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Abstract

Methods for analysing drawdown and buildup data during reservoir monitoring test sequences are described and illustrated. Reservoir monitoring well tests are characterised by a short shut-in period to run pressure gauges followed by a drawdown- period to run pressure gauges followed by a drawdown- buildup sequence.

A new approach to analysing drawdown test data following a short shut-in period during reservoir monitoring efforts is presented. The method employs a variation of the classical Horner plot technique and is applicable to radial flow and pseudo radial flow systems. The flowing bottomhole pressure immediately prior to running the gauges in the hole can be determined from the plot and thus aid in the analysis of the first shut-in period data. The effect of the ratio of the first shut-in period to the test drawdown period on the final period to the test drawdown period on the final shut-in Horner plot slope is presented. The correct method of plotting the final buildup data along with the error involved by using conventional plotting techniques is discussed.

Introduction

Monitoring of well productivities, flow capacities, skin values and reservoir pressures has been an on-going effort for the past decade in the Greater Ekofisk development. More than two hundred drawdown and buildup tests have been conducted area wide, generally on wells that have been producing continuously for an extended period. The reservoir monitoring sequence used in this effort is illustrated in Figs la and lb and consists of (I) the long drawdown (production) phase at essentially constant rate (DD1), (II) a shut-in period to run and set pressure gauges (BU1), period to run and set pressure gauges (BU1), (III) a test drawdown period with specified flow time at the same choke setting as during production (I), if possible (DD2), and (IV) the final shut-in for a specific period (BU2). The second drawdown period (DD2) usually ranges from 8 to 48 hours depending on the well and is intended to return the well back to its producing state at the end of period I. The final shut-in period (IV) is also for a specified period of time to ensure that data exists on the period of time to ensure that data exists on the semi-log straight line of the Horner plot.

Traditional Horner analysis in conjunction with type curve analysis is used to analyse pressure data from the final buildup period. The following questions have to be answered before the analysis can proceed:

  1. What producing time should be used in the Horner time function?

  2. What would be the percent error in the Horner slope if total producing time is used (Periods I + III).

Two values which can be considered for use in the Horner time are total producing time (period I + III) or the period III duration for producing time. Which producing time to use depends heavily on how much pressure recovery occurred during the first shut-in period. The flowing bottomhole pressure prior to first shut-in, Pw fo, (Figure la) and the prior to first shut-in, Pw fo, (Figure la) and the average drainage-area pressure are needed to determine the amount of recovery. The average pressure might be obtained from a Horner plot of the first period's pressure data but Pwfo, cannot be determined accurately pressure data but Pwfo, cannot be determined accurately since it was not measured; consequently it was decided to use the total producing time (Periods I + III) in the Horner time.

The new approach employing a modified Horner plot in analysing the test drawdown (DD2) yields the flowing bottomhole pressure prior to first shut-in, Pwfo Formation flow capacity and skin can also be determined from the plot. It should be understood that this method applies to systems of moderate to high permeability k >1.0 md and where the flow into the permeability k >1.0 md and where the flow into the wellbore can be assumed to be radial or pseudo-radial in configuration.

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