the base of the reservoir from either a production or injection well. MI is injected sequentially into several intervals along the lateral to mobilize EOR oil from previously unswept areas. The lateral well is then returned to normal production or injection service.

Simulation data indicate that the vertical MIST process could recover over 200 Mstb of incremental oil per well at initial rates of up to 1000 stb/D. Calculations for the lateral MIST process indicate potential recovery of more than a million barrels of incremental EOR per lateral.

Field results of the MIST process have been mixed, but overall are encouraging. Incremental EOR response from the first MIST lateral well is currently about 1400 Mstb, resulting in a technical and economic success. Response from three subsequent MIST lateral injectors varied widely. The first vertical well MIST showed little response, but it did provide valuable insight into the performance of the process at Prudhoe Bay. The subsequent vertical well MIST, which had a much larger volume of injected MI, was very successful and has currently recovered over 600 Mstb. Cumulative incremental EOR from the MIST wells through July, 2000, is over 5000 Mstb, with average incremental oil production rates over the past four years of about 4000 stb/D.

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