CO2 Minitest, Little Knife Field, ND: A Case History

Abstract

SUMMARY A joint DOE-Gulf Oil Corporation Minitest of CO2 miscible flooding was conducted in the Mission Canyon formation of the Little Knife Field, North Dakota. In the five-acre minitest area, a central injection and three observation wells were drilled to form a non-producing, inverted four-spot pattern. A 1:1 CO2 WAG injection sequence (preflush water injection, five alternate slugs of CO2 and water, and drive water injection) was implemented. Prior to the CO2 minitest, a detailed reservoir description of the test area was developed through logging, pulse testing and core analysis. Extensive laboratory work had been performed to determine miscibility pressure, swelling and viscosity reduction effects. The minitest included continuous bottom-hole pressure measurement in all test wells and time-lapse logging to monitor saturation changes as alternate slugs of water and CO2 passed the observation wells. Fluid samples from the observation wells were collected periodically to check for tracers and fluid composition. At the conclusion of drive water injection, a fifth well was drilled behind the CO2 front and pressure-cored to measure the residual oil saturation to the CO2 flood. A Compositional Simulator was used to history match the minitest performance and to evaluate the displacement and sweep efficiency in the project area. Reservoir simulation models were used at various stages of the minitest: (a) to determine the volume and rate of water injection required to raise the reservoir pressure above the minimum miscibility pressure, (b) to calculate CO2 slug size, WAG ratio and project life, (c) to select the location of the fourth observation well behind the CO2 front, (d) to predict breakthrough times and concentrations of CO2 and water to decide when to log wells and collect fluid samples, (e) to scale the minitest reservoir description to a typical 160-acre, five-spot pattern for predicting recoveries for a waterflood and the CO2 WAG process, and (f) to perform sensitivity studies for studying the effects of layering, vertical communication and other process parameters.