A mathematical model of the Hawkins Woodbine reservoir was developed to predict individual tract and total fields reserves. Each of the field's predict individual tract and total fields reserves. Each of the field's 362 fault-block sand members was considered a uniform pressure tank, and contact movement was predicted with volume-depth curves. Adequacy of the model was verified by comparing actual and predicted contact movement rates for pre-unit operations.
The objectives of the model-development phase of the Hawkins Unitization Engineering Study were to estimate individual tract and total primary field reserves and to appraise additional recovery possible with unitized operations. In addition, the Engineering possible with unitized operations. In addition, the Engineering Subcommittee recognized that if it developed a computerized field model to meet those objectives, it would have a tool that later would prove valuable in planning unit operations, including forecasts of production, drilling, and recompletion needs for maximum recovery and reservoir control. The need for the study arose when the Hawkins Unitization Working Interest Owners Committee requested the Engineering Subcommittee to estimate reserves. The owners believed that an acceptable field unit participation formula would allocate primary reserves to each tract plus a fair share of the increased recovery resulting from unitized operations. The Engineering Subcommittee recognized that the Hawkins field (having 45 fault blocks, 10 sand members, 327 tracts, and nearly 500 producing wells) would be difficult to model in necessary detail with a conventional finite-difference simulator. A three-phase, three-dimensional simulator would have been necessary for each of the 362 fault-block sand members, most of which are in partial communication with others. The subcommittee also partial communication with others. The subcommittee also recognized that a highly simplified, total-field modeling procedure probably would not provide the necessary detail for individual probably would not provide the necessary detail for individual tract reserves. Accordingly, the subcommittee chose the middle ground. It developed a model that included the detail of each well and each fault-block sand member, but avoided the detailed fluid-displacement calculations of a three-phase, three-dimensional finite-difference reservoir simulator. The model treats each fault-block sand member as a uniform pressure tank. Dimensionality is introduced with a volume-depth pressure tank. Dimensionality is introduced with a volume-depth curve for each sand member. Some areas of the field produce by water drive, others by gas drive, and some by depletion (solution-gas) drive. Certain areas contain only oil, some have gas and oil, and others contain only gas. To handle this wide variety of conditions, residual hydrocarbon saturations for the various drive mechanisms were input in the model, from which recovery efficiencies were determined. The model calculates new fluid-contact positions at the end of each year, using material balances on calculated fluid influxes and effluxes and volume-depth curves. The remainder of this paper describes the modeling procedure in a general way. There is as little reference as possible to specialized, one-of-a-kind challenges offered by Hawkins field and little mathematical detail. This approach reflects our belief that the modeling procedure we describe generally is applicable to geologically complex, multidrive reservoirs.
In this section, we will describe our modeling procedure aided by a logic flow chart (Fig. 1).