A Full Field Model Study of the East Velma West Block Sims Sand Unit Reservoir
Authors: Bolling John D. 
Published: December 2007

A full-field numerical model of the East Velma West Block Sims Sand Unit (EVWBSSU) reservoir was developed. The three-dimensional, four-component, heterogeneous, 3,200-cell model will aid in the evaluation, analysis, and operation of a CO2 flood now in progress. CO2 is being injected in a continuous mode in the updip portion of the steeply dipping reservoir, A specially portion of the steeply dipping reservoir, A specially constructed grid, detailed reservoir description, and multiple fluid property tables were used to account for the variation in the reservoir's structure, geology, and fluid properties. The present state of the reservoir was properties. The present state of the reservoir was determined after 33 years of production history were matched. Methods of production included primary depiction under gravity drainage, secondary production using gas injection for pressure maintenance, and waterflood. From the history-matched model, field performance predictions were made for continued waterflood and various operating scenarios under the present CO2 injection scheme. Results include the effect Of CO2 pipeline supply rate, allocation Of CO2 and water to injection wells, and uncertain parameters on reservoir performance. From these projections it was concluded that the amount of injected projections it was concluded that the amount of injected CO2 required to produce an incremental barrel of oil over waterflood was not strongly dependent on CO2 pipeline supply rate or allocation of injected fluids to pipeline supply rate or allocation of injected fluids to injection wells. This conclusion seems reasonable mechanistically because the miscible displacement was dominated by gravity, the areal variation in predicted updip oil saturation was not great, and the continuous injection coupled with high permeability updip caused uniform distribution of CO2.

Introduction

The EVWBSSU reservoir was studied using a four- component numerical reservoir model. The steeply dipping reservoir is located in Stephens County, OK, and was discovered and developed during 1949–50. Methods of production during the life of the field included primary depletion under gravity drainage (1949–62), secondary production using gas injection for pressure maintenance production using gas injection for pressure maintenance (1962–72), and waterflood (1972–82). Field recovery through all phases of production history was 32.3%. Currently, the field is being flooded with CO2 and water. A pipeline supply of CO2 from a fertilizer plant in Enid, OK, along with recycled CO2 and gas obtained from a field processing plant, is being injected continuously into seven updip injection wells. The field model was developed as a planning tool to aid in the evaluation, analysis, and operation of the CO2 flood. A history match of the field production data yielded an estimate of the present state of the reservoir:current fluid saturations by tracking gross fluid movement during the life of the reservoir andverification of the reservoir and fluid descriptions.

The most important mechanisms to history match for realistic predictions were the gas/oil gravity drainage and waterflood oil-rate decline. The gravity-drainage calculation gives an estimate of the location and movement of the gas/oil contact (GOC) during the first 23 years of production. This information is important in the sweeping production. This information is important in the sweeping of residual oil with the updip CO2 injection scheme. The waterflood oil-rate decline yields a forecast of how the reservoir would decline under present operating conditions. One can then evaluate how much better performance is with CO2 injection. performance is with CO2 injection. From the results of the history match, simulations to analyze the current updip CO2 injection operations were performed. Deliverability Of CO2 from the Enid fertilizer performed. Deliverability Of CO2 from the Enid fertilizer plant has been up to two-thirds less than anticipated. The plant has been up to two-thirds less than anticipated. The model was used to predict the effect of lower CO2 pipeline supply rates on reservoir performance. Then pipeline supply rates on reservoir performance. Then cases were run to evaluate current operations. Finally, sensitivity studies were performed on uncertain model parameters. parameters. Model Development

A complex and carefully constructed reservoir model was required because of EVWBSSU's steep and variable dip, variation of fluid properties, and heterogeneity. A special grid was constructed to account for the field's structural variation. Multiple tables were used for some fluid properties. A detailed geologic description was constructed properties. A detailed geologic description was constructed from core and log data. Incongruent field data were compiled into a form suitable for history matching.

Grid Construction. Gridblocks were constructed by a method that accounts for both the variation in the field's dip and its thickness. In addition, if it is assumed that the reservoir was deposited in layers parallel to the bedding plane, the resulting grid orientation is such that the plane, the resulting grid orientation is such that the permeability tensor contains only diagonal elements. permeability tensor contains only diagonal elements. JPT

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