Theory and Practice of the Closed-Chamber Drillstem Test Method

Abstract

This paper describes a new method of drillstem testing to provide greater safety, precision, flexibility and secrecy than conventional methods. As testing progresses, the surface valve is closed during flow periods, an surface pressures are monitored to compute gas and/or liquid rate. Theoretical development of pertinent equations is discussed, and a field example is given to illustrate their use. Recommended procedures before and during the test are provided. Introduction The objective of drillstem testing (DST) is to obtain information on pressure, well potential, and fluid content of a reservoir. Accurate pressure data is required for hydrodynamic studies and as a starting point for reservoir studies. Well potential is determined from information on pressures and flow rates as a function of time. Fluid pressures and flow rates as a function of time. Fluid content of a reservoir can be established only if sufficient fluid has been produced to be identified in the drillpipe or caught in the sample chambers. This paper describes the closed-chamber technique with which it is possible to monitor the initial flow period of the drillstem test. It provides good field estimates of gas and liquid flow rates during preflow periods. These rates then can be used to determine minimum flow times necessary for fluid recovery and for presetting surface equipment, should the remainder of the test be conventional. Knowledge of the flow rate is useful for estimating the duration of shut-in time necessary to obtain a satisfactory pressure buildup. In addition, it is possible to calculate pressure buildup. In addition, it is possible to calculate permeability from the initial buildup. permeability from the initial buildup. Description of a Closed-Chamber Test A closed-chamber test is similar to a conventional DST in many ways. The major difference is that in the closed-chamber test, the well is closed in at the surface when producing and open at the surface only when shut in at the producing and open at the surface only when shut in at the formation. Instrumentation for the closed-chamber test is such that fluid influx is monitored throughout the test. Flow rates can be estimated as the test progresses. Flow rates and recoveries can be confirmed upon test completion. Fig. 1 shows the surface equipment used in conventional DST'S. Fig. 2 describes the additional gauge manifold needed at the surface for closed-chamber testing. This gauge manifold is used to observe and to record surface pressures during the test. Fig, 3 depicts subsurface equipment for closed-chamber testing. It is the same as that used for conventional testing, except for a pressure recorder located above the test valve. The pressure record is used to calculate fluid influx during the test and to confirm flow rates upon test completion. Theory Most DST's are run with empty drillpipe. When the test valve is open, gas and liquids enter the drillpipe and flow to the surface. Fig. 4 is a schematic representation of a typical test, where liquids are allowed in, but do not flow to surface. Gas is free to flow in and out of the drillpipe. Under normal operating conditions, gas is vented at the same rate as it enters, and no change of mass occurs in the system. However, if the flow rate of gas being vented differs from the entering flow rate, then the system mass will change according to the following mass balance for single-phase gas flow:(1) where mass in system = pMV/RTz mass rate in = pscMqin/RTsc mass rate out = pscMqout/RTsc JPT P. 1539