Production Rate Forecasting Model
PROCAST© provides the tools for predicting rate performance and optimizing field-wide production. PROCAST© facilitates ease of data entry and provides a capacity to track a variety of development scenarios. Also, the program uses system analysis, material balance concepts, and reservoir and well data to forecast reservoir performance. It gives flexibility in scheduling, grouping, and tracking reservoir and well data. Lastly, it illustrates predictions.
PROCAST© is based upon a simple rate forecasting model using deliverability data and a tank reservoir model.
Where PROCAST© differs from what can easily be done in a spreadsheet, is in its handling of complex and extensive gathering systems, and detailed deliverability relationships. Coupled with real-time network and well constraints, the program will provide a flexible and powerful forecasting tool for evaluating facility modifications, production allocation schemes, or even simple well forecasting with minimal data input.
In its simplest form, forecasting the production of a reservoir consists of merging the data from a two independent relationships: the pressure-cumulative production relationship for the reservoir and the well deliverability characteristics. Neither of the two has a component of time, but used together, the synthesis of a rate-time plot is possible.
Beginning with the initial reservoir pressure, a corresponding p/z value is calculated. Using a defined p/z vs. cumulative gas production trend or simply the OGIP, the reservoir pressure at any cumulative gas production can be found. Starting with the initial reservoir pressure, and some assumed flowing tubing pressure, a deliverability value for the well, Q, is calculated. Making an arbitrary time step will produce the well at the pre-defined rate, and deplete Q*time reserves from the reservoir. With the new calculated cumulative gas production, the new reservoir pressure can be calculated from the depletion (p/z) curve.
Following the same (circular) logic, the new reservoir pressure will be lower than the original reservoir pressure and the well deliverability will decline concurrently. Using this new deliverability, the process starts over and a second time step is taken. The current version of the program does not expand upon this forecasting methodology with any of the numerical solution techniques that are available. Any improvement in accuracy will come with a cost in computation time. Release version 1.0.71b takes an intermediate approach by taking a maximum internal time step of one month, regardless of the actual total time step requested.
As program usage increases, improved solution methods can be implemented if the current methodology proves inadequate. In addition to the primary phase, condensate yield vs. pressure curves for each reservoir can be used to predict liquids deliverability. In contrast with the prior GASMAN program, the interpolation routines in PROCAST© are very stable and will provide a value based on a two dimensional surface interpolation using the Lagrange 3 point methodology for internal points to the solution and a two point extrapolation when a value outside the defined range is requested. With a sure value comes possible erroneous values, so negative rates are eliminated and a color coding scheme is used to indicate if a deliverability value is independent of surface pressure (RED) or outside the bounds of the defined deliverability curves (BLUE).