Abstract

A systematic and efficient approach for well calibration against wellhead pressure has been developed and verified for two reservoir simulators. The new approach integrates the producing well conditions into the flow table generation step so that the well productivity can be matched while generating the flow table. The main advantage of this approach is that most of the calibration effort is spent on individual wells, which requires less computational effort. Once the appropriate flow tables are generated, it only takes a few reservoir simulation runs to calibrate all the wells in the model. The computer applications based on this approach yield significant cost savings by reducing the project time from weeks to days, and by improving the quality of the well calibration.

Introduction

Well calibration after a successful history match is an essential but tedious process in reservoir simulation. This procedure ensures that the production wells will give realistic rates during the prediction phase. Hence, it provides reliable predictions on when and how many wells have to be drilled to maintain a field production target rate, and the ultimate recovery from existing wells. The well calibration process is crucial in determining the economics of a recovery scheme.

There are two types of well calibration. The first type is calibration against flowing bottom-hole pressure (FBHP). The flowing bottom-hole pressure for each well was specified and the PI multiplier was adjusted until the simulation rate matches with the measured rate. This procedure is straightforward and will not be described here. The second type is calibration against wellhead pressure (WHP), which is a lot more complicated than the first type because wellbore hydraulics is involved. The procedure for calibrating against WHP is discussed in this paper.

There are several procedures for well calibration in Saudi ARAMCO, and different procedures are used for different simulators. The most vigorous procedure is done manually and requires many trial-and-error reservoir simulation runs. Typically it takes about 2 months to calibrate a model with 400 to 500 wells.

This paper describes a new approach for calibration against WHP, which integrates the producing well conditions into the flow table generation step. One of the advantages of this approach is that the calibration is performed on individual wells, which requires less computational effort. After the appropriate flow tables are generated, it only takes a few reservoir simulation runs to calibrate all the wells in the model. This new approach has been implemented in the ECLIPSE1 and CHEARS 2 reservoir simulators.

Calibration against Wellhead Pressure

Experience shows that well calibration against WHP is much more difficult than calibration against FBHP. A set of best-estimate flow table curves (outflow performance curve) does not guarantee an intersection between the outflow performance curve at flowing conditions and the inflow performance curve. Even if there is an intersection, the simulated PI could be very different from the measured PI.

To overcome these problems, a new approach was devised to integrate the well flowing conditions calculated in reservoir simulation, with the flow table generation using a facility simulator. The procedure is briefly described as follows.

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